Advice on Repairs for Audio Research VT100 Mk II

Yes, you are correct, only the output power tubes need to be biased.

@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

 

Hi Edward. There is a lot of wisdom here on this forum.

 

The first question I would be asking myself is “am I in love with this amplifier”?

 

The first question I would be asking myself is “am I in love with this amplifier”? If the answer is yes, it makes me very happy and I do not want another then proceed with your repairs.
 

The amp is worth about 2500 US in full working condition, and probably about $1000 US for parts or not working. If you have the budget, I would consider selling the amp what I could get an upgrading to a more modern ARC product.

 

The amp is worth about 2500 US in full working condition, and probably about $1000 US for parts or not working. If you have the budget, I would consider selling the app for what I could get an upgrading to a more modern ARC product.

 

 ARC used resistors as fuses for years. I have had a personal experience with my VT 130 blowing up resistors and causing me much frustration.

 

 ARC used resistors as fuses for years. I have had a personal experience with my VT 130 blowing up resistors and causing me much frustration.

If you were working inside a tight budget repairing the amp would be a lot cheaper than buying a newer one. 
 

Tube gear reminds me a lot of being a Ferrari owner. They’re not that expensive to buy but they are terribly expensive to own. That being said there is nothing like them! To the uninitiated or uninformed that statement would make zero sense whatsoever.

another consideration is replacing the resistors your self. That is what I did, and I had a lot of fun working on my ARC amp with the understanding that if I failed, I was screwed anyway, learning about the amplifier and its components was a wonderful journey for me. Your amp is old enough and inexpensive enough to afford the education. 
 

Another consideration is the mess Europe is in right now facing a cold winter with limited energy. You may be able to buy a much better amplifier at a deep discount in February or March.

 

Another consideration is the mess Europe is in right now, facing a cold winter with limited energy. You may be able to buy a much better amplifier at a deep discount in February or March.

 

I understand your frustration as I have been exactly where you are. At first I was very unhappy about it, but it really became quite the adventure and I am better off for it.

 

I hope.  and I am better off for it.

 

I hope this bit of information is helpful.

 

 

I had a VT 100, MK III and loved it.   So my comments are based on experience with that particular iteration.
 

Probably, you’ll want a tech to replace the resistor and test to ensure that everything is copacetic.  
 

But it doesn’t take much skill to replace and bias the tubes yourself.  Bias adjustment instructions can be found at the Audio Research Database (arcdb.ws I believe), so you can read it and see how comfortable you are with the procedure.  
 

You can buy a package of matched tubes specifically for your amp at the Tube Store.  They have good selection and pricing, and their customer service is excellent.

 

I used to retube and bias mine in about an hour.  It’s a wonderful amp and I’d hate for you to give up on it.

Since you mention that it is your first tube amp I want to mention that hopefully you are aware that you should not turn a tube amp on and off and back on quickly without giving the capacitors a couple minutes to discharge.  That can lead to problems such as you describe.

I think I did do that a couple times because I was following another rule that someone told me: always turn on the source (DAC/pre-amp) before turning on the amp. Once or twice I forgot about this and turned on the amp first, so then I turned it off, turned on the DAC/pre-amp, and then turned the amp back on. Doh. 

According to the schematics the VT 100 MKII is a fully balanced design. The offset between pos and neg branch needs to be set to 0 and that's taken care of by some ss circuits between the 6922 (some adj. can be made with RV1/RV2 trim pots).

The 6922 are double triodes and it's of course harder to match those tubes because you need a lot of them in order to find 4 tubes per channel where the two triodes in each tube are also about the same quiescent current. That makes it time consuming and expensive. In general Audio Research biases  their output tubes very hot and they need a lot of tubes to select from in order to reach those goals. That's why it is so expensive if you go to Audio Research for retubing.

Many things can go wrong if one tries to do this buying tubes (even matched) somewhere else unless one fully understands how this amps work. Schematics are available online.

To use the VT100 only makes sense IMO(considering the high cost) if one uses  a fully balanced system (at least preamp to amp).

 

Since you mention that it is your first tube amp I want to mention that hopefully you are aware that you should not turn a tube amp on and off and back on quickly without giving the capacitors a couple minutes to discharge.  That can lead to problems such as you describe.

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...

Thanks again to everyone who responded. The tubes were already installed when I bought it and I had a car share and a short drive, so I didn't bother removing the tubes for the drive home. I think he said that the tubes were fairly new. It was my first tube amp, so I didn't know which questions to ask. The former owner is actually really helpful and offered to help pay for the repair if I get the work done at the tech shop here in Berlin that he regularly uses. I however wanted to get it done at the authorised dealer in Hamburg, which he considers to be overpriced.

Some responses above have suggested eating the costs and moving on, but that would mean selling the amp as defective and I wouldn't expect to get more than EUR 500 for a defective amp. That would mean a loss for me of EUR 2,000. Or do you think I could get maybe EUR 1,000?

So, at this point I think I will either bite the bullet and get it repaired at the ARC authorised dealer or have it sent back from Hamburg (which is another EUR 100) and get it fixed by the tech guy here in Berlin recommended by the former owner. I just don't feel like lugging this beast around any more. I don't have a car, so moving it always entails doing a car sharing or taxi/Uber.

I also sent an email to ARC as suggested by some of the posters, so we'll see how they respond. 

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.

.

Op,

I definitely understand your dilemma. Honestly, I think the repair estimate is absurd. thirteen hours.

 

Also, calling ARC is in no way “going behind your repairman’s back”. That is an absurd quote. 

Audio Research makes some of the best amplifiers… regardless of the technology. You are talking about a 20 year old amp. A couple repairs and it is obviously got problems.

I would not loose faith on tube amps, or Audio Research for that matter. I have owned quite a few Audio Research components over the last forty years and have gotten. Nothing but incredible sound quality and reliability. I would get something more contemporary. I own all Audio Research equipment, and have enjoyed their sound and reliability for decades.

 

@cleeds - thanks for pointing to the warranty, I obviously missed that. 

@jea48 raises a very good point about the caps. 

Sorry for your troubles, OP. It may be throwing good money after bad if that makes any sense. But perhaps worth asking about recap cost-- what else would normally go wrong in these? That old Dual 75a I have has no chips that I'm aware of-- but it was recapped a while ago and will probably need another go-over before I consider running it again. 

I've never heard of biasing the small signal tubes in an amp just power tubes, that design seems very user unfriendly to me.

I agree with you Ralph and have a great Thanksgiving. As I said, ARC made some clunkers. Some of their amps were almost impossible to bias without taking to a good technician. And many dealers were forced to become proficient at replacing power tube resistors. I still maintain that for a large (by relative standards) tube amp specialist, their build quality is good and many have been reliable designs, again all things being relative. I know that your own tube amps are at the far spectrum of reliability. Kudos to you. At the end of the day, any electronic component will eventually need service unless pitched for something else, particularly tube amps. 

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.

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.

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. 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 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?"

... 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 ... this amp already feels like a liability ...

Six months is actually a reasonable warranty for used equipment repair, especially for vacuum tube gear.

I’m sorry for your grief but it sounds like tube gear may not be for you. @jea48 may be right that the best thing for you to do is move on.

@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.

..

I don't know this particular model, but had ARC gear for decades- multiple amps, preamps. I still have the Dual 75a that I bought new in 1975-- it is sitting idle with a fresh tube set I bought from ARC a number of years ago. They were never cheap to maintain but I did get a lot of utility out of them. One question I would ask given the price is what kind of warranty you can expect after having paid to replace all the tubes and associated resistors? I used to deal w/ ARC in Minn. and they stood behind their work. That was quite a few years ago- I don't know what difference it makes that you are dealing w/ an authorized facility in another country or how things may have changed given that the company is in different hands and the amp is relatively old from what I gather. Typically, warranties on tubes themselves are limited, but....

I still run tubes, and periodically have the manufacturer go over the amps, line stage, phono stage, etc. There is a running cost to these things-- I like the DR variant of the 6h30 which, for a vintage quad (used in my line stage) is not only costly but very hard to find these days. Generally, I've found tube gear of this calibre to be pretty reliable other than routine maintenance, tube replacement but cannot speak to whether this particular model is worth investing in for the long run. If it is, bite the bullet, get it done and enjoy it! 

ARC amps are built well and tend to be reliable. Until they're not. I believe most ARC experts and fans will admit that there were some clunkers along the way, in terms of sound, design and reliability. I had a VS110 (which was the lower-tier amp contemporary of the VT110 IIRC) that was remarkably reliable for ten years of constant usage. 

Most-but not all-tube amps are expensive to maintain. My ARC Ref 150SE uses eight KT150's. That means paying $1600 or more every three years more or less to re-tube just the power tubes, closer to $2800 if you opt to buy ARC's "specially screened and matched" tubes. 

Dump it. Buy a ss amp. Not worth the 2k repair charge. 
call it a learning lesson. 

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.

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.

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

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.

@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.

.

Cleeds is right. WTS, you are half in already, so I would ask myself did the amp fit my system and would it be a long term fit that I can live with, if not sell and move on, if it is a keeper then bite the bullet and do the repairs and live happily ever after. BTW those prices for the 6922 are really steep. Enjoy the music

It's a fact of life that most tube amplifiers require periodic maintenance and ARC is no exception. I don't think it makes sense to quibble with your tech any more than it makes sense to quibble with your mechanic. Some ARC owners are comfy swapping their own tubes and replacing resistors themselves and that's a fair approach. But once someone is selected to do the job, they should be paid for the work they specify. That way, if there's an issue later, the burden is on them. And at the same time, you've shown them the respect they deserve. If you want the work done cheaper, you're free to take it elsewhere.