Help with Impedance Matching Switch PLEASE

Hello everybody

I'm building a tube preamp in a pedal. It has a Master Output pot. It's designed to run it as a pre in an amp's return FX loop.
I'd like to be able to place it in front of the amp as a stomp box but it doesn't sound very good.
Asking the designer he tells me: "If using this effect purely as a stomp box and not as a preamp, 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".

I guess I could, but isn't this possible using the same pot and a switch to flip between "modes"?

Thanks a lot for your help
Sono

If you post a schematic it will be a lot easier to help you.

Cheers

Ian

Oooops sorry. Hehe damn right you are. I could swear I had attached it but I obviously didn't....
Here goes:

sube

> in front of the amp as a stomp box but it doesn't sound very good.

It has MUCH more gain than many whole guitar amps. It is all the gain a 5F6a Bassman has before the power bottles. (Lacks PI stage but has an added stage after the first stage.) The gain is HUGE.

I can't even see this working in an effects loop unless both gain pots are WAY down.

You did not note supply voltage. Assuming +250V or more, V2b can easily be swinging 50V peaks (35Vrms). And if you "want distortion", it has to be worked that way. No guitar amp is very happy with a lot more than 0.35Vrms at its input.

You want 100:1 divider between this and an amp!

You can't turn-down 100:1 well with a pot, even common Audio taper. (100:1 is down in the crack between 1.0 and 1.1 on a 0-10 dial.)

The PRES network bothers me. Heavy load on the treble network. And it appears that 1.5K+2.2nF is supersonic?

Here's a 2-resistor change which may make the beast less overwhelming. Now you can turn it UP and not melt your amp.

PRR said:

You did not note supply voltage.

Click to expand...

PRR
Thanks for your time and help and you're damn right and I apologize. Seems like I didn't quite woke up today. Let's begin again ok?

So here's the original schem:


and the PSU:


It's supposed to be like a JCM800 so it WILL be loud but I guess a choice of tubes and maybe some tweaking if it's REALLY a crazy horse is possible. My idea was to have a "plexi in a box" kinda thing but on real tubes and being able to put it either in front or in the FX loop return with the flip of a switch. I'm actaully working on something similar for a "Fender in a box".....

All your help and input is much appreciated.
Cheers
Sono

  The idea of changing the pot is to lower the output impedance, but for the eq to work similarlly you need to keep it's load high.

  To get that and some attenuation you could use 1M resistor and a pot between 10k and 100k depending on the desired output range, you should test that.

The precense network could be worked out then, usually in guitar amps is associated with the feedback across the transformer, not just a dampening zobel or a guitar-like tone control.

  Still, with the 1k5 and the 2n2 network I see more like 1.5kHz than 50kHz, depending on the treble control position, and for that kind of circuits loads of interaction between controls is always expected. I guess a reasonable place to put that feedback network would be between cathodes of V1a and V2b, RC in series, the R with a pot in series for the control, tiny cap (100pF or so), 1M pot, some resistor to limit the range, maybe 10k or 100k.

JS

I apologize but my knowledge of electronics design is very limited. Please let me know if I'm understanding you right.

joaquins said:

The idea of changing the pot is to lower the output impedance, but for the eq to work similarlly you need to keep it's load high.
To get that and some attenuation you could use 1M resistor and a pot between 10k and 100k depending on the desired output range, you should test that.

Click to expand...

Ok. Like so?
Presence pot -> switch pos A -> fx loop position -> 1M resistor -> 10k to 100k pot (to be tested)
Presence pot -> switch pos B -> Front of Amp position -> 100k pot (as recommended by designer)

joaquins said:

The presence network could be worked out then, usually in guitar amps is associated with the feedback across the transformer, not just a dampening zobel or a guitar-like tone control. Still, with the 1k5 and the 2n2 network I see more like 1.5kHz than 50kHz, depending on the treble control position, and for that kind of circuits loads of interaction between controls is always expected. I guess a reasonable place to put that feedback network would be between cathodes of V1a and V2b, RC in series, the R with a pot in series for the control, tiny cap (100pF or so), 1M pot, some resistor to limit the range, maybe 10k or 100k.

Click to expand...

Do you mean that the Presence network should be placed "between cathodes of V1a and V2b"?

I was wondering if it would be possible to use only one tube in this circuit and if it would be ok to do it with a switch like so:

subir imagenes gratis


Thanks a LOT for your time and help

  Too much time since I gave this a thought, I don't have it on the top of my head.

  What I can tell right now is that schematic won't work, the last stage is polarized (DC working point definition) with the previous one. You need at least AC couple it and some way of dealing with the polarization and the previous stage. I wouldn't go that route at all, it takes quite a bit more than just the switch.

  My advice, not for being pessimistic or trying to pop a balloon, but to ease you experience. If you don't understand the basics of electronics don't try to design a complex tube stage. You can probably build something as it is and get it working and just that is a great thing to do. Start modifying simper stuff, if possible look someone else's mods, build them and understand them. start to experiment your self, swap one resistor at the time and see what happens. With time you'll be able to design a complete amplifier if that's what you are for. Some theory comes handy as well, expend some time on that as well. It took me two years modding my amp till I got it as I wanted, tiny mods, adding one bottle at a time till it didn't have more room for a single extra knob, some on the front panel and some on the back! If you try to miss steps on the learning curve is more likely you find yourself with a lot of white bottles than a lot of DIY gear. If you do it step by step you'll find yourself with more gear and simpler tools that come really handy when you are up one big projects.

Over all, enjoy learning, and blowing the occasional cap as well!

JS

Not that I don't listen to your advice but I'm kinda inquisitive when I like something, so how about like this?



If that would work ok for the switching to use only one tube, and after I've done my homework, could you answer my previous questions pretty please?

sonolink said:

Ok. Like so?
Presence pot -> switch pos A -> fx loop position -> 1M resistor -> 10k to 100k pot (to be tested)
Presence pot -> switch pos B -> Front of Amp position -> 100k pot (as recommended by designer)

Do you mean that the Presence network should be placed "between cathodes of V1a and V2b"?

Click to expand...

Thanks again for your time and help

Bump

> Bump

I think you want the tonestack off the *cathode* of V2b, not the grid.

PRR

Thanks for pointing that out. Mistake corrected





Thanks and Merry Christmas
Sono

Could someone please answer me and confirm this right? Please...

Quote from: sonolink on December 07, 2016, 03:38:15 PM

    Ok. Like so?
    Presence pot -> switch pos A -> fx loop position -> 1M resistor -> 10k to 100k pot (to be tested)
    Presence pot -> switch pos B -> Front of Amp position -> 100k pot (as recommended by designer)
   
    Do you mean that the Presence network should be placed "between cathodes of V1a and V2b"?

Bumpety bump

No one? Come on guys, pretty pretty please.....

Ok so I finally came up with these layouts and schems. If no one is interested in commenting or correcting or whatever this project I'll understand and just go ahead to see what happens. If someone does have the time I'll be very grateful.

Thanks anyway to all for all the input and lessons you've given me all this time along


The original schem:


The final idea:


The layout:


The On/Off switch:


The PSU:


Cheers
Sono