Questions on the Peavey VT Series 240T Tube Amp

[TomCarlos]

MEF Members....

This is a follow up to my other post on the thread "Need help with Pevey vt series 240T amp trouble shooting" that was started by drummer125.

When looking at the schematics "and circuit board layout pages (posted on the net and this site) for the Deuce VT series, I noticed that the VT Series 240T was slightly different. Much different? No; slightly different. So please know that when you see the images I post below and you might have the same Series 240T amp.

The amp I am trying to save had a flashing tube on the far left (as you face the back of the amp. I describe it that way because is you look at the image of the board, it shows this tube as V2. I think it might be V1 (according to the schematic and it might be mislabeled on the board diagram). I will let the experts tell me. I found that one of the screen resistors, the one connecting to that far left tube was "open." And yes, the tube in that socket was obviously bad. The other tubes are fine. I then went looking for the other screen resistors. I found 1 on the output board and thought the other two were located somewhere else. But as Enzo pointed out, this amp has only TWO screen resistors.

So that is Question 1 - You'd anticipate that each output tube would have it's own screen resistor (at least I did). How does this amp get away with two of the 6L6 output tubes not having a screen resistor? According to the schematic and what I see on the pc board, the screens for V2 and V4 are connected directly to the +490 vdc connection point.

And for Q2 - I went looking for the Grid resistors and those are mounted on the bottom of the board. I did measure them and they are within spec (47k ohms). But good lord, if you had to replace those resistors, good luck getting that board out. You would have to remove the rivets to the output tubes to remove the entire board? I guess you heat up the solder, push the resistors off the board, then mount the resistors on the trace side?

As for Q3 - If you look at the schematic for the output section, it looks like V1 and V3 are a pair; V2 and V4 are a pair. I am not 100% sure the PC board diagram has the tubes labeled correctly. Can anyone confirm this?

I am intentionally posting the output board diagram image upside down as this is how I see it when working on the amp with the back side nearest me and where the tube socket on the far left is the one that had the issue.

Before I fire up the amp, I will do some additional cleanup work. But for now, I was curious to know who this output section works with just two screen resistors.

Thanks in advance!!

[Enzo]

Q1. On schematic at bottom in notes. Last one: items with * used in Mace ONLY. So R31 for example is not used on your Deuce. That means it is not there. Doesn't mean it is just left out, it means the board is wired without a place for it. Look at the six tube board or Mace and find there is a screen resistor for each tube. If you want to add a couple resistors, cut a couple traces and add them. If you want to make them 470 ohm like Fender, go ahead. If you want them 1k like a lot of others, go ahead. None of this is critical. The 470 and 1k ideas have worked for decades, but so has this. This 40-plus year old model has been working for those decades with this arrangement.

Q2. Well you can thank the lord, as that is exactly how you change them. More likely to change a screen resistor, but same same. I unsolder the legs of the resistor and push the old part out the other side. I fish it out from loose in the chassis, and solder a new part on the foil side. On some amps you need to leave clearance. Seems to me on the 5150, there is a switch or something on the rear panel that gets in the way of one of the screen resistors. So I mount it with an extra bit of lead so I can push it off center. Anyway, this takes just a moment to do, rather than yes, drilling out all the rivets.

Q3. I think you may be a little confused as to what is a "pair". V1 and V3 are drawn opposite each other, but they are no more a pair than V1 and V4. In my mind a pair is two tubes working together. So to me V1 and V2 are a pair. Imagine a rope and a tug of war game. One guy on either end. Now imagine we have four guys, and you want to talk about pairs. WOuld you think of the two guys on one end as a pair - like me - or do you think of two pairs, one on a side? I do agree, I think on your four sockt board they swapped V1 and V2 sockets. On the schematic, V1 has the screen resistor and V2 does not. ON the board layout, "V2" has the resistor and "V1" is direct wired. So swap numbers on those two sockets.

It doesn't matter which tube is across from the other. The ones on the top side all work together and the ones on the lower side all work together. The upper left one does not work individually against the lower left one, and so on. If I had the Mace with its six tubes, and I pulled V1, i could remove any one of the lower side tubes to leave it all "balanced". It matters not which one.

How does it work? The screen resistors are there to limit current, but the amp will work fine without them. The famed Fender 5E3 has no screen resistors at all. Just my opinion, but I always thought the Peavey arrangement was more about stability.

As to the typo on the schematic, I surely never let that concern me. As far as I am concerned, if the left end tube is flashing, I will deal with it. The resistors around that socket, the socket itself, whatever. At that point I really don;t care which number tube they call it.

If you have a different schematic, can you post it just for comparison? Or is the V1/2 thing all that troubles us?

[TomCarlos]

Thanks Enzo...

Let me start with the power amp is working! I did some spot soldering, check connections, etc etc etc. In fact, the amp passes a signal from the inputs! The pots are very dirty and hard to turn, so I will pull the main board and clean everyone. I am not sure if the Phaser will work without a footswitch but that is for another day. For now, there is plenty of volume. I can hear some hum so I am guessing the power supply caps will need to be replaced as a matter of an old amp needing typical old age service.

As for having a layout diagram (or schematic for that matter) as 100% correct, I agree- mistakes will be made. And I believe you validated what I thought - that V1 and V2 need to be swapped on the board diagram- no biggie. If I find a layout diagram that is correct, I will post it here. It is not that the output board is off, it is the main board that is different when comparing the VT vs VT Series 240T, in the lower right corner of the chassis (as shown below). The Power Amp board is fairly accurate.

https://www.thetubestore.com/lib/the...-Schematic.pdf

From your experience, I appreciate hearing about some amps not even having a screen resistor and that other amps like the Fender have different values (ie 470 ohms). I get that. I just wasn't sure why only 2 of 4 tubes would have the 100 ohm resistors. Maybe Peavey did see some instability so it was trial and error where they found adding only 2 resistors solved the problem, who knows. If I have nothing else better to do and as a learning experience, I will cut some traces and add two more 100 ohm resistors to the other tubes and see if there is any difference. After all, this is a "project amp" in my eyes. I sure don't see myself totting this beast around to a gig.

The truth is, I really don't intend to use this as a full feature amp. It might make a nice slave amp for someone. So after the PS section, I will turn my attention to the Power Amp Input section.

And here is an inside look at the chassis.

[Enzo]

Is there something specific you can point out so I can see a difference? The photo shows a board, but it just looks like a big board to me.

[TomCarlos]

One example, with the front panel facing you, I noticed in the lower right corner, next to R110 (Master Reverb Control), there is a 2.2uf cap on the edge of the board. I can see the trace on the layout diagram but I do not see a line showing that the Cap belongs there. For some reason, that is the first thing I noticed. So maybe it is just an error that someone forgot to draw the cap onto the layout diagram. If I have time tomorrow, I am going to pull that board to clean the pots and then I can see if the board has a number on it.

[Enzo]

Just looking at that small corner I can see that trace is the +15v rail, see how it goes to pin 8 of each IC? It could be left off the drawing, or it could be they decided to omit the part in production. Meaning your board is an earlier revision. They would not likely issue a whole new drawing for that little change.

I have to say...good eye.

[TomCarlos]

First things first.... I am going to do the cleanup, caps, pots, etc. I will then worry about the footswitch etc. I can find the footswitch connection points on the board but I think there might be an error on the footswitch schematic showing the pin numbers. Again, for another day.

My aim is to find the correct board layout diagram and schematic for this amp - the Deuce VT Series 240T. I am not sure what year this was built. Serial Number QA-553563. My guess for this amp was early 80's because the speakers are 67-8010. There may not be much different in the 240T version versus the prior.

My amp looks just like this one.

[Enzo]

Serial more likely 0A-553563. Zero rather than Q. Any chance there? That would mean 1980. I've been servicing VTs for thirty some years and never ran into a schematic problem. If there are differences, I never found them.

I often hear from people wanting to know what color wire went where or what pin number on the DIN jack, and frankly all I heard was confusion. I always just verified each connection functionally. For example on the schematic, the FS connections show pin numbers on the DIN connector, but from there wires spread out to the board. Look lower left corner of layout, R79. Associated with Q1 at the Automix input jack. Note R79 just goes to a wire pin and stops. It says pin 1 of the DIN, but who knows what color it is. Also the wire colors in teh FS cable...who knows? I plug the cable into the DIN jack and verify continuity to the wire points on the board from the switch terminals in the FS pedal.

Back in that era PV used a boatload of small 2uf/35v electrolytics in the signal paths. And some other small values. I would be VERY surprised if those were not all worn out. And if you run out of 2uf, 4.7uf seems to work just as well.

[TomCarlos]

It could very well be "0" instead of "q". It is hard to see. And yes, I have bunches of 2.2uf caps. The other thing I will look at before yanking the main board is to see if I can get the Phaser to work. I will hone in on U11, U12, U13.

Thanks again for the help !!!

[Enzo]

They used several TL604 switching ICs. Fo example that allows the phaser output into the mix. If you find a bad one, try swapping with the reverb one. They are hard to find, and there are even little work around boards to replace them. But a short cut is to use the reverb one, and just put a jumper wire in the reverb one socket so the reverb is just always on., like in a Fender.

[TomCarlos]

The Phaser Circuit has been fixed. It turns out someone pulled U7, either to replace, clean socket, who knows. I found that Pin 4 was NOT in the socket.

But this incident pings me to look for some extra TL604 chips to have on hand.

[Enzo]

Good luck. I did have a few in stock, but looks like we finally used them up.

[TomCarlos]

Ok... the amp is working and it's loud !

In addition to previous fix on the outboard board...

• Main board = 053178
• All electrolytic caps replaced.
• Balancing resistors in power supply replaced.
• Potentiometers cleaned (and it took some time as a couple pots were frozen).
• All connectors cleaned, including RCAs on reverb tank and Molex connectors.

The amp sounds a little "hissy" with no input and all volume (gain) knobs turned down - maybe that is normal? I'll try another set of 6L6 tubes now that the amp is fully functional.

With no footswitch and nothing plugged into the amp, I would not expect and do not see the LEDs on the front panel lit up. But is that normal? I cannot seem to find an Owner's Manual for the Peavey Deuce/Mace VT Series 240T (that has the inputs shown in the image below) to tell me how those LEDs are used. I tugged on the cable that goes to the Din connector and those LEDs temporarily lit up. But I cannot recreate that.

Normal Input
Automix Input
Effects Input 1 & 2

[Enzo]

This is the Mace VT "manual".
80345001.pdf (peavey.com)

Your schematic link in post #3 is the one for the VT amp. You can see the 053178 on the layout drawing, just it is backwards as we are looking through the board.

Right now I am watching football, enjoying adult beverages, breathing oxygen, and not really in the best shape for technical stuff, so...


Look at the FS. The channel select switch picks one of two, then through the combiner switch. The selects are channel killers, not enablers. So you pick channel A by killing B, and vice versa. The combiner switch opens the circuit so neither channel is killed, ie both on.

[g1]

According to that manual, LED's only function when automix jack is in use.