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Advice on Repairs for Audio Research VT100 Mk II

Hi everyone,

I purchased a second-hand Audio Research VT100 Mk II for about EUR 2,500. After about 1 month, the fan died, which turned out to be faulty resistors near the fan. After about 6 months, there were some loud pops, and two more resistors were blown near one of the power tubes.

I sent it to the authorised Audio Research repair shop here in Germany and they quoted me the following for the repair work:

  • Replacing 10 Resistors 43100004 (at EUR 2.00 per resistor)
  • Replacing 10 Resistors 43100208 (at EUR 7.50 per resistor)
  • Replacing 8 6550 Tubes with J.J. Tubes (at EUR 20.50 per tube)
  • Replacing 8 6922 Tubes with J.J. Tubes (at EUR 83.95 per tube)
  • 13 hours of labour (at EUR 75 per hour).

For a total of EUR 1,905,60 plus 19% VAT, i.e. EUR 2,267. I pushed back and asked if really all of the tubes needed replacing and they said yes. I also pushed back on the price of EUR 83.95 per tube for the 6922 and they said that these tubes needed to be matched and therefore the costs include labour, shipping, and tuning/matching of the tubes.

Does that seem like a fair price for the work? I’m of course reluctant to spend on repairs what I’ve already spent for the amp itself. 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?

Any advice or comments is much appreciated.

Thanks, Edward

edward78

Showing 3 responses by atmasphere

atmasphere's avatar
atmasphere
11,575 posts
 

There are 8 power tube 100 ohm 5% 3 watt screen resistors. (Part # 43100208) ARC uses the resistors as fuses. The wattage value should never be increased.

@fsonicsmith @jea48 

The resistors should be adequate to survive a tube failure. Series fuses should be employed as well. When the fuses start blowing you know you have a bad tube. The fuses should be placed so that replacing them isn't a royal pain the rear. This is proper engineering; it prevents frustration with the product and allows the user to understand that tubes fail and are mounted in sockets for a reason.

From a manufacturing point of view, the less you see of the product once it ships out the better. When it returns under warranty, it eats into the bottom line and is 100% avoidable in this case. 

Letting the resistor be the fuse isn't (my opinion of course) proper engineering. It requires a trip to the service department to do something that the user should be able to do in the home without tools. It risks damage to the equipment along the way and encourages the user to seek alternatives away from the product. Some users might get frustrated and try to replace the resistors themselves- this can result in all sorts of headaches/liabilities for the manufacturer.

atmasphere's avatar
atmasphere
11,575 posts
 

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.

@edward78 FWIW, there are tube amps out there that don't require resistor replacement (which might mean a trip to the service shop) every time a tube fails!

IMO/IME this is a design thing; it is possible to calculate how much wattage the resistor needs to be rated for when a tube fails and does its worst- its not rocket science. Some manufacturers have sorted out this issue and some haven't.

When looking at a new or used amplifier, its a good idea to ask the manufacturer "what happens if a tube shorts out- what damage (if any) will it cause?"

atmasphere's avatar
atmasphere
11,575 posts
 

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.

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