Wha' Haponed?? Blown 1 OHM bias/current sense resistors

[Gaz]

A musician has a 100W amp I built, and called me up saying that it 'died'. No fuses were blown, but that there was zero sound coming from the speaker.

He lives far away so took it to a tech. Today he got a call from the tech saying that the 1 OHM bias resistors were 'defective'. Clearly what he meant is that that the resistors fused. (Cost my friend $120.00 by the way!) He's had the amp for a year, so clearly they were not "defective" and blew for a reason. The are mil-spec 1W units.

Apparently the tubes are fine, which is surprising, and I was puzzled to know exactly why they blew. After a little detective work, I found out that he was running into two 8 OHM cabs with the impedance selector set to 8 OHM, not only that, but one of the cabs had a non-working speaker, so an amp wanting to 'see' an 8 OHM loading, but getting less than 4 OHMS!

He also noted that he just started doing this after a gear-loving friend told him to do so

I know funny and bad things happen when you mismatch impedances, especially the way he did (into a lower load), but I couldn't technically explain why they blew.

Does it make sense this would cause them to blow, and if so why?

Thanks!

[Firestorm]

I can't think of anything other than a serious tube short that could open those up. I hate it when someone else diagnoses a problem and leaves it at "Oh, yeah, the tubes/transformer /resistor/frangistan was defective."

[Gaz]

Seriously, then it also makes it look like it was my fault!

I should have mentioned that there was 1 resistor per pair of tubes, so it puzzles me that they both went at the same time. Has to be related to the mismatch, right? Or could a single tube short somehow cause that?

[Firestorm]

Mismatch? No. Something else.

[Gaz]

Any other ideas? I guess at least if it happens again, it's the tech's problem entirely

[Randall]

By one ohm bias resistors, do you mean one ohm cathode bias measurement resistors? Find out why both tubes are drawing enough current to open both of those. Perhaps the bias circuit itself is defunct?

First thing I would do is remove the output tubes and see if there is a bias voltage.

[Gaz]

Hey Randall,

Thanks for the reply, but not sure if you read the original post. The amp has already been 'repaired' by another tech who says the power tubes are good. I was inquiring to see if anyone could guess why they fused.

[Randall]

I read the post. I am wanting to know what exactly you mean by bias resistors. Are they one ohm cathode resistors to measure current? Are they something else? A fault in the bias supply intermittent perhaps, could have let the tubes draw too much, popping the cathode resistors, and maybe still look OK, according to said tech.

[overtone]

This could be a communication mix-up, can you not talk to the tech directly?

A tech report along the lines of "tubes now fine (meaning replaced & biased), defective 1 Ohm resistors replaced" could lead the customer to focus on defective resistors.

tony

[Gaz]

This could be a communication mix-up, can you not talk to the tech directly?

A tech report along the lines of "tubes now fine (meaning replaced & biased), defective 1 Ohm resistors replaced" could lead the customer to focus on defective resistors.

tony

Yes, this did cross my mind... I'll see if I can get they tech's email.

@ Randall. I see, yes, they are the current sense resistors in each pair of EL34 power tubes.

[Reeltarded]

Only you, this guy, and the tech know about it? Ever watch Breaking Bad?

I know somebody. Just sayin'...

[Gaz]

Haha, no, and I'm from New Mexico!

[Reeltarded]

Bad things happen to all kinds of people, all I'm saying. There might be an accident, or a happenstance.. or even a situation.