Peavey Heritage output drive xisitors

[Mars Amp Repair]

Say,
Has anyone found some more generic cathode drive transistors for this amp? peavey wants you to use mj4247 (70406465) which I don't have.

I'm going to have to order some, but I have a bunch of the other TO-3 case peavey type transistors & wonder if anyone has found a suitable sub. I tried the more common 7048... & 7047... series with no luck.

I've looked at the specs for the MJ4247 & really don't see what is so special about this particular transistor.

Any help appreciated...thanx, glen

[Enzo]

MJ4247 is a 8 amp 120v common xstrs. I see notjhing special about it either. But Moto characterises its gain at 3 amps. PV is drawing how many MILLIAMPS through it? if MJ15003 or something doesn;t work, I might thinik about how those hefty parts perform at low currents.

When you use some more common part, how does the circuit not work? What is the beef.

And since we are replacing those, what else in the circuit could have failed and we are overlooking? The +v on the control grid, the +screen on the screen grid.

[Mars Amp Repair]

hey Enzo,
the number of transistors I tried just wouldn't bias correctly. Bias was way high & there is no adjustment in this ckt. I also couldn't find one of the resistors in the ckt that might change the parameters & get those xistors to function.

When I replaced the xistors with the called for ones, everything worked properly...so, I'm thinking there is some weird parameter that just isn't matching. I also agree that the higher powered transistors just don't like perfoming on the lower end of their gain curve.

I was hoping someone found another number that had worked for them. Thanx, g

[catstrat]

Peavey

Gene at peavey told me those transistors are sort of unique. Assuming you're talking about the cathode drive. He said they tried a field replacement with variety of TO 3s and none of them worked. The ones i replaced were already replaced in the past so i had a generic number to use and i will try to find it. But i got the replacements from MCM. He told me the guy who designed the amp with a cathode drive circuit did it just to show he could do it. Go and figure.

[Mars Amp Repair]

Isn't that a crock! I guess that's how engineers get their jollies ;-]

I had one Heritage where I just used PV#70481180's or one of those typical output transisitors & it worked just fine...this one will have none of that! Everything else in that ckt checks fine, too.

I've ordered them from Peavey, but it looks like they might be out of them & B/O...so if you can come up with the # I'll certainly appreciate it. glen

[Enzo]

I have made subs in the past, don;t ask me what now. But Glen, if you have made subs work in other similar PV amps, then even though "everything else" is OK< maybe it isn't. The cathode drive circuit is one thing, but is that +15VDC present on the control grids? (orwhatever voltage, something positive)

[Mars Amp Repair]

thanx Enzo.
I hear ya on if it worked on one amp...etc. You may have a point. I was not able to get this particular amp working with the sub xistors but was with some of the orig ones I got lucky & found buried in my stock.

I was really just trying to find a viable sub, but perhaps I already had as this particular amp with the original xistors in it is back with the xistors blown again. So, I guess what you are saying could be viable in this case.

I suppose when I get the stock transistors in & eliminate the drive transistors as a possible source of issues, it probably won't work either if there is something else wrong in there.

The cust said it squealed & then blew the fuse again. Makes ya wonder how it can work perfectly for you & not for the cust...an old story in the repair world, huh.

There must be something else going on there. Thanx Enzo....glen

Thanx man, g

[catstrat]

Peavey Cathode drive transistors

I checked my schematic on that Peavey and i believe the replacements i used were 2N 5884s. I'll try to verify that today. That's been 3 months ago and i've not heard anything from the customer so i assume it's still working. I'm guilty of not keeping very good records of the parts i substitute so need to change that.

[Gunny]

I have the same experience...meaning I fixed one of these using generic substitite transistors but can't recall what ones they were. Did you check to see if other transistors in the same circuit are bad? I found I had to replace 6 of them in total (3 per 'side').

[Enzo]

Three per side in a Heritage????

[Enzo]

I'd look into that Glen, because the circuit is not far off the Music Man drive, and we make those go with subs. The PV Classic VTX used cathode drive with the SJ5884s in essentially the same circuit - those are TO220 MJE15030. And isn;t that what we stuff into Music Man amps?

[Gunny]

I stand corrected - I was writing from memory earlier.

2 per side: Q6, 7 and Q8, 9.

Replacing those 4 got the amp working like a charm.

[Enzo]

Q7,9 are the cathode drivers. Q6,8 are the sense limiters. When the voltage across emitter ballast resistors for Q7,9 gets high enough (due to current through them) it turns on Q6,8 which then shunt the bases of Q7,9 to ground, turning them off. You do need to check all of them.

[Mars Amp Repair]

I'm going to get the original driver transistors in there just to eliminate that as a possibility for a source of problems. I have already replaced the driver limiting xistors Q-6 & 8 & of course checked that barrage of steering/biasing/mesmerizing diodes in that ckt.

Enzo not sure if you got that these drive xistors are T03's, not T0-220's like the Musicman variety, so MJE15030 would not work in here. Perhaps you were just using that for comparison.

I could very well have issues elsewhere in the amp, but it is a bit disconcerting that 2 other Heritage amps worked fine with just about any generic T03 driver transistors & this one would only bias with the originals...and now blown up again.

I'll keep ya all posted 'if' I ever get those xistors in from PV. Thanx, glen

[Enzo]

I am totally aware they are TO3s. I was making comparison. These TO3s are driving 6L6s, the very similar circuit drives 6L6s with the TO220 parts in other amps. I am suggesting the circuit shouldn't be so critical it needs the exact part to function since several divergent parts seem to work in similar circuits elsewhere. I wouldn;t want to be blind to a problem on pin 5 of the tubes for example.