Question about amp mod

[justinhedrick]

a friend of mine and i are taking about modding my VTM 60 to have a jack to use that you could control the pre gain volume using a volume pedal. here is what i'm thinking: using a TRS jack, i would send the signal to a volume pedal using a TRS to TS insert type cable, then back to the amp.

now i have several questions:

could it be wired up to defeat the pre gain volume knob on the amp? could I just use a standard passive volume pedal with a min volume setting to get this done?

does there need to be a buffer in the circuit since i'd be adding about 30 feet (15 feet to and from) into the signal path??

anyone ever done this? or know of any company that has?

[Hemisaurus]

Yes it can be done.

You may need a buffer, but either way you could inject a lot of noise in the cable run.

If you have an effects loop on that VTM, you already have a place to insert the volume pedal, though it won't defeat the master.

Let me look at the ugly ass schematic again 

[justinhedrick]

Yes it can be done.

You may need a buffer, but either way you could inject a lot of noise in the cable run.

If you have an effects loop on that VTM, you already have a place to insert the volume pedal, though it won't defeat the master.

Let me look at the ugly ass schematic again 

thanks for the quick reply hemi! i want to use it to control the PRE amp volume, not the post.

[VOLVO)))]

So, you want it to clean up without losing any volume?

[Hemisaurus]

Looking at it, it's probably a giant waste of time, the pre volume is just a 1M Ohm pot in line before any gain stages, you could fit a switch to either take it out of circuit completely or just short it to the 'full-on' position and use a volume pedal in the regular way.

Is the High Gain input affected by the pre gain atall, looking at the schematic it looks like the High Gain input just bypasses that pot, so Volume Pedal > High Gain might be what you want?

If you didn't want to drill an extra hole in the amp, I'd suggest using a pull pot with a switch on it and replacing the pre control with that.

http://www.tubedepot.com/rv24a01f-10-15r1-a.html

[Hemisaurus]

http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/newamps/peavy_vtm_120.pdf

OK, I officially hate the stupid way Peavey did this schematic, it's too fucking hard to read for me without maximum zoom and lots of scrolling.

Disregard all that previous crap.

If I'm reading it right, and god knows it's hard to read, the High Gain input goes from the front panel board to the first triode of the first tube on the preamp board, then comes back onto the front panel board to be injected into the low gain input.

If you use the High Gain input, what I'd do would be make the Low Input a switching TRS jack, like the High Gain input, and rewire it so that rather than disconnecting the High Gain input to itself when a jack is plugged into low, it actually functions as an insert if jacks are plugged into both, and make it your volume insert.

If you use the Low Gain input, because of the pedals in your chain, put the volume pedal at the end of the chain and don't worry about it.

[justinhedrick]

http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/newamps/peavy_vtm_120.pdf

OK, I officially hate the stupid way Peavey did this schematic, it's too fucking hard to read for me without maximum zoom and lots of scrolling.

Disregard all that previous crap.

If I'm reading it right, and god knows it's hard to read, the High Gain input goes from the front panel board to the first triode of the first tube on the preamp board, then comes back onto the front panel board to be injected into the low gain input.

If you use the High Gain input, what I'd do would be make the Low Input a switching TRS jack, like the High Gain input, and rewire it so that rather than disconnecting the High Gain input to itself when a jack is plugged into low, it actually functions as an insert if jacks are plugged into both, and make it your volume insert.

If you use the Low Gain input, because of the pedals in your chain, put the volume pedal at the end of the chain and don't worry about it.

now, if i'm reading this correctly, it still wouldn't REALLY accomplish what i want to do, would it?

i want to be able to control the "pre gain" knob in a foot pedal. those jacks are still before the volume knobs in the circuit, aren't they?

[Hemisaurus]

Depends how authentic you want it to be.

With the pre jack full on, your signal is connected to rhe far end of that 470K.

If you want to be uber authentic, you'd recreate that circuit on the output jack and have yiur volume pedal in place of the 1M pot.

Just ramming the control full on, and putting the volume pedal in circuit before it will have much the same effect though, depends how much work you want to do.

Personally I wouldn't even bother converting the low jack, and just put my volume pedal at the end of my pedal board.

[justinhedrick]

Depends how authentic you want it to be.

With the pre jack full on, your signal is connected to rhe far end of that 470K.

If you want to be uber authentic, you'd recreate that circuit on the output jack and have yiur volume pedal in place of the 1M pot.

Just ramming the control full on, and putting the volume pedal in circuit before it will have much the same effect though, depends how much work you want to do.

Personally I wouldn't even bother converting the low jack, and just put my volume pedal at the end of my pedal board.

you speak true words of riff rock wisdom, sir.