Peavey Windsor Studio damaged by using wrong power tube

[TheTubeNoob]

Let me start by saying that I am aware of how dumb this was. I have a Peavey Windsor Studio which features an auto biasing capability that allows you to use ALMOST any octal power tube. I also have an old Gibson Duo Medalist that I recently replaced 7591 tubes on, and out of curiosity I put one of the 7591s in the Peavey thinking it was on the list of compatible tubes. The tubes lit up, but no sound. I put the original EL34 back in and now it is still no sound. It has two 12AX7 tubes that light up and the EL34 lights up. The power on indicator light turns on as well. There are 3 fuses on the circuit board and none are blown. There is absolutely no sound coming from the speaker at all, not even a hiss or hum. I hooked the speaker up to another amp and the speaker is fine. Looking at the circuit board there are no components that are visibly damaged. At this point I am thinking maybe since the 7591 pinout is different than an EL34 I must have fried something, just trying to figure out what. Does this sound like a shorted output transformer? Where else should I be looking? Thanks!

[pompeiisneaks]

The Pinouts on those two are very different.  The 7591 has pin 5 that is the ground and negative grid, and pin 8 and 4 are the screen, pin 6 is the grid.  The normal 6v6/6l6/el34 pinouts have 8 as grid, 5 as grid and 4 as screen, therefore you sent HT from pin 8 to ground and it may have burnt up things in that path.  I'd look at the grounds for that, and what may have burnt up.  That amp can only take 6L6 and EL34 from the schematic I see.  I hope its nothing too major. 

~Phil

[PRR]

Welcome.

You blew-up the screen supply.

[PRR]

> Peavey Windsor Studio

http://www.thetubestore.com/lib/thetubestore/schematics/Peavey/Peavey-Windor%20Studio%20Schematic.pdf

Look to R37.

[eleventeen]

Really different basing diagram between 7591 and probably *any* other power tube. Why, I could not tell you, but they are. I am sure you've received good information from the above posts. Wildly different tube basing diagrams.


I got a nice, heavy stereo amp chassis out of a Lowrey organ that uses 7591's. I would like to sell the 7591's and convert the amp to more conventional 6V6/6L6, because the 7591s are fetish items that fetch high prices, and these are the prized Westinghouse ones. I already knew the tube sockets would have to be rewired, but it is not quite that easy. On most Fender amps, pins 1 & 6 of the 6L6/6V6 are dead terminals which can be used and ARE used as tie points for the screen resistor and the grid stopper. Not only are those tube-socket pins tied up for other purposes in the existing amp, it is conceivable that an internal jumper (or common-pin) in the 7591 is being used to make a connection. So this is not just a "move pin 6 to pin 4, move pin 5 to pin 1" kind of deal. It gradually dawned on me as I was contemplating the changeover that it would be easier just to strip the amp and rebuild it for 6V6/6L6. (or, leave it as is, or, build an external "adapter" chassis that would have 4x male octal plugs and 4 chassis-mount octal sockets. = Goofy)

[TheTubeNoob]

Wow PRR, thank you for the info! I just pulled R37 and here is what it looks like underneath, definitely fried!! Checked on the multimeter and no continuity. What are the chances this is the only problem? Thanks again!!

[Ritchie200]

I'd like to know why the fuse didn't pop.  Are you sure they are sized properly?

Jim

[PRR]

> why the fuse didn't pop.

Fused 3 Amps at 120V. Fuse can pass 360 Watts.

I think he only shorted R37 (4K) across B+. B+ not specified, assume 400V. 400V/4K is 0.1 Amps. 0.1 Amps times 400V is 40 Watts.

40 is less than 360.

The "normal" amp demand is probably well under 100 Watts (dunno why they fused for 360W). 100W, 140W, is all the same to a 360W fused supply.

Also, when G2 is held low, plate current is about zero, saving another ~~20W. So it might really have gone 100W to 120W. "So?" says the fuse.

In general you can't size the line fuse to protect against internal faults (or mistakes). The line fuse protects against wall-cord burn-up. Maybe a MAJOR internal PT short, but on a many-winding PT like this, some windings can be shorted without much rise of line current.

[Ritchie200]

PRR, doesn't this one have an HT fuse? Sorry I didn't check the schemo.

Jim

[pompeiisneaks]

I see 4 fuses on the schematics I've seen.

1. right before the bridge rectifier: T1.5A
2. Bias: F1A
3. Pilot assembly, T3A
4. Mains: 5A Fast Blow

~Phil

[PRR]

OK, it has 1A fuse in the B+.

The 4K load pulls maybe 0.1A. Meanwhile the normal 0.050A plate current is down to 0.002A. So not even close to blowing.

[pompeiisneaks]

OK, it has 1A fuse in the B+.

The 4K load pulls maybe 0.1A. Meanwhile the normal 0.050A plate current is down to 0.002A. So not even close to blowing.

As always PRR with the super fast math brain comes to the rescue ;)

~Phil

[Ritchie200]

Haha! amen!

Thank you for the esplanation!

Jim

[TheTubeNoob]

Finally got a replacement in the mail for the fried R37. Got it in and the amp is working perfectly. I must have spent a few days staring at the circuit board trying to figure this out. Posted it here and had the solution in 15 min. Thanks PRR for the solution, and thanks everyone for the discussion!