Interesting mod to share

[rfgordon]

I did this experimental mod on an old Ampeg that I thought I'd share. I've never seen this done before--maybe that just means I'm the last guy to find out it makes the amp explode....

Anyway, in comes an old Ampeg Reverb Rocket, the old one with octal preamp tubes. Years previously, another tech (who's a great guy, BTW) had converted V1B from the tremolo oscillator into a second gain stage. The owner wanted me to find a way to dial back the gain (the amp was a raging monster--great for power chord rock--but he's a tele guy).

At some point in the amp's life two of the three input jacks had been eliminated, leaving empty holes.

The second gain stage had a 2K2 RK and a 25 mF CK. I wired the ground end of the CK to a 1/4 jack in hole #3 for a boost switch and changed RK to 10k. This is not new, it's textbook Dennis Kager from the old Sundown amps.

What I did on the first gain stage, though, was different. I lifted the ground side of that 25 mF RC and connected it to the wiper of a 100K pot (after experimenting with other pots), and grounded one of the other lugs. I mounted the pot into the #2 empty hole.

Thus, at one end of the sweep, the cap goes to ground, and at the other end it goes to ground thru 100k of resistance.

The control yields a very interesting and subtle voice change for the amp. Depending on what guitar you play, it has one or more significant sweet spots.

So now that Ampeg's panel is: Input, Voice control, Boost footswitch jack, Volume, Tone, etc.

The client tried it today and was all smiles. I just luv empowering someone's tone bone!

So anyway, it was an experiment that turned out pretty useful, so I thought I'd pass it on to the group. On a future build I might try higher pot values and other cap values to see what happens.

[novosibir]

A schem sometimes (or almost always) says more than thousand words

Larry

[klingo]

+1

a schem is good for europeans readers too

is it a degenerative feedback control ?

is it effective all along the pot rotation or do you think a 10K pot will act the same?(if RK is a 5/10 time lower value)

thanks for sharing.

olivier

[rfgordon]

Here's a schematic of the cathode cap control, crudely done in Paint. I met with the guy this afternoon (so he could pay me), and he's just raving about this new control. He tried a bunch of guitars with it last night, and the control yields a really focused sweet spot for each, but no two guitars like the same control spot.

So anyway, there ya go. I tried this with 1k and 100k pots, and liked the 100k better. I suppose you could try up or down further in values, and try other cap values, too.

[gearhead]

Guesstimate on my part; Isn't that dialing in some local negative feedback? The more you increase the pot value, the less effective the bypass cap is.

[novosibir]

This is a kind of an active gain control and enables you to cut/boost the gain for about 6dB - the higher the cathode resistor's value, the larger the range.

Larry

[rfgordon]

My idea with this (first with the 1k pot) was to be a gain control--to be able to roll on/off how much of the AC signal gets bypassed to ground around the RK. The thing is, in practice, it doesn't "sound" like a gain control. Rather, at certain spots in the range it really seems to interact with the particular guitar's voice.

Charlie told me today that he was trying to describe it to some fellow pickers, but he was having a hard time doing it. So he's ended up inviting several people over to hear it. I suppose it is regulating the local negative feedback inherent in the cathode bias of the triode. Whatever is happening electronically, it sounds really cool!

This amp uses a 6SL7 for the first two gain stages, and this control is on the first stage. I don't know how differently this would sound on 12AX7s.

If anyone else on the forum experiments with this circuit, I'd like to hear your opinions on the effect.

[novosibir]

If anyone else on the forum experiments with this circuit, I'd like to hear your opinions on the effect.

This is one of my 'more special' Marshall mods for the 2203 model already since a long time.

- First I swap the 10K cathode resistor on the 2-nd gain stage with a 5.6K
- Then I remove the low input and mount a 50KRA (neg. log.) pot there
- Finally a .68 cap from the cathode is wired to the pot and from there to ground

This is now the amp's 2-nd gain control, but to obtain a useful overall sound shape, the .001 across the Preamp Volume has to be removed, also the 470p across the upper 470K voltage divider's R in front of the 3-th gain stage. Otherwise you'd get a hiss-machine or - in the worst case - a squealing pig

Btw. I don't recommend this mod on the 3-th gain stage's cathode of a Wreck!

Larry

[klingo]

novosibir wrote

Btw. I don't recommend this mod on the 3-th gain stage's cathode of a Wreck!

heu... why not such a control on the first & second Wreck's cathode caps...with a 47k stereo pot...ok,ok, i stop blaspheming