Peavy Heritage VTX what is this output circuit?

[Enzo]

Does it follow the tube or stay with the socket?

Assuming OK tubes, I am back to a bad cathode transistor for that side.

[g1]

Ok then, check for a shorted/leaky transistor Q7 or Q9, whichever side has the fault.

[J M Fahey]

OK, with tubes in for more than 10 seconds, the side that was blue is actually red plating in a big way. Before, I didn't leave it powered uplong enough for this to occur.

Well, that´s actually "good" ... because it´s what we expected.

We were all very intrigued by those Physics Laws defying tubes .

Suspect shorted drive transistors as said above, but you should also use a fresh confirmed good pair of tubes there; I´d HATE blowing a perfectly good pair of transistors just because one (or both) tube has an internal short or something.

If VERY curious, test those somewhere else, in a conventional amplifier.

Personally I have very little faith in their health and even if they "work", they will be damaged or worn to death anyway.

[Randall]

Enzo, problem stays with sockets.

Juan, tubes are fresh, as mentioned they are recent replacements for two that failed.

I am waiting on word from customer whether to procede with estimated work. Both of the little transistors ahead of Q7 nad Q9 test the same in circuit, so I too am inclined to suspect the big Q7 or Q9.

While I wait for word from customer, the transistors are labeled simply 6465 on schematic, but are actually Hitachi 2SC1030 in the amp. Is this a potential issue?

[Enzo]

Stays with socket is another vote for bad transistor.


2SC1030 is probably a good sub.

The original is MJ4247, a 120v 8A part. Your 2SC is a 150v 6A part...close enough.


I would probably consider an MJE15032 an 8A 250v part. It is only 50 watt, but should be good. It is a TO220, but can be mounted over TO3 holes.

[J M Fahey]

Dunno.
Compare their specs to those used in Musicman amps and post them side by side here.

FWIW I have "temporarily" replaced the MM ones (these should be roughly same, since both drive same kind of tubes and voltages) with plain TIP31C ; "just to save the weekend show" and with instructions to Musicians "bring this back on Monday for proper replacement" ; because I find TIP strong enough both in voltage (<100V) , current (<1A) and dissipation (around 20W) BUT weak on Hfe (dc current gain @ 1A peak current) BUT Musicians never bring them back ("hey Juan, they work fine") so practice say7s they are not THAT critical.

Just beware TIP pinout (BCE) does not match Japanese one, which I don´t remember by heart but guess is EBC or CBE , check datasheets and actual PCB.

[Enzo]

Juan, check out my MJE15032, I used it in a ton of Music Man and PV amps.

[J M Fahey]

Thanks

[Randall]

"2SC1030 is probably a good sub.

The original is MJ4247, a 120v 8A part. Your 2SC is a 150v 6A part...close enough."

Enzo, how did you get MJ4247 from the 6465 on the print? I found some NOS 2SC1030 here, would this be a good route to go?

https://www.rfparts.com/2sc1030.html

[Enzo]

Randall, Peavey used house numbers on their power transistors, but they were not secret. PV published a semoconductor cross refereence which spelled out what the underlying type was for each number, so the part number is 6465. On the reference, it shows that is an industry type Motorola MJ4247.

I then look up the specs on that Moto part, et voila...


An NTE280 should work. I don't usually like to use NTE, but it ain't like there are a hundred of them here.

But that 2SC1030 ought to work too.


Peavey customer service will send that cross guide, but I know we have posted it here before.

[Enzo]

Here, open this thread, and scroll down to post #10.

https://music-electronics-forum.com/...ross+reference

[Randall]

Thank you Enzo for that clarification. It makes sense now. Knowing your adversity to NTE, I feel confident that the NTE280 is a safe choice as well.

So, I wonder if these 2SC1030's in the amp are replacements from a previous repair, and if so should we be wondering why they need to be replaced again?

[Enzo]

Well, the amp is almost 40 years old, who knows how many decades ago those transistors might have been replaced? Things fail. A bad tube could have taken out a transistor or the thing might just simply expire. I have had to replace them from time to time, but I haven't seen then failing in numbers, just random units.


I'll repeat my NTE stand. I don't like them, don't want to use them. But it isn't because they are bad parts. I object to their high cost. A $4 power transistor might cost $12. That is a lot for adding a poly-bag with the specs printed on it. Yeah we pass the cost along, but still.

Also NTE is not exact the same as the real part. It is something they feel will work in place of the real part. In a one-off, this is probably OK, usually OK. But guys use them in power amps where three or four transistors are in parallel at the output. One NTE mixed with three originals does not guarantee they will all share current or will track thermally. SO unless we plan to replace the entire set of outputs, we don't want to mix them.


By the way, on these PV amps and the Music Mans too, always replace the drive transistors in pairs. SO NTE would work, and as long as both sides get them, then they will be the same and should track.

[Randall]

The NTE280 is a 140v, 12A, 100watt NPN, so I feel pretty good about subbing a pair of those. At $12.50 a pair plus shipping it shouldn't upset anyone.

https://www.newark.com/nte-electroni...C-GUSA-SKU-MDC

Customer gave the go ahead, I pulled them and one is shorted, so I have ordered up a pair, and we shall see.

[Randall]

I installed a pair of NTE 280 today, and as predicted by this sage group, amp is up and running, and sounding pretty darn good. Thanks again guys!