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

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?"

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.

There are 8 power tube plate 1 ohm 2 watt 5% BLUE LINE resistors. (Part #43100004.)

https://www.arcdb.ws/Database/VT100/ARC_VT100MKII_schematic_and_parts_list.pdf  

https://www.arcdb.ws/model/VT100

 

@edward78

Did you or the guy you bought this amp from install new 6922 tubes in the Amp?

Or maybe did the guy you bought the amp from pull all the tubes from the amp for shipping? If yes each tube would have to have been marked for each tube socket they were removed from and reinstalled in the same tube sockets they were removed from...

I agree with Cleeds......I sent back my Ayre solid state amp and it cost me about that for a couple of resistors that were a bit out of spec.

There are 8 power tube 100 ohm 5% 3 watt screen resistors. (Part # 43100208) ARC uses the resistors as fuses.

Yes, in contrast to what Ralph said this was not an engineering goof (though some would maintain it is) but a conscious design choice by ARC, allegedly for the sake of sound quality. Finally, with ARC’s latest Ref series of amps, they feature fuses for the power tubes rather than resistors, soft-start with relays, and auto-bias. They threw in switchable pentode/triode to boot. They claim-and I have no reason to disagree-that they have simplified the circuitry enabling them to decrease the NFB. So FINALLY ARC has made a real effort to spare the customer of much of the previous ARC-induced pain. Now if they would only give up on the Tungsol KT150 as being inherently unreliable and unjustifiably expensive and model their top tier amps to accept more reliable power tubes :-)

I believe that ARC's long-time love and appreciation for nearby Magnepan and its loudspeakers has had an unfortunate influence on ARC's amp designs. ARC amps are designed with difficult load handling as a top criterion. Neutrality is now another top criterion. The KT150 is king of the heap for power, low end grunt, and neutrality in the tube world. 

Don’t get me wrong. I love ARC. I am in the "the preamp is the heart of a good system" camp and imho the Ref 6 and Ref 6 SE are as good as a preamp gets. There are different but equally good preamps, there may be some marginally better preamps at far greater cost, but not by much. I am not quite so bullish on ARC amps as I am on their preamp designs. I stick with ARC amps for the sake of optimum matching with my beloved Ref 6 pre. If the situation with the dreaded KT150 does not improve next time I need to re-tube, I may very well switch to a different brand of amp.

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.