Blown 6550 tubes and resistors in ARC VT 200

Yes, this seems to have been ARC's methodology to use power resistors as fuses for the VT series amps. I went through this with the VT130 a few times and after the second time, I bought a dozen resistors so I would be ready when the next power tube failed. It was an easy task to remove the amp's bottom plate and solder in a replacement resistor .... then a few more minutes to re-bias the amp and then back in business. It was quite a loud explosion when the resistor blew up!

It won't totally eliminate the potential for this occurance, but keeping a closer check on the bias will go a long way towards reducing this problem.

And the bias current is still affected by AC line voltage even though ARC has added some circuitry to help reduce this effect. So using a power regenerator, like an Exactpower EP-15A for instance, can eliminate that part of the problem.

Another factor that affects bias is ambient temperature. Make sure your amps have plenty of unobstructed ventilation. And when room temps start to rise (like summer here in Tucson!) it's always good to re-check the bias and adjust it downward if necessary.

I never owned a VT series amp, but had the same (blown resistor) problem with my (fan cooled) M300s, and after taking the above steps, it never happened again.

I am now enlightened. However, before leaving this thread, could someone provide, in simple English instructions, steps on biasing the driver tubes (6922) for the Audio Research VT 200? I have read the ARC instructions, however, I am of the belief that someone in audio world could provide a simpler and similar explanation. I have already biased the power tubes. I would'nt getting some tweak tips here also. No, I do not want to send it to ARC for biasing. It is too costly, tubes and all about $1499 plus two way shiping!!!!

Jaguar, it wouldn't really do any good to send them to ARC, even if you didn't mind the expense, because the amps should be biased, after warm-up, while idling (no input signal connected) using the AC they normally use.

I'm sure someone here is experienced with biasing the VT 200. If all fails, ask for Chris Osanna at Audio Research (Service Mgr.) He's a very nice guy. Arrange a time with him and I'm sure he'll be happy to walk you through it.