Smoking 7.5k grid resistors on AC30/4 build - Oh, the Stink.

[chopstuck]

I'm building an AC30/4 from an old Lafeyette 4x EL84 chassis and on initial start up I get a hum and a sharp triangle sounding wave that goes up in freq. as the amp warms up on my bulb limiter. I have only the recto and power tubes in so far. When I hit full power the EL84 pin 2 resistor smokes almost instantly on any installed power tube. High pitched whine goes up still higher. I'm using the Stank-ray schematic and I can't get the voltages to stabilize with the power tubes in. Any hints?

[Bob-I]

You've got an oscillation going on. Look for the source of the triangle wave with your scope by checking at the input and output of every stage. Once you locate it you can troubleshoot the lead dress in that area. You won't get a stable voltage until this is fixed. Pictures may help.

[stubbyfex]

Hello!
I would try reversing the plate wires from the primary of the output transformer to the power tube sockets. If they were reversed, then the negative feedback would become positive feedback, and it would oscillate. Of course if you don't have feedback then it would not matter.

[chopstuck]

Yup, no feedback. I only have the recto and power tubes in. They all work in other amps. The PI and preamp tubes aren't in.

[Structo]

Do you have a speaker load connected?

[chopstuck]

Yes, correct 8 ohm load per selector.

[Bob-I]

I stick to my assessment. Been there, done that, got a T-shirt.

Since you seem to have a scope, just place it on the plate of the first stage, if you see the triangle wave there's your trouble stage, if not, move to the next stage. Once you locate the source you can chopstick wires around and locate the trouble.

[Stanz]

Hello!
I would try reversing the plate wires from the primary of the output transformer to the power tube sockets. If they were reversed, then the negative feedback would become positive feedback, and it would oscillate. Of course if you don't have feedback then it would not matter.

Not sure how this amp was built, but no feedback in an AC30. So if there is feedback, THAT would definitely be a problem!!

[Phil_S]

I dunno, I'm having some difficulty with the idea that oscillation will smoke a resistor. That thing smokes because there is a short somewhere. That's my thinking. I could be wrong.

As long as you 've got a scope and know how to use it, I'd cure the triangle wave problem first. You don't need power tubes in for that, as you suggest it originates in the preamp.

I'd also be looking for a serious miswire somewhere. Something grounded that shouldn't be grounded? Something that's got B+ but shouldn't have it? Leave the power tubes out and poke around with your volt meter. Be careful!

[Blindog]

I dunno, I'm having some difficulty with the idea that oscillation will smoke a resistor. That thing smokes because there is a short somewhere. That's my thinking. I could be wrong.

As long as you 've got a scope and know how to use it, I'd cure the triangle wave problem first. You don't need power tubes in for that, as you suggest it originates in the preamp.

I'd also be looking for a serious miswire somewhere. Something grounded that shouldn't be grounded? Something that's got B+ but shouldn't have it? Leave the power tubes out and poke around with your volt meter. Be careful!

I believe what happens is the oscillation causes excess current draw on the output tubes therefore toasting the resistor. Is that right Bob?
Hope you get it fixed.

Mark

[jjman]

Also check the grid leak resistors (220k ohms.) Are the 7.5k grid stoppers connected directly to the socket? Why 7.5K and not 1.5K ?

[Bob-I]

I believe what happens is the oscillation causes excess current draw on the output tubes therefore toasting the resistor. Is that right Bob?
Hope you get it fixed.

Mark

Exactly.

[tonestack]

Did you use pin #1 as a tie point for the screen grid resistor? Are the tubes old U.S. production? If so, you may have 7189As (super EL84s). Pin #1 is internally wired to pin #2 on the 7189A, which means that pin #2 is at screen grid potential. I have run across European tubes that have pin #1 internally wired to pin #2. My rule of thumb is to never use pin #1 as a tie point on EL84-based amps.

[Phil_S]

I believe what happens is the oscillation causes excess current draw on the output tubes therefore toasting the resistor. Is that right Bob?
Mark

Exactly.

OK, thanks. Lesson taken, and I appreciate that you took the time to respond.
Phil

[Structo]

I'll bet that is what it is.

Many times we tie pins 1 & 8 together.