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General Discussions => Drawing Board => Topic started by: sonolink on November 28, 2018, 07:48:47 AM

Title: Help with modifying Circuit please
Post by: sonolink on November 28, 2018, 07:48:47 AM
Hello everybody

I am trying to modify a guitar preamp circuit, adding some switches to create different distortions or channels.
This is the original schem by GrindFX:

(https://i.postimg.cc/xJctj52p/STM800.jpg) (https://postimg.cc/xJctj52p)

This is my modded schem:

(https://i.postimg.cc/G8wH9hw0/Modded-Schem.jpg) (https://postimg.cc/G8wH9hw0)

I've added True Bypass Switch and Clean/Crunch/Lead Channels.

My main questions are about the 22n Caps connected to the plate resistors:
1-should they go before the switch (as C3) or after (like C5)?
2-should C13 go after SW4.B? SW4 is supposed to allow the player to choose to place the preamp in front of an amp (through 100k pot) or in its FX loop (through Presence circuit and 1M pot)

Plate voltages are 190V unloaded.
Thanks in advance for your time and help
Cheers
Sono

Title: Re: Help with modifying Circuit please
Post by: sonolink on November 30, 2018, 08:11:20 AM
pretty please.....
Title: Re: Help with modifying Circuit please
Post by: dmp on November 30, 2018, 10:27:33 AM
My main questions are about the 22n Caps connected to the plate resistors:
The 22n caps are there to block DC voltage. You need to understand and analyze the circuit before you add switches. For several reasons, like safety (you probably don't want 190V on a switch) and whether the switch will pop and blow your speaker from a large DC voltage jump. Generally you want the switches to be where the DC voltage is 0v.

You have a triode gain stage (v1a v1b) and a DC coupled cathode follower (V2). The triode gain stage has  a high voltage on the plate and uses the 22n cap to block that voltage from the grid of the next stage.  The DC coupled cathode follower, however, uses the plate voltage of the 1st triode to bias the 2nd stage, with the output off the cathode. 
You can read about these basic circuits and understand the voltages at the different plates.

With SW3 you are bringing the cathode follower a stage earlier  and switching a pretty high voltage. That looks like a bad idea.

I'd suggest you read a lot more about tube gain stages before building anything. There are a lot of amps with switching so search for schematics for ideas of how to do it safely. And it might not be a bad idea to work on a low voltage transistor pedal first instead of tubes. Once you get that working, move on to tubes.
Title: Re: Help with modifying Circuit please
Post by: sonolink on November 30, 2018, 10:52:08 AM
Hi Dmp

Thanks for taking the time to check the schems and for your reply :)

I have built many tube amps and pedals, although this is the very first time I try to mod one. 
I am generally very cautious (you're never safe enough) and love electronics. I wish I knew more about it although I try to learn as much as I can. That's the reason why I post here. I have learned quite a bit from this forum :)
I will check through everything you say and try to come up with a solution or a different approach.

With SW3 you are bringing the cathode follower a stage earlier  and switching a pretty high voltage. That looks like a bad idea.
What is the bad idea? Bringing the cathode follower a stage earlier, switching a pretty high voltage? Or both? :)
If high voltage is the problem wouldn't a 250VDC capable switch be a solution?
I suppose you are aware of this, but the idea behind this switch is to have different levels of distortion, of course

Again, I appreciate your time and help :)
Title: Re: Help with modifying Circuit please
Post by: dmp on November 30, 2018, 11:04:33 AM
What is the bad idea? Bringing the cathode follower a stage earlier, switching a pretty high voltage? Or both? :)
I suppose you are aware of this, but the idea behind this switch is to have different levels of distortion, of course
I think the DC coupled cathode follower should be left alone, since I'm don't think you could switch the DC without putting high voltage on the switch and a loud pop. This means leaving V2A & B unswitched.
If you want to reduce gain, you would probably want to switch out the gain stage, v1a perhaps. But you should do some research as I haven't built an amp that does switching out gain stages.  There are a lot of schematics around that have switching in them. You just need to do a lot of research.

For instance, if you scroll down the page here:
https://el34world.com/Hoffman/files/Hoffman_Plexi50.pdf (https://el34world.com/Hoffman/files/Hoffman_Plexi50.pdf)

You'll see a 'hot switch' schematic that switches an extra gain stage in and out of a plexi style preamp. Notice the DC coupled cathode follower is left alone and the relay switch operates on places of the circuit that are at 0v DC
Title: Re: Help with modifying Circuit please
Post by: sonolink on November 30, 2018, 11:24:28 AM
Thanks a lot for the link!! I'll look into that idea and see what I come up with :)
Thanks again! ;)
Title: Re: Help with modifying Circuit please
Post by: sonolink on November 30, 2018, 02:12:10 PM
How about this?
(https://i.postimg.cc/pmJRVHfr/Angry-Boy-Hoffman.jpg) (https://postimg.cc/pmJRVHfr)

Is that better? Maybe I'm missing out some DC blocking cap? Thanks again for that link. Great piece of document, layout included :)

Cheers
Sono
Title: Re: Help with modifying Circuit please
Post by: dmp on November 30, 2018, 02:25:28 PM
The switches look better for DC voltage issues.

Do you expect to be able to get similar levels as you switch?  Because I'm not sure you'll be able to do that.

And do you want the tone stack to only affect the 'dirty' channel? Seems you might like a tone stack on the clean channel as well. fwiw.

Title: Re: Help with modifying Circuit please
Post by: sonolink on November 30, 2018, 05:40:27 PM
The switches look better for DC voltage issues.
Excellent! Thanks 8)

Do you expect to be able to get similar levels as you switch?  Because I'm not sure you'll be able to do that.
The Gain Pot controls both "dirty" stages so I don't know how much difference there will be between the 2 but if the difference is HUGE maybe it could be compensated with some fixed resistor? Just a guess...
I would hope it to be the usual difference between a crunch and a lead channel, but I won't know until I fire it up I guess :)

And do you want the tone stack to only affect the 'dirty' channel? Seems you might like a tone stack on the clean channel as well. fwiw.
Oooooops, thank you for pointing that out! Geez... I'm not at home so I can't redraw the schem right now but I guess I should place the tone stack between SW3 and SW2 right?

Thanks again for your time and help mate ;)
Cheers
Sono

Title: Re: Help with modifying Circuit please
Post by: sonolink on December 01, 2018, 04:49:54 AM
Like so?

(https://i.postimg.cc/kBYWM3gq/Angry-Boy-Hoffman-Tone-Stack.jpg) (https://postimg.cc/kBYWM3gq)

Cheers
Sono
Title: Re: Help with modifying Circuit please
Post by: sonolink on December 01, 2018, 01:41:57 PM
I was thinking of using this bypass switch for all the switching except the impedance matcher (SW2) but I just realized I won't be able to use it to switch from Clean to "Dirty", right? :(

(https://i.postimg.cc/7b7pRWzL/Opto.jpg) (https://postimg.cc/7b7pRWzL)

If so will a Stomp DPDT switch make noise when switching and is there a fix for that? It has to be a Stomp switch because this thing is to be built inside a pedal...

Cheers
Sono
Title: Re: Help with modifying Circuit please
Post by: PRR on December 01, 2018, 08:21:36 PM
3P2T switches are now common in stomp-world.
Title: Re: Help with modifying Circuit please
Post by: sonolink on December 02, 2018, 05:26:19 AM
Thanks for your input PRR.

3P2T switches are now common in stomp-world.

Thanks for your input PRR.
I'm aware of that, but won't I have popping noises?
I guess it's not possible to use a DPDT for that unless I give up the LED right?

Cheers
Sono

Title: Re: Help with modifying Circuit please
Post by: PRR on December 02, 2018, 03:57:11 PM
> won't I have popping noises?

Popping is a very different problem, not about the number of poles.

In any case, the total cost is like buying a lunch. Build it, try it, don't over-think in advance.
Title: Re: Help with modifying Circuit please
Post by: sonolink on December 02, 2018, 05:56:08 PM
Popping is a very different problem, not about the number of poles.

Sorry for the misunderstanding, I didn't mean it like that. It was 2 different questions really. Just trying to learn and understand.
Anyway I'll build it and see if it pops or not. Thanks.

Cheers
Sono
Title: Re: Help with modifying Circuit please
Post by: dmp on December 03, 2018, 11:02:14 AM
If you want the same 'plexi' sound in the dirty and clean channels, then the tone stack should be driven by the cathode follower in both channels.
Title: Re: Help with modifying Circuit please
Post by: sonolink on December 03, 2018, 01:24:00 PM
If you want the same 'plexi' sound in the dirty and clean channels, then the tone stack should be driven by the cathode follower in both channels.

Ok. So I've read a bit about what a cathode follower is and I think I see what you're saying. If I understand you correctly, basically in the schem as it is, the tone stack when in Clean channel, is driven by V1A Anode, whereas when in Dirty channel, it'll be driven by the Cathode of V2B.

According to what I've understood from my reading, a cathode follower is a unity-gain stage placed between the preamp stages and the tone stage to buffer them from each other. It provides the ability to source extra current to drive the tone stack and won't add any gain (since it's unity gain). Originally I took the "Clean Channel" from the Matchless HotBox: http://diy-fever.com/wordpress/wp-content/uploads/2009/12/tube_reactor.gif
In that schem the design gives up the tone stack for the Clean channel, but as I'm a complete ignorant in designing I didn't even realize that. Thanks to you I know a tiny bit more about the amps I build for a hobby :)
My first thought was to place the Gain pot after V1A cathode, but something told me that's wrong and I should look at things the other way around. So, I imagine the switching should occur in a different place to keep the tone. How about this? I've added a cap before V2B's grid to block any DC coming from pin1. Am I right?

(https://i.postimg.cc/qg2gGTpM/Angry-Boy-Hoffman-Tone-Stack-2.jpg) (https://postimg.cc/qg2gGTpM)

Cheers
Sono
Title: Re: Help with modifying Circuit please
Post by: dmp on December 03, 2018, 03:03:30 PM
That circuit uses a DC coupled cathode follower.
Title: Re: Help with modifying Circuit please
Post by: sonolink on December 03, 2018, 03:15:52 PM
Sorry, I don't understand your reply :(
Title: Re: Help with modifying Circuit please
Post by: dmp on December 03, 2018, 03:24:37 PM

Google "DC coupled cathode follower"
1st result: http://www.valvewizard.co.uk/dccf.html (http://www.valvewizard.co.uk/dccf.html)

Read that page, you'll see the in the image that a DC coupled cathode follower uses two triodes. You can't split them and expect the circuit to work.

Title: Re: Help with modifying Circuit please
Post by: sonolink on December 03, 2018, 03:45:26 PM
Ok. I'll read the article and report back ;)

Cheers
Sono
Title: Re: Help with modifying Circuit please
Post by: sonolink on December 03, 2018, 07:25:34 PM
Ok. I've read that page and "sort of" got the idea but I can't figure out a real solution to have the tone stack for both clean and dirty channels driven by the DC coupled cathode follower...the only things I can think of is using V1A as gain stage of V2B but I don't know if that's possible and would imply quite a bit of arrangement or using V2A instead of V1A in the Clean Channel.....
Anyway, I came up with this which is probably wrong because of the Clean pot placement before the stage but I can't come up with more ideas right now :)
(https://i.postimg.cc/23gr84SK/Angry-Boy-Hoffman-Tone-Stack-3.jpg) (https://postimg.cc/23gr84SK)

Maybe using a 4P3T footswitch would allow to switch in the Clean Pot (between the Cathode Follower and it's gain stage) but unless there really is no other way I'd rather look for some other option.

Some hints/leads would be seriously appreciated :)

Cheers
Sono
Title: Re: Help with modifying Circuit please
Post by: sonolink on December 04, 2018, 03:35:47 PM
Some help pretty please.....

I found another similar schem but the clean channel is the same as my first one...
http://guitar-gear.ru/wp-content/uploads/2016/04/Hot_Box_scheme.jpg
Title: Re: Help with modifying Circuit please
Post by: sonolink on December 05, 2018, 04:00:24 PM
Bump...
Title: Re: Help with modifying Circuit please
Post by: sonolink on December 08, 2018, 06:27:11 AM
Ok, does this make sense?

(https://i.postimg.cc/ZCQT4j57/Angry-Hoffman-Tone-Stack-Switched.jpg) (https://postimg.cc/ZCQT4j57)
Title: Re: Help with modifying Circuit please
Post by: abbey road d enfer on December 09, 2018, 03:18:35 AM
 I have to say the way the tone stack is loaded with a 25k presence pot is quite surprizing. However, it seems it should do what it is intended to do, except it attenuates the signal by about 15dB. Is it intended? I don't know...
Now, SW3 bypasses one stage and there is no way to compensate the gain difference. I would think a pot at the output of V1b would be desirable.
Also SW4A1&B1 switch the grids, which don't have a grid leak, so you can expect a huge pop when switching.
Title: Re: Help with modifying Circuit please
Post by: sonolink on December 09, 2018, 05:49:25 AM
Abbey Road,

First of all, THANKS for taking the time to examine the schem and help me out on this one :)

I have to say the way the tone stack is loaded with a 25k presence pot is quite surprizing. However, it seems it should do what it is intended to do, except it attenuates the signal by about 15dB. Is it intended? I don't know...
This circuit is essentially a preamp in a stompbox that can be put in front of the preamp of an amp. In such position it is going to add many gain stages in front of however many gain stages the amp already has and it’s going to be LOUD. OR it can be plugged into a power amp or into the return of an amp’s loop (bypassing the internal pre)...

This is where the presence control comes into play: it will probably be a bit bright when used as a stomp. The presence trimmer can be used to dial this back. As a pre the power stage is likely going to deal with the brightness of the pre so it can be turned up or just tweaked to taste.

BTW, I just realized that SW4.A should probably be placed AFTER the Presence trimmer (since its function is to choose the position of the circuit) and both trimmer and switch can be set according to the position choosed (in front of an amp's preamp or in its FX loop) as a "set and forget" setting. Am I right?

Now, SW3 bypasses one stage and there is no way to compensate the gain difference. I would think a pot at the output of V1b would be desirable.
Ok. I thought R18 would compensate for gain as a 1M pot half way. Maybe I should convert it into a 1M pot then? Or the whole R17/R18/C16 cluster is wrong?  ???

Also SW4A1&B1 switch the grids, which don't have a grid leak, so you can expect a huge pop when switching.
Ooops, maybe I should have put C1/22nF-R4/1M-R3/1K between SW4.B1 and V2A's grid? Like so:
(https://i.postimg.cc/YvFqsDWH/Grid-Leak.jpg) (https://postimg.cc/YvFqsDWH)

And replicate C1/22nF-R4/1M-R3/1K between SW4.A1 and V1A's grid?

Again, I really appreciate your time and help :)
Cheers
sonolink
Title: Re: Help with modifying Circuit please
Post by: abbey road d enfer on December 09, 2018, 07:49:17 AM
This circuit is essentially a preamp in a stompbox that can be put in front of the preamp of an amp. In such position it is going to add many gain stages in front of however many gain stages the amp already has and it’s going to be LOUD. OR it can be plugged into a power amp or into the return of an amp’s loop (bypassing the internal pre)...

This is where the presence control comes into play: it will probably be a bit bright when used as a stomp. The presence trimmer can be used to dial this back. As a pre the power stage is likely going to deal with the brightness of the pre so it can be turned up or just tweaked to taste.
I understand all that, but my concern is that when you bypass it (going through the pot IMP-Matcher) there is a possivle huge level bump.

BTW, I just realized that SW4.A should probably be placed AFTER the Presence trimmer (since its function is to choose the position of the circuit) and both trimmer and switch can be set according to the position choosed (in front of an amp's preamp or in its FX loop) as a "set and forget" setting. Am I right? [/quote]


Quote
I thought R18 would compensate for gain as a 1M pot half way. Maybe I should convert it into a 1M pot then? Or the whole R17/R18/C16 cluster is wrong?  ???
Gain around V1b is about 8-9. If you want to compensate, you need a divider of about the same. I would say 470k/68k. Remember that an audio pot at mid-rotation attenuates by about 20dB (10:1).

Quote
Ooops, maybe I should have put C1/22nF-R4/1M-R3/1K between SW4.B1 and V2A's grid?
Yes, but you don't need C1.

Quote
And replicate C1/22nF-R4/1M-R3/1K between SW4.A1 and V1A's grid?
Yes, and again you don't need the cap.
Title: Re: Help with modifying Circuit please
Post by: sonolink on December 09, 2018, 09:44:50 AM
I understand all that, but my concern is that when you bypass it (going through the pot IMP-Matcher) there is a possivle huge level bump.

You're probably right but that doesn't really worry me. The idea of that switch is to:
1-decide to use the stompbox in front of an amp OR in front of a Poweramp.
2-Put the switch in the appropriate position
3-Switch on stompbox and amp
4-Adjust Presence pot to taste
5-Play :)

Hence my comment "I just realized that SW4.A should probably be placed AFTER the Presence trimmer (since its function is to choose the position of the circuit).

If you decide to change the position of the stompbox you would have to switch the amp off anyway to avoid bumps.
It's a "set and forget" thing. It's purpose is not to switch while playing.

I will correct the schem according your suggestions and report back.

Thanks again a LOT for your input Abbey :)
Title: Re: Help with modifying Circuit please
Post by: abbey road d enfer on December 09, 2018, 12:24:03 PM
Have you really tested the circuit with the presence pot?
My sim shows that the Treble control acts as a "rocker" between bass and treble, which may be acceptable, but teh Bass control acts more or less like a switch, with the variation concentrated on the first percent of rotation.
IMO this circuit should be placed at the anode of V2A, with a 500k pot and a 1nF cap.
Title: Re: Help with modifying Circuit please
Post by: sonolink on December 09, 2018, 01:44:47 PM
Hi Abbey

Have you really tested the circuit with the presence pot?
Not yet. I was about to build a proto when DMP forum member brought up some points that I corrected. Initially the Clean channel didn't have a tone stack, like the original circuit schem:
(https://i.postimg.cc/xJctj52p/STM800.jpg) (https://postimg.cc/xJctj52p)

My sim shows that the Treble control acts as a "rocker" between bass and treble, which may be acceptable, but the Bass control acts more or less like a switch, with the variation concentrated on the first percent of rotation.
IMO this circuit should be placed at the anode of V2A, with a 500k pot and a 1nF cap.
You mean the whole tone stack or just the Bass control?
Not that I'm sure this is of any use, but the circuit is supposed to be a replica of the Marshall Plexi, and its tone stack is a copy of the Plexi like this one for instance:
https://el34world.com/Hoffman/files/Hoffman_Plexi50.pdf
I'm not meaning to say you're not right, just saying that it seems to be a known circuit and that it sounds great. Of course that doesn't mean it's right, that the bass control is actually "usable" or can't be improved... :)
Maybe I should build it and then report back? 8)

Cheers
Sono
Title: Re: Help with modifying Circuit please
Post by: abbey road d enfer on December 09, 2018, 06:17:44 PM
just saying that it seems to be a known circuit and that it sounds great. 
Indeed, but the Marshall/Fender tone stack is meant to be loaded with a rather high impedance, typically 1 Megohm. You load it with 25 k. That makes it behave badly.
Title: Re: Help with modifying Circuit please
Post by: sonolink on December 10, 2018, 06:38:00 AM
Indeed, but the Marshall/Fender tone stack is meant to be loaded with a rather high impedance, typically 1 Megohm. You load it with 25 k. That makes it behave badly.

I see what you mean....that's no good. I downloaded Duncan's Tone Stack Calculator and loaded the Marshall Tone Stack. When I input 25K as Load Resistance (I understand that that would be the Presence trimmer) I see what you mean by bass control acting as a switch. I think thta originally GrindFX put that 25K trimmer there to be able to adjust the brightness produced when the stompbox is in front of an amp's preamp but then again, maybe that's not the best approach?
Any suggestions? Or maybe I should just build a proto and see?

Cheers
Sono



Title: Re: Help with modifying Circuit please
Post by: abbey road d enfer on December 10, 2018, 07:21:15 AM
I see what you mean....that's no good. I downloaded Duncan's Tone Stack Calculator and loaded the Marshall Tone Stack. When I input 25K as Load Resistance (I understand that that would be the Presence trimmer) I see what you mean by bass control acting as a switch. I think thta originally GrindFX put that 25K trimmer there to be able to adjust the brightness produced when the stompbox is in front of an amp's preamp but then again, maybe that's not the best approach?
Any suggestions? Or maybe I should just build a proto and see?

Cheers
Sono
I already answered that: "IMO this circuit should be placed at the anode of V2A, with a 500k pot and a 1nF cap."
Title: Re: Help with modifying Circuit please
Post by: sonolink on December 10, 2018, 07:38:47 AM
I already answered that: "IMO this circuit should be placed at the anode of V2A, with a 500k pot and a 1nF cap."

You did indeed, sorry :)
I'll draw it and report back

Cheers
Sono
Title: Re: Help with modifying Circuit please
Post by: sonolink on December 10, 2018, 10:02:37 AM
I've made the changes for gain compensation in SW3, avoiding pops when switching grids, and relocation of Presence circuit after V2A anode.
Here goes:
(https://i.postimg.cc/S2RP3txr/Angry-Boy-Abbey.jpg) (https://postimg.cc/S2RP3txr)

Would that be correct?
Cheers
Sono
Title: Re: Help with modifying Circuit please
Post by: abbey road d enfer on December 10, 2018, 11:01:58 AM
You need a 1Meg grid leak resistor on V1b.
You don't need C21.
You may want to swap C20 and the presence pot, so you don't have high voltage on the pot.
You may need grid stoppers on V1b and V2b.
In general, 1k is a bit low for grid stoppers.
Title: Re: Help with modifying Circuit please
Post by: sonolink on December 10, 2018, 03:35:09 PM
Updated schem:
(https://i.postimg.cc/yg08Dz5f/Angry-Boy-Abbey-02.jpg) (https://postimg.cc/yg08Dz5f)

added 10K grid stoppers with a 470pF cap to ground to avoid hiss
added 1M grid leak resistor on V1b
Swapped C20 and the presence pot to avoid high voltage on the pot

I wonder why the original GrindFX schem has C10 and C11 for...
(https://i.postimg.cc/xJctj52p/STM800.jpg) (https://postimg.cc/xJctj52p)

Would changing the 68k input resistor to 33K change something? Gain I suppose?
Do you reckon it's ready to proto? :)

I'm really grateful for your help :)
Cheers
Sono


Title: Re: Help with modifying Circuit please
Post by: abbey road d enfer on December 10, 2018, 04:45:27 PM
added 10K grid stoppers with a 470pF cap to ground to avoid hiss
What hiss? These 470pF will ruin the response, for no sensible gain.

Quote
I wonder why the original GrindFX schem has C10 and C11 for...
C10 is necessary for the presence control to work. C11 may be there to isolate DC coming from the outside. Just guessing...


Quote
Would changing the 68k input resistor to 33K change something? Gain I suppose?
It would change the input stage's HF response, in an insignificant way.
I think you should melt some solder soon.
Title: Re: Help with modifying Circuit please
Post by: sonolink on December 11, 2018, 04:27:32 AM
What hiss? These 470pF will ruin the response, for no sensible gain.
 

When you tell me things I try to learn because I love understanding electronics and many times I'm blind (or severely short sighted), so when you said I needed grid stoppers I searched and read and came across this very interesting paper:

"To find a suitable value for the grid-stopper, simply apply the formula for an RC filter. To get a cut-off frequency of 20kHz we need a grid stopper of: Rg = 1 / (2 pi f C) = 1 / (2 pi × 20000 × 200 × 10^-12) = 39789 ohms
Fender commonly used two jack sockets each connected to 68k grid stoppers which appeard in parallel when using only the 'hi' input, making 34k. Close enough.
However, the input grid stopper adds the most amount of Johnson noise (hiss) of any resistor in the amp. (A 68k stopper generates at least four times more hiss than a typical 12AX7!) Can we use a smaller resistor to reduce noise, but still keep the bandwidth the same? Yes, it's easy. We simply add a little extra capacitance to make up for the lower resistance. I recommend a 10k resistor, which will make the amp much less hissy, together with an extra 100pF to 470pF capacitor from grid to ground."

http://www.valvewizard.co.uk/gridstopper.html

The bit about hiss kinda made sense in my head. I didn't realize that the combination of both components would affect the spectrum.
   
C10 is necessary for the presence control to work.
 
OK, So the if the cap is placed before or after the Presence circuit doens't make a difference except for blocking DC when it's before?

C11 may be there to isolate DC coming from the outside. Just guessing...
Wouldn't my modded circuit benefit from that? Or it doesn't really matter or affects tone? Or in other words I'd like to understand why you told me I don't need it :)

BTW, just wondering: aren't R5 and R6 "repeating" R1 and R2 somehow?

It would change the input stage's HF response, in an insignificant way.
I think you should melt some solder soon.
I can't wait for it :)
I will do the final corrections, post them and if all is fine, I'll draw the layout :)
Title: Re: Help with modifying Circuit please
Post by: abbey road d enfer on December 11, 2018, 05:25:57 AM
"To find a suitable value for the grid-stopper, simply apply the formula for an RC filter. To get a cut-off frequency of 20kHz we need a grid stopper of: Rg = 1 / (2 pi f C) = 1 / (2 pi × 20000 × 200 × 10^-12) = 39789 ohms
Fender commonly used two jack sockets each connected to 68k grid stoppers which appeard in parallel when using only the 'hi' input, making 34k. Close enough.
However, the input grid stopper adds the most amount of Johnson noise (hiss) of any resistor in the amp. (A 68k stopper generates at least four times more hiss than a typical 12AX7!) Can we use a smaller resistor to reduce noise, but still keep the bandwidth the same? Yes, it's easy. We simply add a little extra capacitance to make up for the lower resistance. I recommend a 10k resistor, which will make the amp much less hissy, together with an extra 100pF to 470pF capacitor from grid to ground."
All he says is correct, except he forgets two things:

Specifically, if we take the example of V1b, it sees the R14, C14, the 1Meg pot and the output Z of V1a. This results in between 100k and 250k, except when fully CCW. Then you see that whatever the grid-stopper, 10k or 33k, is negligible in terms of noise.
However, in terms of frequency response, the 470pF grid cap is not negligible, creating an HF cut-off at 2.8kHz in the worst case. Cascading this effect in the 4 stages ends up with a cut-off frequency of about 1.7kHz.

 
Quote
    So the if the cap is placed before or after the Presence circuit doens't make a difference except for blocking DC when it's before?
Correct.

Quote
C11. Wouldn't my modded circuit benefit from that? Or it doesn't really matter or affects tone? Or in other words I'd like to understand why you told me I don't need it :)
If there is no DC coming from the ouside world, it is not necessary. It may somehow affect the LF response, if driving a low-ish impedance. Don't need is exactly what I mean, whether it's here or not won't change a thing in practice. You know, I could probably add 50 components to your schemo that would not affect tone in any way 8). That's what some so-called gurus do, claiming all sorts of unmeasurable benefits.

Quote
BTW, just wondering: aren't R5 and R6 "repeating" R1 and R2 somehow?
I agree, you could dispense with them.

One thing I had not noticed; the input is permanently loaded with the 250k Clean pot. I would probably up it to 500k or even 1Meg. Indeed, C23 & 24 may need to be adjusted accordingly. This is something you'll need to do when tuning the circuit.
Title: Re: Help with modifying Circuit please
Post by: sonolink on December 11, 2018, 12:06:19 PM
Cascading this effect in the 4 stages ends up with a cut-off frequency of about 1.7kHz.
OUCH!! Not good...  ;D

One thing I had not noticed; the input is permanently loaded with the 250k Clean pot. I would probably up it to 500k or even 1Meg. Indeed, C23 & 24 may need to be adjusted accordingly. This is something you'll need to do when tuning the circuit.

Actually you're right (as usual...). How about this alternative switching to avoid having the input permanently loaded with the 250k Clean pot? I've added R5 between the switch and V1A so that V1A's grid goes to GND when Clean channel is engaged. Does that make any sense? Or is the previous switching scheme preferable?
(https://i.postimg.cc/QHFK2fjM/Angry-Boy-Abbey-02-ALTSwitch.jpg) (https://postimg.cc/QHFK2fjM)

I'll begin drawing the layout this evening. I can't wait to build this little beast :)
Cheers
Sono
Title: Re: Help with modifying Circuit please
Post by: abbey road d enfer on December 11, 2018, 12:18:14 PM
Actually you're right (as usual...). How about this alternative switching to avoid having the input permanently loaded with the 250k Clean pot? I've added R5 between the switch and V1A so that V1A's grid goes to GND when Clean channel is engaged. Does that make any sense? Or is the previous switching scheme preferable?
(https://i.postimg.cc/QHFK2fjM/Angry-Boy-Abbey-02-ALTSwitch.jpg) (https://postimg.cc/QHFK2fjM)
That is much better.
Title: Re: Help with modifying Circuit please
Post by: sonolink on December 11, 2018, 12:27:40 PM
That is much better.

Yesss!! Excellent!! Thanks a lot for the help and motivation Abbey!!
I'll get back with a layout in no time 8)
Title: Re: Help with modifying Circuit please
Post by: sonolink on December 11, 2018, 03:43:25 PM
Layout question please: I always place the input grid resistor as close as possible to the grid. In fact I usually solder it directly on the socket. Keeping this in mind, I have drawn the bypass switch I was planning to use into the schem. That led to duplicating the 68K grid resistor and the 1M to GND on the Clean channel so that when it disengages it (the Clean channel), the channel is brought to GND. I hope all that makes sense . Do I actually need R6? Is all this ok or I'm just overdoing things?  :-\

(https://i.postimg.cc/BtkgpFP9/AB-input-layout.jpg) (https://postimg.cc/BtkgpFP9)

Cheers
Sono
Title: Re: Help with modifying Circuit please
Post by: abbey road d enfer on December 11, 2018, 04:35:02 PM
Layout question please: I always place the input grid resistor as close as possible to the grid. In fact I usually solder it directly on the socket.
That is not necessary. The grid-stopper must be close-mounted, but the grid leak can be put almost anywhere, within reason, since its role is conducting DC.

Quote
Keeping this in mind, I have drawn the bypass switch I was planning to use into the schem. That led to duplicating the 68K grid resistor and the 1M to GND on the Clean channel so that when it disengages it (the Clean channel), the channel is brought to GND. I hope all that makes sense . Do I actually need R6?
You don't need R6, neither R1, since galvanic continuity is ensured by the 250k pot.
Title: Re: Help with modifying Circuit please
Post by: sonolink on December 11, 2018, 04:39:34 PM
Excellent news! Thanks Abbey  ;)
Title: Re: Help with modifying Circuit please
Post by: sonolink on December 22, 2018, 08:20:01 PM
Here's the layout I've come up with:

(https://i.postimg.cc/nMBRGbm7/Angry-Boy-Final-Layout-10-A.jpg) (https://postimg.cc/nMBRGbm7)

Under the tubes is the preamp board, and under this are the PSU and the Bypass opto switch. The PSU is fed with a 9V/2A supply.
I will be building this thing these holidays.

To connect this device to a mixer (without using a power amp/speaker) I would have to use a D.I. right?
To get a nice guitar sound I would probably also need to put it through a speaker emulator or an EQ before the D.I. right?

Cheers and Merry Christmas!!
Title: Re: Help with modifying Circuit please
Post by: abbey road d enfer on December 23, 2018, 04:24:38 AM
One thing that was not obvious on the schemo is that the master pots are in fact a dual type.
First, it is very difficult today to find custom pots.
Second, why the different values?
Third, why not making them separate? that would increase the versatility of the box.
Title: Re: Help with modifying Circuit please
Post by: sonolink on December 23, 2018, 06:46:20 AM
One thing that was not obvious on the schemo is that the master pots are in fact a dual type.
Quite right. But I have to say it's something I implemented later to avoid redundant pots (please keep reading)

First, it is very difficult today to find custom pots.
True. But I've customized them :)
I bought double gang 1M and 100k pots and swapped wipers. It's quite easy to do actually, at least with the brand I got hold of.

Second, why the different values?
Third, why not making them separate? that would increase the versatility of the box.
The original circuit ((https://i.postimg.cc/xJctj52p/STM800.jpg) (https://postimg.cc/xJctj52p)), is meant to be used in an amp's FX loop return. According to the designer "If using this effect purely as a stomp-box and not as a pre-amp, then you may wish to substitute the A1M master pot for an A100K. It’ll make it a little less in your face. Or you could just turn it down. Because if you buy an amp later with a loop in it, you might want to run it as a pre into its return."

So in my schemo (actually our schemo, hehe), SW4 is meant as an Output Impedance Switch. When the box is placed in front of an amp it should be operated through the 100K Master Output pot and when placed in an amp's FX loop through the 1M Master Output pot. Since this is a setting that won't be changed while you play, the switch, instead of being a footswitch, will be on the side of the box as a slider. At that point I found myself with 2 Master pots, hence my idea of using a double pot. All this also allows more room for the other pots and boards.
I hope that answers your questions :)
And the next thing I thought about, since many guitarists are more incline nowadays to carry less gear :), was to use the box WITHOUT an amp, directly to a PA or a mixer. I guess one of the outputs or both would work into a D.I. box but I'd probably need to EQ the output to make it sound like a guitar speaker or it'll sound way too fizzy.
If all this is right, a speaker emulator is an EQ or something more is involved?
Cheers
Sono
Title: Re: Help with modifying Circuit please
Post by: abbey road d enfer on December 23, 2018, 08:26:46 AM
Quite right. But I have to say it's something I implemented later to avoid redundant pots (please keep reading)
True. But I've customized them :)
I bought double gang 1M and 100k pots and swapped wipers. It's quite easy to do actually, at least with the brand I got hold of.
The original circuit ((https://i.postimg.cc/xJctj52p/STM800.jpg) (https://postimg.cc/xJctj52p)), is meant to be used in an amp's FX loop return. According to the designer "If using this effect purely as a stomp-box and not as a pre-amp, then you may wish to substitute the A1M master pot for an A100K. It’ll make it a little less in your face. Or you could just turn it down. Because if you buy an amp later with a loop in it, you might want to run it as a pre into its return."

So in my schemo (actually our schemo, hehe), SW4 is meant as an Output Impedance Switch. When the box is placed in front of an amp it should be operated through the 100K Master Output pot and when placed in an amp's FX loop through the 1M Master Output pot. Since this is a setting that won't be changed while you play, the switch, instead of being a footswitch, will be on the side of the box as a slider. At that point I found myself with 2 Master pots, hence my idea of using a double pot. All this also allows more room for the other pots and boards.
I hope that answers your questions :)
OK; I just wanted to make sure you have thought about these issues. I see you've covered your bases.

Quote
And the next thing I thought about, since many guitarists are more incline nowadays to carry less gear :), was to use the box WITHOUT an amp, directly to a PA or a mixer. I guess one of the outputs or both would work into a D.I. box but I'd probably need to EQ the output to make it sound like a guitar speaker or it'll sound way too fizzy.
If all this is right, a speaker emulator is an EQ or something more is involved?
There's all sorts of speaker emulators, some have just basic EQ, others have some kind of distorsion emulation.
I play quite oten in a place where amps are forbidden. I have tried many speaker/amp emulators. One of the best is the venerable Sans Amp classic (mine is a middle-90's) but I put it out of service because it became unreliable (scratchy pots and switches), I had a Palmer Pocket Amp, which sounded good too, but developed hiss, which made it unusable, and I couldn't fix it, because lack of schemo combined with SMD's. Then I tried a Boss FBM1 that emulates a 59 Bassman, didn't like it, too crunchy for me, then a Boss FDR1, that emulates a Fender Deluxe, I liked it, but found it was too fiddly on stage. For sometime I used a B.......r PB100, which was actually not bad, although iy is just a basic EQ.
Now I use a Line6 Pocket Pod. Considering the price, I think it's a good compromise.
Indeed, if I was very picky, I would use something like a Torpedo or Helix.
For recording, I use DI only for pre-prod, then use an amp to move air. Only once I did use DI, in order to fix a blurred note on a track that had been recorded with a Vox Tonelab. I DI'ed the track clean, and used the convolution tool in Samplitude to impose the same sonic signature.
Title: Re: Help with modifying Circuit please
Post by: sonolink on December 23, 2018, 02:53:08 PM
OK; I just wanted to make sure you have thought about these issues.
And I thank you for it :)

I play quite often in a place where amps are forbidden.
I had no idea you play guitar! That's great to know and I'm glad you do :)

There's all sorts of speaker emulators, some have just basic EQ, others have some kind of distorsion emulation.
I play quite oten in a place where amps are forbidden. I have tried many speaker/amp emulators. One of the best is the venerable Sans Amp classic (mine is a middle-90's) but I put it out of service because it became unreliable (scratchy pots and switches), I had a Palmer Pocket Amp, which sounded good too, but developed hiss, which made it unusable, and I couldn't fix it, because lack of schemo combined with SMD's. Then I tried a Boss FBM1 that emulates a 59 Bassman, didn't like it, too crunchy for me, then a Boss FDR1, that emulates a Fender Deluxe, I liked it, but found it was too fiddly on stage. For sometime I used a B.......r PB100, which was actually not bad, although iy is just a basic EQ.
Now I use a Line6 Pocket Pod. Considering the price, I think it's a good compromise.
Indeed, if I was very picky, I would use something like a Torpedo or Helix.
For recording, I use DI only for pre-prod, then use an amp to move air. Only once I did use DI, in order to fix a blurred note on a track that had been recorded with a Vox Tonelab. I DI'ed the track clean, and used the convolution tool in Samplitude to impose the same sonic signature.
Do you reckon the box as it is now would sound ok into a mixer/PA?

Cheers
Sono
Title: Re: Help with modifying Circuit please
Post by: abbey road d enfer on December 23, 2018, 05:24:49 PM
I had no idea you play guitar! That's great to know and I'm glad you do :)
You'd be shocked learning that several of us who are posing as respectable members of society are in fact busying themselves with the task of killing the hair cells in their fellow citizens' organ of Corti.  8)

Quote
Do you reckon the box as it is now would sound ok into a mixer/PA?
You may have to do some adjustment on the voicing, but I think you can find a satisfoctory setting.
Title: Re: Help with modifying Circuit please
Post by: sonolink on December 25, 2018, 06:28:30 AM
You'd be shocked learning that several of us who are posing as respectable members of society are in fact busying themselves with the task of killing the hair cells in their fellow citizens' organ of Corti.  8)
 
All the more respectable to me you guys are 8) A world with more music and true Rock'n'Roll would indeed be a better one  ;)

You may have to do some adjustment on the voicing, but I think you can find a satisfoctory setting.
You mean adjusting the caps and resistor values in the tone stack?

Merry Christmas and thanks again for your help :)
Cheers
Sono
Title: Re: Help with modifying Circuit please
Post by: abbey road d enfer on December 25, 2018, 06:57:35 AM
You mean adjusting the caps and resistor values in the tone stack?
Not only in the tone stack; you may want to alter C10, R14 & C14, R17 & C16, R24 and C3 & C24, they all shape the response.
BTW, I just noticed the presence control is not correct. Check attachment for correct connection.
Title: Re: Help with modifying Circuit please
Post by: sonolink on December 25, 2018, 07:13:13 AM
BTW, I just noticed the presence control is not correct. Check attachment for correct connection.
Oh thanks for checking that :)

Not only in the tone stack; you may want to alter C10, R14 & C14, R17 & C16, R24 and C3 & C24, they all shape the response.
Ok. I should probably first build it and hear what it sounds like on an amp and then try the D.I. thing and go from there.
Cheers
Sono

Title: Re: Help with modifying Circuit please
Post by: sonolink on January 06, 2019, 11:15:01 AM
Is the value of C10 (0.68uF) critical? Is it ok to replace it with a 1uF/50v electrolytic? I'm asking because the 0.68uF film caps I have are 250VDC and a bit bulky...

Cheers
Sono
Title: Re: Help with modifying Circuit please
Post by: abbey road d enfer on January 06, 2019, 12:43:37 PM
Is the value of C10 (0.68uF) critical? Is it ok to replace it with a 1uF/50v electrolytic? I'm asking because the 0.68uF film caps I have are 250VDC and a bit bulky...
Indeed you can; however electrolytic caps have a larger tolerance than film, and their value drifts with age. It may probably not matter much.
Title: Re: Help with modifying Circuit please
Post by: sonolink on January 06, 2019, 12:45:02 PM
Excellent. I'll use it for the proto then ;)
Title: Re: Help with modifying Circuit please
Post by: abbey road d enfer on January 06, 2019, 12:59:47 PM
Excellent. I'll use it for the proto then ;)
Check polarity; positive to cath, negative to ground.
Title: Re: Help with modifying Circuit please
Post by: sonolink on January 06, 2019, 01:10:20 PM
Of course. Thanks for checking. I've had a few smokey missiles in the past hehe ;)
Title: Re: Help with modifying Circuit please
Post by: sonolink on January 17, 2019, 01:41:58 PM
Hi again,

A quick and dumb question please, as I suspect I wired the pots the wrong way around :( But before rewiring them I'd like to be sure...
To have an increase in Gain, I should read a decreasing resistance on my DMM as I turn the pot clockwise, right?

So if I look at the pot with the shaft up and the pins looking at me, and I number them 1 to 3 from left to right, I should connect GND to pin 1, and the input and output to the other two pins (in doesn't matter in which order), right?

Ok, that's 2 questions :)
Cheers
Sono
Title: Re: Help with modifying Circuit please
Post by: abbey road d enfer on January 17, 2019, 01:54:21 PM
To have an increase in Gain, I should read a decreasing resistance on my DMM as I turn the pot clockwise, right? 
Depends where your DMM is connected...

[/quote] So if I look at the pot with the shaft up and the pins looking at me, and I number them 1 to 3 from left to right, I should connect GND to pin 1, [/quote] That's the case with most potentiometers, yes

 and the input and output to the other two pins (in doesn't matter in which order), right?
[/quote] It certainly does matter! The wiper is generally the middle pin.
Title: Re: Help with modifying Circuit please
Post by: sonolink on January 17, 2019, 02:24:08 PM
Sorry if I didn't explain myself properly. What I meant to say was:

-if I want an increase in gain I should connect the pot so that I have a decrease in resistance right?

It certainly does matter! The wiper is generally the middle pin.

Ok. I would've thought so and always thought one was the input and the other the output but I've checked several schematics and some places the input on pin2 and others on pin3, and if I connect the DMM terminals to pins 2 and 3 either way around I get the same resistance reading...

I also noticed that on the layout I draw the pots connections differently than on the schem but then again some people count pin numbers left to right and others the opposite way...  :o

A bit confusing innit?? ;)

Cheers
Sono
Title: Re: Help with modifying Circuit please
Post by: abbey road d enfer on January 17, 2019, 02:29:39 PM
Ok. I would've thought so and always thought one was the input and the other the output but I've checked several schematics and some places the input on pin2 and others on pin3, 
You may see input on wiper (pin 2) particularly on some Gibson guitars, but also in some cheap mixers.

and if I connect the DMM terminals to pins 2 and 3 either way around I get the same resistance reading...[/quote] That's correct.
Title: Re: Help with modifying Circuit please
Post by: sonolink on January 17, 2019, 03:00:13 PM
You may see input on wiper (pin 2) particularly on some Gibson guitars, but also in some cheap mixers.

Ok. I'll connect GND to pin1, input to pin 3 and output to the wiper then :)

Cheers
Sono
Title: Re: Help with modifying Circuit please
Post by: sonolink on January 20, 2019, 06:44:30 PM
I've built a PSU for the little beast. It should be fed with 9V/1A and provide 2 lines of HV (190VDC) and a 6VDC line for Heaters. I get everything ok except that one of the HV lines reads aprox 80VDC instead of 190 like the other one.
Any ideas as why this could happen and where to begin looking?

Thanks a lot
Sono

(https://i.postimg.cc/Mv6tMtYp/PSU-Schem.png) (https://postimg.cc/Mv6tMtYp)

(https://i.postimg.cc/fVkK5Lp4/PSU-Vero.png) (https://postimg.cc/fVkK5Lp4)

Title: Re: Help with modifying Circuit please
Post by: abbey road d enfer on January 20, 2019, 10:24:00 PM
I've built a PSU for the little beast. It should be fed with 9V/1A and provide 2 lines of HV (190VDC) and a 6VDC line for Heaters. I get everything ok except that one of the HV lines reads aprox 80VDC instead of 190 like the other one.
Any ideas as why this could happen and where to begin looking?
Do you mean HV1 is lower than HV2? I would think it's normal since HV1 has a 10k resistor in series. If you draw 10mA from this rail, the voltage drops by about 100V.
Title: Re: Help with modifying Circuit please
Post by: sonolink on January 21, 2019, 03:03:55 AM
Mmmm makes sense....but there must be something wrong in my build. When I first built and tested the PSU, before building the pedal, I had 190V on both HV terminals and could adjust more or less voltage with the pot...looking at the schem I thought the problem would be somewhere after the fuse (bad 47uF cap) but what you say also makes sense....but I didn't dream the test readings so this is a bit confusing :)

This is the original schem: (https://i.postimg.cc/zLcH5sC4/PSU-Cleggster.jpg) (https://postimg.cc/zLcH5sC4)

I'll check the components, solder joints and connections to make sure they're ok and hope that fixes it ;)

Cheers
Sono
Title: Re: Help with modifying Circuit please
Post by: abbey road d enfer on January 21, 2019, 03:24:59 AM
This is the original schem:
Can't read it; way too small.
Title: Re: Help with modifying Circuit please
Post by: sonolink on January 21, 2019, 03:30:06 AM
Sorry. here you go:

(https://i.postimg.cc/ydVmhBk6/PSU-Cleggster.jpg) (https://postimg.cc/zLcH5sC4)
Title: Re: Help with modifying Circuit please
Post by: abbey road d enfer on January 21, 2019, 05:06:51 AM
Sorry. here you go:
Clearly, if you have 190V at HVA, you can't have 190V at HVB, unless the current draw is zero.
Title: Re: Help with modifying Circuit please
Post by: sonolink on January 21, 2019, 08:15:51 AM
Ok. I've reflowed some joints and rechecked and now all seems fine except for one thing: when I put my DMM between say HV1 and GND the voltage slowly drops down. If I disconnect the DMM terminals and reconnect them the voltage is back up to 190v and drops down slowly again. This doesn't happen at the Heaters terminal. There a get steady 6V...
This happens with no load.
Is that normal?

Cheers
Sono
Title: Re: Help with modifying Circuit please
Post by: abbey road d enfer on January 21, 2019, 10:04:47 AM
Is that normal?
Yes; check Ohm's law.
Title: Re: Help with modifying Circuit please
Post by: sonolink on January 21, 2019, 07:08:32 PM
Check Ohm's law.
Ok. I'm not very good at this but I'll try :)
Ohm's Law states V=I*R
So, if Resistance is a constant, voltage is slowly dropping because current is diminishing?
But I still don't know why this happens. Maybe because there is no load connected? But then, why is it not happening on the Heaters terminals?

Cheers
Sono
Title: Re: Help with modifying Circuit please
Post by: abbey road d enfer on January 21, 2019, 11:44:12 PM
Ok. I'm not very good at this but I'll try :)
Ohm's Law states V=I*R
So, if Resistance is a constant, voltage is slowly dropping because current is diminishing?
The voltage drops, but because there is a capacitor it takes some time; this time is governed by the current draw and the capacitor value.

Quote
Maybe because there is no load connected?
When you connect your meter, it draws some current, admittedly very small, so it takes a long time to discharge the capacitor.

Quote
But then, why is it not happening on the Heaters terminals?
Because the voltage is regulated.
Title: Re: Help with modifying Circuit please
Post by: sonolink on January 22, 2019, 03:41:03 AM
The voltage drops, but because there is a capacitor it takes some time; this time is governed by the current draw and the capacitor value.
I told you I wasn't very good at this..... ;)
Mmmm I see... I thought that the cap would hold the charge since the circuit is connected but I can see it's a silly thought since that wouldn't be very useful...
So, if the current draw is bigger the cap wil discharge quicker and when completely discharged will just conduct like a wire so to say, until the current draw stops or diminishes, giving it time to recharge.....or something like that?
In other words, because the cap blocks DC it will charge unless there is a current draw?

Because the voltage is regulated.
Of course...the 7806...silly me...

Thanks a lot for your explanations and excuse my inquisitiveness  ;)
I hope I can finish the build this evening

Cheers
Sono
Title: Re: Help with modifying Circuit please
Post by: sonolink on January 28, 2019, 09:15:03 AM
So I finally got some time to get to build this little beast :)
I plugged it into a sound card (Hi-Z input) but I get a huge steady signal (I haven't heard it, only got a visual on the meter). My guess is I have some bad gnd connection or massive gnd loops. I have shielded all the signal path (well I tried anyway) The shielded cables are the purple and blue ones. They are connected to GND only on one end. Maybe it's not worth shielding ALL the path?

(https://i.postimg.cc/nMBRGbm7/Angry-Boy-Final-Layout-10-A.jpg) (https://postimg.cc/nMBRGbm7)
https://postimg.cc/nMBRGbm7

Thanks
Sono
Title: Re: Help with modifying Circuit please
Post by: DaveP on January 29, 2019, 05:08:29 PM
The DC coupled cathode follower is crucial to the best sounding amps as that article will explain....do not change it!

DaveP
Title: Re: Help with modifying Circuit please
Post by: sonolink on January 29, 2019, 05:29:02 PM
The DC coupled cathode follower is crucial to the best sounding amps as that article will explain....do not change it!

DaveP


Sorry, what do you mean? What article?
Title: Re: Help with modifying Circuit please
Post by: DaveP on January 29, 2019, 05:32:46 PM
Looks like it was just deleted, this one.......

http://www.valvewizard.co.uk/dccf.html

DaveP
Title: Re: Help with modifying Circuit please
Post by: sonolink on January 29, 2019, 05:41:16 PM
The DC coupled cathode follower is crucial to the best sounding amps as that article will explain....do not change it!

DaveP

Sorry I don't understand. Doesn't the modified circuit has a cathode follower just before the tone stack?

Cheers
Sono
Title: Re: Help with modifying Circuit please
Post by: sonolink on January 31, 2019, 08:23:37 AM
I have shielded all the signal path (well I tried anyway) The shielded cables are the purple and blue ones. They are connected to GND only on one end.
(https://i.postimg.cc/nMBRGbm7/Angry-Boy-Final-Layout-10-A.jpg) (https://postimg.cc/nMBRGbm7)
https://postimg.cc/nMBRGbm7

Any thoughts on this please? Is it worth shielding the whole thing as in the pic (purple and blue cables) or just using shielded cable between the input and the first tube's screen as done on amps would be enough?

Thanks!
Sono
Title: Re: Help with modifying Circuit please
Post by: DaveP on January 31, 2019, 08:45:59 AM
Screening is not usually necessary when the signal is over a volt or two, unless it is very near a strong magnetic field like a PT or choke IMHO.

Your biggest danger will be from earth loops, make sure you place the signal earth carefully to minimise loops.

DaveP
Title: Re: Help with modifying Circuit please
Post by: sonolink on January 31, 2019, 08:50:53 AM
Thanks for your reply DaveP.
That means that I should be ok screening between the input and the first stage only. But should I worry about the inductor in the PSU?

Cheers
Sono
Title: Re: Help with modifying Circuit please
Post by: DaveP on January 31, 2019, 05:05:13 PM
Quote
But should I worry about the inductor in the PSU?
That depends.

It should be placed well away from the input stage as part of normal practice,  if that is not possible, then a steel shield may help.  It is often possible to turn these components sideways to orientate the magnetic field in another plane, you can experiment with that later.

The earthing of centre taps and the power section bridge and cap earth should all be kept to the shortest dimensions and the signal earth including the power supply earth should only contact the chassis at the input socket.  The power cord earth should go straight to the chassis where it enters, this will minimise hum.

DaveP
Title: Re: Help with modifying Circuit please
Post by: sonolink on January 31, 2019, 06:22:44 PM
You do realize this is supposed to be built inside a pedal don't you? The enclosure is about 140x120mm....the PSU is a switched mode thingy on veroboard...hence my first idea of shielding everything...
Cheers
Sono
Title: Re: Help with modifying Circuit please
Post by: DaveP on January 31, 2019, 06:42:23 PM
My mistake ::) :-[

DaveP
Title: Re: Help with modifying Circuit please
Post by: sonolink on February 01, 2019, 06:07:24 AM
I guess I can build it the simple way, with standard screening and see what happens :)
Title: Re: Help with modifying Circuit please
Post by: abbey road d enfer on February 01, 2019, 06:45:24 AM
But should I worry about the inductor in the PSU?
You'll probably have to find the orientation where noise is minimized. Make sure you have the possibility to move it around before attaching it definitely.
Title: Re: Help with modifying Circuit please
Post by: sonolink on February 01, 2019, 06:51:28 AM
Well it's a little too late for that now...if the inductor is a problem and has to be moved I'll have to redesign the veroboard.

I think I'll build it with shielded cable between input and the first stage and see what happens...

Cheers
Sono

Title: Re: Help with modifying Circuit please
Post by: sonolink on February 09, 2019, 08:30:59 AM
Hello again,

So I built this thing finally and here's what it looks like:

(https://i.postimg.cc/q63YP6YJ/01-Outside.jpg) (https://postimg.cc/q63YP6YJ)

I ended up shielding all the signal path (grey cables on pic, purple and blue cables on layout diagram)

(https://i.postimg.cc/9DpSHhx7/02-Inside.jpg) (https://postimg.cc/9DpSHhx7)
(https://i.postimg.cc/nMBRGbm7/Angry-Boy-Final-Layout-10-A.jpg) (https://postimg.cc/nMBRGbm7)

I checked voltages as follows:

V1:
p1:140v
p2:0v
p3:1v
p4-5:0v
p6:158v
p7:0v
p8:1.6v
p9:6.5v

V2:
p1:88v
p2:0v
p3:0.6v
p4-5:0v
p6:158v
p7:87v
p8:88v
p9:6.5v

I applied my scope with a 10x probe between jack input and GND and then jack output and gnd and I got this:

Jack input no jack
(https://i.postimg.cc/PChc7xMH/03-Input-Jack.jpg) (https://postimg.cc/PChc7xMH)

Jack Output no jack
(https://i.postimg.cc/LnjwvfLv/04-Output-Jack.jpg) (https://postimg.cc/LnjwvfLv)

Jack Output jack plugged in
(https://i.postimg.cc/yWM2Fngk/05-Output-Jack-In.jpg) (https://postimg.cc/yWM2Fngk)

Any thoughts?

Cheers
Sono
Title: Re: Help with modifying Circuit please
Post by: DaveP on February 09, 2019, 05:31:18 PM
50Hz looks like a hum loop,  the high frequency might be pick-up from fluorescent lighting, the unplugged noise looks around normal.

Nice job by the way.

DaveP
Title: Re: Help with modifying Circuit please
Post by: sonolink on February 09, 2019, 05:54:39 PM
Thanks :)

the high frequency might be pick-up from fluorescent lighting, the unplugged noise looks around normal.

There's no fluorescent around though. Maybe coming from the inductor?

I guess the loop comes from the shielding so I'll check that first.

Cheers
Sono
Title: Re: Help with modifying Circuit please
Post by: sonolink on February 12, 2019, 03:36:19 AM
Btw, any way to locate the loop without unwiring the whole thing? Just hoping.... :)

Cheers
Sono
Title: Re: Help with modifying Circuit please
Post by: sonolink on February 12, 2019, 12:57:17 PM
Ignore my previous post. I'm a retard and forgot to connect GND to the enclosure. Once that done hum is GONE :)

Unfortunately, I've got intermittent sound and after some chopsticking I've detected some bad solder joints.

So I'm gonna go through this little mofo again and I'll report back :)

Cheers
Sono

Title: Re: Help with modifying Circuit please
Post by: sonolink on May 04, 2020, 09:20:18 PM
Hi again,

Sorry for not posting more about this. To make a long story short, I finally went through all the circuit checking everything and got it to work. Quiet as a mouse :)

BUT

I don't really like it very much :(
The sound is a bit fizzy and not very good. I think all the switches and combinations "denatured" the circuit (or something like that anyway).
So I decided to go back to square 1 and built a Fender AB763. This one sounded great and was quiet too. Great sound and dynamic range, and organic response...

BUT

I missed some distortion options. I does have distortion but only with the gain all the way up.

So, I built this: https://groupdiy.com/index.php?topic=74809.msg950841#msg950841

I just wanted to close this thread and thank again all the input and advice, especially Abbey Road d Enfer ;)

Cheers
Sono