Peavey MX VTX 112BW - Blowing Power Transistors

[kdawg]

Hello - I have a Peavey MX VTX that had one of the SJ6465 power transistors (Q5,Q7) and the driver transistor (Q4,Q6) shorted. I replaced all 4 with the same transistors and powered it up - I was getting audio out on Low power, then I switched to High and it blew a power transistor again (I should have been monitoring Vi current, dumb move). So I replaced transistors again. Now in standby - Vi current is 38mv and 42mv. Switching to Low power, it shoots up to 75mv and 80mv. I'm only doing this briefly now because I don't know what it can handle before blowing. I've checked all tubes with tube tester, grid resistors, and voltages from the schematic:
- both 0.522 V are correct
- R73 is 1.2 V

6L6GC voltages on "Low" Power (all 4 the same):
1,8 (Vk) - 40V and 38V on each pair
3 - 265 V on all 4
4 - 265 V on all 4
5 - 15 V on all 4

Vk says it should be between 75 - 90 V, is that for high power, seems that 40 and 38 are about 1/2 of that for low power?

So my question is, does this look normal or too much current? It's passing audio again on low but I don't want to blow any more transistors. I'm scared to try "High" power again until I know... The R73 4.3k is a bias adjustment right? Do I need to reduce that value to get Vi lower?

Schematic for this version amp:
https://irationaudio.files.wordpress...-schematic.jpg

Thanks!

[Enzo]

In no particular order:

Q4,6 are limiters, not drivers. If you removed them the amp would still function. But don't

If Q4,6 appear shorted, they are more or less parallel with Q5,7, the tube drivers. SO if Q5,7 are shorted, they can make Q4,6 appear so. test them again with the bad large transistor removed.

Where did you find the 6465s?

Find out what is wrong with the amp first before you think about modifying it.

All voltages assume high power mode. I would not assume any low power values.

The MJ4247 is an 8 amp transistor, I don't think current is an issue. On the other hand it is only rated 120v. In my world I would likely manipulate an MJE15032 in its place - like Music Man did. It is a 250v 8 amp part.

test those 6L6s in another amp, not a tube tester. testers can verify a bad tube, but cannot tell you a tube is good.

Pull the power tubes, set to high power. What voltages are around the socket? Got +15v on all the pin 5? Got 500v or so on all pins 3 and 4? Any pin 8s shorted to ground?

Your driver transistors have 3 ohm 5w ballast resistors in their emitter ckt. Are they healthy?

Once you have verified the 6L6s will work in another amp. Pick ONE tube and stick it in one socket. Does the amp power up? If so, move that tube down one socket, same test? Check all four sockets. Now pick one socket and try each of the four tubes in it. A lot of cross verifying, but you want the amp to work.

[kdawg]

Thanks again Enzo for such a detailed and quick reply... I ran through all your steps/checks and everything checked out, I got the 6465s from cedist.com but looks like they're no longer in stock. I should probably switch to MJE15032 to be safe, but I did power everything up with the old tubes and it seems to be working OK in high/low. There seems to be some low volume not normal noise/distortion on decaying guitar note, I'm thinking it needs a new 6L6 set (they are very old looking Peavey tubes) - I didn't see it on the scope out of the power transistors.

[Enzo]

I trust CEDist, if you already have them, great. Anyone who has them will run out sooner or later as PV hasn't made them in years.

Just to be sure, connect different speakers to the amp. Noise on note decay is a classic sign of rubbing speaker voice coils.

[g1]

Also, are your measurements meeting the Vi and Vk requirements stated in the schematic notes?

[kdawg]

I found the distortion was actually something pressing on the speaker just enough, so that's good. Vk voltages look within range for High Power, but Vi looks like more than double. I'm just measuring across the 3 ohm resisors. Here's the Vk / Vi for low/high power:

Low: (for each set of tubes)
Vk - 43v, 41v
Vi - 78mv, 72mv

High:
Vk - 80v, 77v
Vi - 88mv, 82mv

It's sounding fine, just want to make sure it won't blow itself up again when dig in to it.

Thanks

[g1]

I think you should be fine. That works out to around 6Watts at idle per tube.
I'm not sure where they got their numbers from but they seem kind of odd. At the min. Vi numbers they give, it would be 1.5W per tube, and the Vi numbers don't seem to correspond to what you are getting for Vk, which seem correct.