sonolink

JTM45 Tube Preamp in StompBox Help
« on: April 10, 2020, 02:40:50 PM »
Hello,

This is my schematic for a 2 channels tube pedal guitar preamp, consisting of a Matchless HotBox Clean Channel and a JTM45 Channel with Fat Switch
The PSU delivers HV:195VDC, Heaters: 6.3VDC.

Originally it uses ECC83s but I'm thinking of making it compatible with 6NxP Russian (or western 6vdc heater) twin triode preamp tubes and Russian 6n16b, or 6n17b subminiature tubes.

I am not an EE so if you see any obvious design errors, or have any comments/advice I would be most grateful.




Thanks in advance
Cheers
Sono
« Last Edit: April 12, 2020, 12:01:33 PM by sonolink »
Why sleep when you can mix?
M.P.Stavrou


Matador

Re: JTM45 Tube Preamp in StompBox Help
« Reply #1 on: April 10, 2020, 03:31:44 PM »
I can't make out anything on the schematic: it's encoded in a very fuzzy mode of JPEG compression.

Can you print your schematic as a PDF?

sonolink

Re: JTM45 Tube Preamp in StompBox Help
« Reply #2 on: April 10, 2020, 04:09:55 PM »
Sorry. Try now please:

« Last Edit: April 12, 2020, 12:02:05 PM by sonolink »
Why sleep when you can mix?
M.P.Stavrou

ruffrecords

Re: JTM45 Tube Preamp in StompBox Help
« Reply #3 on: April 10, 2020, 04:29:06 PM »
A circuit such as this designed for ECC83 tubes does not translate well to Russian 6N16B types because their characteristics are very different. The tone shaping networks in particular will act completely differently, the biasing will be wrong and the gain much lower. You might have better luck with the 6N17B as it differs less from the ECC88 but characteristics of the circuit will change. If you want to make a version compatible with these types you really need to re-calculate most of the component values.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

sonolink

Re: JTM45 Tube Preamp in StompBox Help
« Reply #4 on: April 10, 2020, 04:41:43 PM »
Thanks Ian :)

I'll definitely check that out after getting it to work with ECC83s.

Do you see any obvious errors otherwise?
I'd rather "measure twice before cutting" if you know what I mean 😉

Thanks a lot for your time and help.
Sono

Why sleep when you can mix?
M.P.Stavrou

sonolink

Re: JTM45 Tube Preamp in StompBox Help
« Reply #5 on: April 11, 2020, 12:21:35 PM »
Nobody?


Pretty please....

Why sleep when you can mix?
M.P.Stavrou

PRR

Re: JTM45 Tube Preamp in StompBox Help
« Reply #6 on: April 11, 2020, 04:12:03 PM »
> Nobody?

Presence network is awful low impedance for where it is stuck-in.

That image is hard to read, and when zoomed it turns fuzzy. At some point you gotta stop fretting and start soldering.

sonolink

Re: JTM45 Tube Preamp in StompBox Help
« Reply #7 on: April 11, 2020, 04:52:00 PM »
Thanks for your reply PRR :)

Sorry about the image. I've uploaded another one and updated links. I hope this one works:




Presence network is awful low impedance for where it is stuck-in.

I might just leave the Presence circuit out for now then. It belongs to the power section of the original circuit really, so maybe I don't really need it.

I'm thinking of implementing an assymetrical LED distortion circuit but I don't know where to place it. I'm going to build it first, and report back.

Thanks a lot for chiming in!!
Cheers
Sono
« Last Edit: April 12, 2020, 12:02:30 PM by sonolink »
Why sleep when you can mix?
M.P.Stavrou

john12ax7

Re: JTM45 Tube Preamp in StompBox Help
« Reply #8 on: April 11, 2020, 05:54:56 PM »
RY at 270k will roll off a lot of high end.  This is useful for high gain amps so the sound isn't too fizzy,  but you might find the sound dull in your application with only 2 gain stages.

You also will not get too much distortion with only 2 gain stages (if that was your goal).

ruffrecords

Re: JTM45 Tube Preamp in StompBox Help
« Reply #9 on: April 11, 2020, 05:56:02 PM »
Thanks Ian :)

I'll definitely check that out after getting it to work with ECC83s.

Do you see any obvious errors otherwise?

Nothing obvious other than it clearly needs to fed into a similarly high impedance stage which I think it is safe to assume it is.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'


sonolink

Re: JTM45 Tube Preamp in StompBox Help
« Reply #10 on: April 11, 2020, 07:36:01 PM »
I'm not sure I mentioned it's supposed to be a pedal, so it will be plugged in front of an amp or in it's FX loop return.


Thanks a lot to everyone for your input and for your time :)
I'll have it built in a couple of days and report back. I'm sure it'll need some tuning :)

Cheers
Sono

Why sleep when you can mix?
M.P.Stavrou

sonolink

Re: JTM45 Tube Preamp in StompBox Help
« Reply #11 on: April 12, 2020, 09:49:16 AM »
Is it a good idea to place a switch between the grid resistor and the grid? Will it pop?

Why sleep when you can mix?
M.P.Stavrou

sonolink

Re: JTM45 Tube Preamp in StompBox Help
« Reply #12 on: April 12, 2020, 09:49:58 AM »
I mean doing this:

Why sleep when you can mix?
M.P.Stavrou

sonolink

Re: JTM45 Tube Preamp in StompBox Help
« Reply #13 on: April 12, 2020, 09:50:38 AM »
Instead of this:
Why sleep when you can mix?
M.P.Stavrou

andyfromdenver

Re: JTM45 Tube Preamp in StompBox Help
« Reply #14 on: April 12, 2020, 10:49:52 AM »
you'll lose the ground reference on the V1 grids if you implement that change.

My thoughts:  experimenting on tube circuits is tricky due to the commitment nature of the testing phase.  I was just sort of thinking, maybe use 220k plate resistors on the first stage and 2.2k cathode resistors then you can at least parallel onto those to mess with different bias configurations. 
I am a less is more person and the construction is highly important because I am not an EE, just a passionate hobbyist. I am trying to imagine what kind of "box" this is all going in. 
I just also wanted to add that if you were to try different tubes as per your original post, be sure to account for heater to cathode voltage with the DCCF in the fender/marshall tone stack.  But as another has said, you probably want to maximize your gain so 12ax7 is a likely better choice.  I was also going to say, maybe have a switch to parallel the treble cap with another 250pf to get that 2204 midsy punch.
Only last thought is do you need the exit coupling cap? 

Also,
Best wishes  ;D
« Last Edit: April 12, 2020, 10:57:07 AM by andyfromdenver »
my la2a scratch build: https://groupdiy.com/index.php?topic=65716.0
my orig tube DI w/ EQ build: https://groupdiy.com/index.php?topic=73834.0
2020 fav artist: Kate Bush

sonolink

Re: JTM45 Tube Preamp in StompBox Help
« Reply #15 on: April 12, 2020, 12:00:49 PM »
Hi Andy,

Thanks for your kind reply. Let me please put all this in context. I built an Alembic Bass preamp (essentially a Fender Showman preamp) some time ago. Then I put it in a stomp box. The project was largely based on this:



The sound was great both in front of the amp as well as in the FX loop. Then I stumbled upon the STM800 (a JCM800 preamp in a box) and I built it with several mods suggested mainly by member Abbey Road d'Enfer (which I still thank infinitely for his patience and wisdom). The thread is here in case you want to check it out: https://groupdiy.com/index.php?topic=70561.0
The STM800 schematic is here:



The short version is, I got back to the original STM800 stomp box project for several reasons, but the "JTM45 version". This is the schematic:



Then I added to that schem the Matchless Hot Box Clean Channel from here:



And ended up with this (some mistakes are now corrected like plate resistor on V1-A):



After building it I will probaby want to try adding LED clipping distortion somewhere, but as PRR said, I need to solder first and listen to what comes out :)

My previous question was to see if I could spare duplicating things but as you said that's not a good idea since I'd loose the ground reference when switching.

Regarding your other thoughts, which I appreciate, I am a passionate hobbyist aswell :)
Actually I'm a sound engineer and guitar player and I build things on my free time trying to understand the magic of electronics :)

I was just sort of thinking, maybe use 220k plate resistors on the first stage and 2.2k cathode resistors then you can at least parallel onto those to mess with different bias configurations. 

V1-A seems to have that configuration. You can see it on the now corrected schematic. Or do you mean another stage?


Only last thought is do you need the exit coupling cap? 

Actually that was going to be another 2 questions I have:
- Should I place the switch after the exit coupling cap BEFORE the Master pot?
- Do I really need the last cap? The cap is present on the original STM800 schem, and on very the first box I built (the AB273 modded circuit) that's why I kept it, but do I need it?


Thanks a lot for your time and help
Greetings
Sono

« Last Edit: April 12, 2020, 12:06:12 PM by sonolink »
Why sleep when you can mix?
M.P.Stavrou

andyfromdenver

Re: JTM45 Tube Preamp in StompBox Help
« Reply #16 on: April 12, 2020, 05:47:59 PM »
oh I see that the lower circuit has the 220k, 2.2k.  I was focusing on the jtm45 path.  Re: the coupling cap, I'm not sure what the common practice is for guitar pedals, but if you were plugging this into your guitar amp, you wouldn't need it. 
I wouldn't be too determined to recreate the original schemes, this is your chance to make it your own!  You could lower the input grid resistors from 68k down to 10k. I think someone already said to remove RY or at least be prepared to jump it in testing, if you included it because of the original two channel scheme, it's purpose was to mix the two signals/ minimize interference. 

My "invention" for experimenting on tube circuits is in my tube di with eq in the link.  If you can construct a filtered power source with heater voltages, you can construct tube circuits on cardboard like in the pics and try different things more easily.  It seems this design can be constructed straight off the jacks, tube sockets, pots, and maybe a tag strip or two.   
As I know you know, this all needs to be done with a very clear head! 
my la2a scratch build: https://groupdiy.com/index.php?topic=65716.0
my orig tube DI w/ EQ build: https://groupdiy.com/index.php?topic=73834.0
2020 fav artist: Kate Bush

sonolink

Re: JTM45 Tube Preamp in StompBox Help
« Reply #17 on: April 13, 2020, 08:02:41 PM »
the coupling cap, I'm not sure what the common practice is for guitar pedals, but if you were plugging this into your guitar amp, you wouldn't need it. 

Can you tell me in what case I would need it? I'm asking because one thing I want to do at a later stage is to give it a DI output with speaker emulation  8)

I wouldn't be too determined to recreate the original schemes, this is your chance to make it your own!  You could lower the input grid resistors from 68k down to 10k. I think someone already said to remove RY or at least be prepared to jump it in testing, if you included it because of the original two channel scheme, it's purpose was to mix the two signals/ minimize interference. 

I am not really determined in recreating original schemes. I know the sound I'm after. But to get there I think it's better to start from a "known point", listen to it and then begin tweaking.

As I know you know, this all needs to be done with a very clear head! 

Definately! That's why I like to check here before heating up solder ;)
Thanks a lot for your input. I'll check your DI.
I'll be reporting back in a few days with pics.

Cheers
Sono
Why sleep when you can mix?
M.P.Stavrou

sonolink

Re: JTM45 Tube Preamp in StompBox Help
« Reply #18 on: May 04, 2020, 09:02:05 PM »
Hello again people,

I just finished building it. It's late here so I won't be able to hear it until tomorrow but I can show pics and voltages at least :)









WITHOUT tubes:
V1: p1: 187, p6: 189, p9: 6.3
V2: p1 & 7: 189, p6: 191, p9: 6.3

WITH tubes:
V1: p1:118, p6:133, p9: 6.3
V2: p1 & 7:97, p6: 171, p8:98, p9:6.3

If I'm not wrong, everything looks ok, right?
Tomorrow I'll plug it into an amp and check the mojo ;)

Thanks a lot for all your input. All comments and suggestions are very welcome as usual :)
Cheers
Sono
Why sleep when you can mix?
M.P.Stavrou

sonolink

Re: JTM45 Tube Preamp in StompBox Help
« Reply #19 on: May 08, 2020, 08:45:24 AM »
So I finally heard this thing....overall it sounds GREAT!!

In front of a clean amp it's just perfect. In the FX Return it's cleaner but creamy.
I'm very happy with it.
I finally took out the final condensor.

Thanks a lot for all the input and help

Cheers
Sono


Why sleep when you can mix?
M.P.Stavrou


 

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