Issue with ARC Ref 110 amp

Thanks guys. After replacing speaker cables and moving IC cables to a different preamp input, the fuse doesn’t blow anymore but I can hear a subtle, high-pitch hiss on the right twitter. It’s not there all the time but comes and goes and starts to appear after a while, I guess when the amp starts to heat. Any idea?

The hiss could be a tube or the speaker.  At least the amp is not blowing fuses.  Switch the speaker cables to see if the hiss moves.  If it moves, the problem is likely the amp.  If the hiss does not move, it is likely the speaker.

If the hiss moves, and the focus is back to the amp, check and adjust as needed tube bias.  Hopefully, you can adjust bias within ARC spec, which I believe is 65mV per tube. 

If the Ref 110 only permits you to set the bias on one tube in a pair and the other tube is a slave, I believe spec for the slave ranges between 57mV to 73mV.  That said, I try to source power tubes that match within 2 or 3mV to each other.

Assuming you can bias the tubes, that tells me you probably did not fry a bias resister.  If you come up with one or more wacko bias reading, then you probably fried one or more bias resisters.  If you know how to SAFELY do electronic repairs, you can change the bias resisters yourself.  If not, take the amp to a tech.  This is a "low-tech" fix.  Call ARC for ARC sourced bias resisters. 

WARNING:  there is lethal high voltage lurking around the innards of the amp.  If you do not know what you are doing, find a tech to replace the bias resisters.

OK, ... assuming you have the bias situation under control, tell me, ... how many hours have you racked up on your tubes.  If you are using 6550s or KT-120s, the useful life is 2000 hours.  The 6H30 driver tubes have a 3500 to 4000 hour useful life.  Do not be cheap, always use fresh tubes.

Last point, staying with the tubes.  Assuming your problem is not the speaker, and biasing is ok, you may have simple tube hiss.  FYI:  regardless of the age, tubes can hiss.  I just bought a Ref 6SE with brand new tubes.  One or more 6H30 audio circuit tubes in the right channel made a high frequency hiss.  ARC replaced the whole bank and all was good.

So, go back to my original suggestion.  If you are sure the hiss is not coming from the speaker and bias is ok, and you have localized it to the right channel in the amp, switch the tubes from the right bank to the left bank. Start with the driver tubes first.  If the problem persists, switch the power tubes. 

My guess is that if you eliminated the speakers as the problem, and you can bias the power tubes, you have one more hissing tubes. 

It is always possible that another passive in the circuit is causing a problem, but IME, tubes are a likely culprit.

Good luck and I hope this helps.  

BIF
                        

Thanks BIF I’m pretty sure it’s one of the driver tubes and I’ve resolved to change all tubes. Hopefully that’ll solve the issue.