Tube amp master volume expression pedal mod

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Tube amp master volume expression pedal mod

Postby ccaviris89 on Tue Dec 17, 2013 8:04 am

I have a custom built tube amp that I love. I have a modification that I would like to get done, but before I take it back to the guy who built it and ask him about it, I figured I'd run it by a forum to see if it's possible before I waste his time.

Background (you can skip and jump strait to my question of you want):
The amp had an EQ bypass switch, which I recently had changed out to a jack that can be used with an external foot switch (when nothing is plugged into it, the EQ is allowed to go to ground and enabled, when a pedal is plunged in, it can either allow the EQ to go to ground or breaks the ground bypassing the EQ). I love the sound of the EQ bypassed! Obviously, when bypassed the volume level also goes up. If I was alternating between lead and rhythm, this might be ideal. However, I typically just play rhythm and often play without a band (so a more constant volume level would be desired. The ideal usage for me would be a miens by which I can enable the EQ when I want a clean sound, then bypass the EQ to get something a little more open that breaks up a little, but without having too significant a change in volume.

Question:
My amp has a master volume control. Would it be possible, without significant modification to the amp, to either have an expression pedal that would just drop the "master volume" a little or to have an external expression pedal that could be used instead of the master volume knob (So, the master volume knob is enabled when nothing plugged in. When Something is plugged in, the expression pedal sets volume and the master volume knob on the amp is bypassed). The two ideas I had were as follows:

1. Expression pedal (first idea): Put a stereo 1/4" jack in the amp that has a little built in switch. The master volume (from what I understand) essentially acts as a variable potentiometer between the preamp stage and the output stage. So, I would have an expression pedal with a potentiometer that matches the one in the master volume of the amp and essentially run everything through a shielded TRS cable as shown in the picture bellow.
Concerns: Running the signal through a 10 ft cable could drain the tone. In fact, I think this is a bad idea for just that reason...

2: Foot-switch (second idea): On the signal going to the master volume, solder a resistor (not sure what volume) to the ring of the already existing jack that handles EQ Bypass. Replace the current mono cable on the foot-switch that I have with a shielded TRS cable, and have the switch alternate between grounding/enabling the EQ and grounding the resistor connected to the signal going into the master volume (which should attenuate it without making lots of smoke and fire I hope?). See picture bellow.
Concern: Adding a resistor to ground may cause unforeseen consequences.

3. Combination of 1 and 2 (not pictured): Solder a resistor to one end of the master volume to a 1/4" jack. Use an expression pedal with a potentiometer, one end soldered to ground and the middle lug soldered to a foot switch going to the wire leading to the resistor soldered to the master volume pot. The resistor will prevent the master volume from going completely to ground, the potentiometer would allow to vary how much the signal level is dropped, and the bypass switch would enable full volume.
Concern: Same as second idea

Thank you for taking the time to read this and I look forward to hearing from you.

PS: I know some basics, but I am NOT someone who is qualified to just go messing around inside a tube amp. I am NOT planing on doing any of these modifications on my own without first speaking to the person who designed and built this amp. I just want to know if these ideas seem like they could work and if anyone has a different idea.

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Re: Tube amp master volume expression pedal mod

Postby Papa Dog on Wed Dec 18, 2013 6:10 pm

gotta schematic for the amp?
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Re: Tube amp master volume expression pedal mod

Postby ccaviris89 on Wed Dec 18, 2013 6:32 pm

No, I'm asking more in general terms if the idea seems like something that would work. The amp has a master volume which, I believe, acts as a variable resistor between stages. I really don't know much beyond that.
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Re: Tube amp master volume expression pedal mod

Postby Clanger on Wed Dec 18, 2013 8:30 pm

A more usual approach would to have two channels, one with EQ and one without(bypass) each with their own volumes control. It least that is my design of homebrew.

Sending signal back out from post preamp stages is ok so long as it's buffered in some way, but otherwise heaven forbid if a decoupling cap were to go and you get anode voltage out to your pedal.

Master Vol implementations usually have a pot leg to earth, this is what makes the wiper a voltage divider. It's not so much about the resistive value as the voltage division.

Sorry may not have helped there much?

Tell us about the amp , it sounds good, I am a big fan of EQ bypass.
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Re: Tube amp master volume expression pedal mod

Postby ccaviris89 on Wed Dec 18, 2013 11:27 pm

Clanger wrote:A more usual approach would to have two channels, one with EQ and one without(bypass) each with their own volumes control. It least that is my design of homebrew.

Sending signal back out from post preamp stages is ok so long as it's buffered in some way, but otherwise heaven forbid if a decoupling cap were to go and you get anode voltage out to your pedal.

Master Vol implementations usually have a pot leg to earth, this is what makes the wiper a voltage divider. It's not so much about the resistive value as the voltage division.

Sorry may not have helped there much?

Tell us about the amp , it sounds good, I am a big fan of EQ bypass.


It is a tad unusual, I know because I couldn't find anyone else mentioning it.

Yea I agree, I don't think sending the signal out is a good idea, like I said, worried it'd drain the tone (what was an inch or two of wire becoming 6 ft each way...no good). The voltage shouldn't be a problem because it would have a ground shielding the pedal. I happened to speak to the builder the other day, he also agreed that sending the signal to a pot in a pedal will probably suck the tone out of the amp. Also, he said that the voltage at the point I was thinking of adding the pedal shouldn't be dangerous. He did think the second idea might be interesting and worth trying (basically, put a resistor from one lug of the master volume to a foot switch and either ground it or lift it. Otherwise, leaving the master volume exactly as it. My second idea). The only signal that would be running through the long cable would be to ground, so the main concern might be introducing a little noise on the higher (ground lifted) volume setting. I'm not going to do that by myself, but I might take it down to him next time I'm in the studio and do it under his supervision.

Considering how little I've said about the amp, you're quite helpful ;)

The amp is AMAZING!!! I don't know much on specifics, but it's based on a variety of amps, especially designs inspired by Ken Fischer. The amp itself is a prototype built by Jim Sabella of Sabella Recording Studios in Long Island NY. I do there web development and odds and ends there. I'd heard the amp as he was building it and wanted it, so made him and offer and bought it. He made a demo video of it a few months back:
http://www.youtube.com/watch?v=Hq1TUBPMNUI
That was filmed before I had him add in the EQ bypass and shortly after he added an effects loop. However, for that video, he temperately removed the effects loop.
The one downside of it being a prototype is that it's a bit cramped inside. If you look at 1:46 of the video, you can see the guts. Adding two separate channels with their own gain and switching between them would almost defiantly not fit. Also, I think my design might be less intrusive then a channel switcher. Adding a channel switching mechanism to an amp can often take a little bit away from the tone (as does adding anything, even my effects loop). With my approach, if the pedal controller isn't plunged into the amp it should have 0 effect on the electronics. Depending on hoe my second idea works, if I'm right on my thinking, the ground thing might just work...
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Re: Tube amp master volume expression pedal mod

Postby Papa Dog on Thu Dec 19, 2013 2:41 pm

SMF used a swiching system where the single was shunted to ground to "disable" either one channel or the other.
I dunno if there are any other amps which use so elegant a system and it doesn'rt require a relay.
Image
SMF pre-amp

I do not think that is what you are after, though.
Using the same kind of circuit for a volume drop should be possible.
I think that I might would try using a "trim pot" after the cap shown in the clean channel of the SMF and then to ground. That way you could select the amount of volume drop you would get. There is some fairly advanced and complex math that goes into describing exactly which values you would need, and you might have to change a couple other resistors between the pre-amp stage and power amp, but I do not know how you amp is built and the math is way above my skill set.

You might wanna present the SMF schematic to your tech and see if he can implement something like that easily and cheaply, without major changes to the amp or it's tone.

Good luck in your search and implementation.
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Re: Tube amp master volume expression pedal mod

Postby ccaviris89 on Fri Dec 20, 2013 6:12 am

Now that's certainly an interesting idea! Thanks for your insight Papa Dog!
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Re: Tube amp master volume expression pedal mod

Postby Papa Dog on Sat Dec 21, 2013 5:39 am

your welcome.
There is one other method which occured to me tonight.
You could have your tech put a trimmer between the cathode resistor and ground on the tube which does mast volume duty, and put a foot switchabe by-pass on that trimmer.
Image
similar to V3b in this schematic
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Re: Tube amp master volume expression pedal mod

Postby gusfinley on Tue Dec 24, 2013 12:22 am

It looks like the amp has a tube buffered effects loop. Do you run any effects in the loop? Try your expression pedal in there as a master volume. If it's a passive expression pedal it might work.

I have a buddy that wanted a master volume in his amp and we put his expression pedal in his loop and he now uses it for a master volume control.

Its hard to make specific suggestions, without seeing a schematic.

You could make a custom footswitch with two conditions:

A) clean with EQ no attenuation

b) EQ bypass with internal or external adjustment for balancing channel volume

You could run one instrument cable from your EQ bypass Jack to the Footswitch, Then run the effect loop to the footswitch, for your added attenation for the EQ bypass channel. Using one switch you could make both changes, then run the output to the expression pedal for a master volume then back to the return of effects loop.
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