Peavey Heritage VTX driver checking?

[nashvillebill]

Customer brought over a Peavey Heritage VTX. Found a couple of bad opamps, replaced those. Now have clean signal all the way to Pins 1 and 7 of U9, and through the caps to the collectors of Q6 and Q8 and thus to the bases of Q7 and Q9. Don't have any tubes in. Signal at Points 2 and 3 after Q7 and Q9 is real ugly. Is this to be expected without tubes, or does that mean Q7 and Q9 are bad (or perhaps Q6 and Q8) I see Q8 has been replaced in the past.

I assume the board must be pulled--that is, Q7 and Q9 must be soldered to the board, would've been nice if they were socketed. Shucks.

What's the latest advice on subs for the 6465 cathode drivers?

[nashvillebill]

I dunno why, but my schematic shows up as a blank screen, I've changed the file name, this forum software is very clumsy to post something with.

[Enzo]

It was probably one of the dead files, you need to either zip it or alter it. But here:
http://bmamps.com/Schematics/Peavey/...tx-sch+pcb.pdf

Parts are so seldom changed, socketing would just reduce reliability.

Without the tubes, Q7,9 have no circuit, and any readings are meaningless. Put some tubes in there and see what the circuit does. Don't start planning on replacing Q7.9 until you find out they are defective. If they test OK on a diode test on your meter, they are likely OK. The schematic has voltage ranges for the tube cathodes. The voltage across the 3 ohm emitter resistors tells the tale.

make sure you have +15v on the grids, and plate and screen voltages are present.

MJE15030 or 15032 are 8A 50w TO220, but would probably work there, as the very similar Music Man circuit uses them. A To3 sub might be the (ugh)NTE280.

[nashvillebill]

I pulled them, both the 6465 drivers are bad (measured on my little Peak transistor tester). Q6 and Q8 measured good on my tester but they may still be leaky. I have some 2N2222 that should work there I reckon. The 47 and 120 ohm resistors measured OK.

What about using some 2N3055 to replace the 6465? I've got a bunch of them and could try to match hfe between the two 2N3055.

Thanks!

[Enzo]

2N3055 is a 60v part. Note right on your schematic that the idle voltage at the cathode is as high as 90v, and certainly can go higher during operation. The subs I mentioned are higher voltage parts.

The little ones are probably OK.

[J M Fahey]

Wow !!!!! Q7/Q9 are TO3 metallic case?

MM uses perfectly adequate plastic TO220 cased ones there, but Peavey *always* built stuff strong and to last forever.

Incredible

[km6xz]

They build everything in China now so they are not long term investments anymore. T0-3 is becoming obsolete new, even TO-92 is being dropped, such a ON Corp(ex-Motorola) so many of those units we assumed could be repair far into the future will have to be done with scrap parts out of junkers

[nashvillebill]

2N3055 is a 60v part. Note right on your schematic that the idle voltage at the cathode is as high as 90v, and certainly can go higher during operation. The subs I mentioned are higher voltage parts.

The little ones are probably OK.

Thanks for pointing out the 90V on the cathode, I didn't print out the full schematic and it's easy to miss stuff looking at the schematic on the computer screen.

[J M Fahey]

They build everything in China now

I was talking about the 1981 model shown
As our Guru would say: "a Chinese Peavey is not a Peavey .... "

Can't post the MP3 here out of respect for Forumite ears and mental peace, but imagine me singing:
"I said built .... he said build ..... "
to the music of:

[Enzo]

Peavey did like to overspecify transistors, they put extra ones in their power amps.

Those particular TO3s have been obsolete for decades. They used the TO3s for their 100 watt VTX amps - four 6L6. The 50w Classic VX used a TO220 MJE15030 The TO3 was an MJ4247, an 8A 120v part. MJE15032 is a 8A 250v part.


The voltage ratings are in the notes lower right next to the power supply circuits.

[nashvillebill]

Customer doesn't want to modify the heat sink to use the TO-220, so as much as I don't like to use NTE, looks like I'll get the NTE280's.

I did find a bad tube, real shame...was a quartet of old RCA's. I'll pop a set of Sovteks in there for him and have him save a pair of the good RCA's for his '65 Bassman AA165. (If he ever sells it, the old RCA's would add that mystical "mojo" that people will pay more money for, not sure they actually would sound any better than the newer tubes he's got in it now)

I should've noticed the cathode voltage on the notes, also should've remembered that's how the amp was designed. Darned CRS.

[Enzo]

I don't recall, did we ask Peavey if they had anything to send?

The reasons I hate NTE are not that they don't work, so I'd bite that bullet myself.

The heat sink probably need no modification, the TO220 lies flat where the TO3 went, bend the two end legs backwards to fit down the B and E holes and bolt the collector to the heatsink. If the screw cannot be made to act as connection, a solderlug and short wire will make the connection.

Whatever you do, don't forget to check the two 3 ohm resistors. When a transistor fails, it sometimes burns them open.

[nashvillebill]

Ok, for others who may find this thread in the future, amp is now fixed. The customer agreed to use substitute TO-220's so I got the MJE15030G for Q7 and Q9. I laid them flat, also added a finned heat sink underneath the MJE15030G. I made insulators between the new heat sink and existing aluminum plate, out of a mica sheet. The MJE15030G screws into the not-connected screw hole. One leg was able to bend down into the existing hole, (the emitter I believe), I made flying leads for the base and collector. I replaced Q6 and Q8 even though they tested OK, Antique Electric had MPS6531's. I also replaced the 47 ohm, 120 ohm, and 1.5K resistors around them, since one of the 47's look a little heated though it tested good.

Before installing tubes I checked for 15 volts on the grids, looked good. I installed new Sovtek 6L6's and hooked up 4 ohm dummy load. Hooked scope up to output...no signal. WTF? After a minute or so it dawned on me, I had forgotten to plug the sig gen back into the amp input. After that, signal looked good on output. Amazing how that works huh?

Damn that amp is loud. That's a lot of oomph out of two pairs of 6L6's.

Thanks for everyone's input, Enzo is one of the Internet's most valuable resources.