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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@edward78

The VT100 MK II first came out in 1998 though 2000, (maybe through the spring of 2001). The amp is at least 21 years old. I am surprised the Tech that gave you the price for the repairs didn’t mention that. How about the electrolytic caps in the power supply? They are probably original. They may still be ok and last for many more years. But... I would suggest you call the Tech and ask him what it would cost to replace the caps.

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

If it were me, I would lick my wounds and move on and caulk it up to experience learned... You will end with a whole lot more money in the amp than it is worth.

FWIW, there is a very good chance the guy you bought the amp from knew the amp was ready for a complete re tube.

..

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...

Owners Manual for the VT100 and VT100 MK II.

https://www.arcdb.ws/Database/VT100/ARC_VT100MKII_manual.pdf

Note, page 1:

Diagram indicates relative positions of all (16) tubes on the two circuit boards as viewed from the front and looking down from above the amplifier. 

Next go to page 2:

2) ........ match its location "V' number (written on the base of the tube) to the "V" number printed next to each socket on the circuit board. (see accompanying tube location diagram.) Firmly seat each tube in its matching socket.

Next go to page 4:

Output Tube Bias Adjustment

Note, no mention of the input and driver tubes Bias Adjustments.

Just a guess most of the VT50, VT100, VT100 MK II, and VT 200 amps were damaged by their owners when they bought their own tubes and did a total re-tube of the amp. If the owner only replaced the power tubes there was no problem... Problem was when the 6922 tubes were replaced... Nothing in the owner manual about needing to bias the 6922 tubes. Just the output power tubes needed to be biased... Problem is if the new 6922 tubes Triode sections were not tightly, closely, matched and the new tubes were not biased to ARC specs, (especially the driver tubes), damage to the amp will be the result. and sometimes the damage is extensive.

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@77jovian

The ARC VT100 MK III does not require the 6922 input and driver tubes to be biased. Only the output power tubes need to be biased...

It can take hours to bias the 6922 tubes in a VT100 or VT100 MK II...

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