Peavey amp

Tube or solid-state, this section goes to eleven!
User avatar
Jmpj48
Information
Posts: 32
Joined: 11 Nov 2016, 21:08
Has thanked: 1 time
Been thanked: 2 times

Post by Jmpj48 »

It’s a good amp got a good gain channel on it. Been nodding it to Try to get in the high gain territory. Sucks not having a tone stack on the gain channel can’t get a scooped sound.

User avatar
george giblet
Resistor Ronker
Information
Posts: 404
Joined: 30 Dec 2008, 17:27
Has thanked: 206 times
Been thanked: 309 times

Post by george giblet »

It’s a good amp got a good gain channel on it. Been nodding it to Try to get in the high gain territory. Sucks not having a tone stack on the gain channel can’t get a scooped sound.
It should be possible to get the tone control in the signal chain. The problem is you want to get a scooped sound but you don't want to affect the tone. To some degree that's a contradiction since we have to affect the tone to get the scoop. IMHO the best plan of action would be to go for the idea in the patent where you only tweak the treble cap. Perhaps even use a large R7 value (on VSS-20 schematic) and tweak C5 to taste (VSS-20 schematic).

While that's straight forward, you can't just change those values (not yet).

The problem is I don't trust the VSS-20 schematic ATM. If we don't know what the existing circuit really is it's hard to suggest a mod. I'm fairly sure the VSS 20 schematic has an error around relay contact SW1-A. The SW1A-wiring is wrong and it is not clear what it is. If your PCB has two 22K's around the tone controls then the circuit is likely to be close to the Bandit 75 and not the supersat patent.

I've attached what I believe is the correct relay (Peavey Relay 71322277 OMRON G6A-274P 12V). The contacts are on page 8.
You can see pins 4 and 13 are the common contacts. On your VSS 20 schematic it doesn't even show pin 13. So clearly something is wrong.
In gain mode pins 8 and 9 and selected. So the bottom contact SW1-B (on VSS 20 schematic) actives the second stage stage. However, for SW1-A it connects pin 13 to pin 9, pin 9 goes to C5 and R7. I have a feeling pin 11 is not connected and that pin 13 might go to R27 and C10, not to the treble cap C8 and treble pot R14.

You need to trace your board to get an idea what's going in. Especially confirm pin 11 of the relay is not used and that pin 13 of the relay is going to R27 & C10 (or C8 and pot R14 - whatever circuit you have).

You can see there's two possibilities for the existing circuit,

If you look at the supersat patent the SW-2 relay contact (SW1-A on the VSS 20 schematic) switches the cap and resistor to the top connection on the treble pot.

If you look at the Bandit 75 schematic the switch bypasses the whole tone control. The switch is drawn nicely in that when the relay connections are to the right it is in GAIN mode. The 68nF cap and 22k resistor connect to the second 22k resistor. This effectively takes the output of the previous opamp (U1A) on connects through to the input of IC2A, effectively bypassing the tones controls. The matches-up very well with the block diagram.
Attachments
Peavey Relay 71322277 OMRON G6A-274P 12V en-g6a.pdf
(1.3 MiB) Downloaded 168 times

User avatar
george giblet
Resistor Ronker
Information
Posts: 404
Joined: 30 Dec 2008, 17:27
Has thanked: 206 times
Been thanked: 309 times

Post by george giblet »

Something occurred to me about the goal of getting a notched EQ.

On the Peavey circuit the tone control is *before* the clipper, so even if we get the Tone control working in gain mode you probably won't get the notched sound you are looking for. For that you would need to add a notch circuit at the output of the clipper, something like a bridge-T network. That will EQ the *clipped* signal.

User avatar
Jmpj48
Information
Posts: 32
Joined: 11 Nov 2016, 21:08
Has thanked: 1 time
Been thanked: 2 times

Post by Jmpj48 »

Yes I definitely think there is something missing in the vss schematic. Your right the bandit 75 schematic looks just like the block diagram I posted from the vss manual.

User avatar
teemuk
Breadboard Brother
Information
Posts: 147
Joined: 17 Aug 2009, 19:54
Has thanked: 3 times
Been thanked: 55 times

Post by teemuk »

The "Supersaturation" generation of Peavey amps didn't generally have any notch EQ after the tonestack - except for the effects of current feedback in power amp that emphasizes high and low end response.
The Bottom/Body/Punch tone controls (featured in later amps) provided level response with controls at noon. The Supersat channel has very distinct mid-range emphasis (prior to distortion) on its own and rolls down higher frequencies post. Tone controls (if featured) were for finer tuning not for additional coloration as is.

User avatar
Jmpj48
Information
Posts: 32
Joined: 11 Nov 2016, 21:08
Has thanked: 1 time
Been thanked: 2 times

Post by Jmpj48 »

Good to know I’m learning a lot about peavey circuits thanks.

User avatar
george giblet
Resistor Ronker
Information
Posts: 404
Joined: 30 Dec 2008, 17:27
Has thanked: 206 times
Been thanked: 309 times

Post by george giblet »

The other day played around with getting the Tone stack to have an effect in Gain mode. I tried a couple of things one was along the lines of tweaking the parts in the Supersat patent. A few conclusions we clear,

1) With the tone control it attenuates a good deal of the lower mids.
2) With the larger treble cap switched in, it causes the mid control to affect much lower frequencies say 200Hz.
You won't get a classical sucked-out mid anywhere near the 400Hz to 1kHz region.

It may be possible to counteract (1) to some degree. In the original circuit the C5,R7, R27 creates a high-pass filter around 210Hz. C25 and R49 creates a high-pass filter around 720Hz. With the mods the tone control is already taking out a good deal of 200Hz to 700Hz, so the idea is
to flip the roles of high-pass filters (C5,R7, C27) and (C25, R49). You let the modified tone control approximate the 700Hz roll-off then you increase C25 by a factor of 3.4 to move the cut-off down to take the place of the 210Hz roll-off. You will need to adjust the actual values to tweak the tone for sure.

That lets the tone control have an effect and tries to keep the original tone but it still doesn't allow the mid control to give a suck-out sound, and the mid control is before the clipper not after. So even with all this juggling about it still might not do what you want.

User avatar
Jmpj48
Information
Posts: 32
Joined: 11 Nov 2016, 21:08
Has thanked: 1 time
Been thanked: 2 times

Post by Jmpj48 »

Thanks for messing around with it to see what you could come up with. It’s just weird why they would build a amp that u can’t have a tone stack on the gain channel. Don’t make no since. They could of least put a fx loop on it were u can run some kind of outboard eq. Seams like peavy like this tone stack they used it a lot in there 80’s amps. If u look at the peavey tg raxx Schematic pretty much same circuit accept his has tubes. Same tone stack in the front. Aleast they put there famous active eq after the gain stage lol.

User avatar
george giblet
Resistor Ronker
Information
Posts: 404
Joined: 30 Dec 2008, 17:27
Has thanked: 206 times
Been thanked: 309 times

Post by george giblet »

It’s just weird why they would build a amp that u can’t have a tone stack on the gain channel. Don’t make no since. They could of least put a fx loop on it were u can run some kind of outboard eq.
I suppose it all boils down to economy. The early 80's and 70's Peavey amps were mostly dual channel. They obviously tried to strip things back on the Bandits and the VSS-20. The presence on the Special kind of edges up by adding one more control. However, ultimately the economizing didn't work out as the later Bandits go back to separate tone controls.

Fx loops weren't common on any of the earlier Peavey stuff. Some of the late 70's/early 80's amps had Pre Out/Power In sockets where you could break out the signal.
Seams like peavy like this tone stack they used it a lot in there 80’s amps.
Yes it was used very early on. The earliest example I know of is the Peavey Deuce 2x12 Combo, about 1976. It has aluminium knobs, before the blue/grey plastic knob stuff. It was also one of the first models with the hybrid tube-transistor power amps. The preamp is transistor. You can see the amp has two version of the tone stack Bass/Middle/Treble and and Bass/Treble version. The schematic is on the last page of the user manual,

https://assets.peavey.com/literature/manuals/deuce.pdf

IIRC, there was an early Mace model which was similar.

User avatar
Jmpj48
Information
Posts: 32
Joined: 11 Nov 2016, 21:08
Has thanked: 1 time
Been thanked: 2 times

Post by Jmpj48 »

The vss is a really good sounding amp with the chorus kicked in. And it’s really really loud for only 20 watts. It would make a really good recording amp. Hey if I add a fx loop do u know were I would put it. After the post volume? If I was using a delay pedal would it come out in stereo. Since it’s a stereo circuit.

User avatar
george giblet
Resistor Ronker
Information
Posts: 404
Joined: 30 Dec 2008, 17:27
Has thanked: 206 times
Been thanked: 309 times

Post by george giblet »

Hey if I add a fx loop do u know were I would put it. After the post volume?
Checkout the Bandit 75 schematic, it actually has the FX loop.

User avatar
Jmpj48
Information
Posts: 32
Joined: 11 Nov 2016, 21:08
Has thanked: 1 time
Been thanked: 2 times

Post by Jmpj48 »

I don't know what resistors or capacitors i would have to add or takeout to add the fx loop.

User avatar
george giblet
Resistor Ronker
Information
Posts: 404
Joined: 30 Dec 2008, 17:27
Has thanked: 206 times
Been thanked: 309 times

Post by george giblet »

I don't know what resistors or capacitors i would have to add or takeout to add the fx loop.
You will need: 10uf electrolytic cap, 1M resistor, 47k resistor
2x 6.5mm Jacks, one with a normally closed switch contact.

Using your VSS 20 schematic as reference,

- Lift the negative terminal of C31 (2u2)
- add a 1M resistor from the negative terminal of C31 to ground
- Wire the negative terminal of C31 to the FX Return Jack.

- connect the negative end of a new 10uF electrolytic cap to the PCB hole where C31 used to go ie. to the track that goes to the POST gain pot wiper.
- add a 47k resistor from the positive end of the 10uF to ground
- wire the positive end of the 10uF cap to the FX Send Jack
and also to a normally closed contact of the FX return Jack.
You need a normally closed switch on the FX Return Jack so it bridges
the FX Send to FX Return when there are no jack inserted.

If you look at the Bandit 75 schematic is follows what I've described here quite closely.
The only difference is the Send on the Bandit goes to an opamp whereas the Send
of the mods has to go to the POST gain pot.

User avatar
Jmpj48
Information
Posts: 32
Joined: 11 Nov 2016, 21:08
Has thanked: 1 time
Been thanked: 2 times

Post by Jmpj48 »

Thanks for the info I’ll let you know how it turns out.

Post Reply