Advice on Repairs for Audio Research VT100 Mk II

@edward78 said:

I’m fine with the price of the resistors, 6550 tubes, and labour but the price for the 6992 tubes seems too high. On the JJ Tubes website, these tubes sell for about EUR 20 a piece and they do not even offer matched sets of 6992. Do the 6992 input tubes really need to be matched?

The VT100 and Vt100 MK II are good sounding amps. But they are not user friendly when it’s time to retube the 6922 signal tubes. The average audiophile is not qualified to bias the 6922 signal tubes. If not done properly, biased within ARC specs, damage will done to the amp. It can damage power tubes, will cause power tube(s) screen resistor(s) to blow. That can damage circuit traces when the resistor(s) blow.

Do the 6992 input tubes really need to be matched?

Not just matched... Both sections of the 6922 tubes must be tightly, closely, matched... Especially the driver 6922 tubes. IF not tightly, closely, matced it will be impossible to set the ARC spec bias voltage within specs.

Link Below will show you just how involved it is to bias the 6922 to ARC’s specs.

http://mycollins.net/audio/artube1.html

ARC complete retube bias instructions:

https://www.arcdb.ws/Database/VT100/ARC_VT100MKII_bias_adjustment.pdf

FWIW: Not every tube vendor out there will take the time to sort through umpteen 6922 tubes to find tubes that have tightly, closely, matched Triode sections. Close won’t get it!.. Even if the sections are tightly, closely, matched the tech may still spend hours moving the 6922 tubes around to bias the tubes to meet the specs for each channel of the amp.

I don’t know how much you paid for the amp, but you may want to buy something else.

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Yikes.

13 hours of time seems excessive for the damage. The resistors were damaged by the power tubes FWIW. Its reasonable to expect that every time a power tube fails a resistor could be damaged- and so will need another trip back to the shop...

If the tubes are really that difficult to match, I'd be looking elsewhere- this amp sounds like a liability.

You could try reaching out to ARC service department. They have been very responsive and helpful in my experience.

First get more information directly from ARC to make certain that this is not a hard amp to maintain, and that it would not demand more than it could offer in the long run. You bought it, you cannot change that, but you can negotiate with your tech, fix it and then try to sell it minimising your loss. Unless you want to keep it, in that case you have to learn how to. All above are solid answers to your question.

Many thanks to everyone for the very helpful responses.

As to dealing with local tech shop, I also prefer just trusting the tech guy to give me a fair price, but when we're talking EUR 2,300, I want to understand what I'm paying for and why. I don't think that's quibbling or disrespectful. As to the burden if an issue comes up later, the shop will bear this burden for only 6 months, which is the guarantee they're offering. It's better than nothing, but not exactly confidence inspiring.

I didn't realise that the input tube matching was so complex, as described in  Jea48's post. This was really helpful and makes me think that the price isn't so bad after all. 

I also thought about contacting ARC directly, but that feels a bit like going behind the back of the local authorised repair shop. I would also expect ARC to defer to the opinion of the local shop. But maybe they can give me an opinion on the expected cost/benefit going forward if I go ahead with the repair, as petg60 pointed out. 

And yes, this amp already feels like a liability. I love the sound but the amp has needed two repairs within the first 6 months, so it's really put me off the whole tube amp thing. The first repair was replacing two fan resistors, which I did myself and was actually kind of fun, but I didn't enjoy it enough to say, yeah, I'm going to invest the time and energy to learn DIY tube amp repair and replace 20 resistors. Thus, my idea was to get the amp repaired, sell it, and go back to transistors. But if I go ahead with the repairs, I will have invested EUR 4,700 and I'm sure I wouldn't get that much on the second hand market. So either I accept the loss and move on or I use the amp for (hopefully) a few years before the next repairs are needed. If I knew I could get 4 years out of it without another repair, I would choose this option.