Peavey Delta Blues boost mod

[Ian S]

Hi everybody,

I have been reading on this forum for years and learned a lot. A lot of knowledge is available here. Thank you all for that.

So now it’s time for a question:
I recently acquired an older Peavey Delta Blues amp (1999-2000) . It has footswitchable channels and tremolo but the boost function can only be activated with the button on the amp.
I would like to be able to operate the boost with a footswitch but the only mod I have found is on the Blue Guitar site and is rather complex, adding a relay (which needs power).
I am looking for an easier way to create a foot switchable boost function. With easier I mean the switch should only make/brake a ground connection. That way it will be easy to build and also be save.

I think I found the solution in my VHT Special 6. It has a volume boost switch that is implemented rather simple. The (fixed) tone stack is grounded with 68K resistor and the footswitch just bypasses this resistor. Since both tonestacks are Fenderish I think a similar set up should also work with the Delta Blues. Just lift the grounded leg of the mid pot and connect it to ground with a 68K resistor on an extra jack at the back of the amp. The footswich wil then bypass this resistor. The effect will be different from the factory boost which is a midboost, but that’s not an issue for me.
Also without the footswitch connected it will not be switchable but I could even use a 60 K audio pot with a push/pull switch on the back of the amp (instead of a resistor) to make the boost adjustable and also switchable.

Do you guys think this will work?

Delta Blues schematic: http://www.prowessamplifiers.com/sch...elta_blues.pdf
VHT Special 6 schematic : http://www.vhtamp.com/pdf/VHT_Specia...ic_5-17-10.pdf

ian

[Steve A.]

I forget who it was but someone suggested using a pneumatic squeeze bulb like those used with an old camera to manually activate the Boost switch. A metal bracket could be mounted under the nut and washer holding down the adjacent pot. You can buy a shutter release bulb for $12.95 plus $3.50 s/h.





https://www.bhphotovideo.com/bnh/con...D&Q=&A=details

Squeeze bulb - Camera-wiki.org - The free camera encyclopedia

Steve A.

P.S. I find the stock boost circuit to be unusable as it bypasses the treble cap with a 0.047uF cap for a HUGE mid boost. I preferred the subtle boost of a 390pF cap but you could use a larger cap like .001uF or even .01uF. Because of the way the Classic 30/Delta Blues is constructed with 3 circuit boards folded in a U-shape it can be very tricky to work on.

One solution that requires no mods is to run a clean boost or graphic equalizer in the FX loop.



https://www.thegearpage.net/board/in...switch.308296/

[Enzo]

The solutions using a relay are switched by a simple pedal to ground. The switch is in the middle of the tone stack, so I would not recommend running a shielded cable from it down to a pedal, hence the relay.

[eschertron]

A footswitch breaking the ground connection at the bottom of the stack works for a simple boost (search for 'tone stack lift'). Adding a high-value resistor/pot to the circuit allows for additional control over the amount of boost. Try it and see.

[Enzo]

There is that. Do you just want a boost function or do you want to remote the existing boost?

[Ian S]

Thanks for the fast replys.

To answer Enzo's question: My first Idea was to remote the existing boost but it turned out to be somewhat complicated and the mid boost is not the tone I really like for a lead sound.
So I am going to try the "tone stack lift" and see if that's what I like.

Ian

[Steve A.]

Thanks for the fast replys.

To answer Enzo's question: My first Idea was to remote the existing boost but it turned out to be somewhat complicated and the mid boost is not the tone I really like for a lead sound.
So I am going to try the "tone stack lift" and see if that's what I like.

Ian

In most PCB tube amps it would be a fairly simple matter to disconnect the tone stack from ground to see if that is indeed the sound that you are looking for. However with the Peavey Classic 30 or Delta Blues there are many extra steps required because of the way the amp is constructed.

You need to:
• remove the chassis from the cabinet,
• disconnect and label all of the flying leads,
• remove the control knobs and nuts and washers for all of the pots and jacks,
• remove the 3 boards folded together in a U-shape,
• unfold them with breaking any of the hinged jumper wires,
• figure out the simplest way to break the circuit from the tone stack to ground and then
• perform the work which would probably be desoldering one of the leads of a cap.

Before you can test the results to see if you like the tone stack lift you then need to reverse most of the previous steps:
• carefully fold the 3 boards back together w/o breaking any of the hinged jumper wires,
• reinstall the circuit boards in the chassis being sure to reestablish the necessary grounds by putting the nuts and washers back on the jacks and enough of the pots to hold the 3 circuit boards securely and then
• put the chassis back into the cabinet.

All of that just to see if the tone stack lift boost is what you are looking for. If it isn't then you need to repeat all of the steps to reconnect the tone stack to ground. If it is what you are looking for then you need to repeat all of those steps while performing the actual modifications.

In a typical Fender PCB tube amp the whole job would be much easier:
• you need to remove the chassis from the cabinet,
• locate the simplest point to disconnect the tone stack from ground,
• desolder the lead from the cap or jumper and lift it up from the circuit board.

You can now plug the amp back in after reconnecting the speaker to see if you like the results. If not you then resolder the lead you lifted and reassemble the amp. Total time at this point would be 15 or 20 minutes compared to perhaps an hour for the Peavey (or much longer if you happened to break one of the hinged jumper wires.)

If you like the effect in the Fender PCB amp it might take another half hour to drill a hole in the chassis for the footswitch jack which you wire to the circuit board, test the amp and then put the chassis back into the cabinet.

Steve Ahola

P.S. When I first started modding my Peavey Classic 30 amp in 1997 there were not all of the great overdrive and distortion pedals which are available today. I had just installed a mod kit in my 1965 Pro Reverb making it a 2 channel amp with overdrive and relay switching along with a tube driven effects loop. The rebuilt amp sounded so much better than my Classic 30 that I decided to experiment with similar mods on the Peavey since at that point my other option would have been to sell it back to the store at half the price I had paid for it a few years earlier.

With all of the great overdrive and distortion pedals available today I don't recommend any mods on the Classic 30 besides replacing the tubes and speaker. Basically use the clean channel with pedals for overdrive and distortion. You can set the overdrive channel for a nice light crunch for alternate sounds with and without pedals.

[Enzo]

Look at the schematic. Use the Classic 30. The amp is the same circuit. The bottom of the tone stack is the pin 1 end of the MIDs pot. On the C30 layout you can see the traces. The ground in that area runs along the board edge, and there is a short trace from the pot leg to that ground strip. You would cut that little short trace. Then run a pair of wires to either side of that new gap. Those wires now wire to a switch.

making that change - cutting the tone stack ground - is very simple. Where the difficulty comes in is making it a footswitchable thing. You could use shielded cables from the cut trace out through a jack to a foot switch. But I don't recommend the signal path leaving the amp, that is just asking for noise. And so that brings us back to using a relay. Mount a tiny relay near the cut trace. There is no problem coming up with a DC power supply for the relay, after all there is already a relay there for channel switching. It runs off the -36v supply, so would our relay. That way the foot switch just handles a small DC current, no signal.

[Ian S]

Steve, Enzo, thanks again for the detailed answers!

I read the warnings about removing the pcb's from the chassis. I will remove the amp from the cabinet this weekend and see how scary it looks

Enzo's recipe is what I have in mind. Wire the gap and mount the pcb´s back in and then experiment a bit to see how it sounds and if its worthwhile to remote it to a footswitch.

What makes me wonder: I did not find a tone stack lift as a mod for the Classic or Delta Blues on the internet. If its a common thing then I would have expected someone has tried this before. Or did I miss them?

Ian

[Enzo]

It isn't something most people would do. it will give you a boost, as in louder, but you would lose all tone control. It was offered up as an idea to try and see if it went the direction you like.

[Steve A.]

[The tone stack lift mod] isn't something most people would do. it will give you a boost, as in louder, but you would lose all tone control. It was offered up as an idea to try and see if it went the direction you like.

Ian, if you are going to add a relay to lift the tone stack I'd suggest instead that you just wire the stock C30/DB boost switch to a relay. In 1997 I had the questionable idea of using the reverb relay and footswitch circuit for my modified boost circuit which involved cutting traces and rerouting signal paths.

Enzo pointed out a much simpler way to do that: wire the boost switch to an added relay "mounted like a bug with its legs sticking up" on an unused area of the circuit board. Tap the relay coil supply voltage from the other relays and wire it to an added footswitch jack. Wham bam and you're done- much easier than the convoluted route I took.

I wish Enzo had been around here 20 years ago when it was still AMPAGE because at that time none of the amp techs here wanted to have anything to do with a poorly engineered PCB amp like the Classic 30 - I don't think that you can unfold and refold those circuit boards more than 3 or 4 times before the hinged jumper wires start breaking - so I was on my own in uncharted territory.

Here is an edited drawing of the C30 tone stack; I removed the second set of contacts on the boost switch for clarity. I think that you would want to insert the relay contacts between the common contacts of the Boost switch and the CW terminal of the treble pot.



Perhaps Enzo has some suggestions on replacements for C11(.047uF) and/or R12(220k). This is the same boost circuit as used on the Classic 20 which I think is appropriate but might be overkill on the C30. (I suspect that R12 is used mainly to prevent pops when engaging the boost. 1M or even 4M7 should work just as well with less of an effect on the sound.)

Here is an annotated drawing of V1b and the tone stack. I realized much later that the 2M2 resistor and .047uF coupling cap act as a local feedback loop identical to that shown on the Dumble ODS schem sold by Pat's Schematics in the 90's (later schematics showed a 22M resistor.) With the 2M2 resistor the local feedback adds more compression to the signal.





Steve Ahola

[Enzo]

R12 just keeps C11 charged, so it can be whatever higher value you like. R12 and C11 would form a rolloff when boost is off bit for the size of the resistor. C11 just bypasses the treble cap to make it big and fat.

You could wire the relay contacts in series with S2A, yes. Then you have to be careful to leave the boost panel switch open for the FS to work. Or you could wire relay in parallel with the switch, but again it matters which position the switch is left in.

I was around for the C30 cathode bias mods which were on your web site ( I don't know if they still are), that was almost 20 years ago wasn't it?

[Steve A.]

R12 just keeps C11 charged, so it can be whatever higher value you like. R12 and C11 would form a rolloff when boost is off bit for the size of the resistor. C11 just bypasses the treble cap to make it big and fat.

You could wire the relay contacts in series with S2A, yes. Then you have to be careful to leave the boost panel switch open for the FS to work. Or you could wire relay in parallel with the switch, but again it matters which position the switch is left in.

I was around for the C30 cathode bias mods which were on your web site ( I don't know if they still are), that was almost 20 years ago wasn't it?

I just checked the AMPAGE archives here and it looks like you started posting in 2003.

I think it makes more sense to have the footswitch work only if the boost switch is toggled on. That way when the boost switch is off it can't get accidentally turned on.

Hmmm... at the time I thought that the .047uF boost cap was excessively large but it is wired in series with another .047uF cap on the plate of V1b reducing the effective value to ~.023uF which is not that different from the .022uF treble cap bypass boost cap used in the ODS. It makes me wonder if James Brown at Peavey had access to a reverse-engineered Dumble schematic, especially with the .047uF/2M2 local feedback loop not seen in many amps besides Dumbles in the early 90's. Perhaps even the same drawing that Pat's Schematics was selling...?

Steve A.

[Ian S]

I had the amp out of the cab a few days ago and the jumper wire construction sure looks fragile and you have to squeeze the pcb'stogether conciderably to get the pot shafts out.

I want to use the amp at a gig next week. Meanwhile I'll use a tubescreamer for the boost. See if I like it. Maybe that's a better option as Steve already mentioned.

Ian.

[Steve A.]

Adding a graphic EQ to the FX loop would further increase the sonic options of your amp. Or take it a step further with an Empress Para EQ. (While it lacks fully variable Q controls it does have a 3 position Q switch on each of the 3 bands.)

https://empresseffects.com/products/paraeq

Steve A.