Problems with a Peavey Classic VTX 65 watt amp.

[THumming]

Hello, I am kind of new to posting a question, so here goes.

I recently acquired a Peavey Classic VTX 65 watt amp. It’s looking like there are some problems with it.

There are 2 channels on the amp. “Normal gain” and “Lead gain”. By default the Lead channel is active when you do not have the foot switch. To activate the Normal channel, you need the foot switch. Unfortunately I do not have the footswitch to this amp. The way I was able to activate the Normal Gain channel was to jump a wire on the rear DIN connector(6 pin version) from pin 4 to ground. I’m also able to turn reverb off by grounding pin 2 and for the phase I’m able to ground pin 3. I still have not been able to determine how to activate both channels simultaneously. I think that is one of the features of this amp. I have also heard of this being referred to “warping”. I look to build a footswitch for this amp. I noticed a couple issues.

1. When Normal Gain is active, the volume for that channel is acting strange. I turn it to position 1 and its immediately loud. Then form position 1 – 6 there is no variation in the volume. Then from pos 6 – 7, it gets super loud.
2. The second issue is with the phase. When I activate it by turning up the depth and range, I can hear slight noisy oscillation but there is no depth. When I play a chord, the effect is next to nothing. I think the circuit is oscillating but there is something wrong with the depth control. Pots for both controls seem to be good according to an ohmmeter.

I can’t seem to find the schematic online anywhere for this amp. I can find the VTX Heritage schematic but not the VTX Classic series. It looks like the Heritage might be similar to the Classic but I can’t be sure.

One problem was the Potentiometers were very dirty. I removed the pre amp board and cleaned all the pots. No more crackles and pops now. I was kind of hoping the cleaning of the pots would fix the phasing and the issue with the Normal Gain volume. Both issues are still there. When I use the default “Lead gain”, the volume on that channel behaves correctly. The problem with the Phase is still present on the “Lead gain” channel.

The next step for me will be to contact Peavey for a schematic. From Internet searches, Peavey has a very good reputation with providing schematics.

Has any one here had similar issues with this amp?

[Enzo]

This amp is not made to have both "channels" on at once. One could perhaps add a mixing stage then replace the switching JFETs, and shunt across Q8 to leave the post always active.

PV schematics are not in abundance online because they will just email them to you when requested.

The very simple FS wiring is part of the schematic.

Pull the funny volume control and check it with an ohm meter. You may have a broken element inside.

Phasor? First thing to my mind is the little 2uf e-caps at the in and out ends of the delay string. Dried up? All the phasor is is a side path through a delay line and then mixed back in at the depth pot. Make sure the depth pot is OK, then simply trace the signal through the delay stages. Missing anywhere? There are no special OTAs or any TL604s in this circuit, just plain old vanilla op amps.

[THumming]

Thank you very much for the schematic!!

I'm have not heard back from Peavey yet. I'm sure I will hear from them tomorrow.

So, this amp does not mix the two channels together. That would explain why I couldn’t seem to find the right pin combination . Per the schematic, it just has Channel / Reverb / Phase. That works.

I did pull the amp apart again. On your suggestion, I desoldered the pot and checked it again. I even saw it was a linear pot by setting it at the center and checking each side. Each side was about 120K – 130K. On the back it says it’s a 250K linear pot. I actually thought it was a little odd it was a linear pot. I’m used to seeing audio taper pots with volume controls. I checked against the schematic you sent and low and behold, it says the “Norm Gain” volume is a 10K linear pot. Now I’m wondering where do I get a 10K pot. I start checking the pots on board against the schematic and I see the 10K linear pot is soldered in where the “equalization / high” control is. The two pots were interchanged. I desoldered the 10K and put it in the “Master Gain” control and the 250K in the “equalization / high” spot. I put the amp back together and now the volumes on both channels work well.

As for the phase, I verified the correct value caps were in the indicted positions in respect to the parts placement diagram and the schematic. I will have to chase the signal through the delay bridge as you suggested. The master volume was more important to me.

Oddly enough, I did not notice any problem with the “equalization / high” control. Never felt like I needed more from the pot. I guess I usually keep the tone on the low side.

Having the schematic made a huge difference. Thanks again. When I originally check the pot, I had no reason to believe it was in the wrong spot. Thinking about what I was hearing and describing, it kind of makes sense. It just never occurred to me that pot value is incorrect.

I’m going to go after the phase next. I might invest in some new speakers. The cones on the Scorpions are crude up with some thick hardened dust.

[Enzo]

PV will send you any schematic you request. Calling is the best way for quick response.

COne dirt, tooth brush.

You verified the correct value caps were in place, but did you verify they were not dried out? You probably do not have any 2.2uf caps, but slap a 4.7 in there if you have it. Much more common value. 2uf caps are more common in older transistor amps though, so if you do that sort of work, buy a handful to have.

[THumming]

I took another pass at addressing the phase issue. On your suggestion, I checked the caps and they seemed to be good. I started chasing the signal through the delay stages with a scope. It seemed that I lost the signal at pin 1 of U4A (4741). I checked what I had laying around and found a LM324 op amp still in the radio shack package. The pins match the 4741 so I swapped U4 with a LM324. Things quieted down. I now could adjust the depth and hear the sound change. I still had no range. I was looking at the schematic and noticed that the range was being controlled by part of U5. I did have another quad op amp chip in my parts bucket. The number was mc3403p. With some goggle searching, it looked like the pins matched. So… I swapped it out for U5 (another 4741). The Phasor is working correctly now. I’ll probably grab another LM324 from radio shack (if they still sell them). Maybe even order a couple of 4741s from digi-key. You mentioned that there was nothing special about the op-amps, do you think a LM324 will hold up? I played around with the amp for about an hour, the chips were not warm. The volume was rather low.

At this point, I thinking I have a nice Peavey amp. I probably won’t use the phasor too much but it would bug me that there were bad parts in the amp. Who knows, maybe over time they could cause problems elsewhere. Something that really impressed me was all the chips are in sockets. It makes changing them a breeze. You don’t see that in most consumer electronics. It only costs a few cents if that. I wish more manufactures would do that. Desoldering chips is an art within itself. Hats off to Peavey for doing that.

Thanks again for the help Enzo !!

[Enzo]

If a LM324 is working, I suppose it will continue to do so. I generally just put a TL074 in the socket. I sure wouldn;t bother tracking down an old 4741

[derull]

peavey classic vtx

thanks to you folks for the useful information! i myself own a classic vtx for about 6 months now and it immediately turned out as the best live amp i ever had (and i had lots of marshall, fender, peaveys, diys etc).

i will check the circuit, because the one thing that is bothering me is the post volume that - in my amp - works on both channels and thus does not allow to adjust the lead and normal channel independently. it would make the amp much more flexible (i like the distortion of the lead channel), although i use the vtx primarily with pedals and switch from normal for rhythm guitar to lead for solo, adding just power and a little additional "poweramp-style" distortion by turning up the saturation pot. that works fine

one recommendation: adding an open back cab with celestion v30 type speaker(s) gives the amp a much more "classic british rock tone" and smoothens the (kind of extreme) beam of the stock speakers significantly.

regards

arndt