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I turned on my VT100 for some normal listening and noticed that the center image had shifted to the left about 3/4 of the way. I swapped speaker cables, and the image moved to the other side. I swapped interconnects to the amp and nothing changed. I then knew that something was probably wrong with the amp.I swapped the left and right quads of output tubes that I had recently replaced. The sound did not change. I then, with great hesitation, swapped the small driver tubes. The image shifted to the right about 3/4 of the way. I swapped them back, the image shifted back to the original position (Skewed to the left). Now in addition there was a microphonic squeal in the right channel (the weak channel) that sounded like a guitar amp with a bad tube or a Conrad Johnson PV series preamp with a microphonic tube. I immediately shut the amp down.
I had been lucky enough to stumble across Abe Collin's site on VT100 retubing so I felt like I could handle the retube myself. Before reading ahead, you make want to check out his page or a lot of this won't make sense. Here is the link:
http://mycollins.net/audio/artube3.htmlI called Upscale Audio and got 12 tubes sent to me (8 for the amp, 4 for the preamp). I called Audio Research and they faxed me instructions and a schematic. I then aquired two Fluke multimeters and a set of clip adaptors from Radio Shack to latch onto the small parts while setting the voltages.
I got the tubes and tried my best to group them closely into two sets of quads. According to the instructions, you should remove the output tubes when adjusting the amp, but you should never turn on the amp without the preamp tubes. Also, you can run the VT100 without a set of speakers attached.
I installed the quads and turned on the amp. I attached leads as shown in the photos on the Collins site. I let the amp warm up for about 20 minutes. I adjusted the driver tubes until they tracked within .001 volt of each other. The is not as easy as sounds. Each tube reacts to the other and getting them together takes a little trial and error, but eventually you will learn to bring one up too high and then use the other to bring it down and the two tubes will get closer to unity. Even worse, the tubes drift and are sometimes slow to respond to small adjustments. This won't make sense until you see it done. I set them each to around .100 volts and then checked the +60V on the board. I then adjusted each driver tube half up or down until the +60V reading had gone half the distance to the 60V spec and then rechecked their balance with each other. Once I had the two driver tubes withing .001 volts of each other and the +60V within .01, I checked the balance between the two Infinicaps.
The spec on these has to be within 20V of each other and around 160V. I ended up with 22V apart on the left and 30V apart on the right. The only thing you can do is power the amp down, swap some tubes, and start the process over. Any changes will affect the balance between the driver tubes, the +60V spec, and the balance between the infinicaps. After many hours and many changes I had the left channel within .03 volts of each other and the right channel within 2 volts of each other. I put all of the covers back in place, let the amp run for another hour, and then checked everything again. I made sure that the drivers were at unity, I made sure the +60V on the left and right channel matched, and I made sure that the voltage on the two infinicaps was still close to the same. Everything stayed in spec.
Unfortunately, getting the tubes in the right places to make those two infinicaps match up was pure luck. There may be a method to the driver tube placement, but I unfortuantely did not keep up with which tubes came from which boxes that they were shipped in. The boxes had the triode measurements that would have told me a lot about the final order. Maybe I will keep up with them in 5-10 years when its time to do this again and develop a method to save a lot of time.
Anyhow, I replaced the output tubes, warmed up the amp, and biased the output to 130 on each side.
The sound was wonderful.
I learned a few things about retubing this amp. First off, if you are not sure of your skills with a multimeter, being able to identify components, being able to read a circuit diagram, and patience then don't try this job. You will need AT LEAST TWO digital multimeters, insulated clip adaptors, steady hands, and a LOT of patience.
This was not an easy job and half-assing it could leave you with a broken amp or worse.
In my case the time invested was well worth it. The NOS Sovtek 6922 type 2's did a wonderful job. Vocals are now much more detailed. The soundstage is as good or better than it was before. The highs are better. The bass still kicks ass, maybe better than before. Best of all, I was able to do this myself with a little help from Abe Collins' website, Leonard at Audio Research, and Kevin at Upscale Audio. I want to thank all of them.
Follow Ups:
I'm glad my pics and website helped some. If there were any areas that need clarification, please let me know. Congrats!
I've got a friend down here who recently purchased a used VT 100 mkII, some Alon Circe's and a Audio Aero Capitole mkI CDP. A couple of months ago, when we first met, he asked me to help him set up his system, being more into HT. I went down and we set everything up, but prior turning everything on for the first time, I tested the output and input tubes on my Triplett 3423.When we finally got things squared away and turned the amp on for the first time, we got a transient that seemed to pop a 110 ohm resistor for some reason. Although the amp fired up, the right channel was only running on three tubes. We ordered the part right then and there. Due to his family coming, it was a month before he brought it back and I soldered it on. I went down that week and did a power tube bias and things worked well. I used Abe's site for pointers, but I really never tried to fiddle with the input drivers and their bias. If it were my own amp, I'd muck with it, but I don't want to screw around with a delicate procedure on somebody else's piece of kit.
What got me was the complete kit comprised of 8 6550Cs and 8 6922 Sovteks selling through AR for like $1650. Anyway, things seem to be running well for him and he's happy. It's just a lot of cash to retube those suckers using NOS tubes.
I paid $29 a piece for my 6550C's and got a matched Octet. I also got the highest grade 6922's that are older Sovtek's from the 70's for $25 a piece. The whole tube set was $432.00 plus shipping. That seems to be 1/4th the price of tubes from ARC. They do fax the schematic for free.I know that I could have saved money by going with other 6922's, but as hard as this thing was to get balanced, I am glad that they were all within 10% or less with the individual triode balance. I also trust that the tubes I get from Upscale will work and last. I also feel that if something went wrong or was defective, then I would be taken care of. I have ARC and CJ amps and preamps and have always recieved good advice and service from Upscale. Other stores may offer as much service, but I am sticking with what I know and I have never been let down.
The other reason I am glad to have spent a little more on the NOS Sovteks is the sound. A lot of it could be due to having 8 new tubes in the amp and 4 in the preamp or that the amp is back into spec, but man the setup sounds good right now. I was just happy to get my amp back up and running, but all night, vocals have been better than ever. Its just spooky how they are there in space. I kept looking up and noticing how good the sound was while I was trying ot read. I really did not expect this much of an improvement, but it is there.
The last test for these tubes will come this next weekend when I pull the driver tubes, open the side panels and check to see how much the 6922's drifted in the first 6 days. I also plan to grab the digi-cam and take some snapshots of how I did the measurements and set up the tubes. I also plan on trying to go into a little more detail on how I did the settings. I may see if Mr. Collins wants to add to his setup pages or link mine. Either way, new pics, hints, and even part numbers will be up some time next week.
At $110 a quad for 6550C and $12 each for the 8 Sovteks, (retail) I get $300. Buying in bulk, ARC pays 25% less... $225. 800% profit for the service of matching (which is done for free by Uncle Ned and thetubestore.com) the 6550C and testing the 6DJ8s for microphonics? 2 (arguably 4)of the 6DJ8s are nearly incapable of even being microphonic, and again Ned can supply matched, section balanced tubes. There are those who will buy ARC or other "selected " tubes and not even WZ Johnson himself, in a bout of temporary insanity, could dissuade this. I am not criticizing those who buy selected tubes... I just am becoming increasingly soured on manufacturers like ARC who have taken their cue from the auto industry and realized that customers could be raked over the coals on the initial purchase, but the BIG $ is in parts and service. Uncle Ned and Jim McShane can (or could.. his stock may be gone) supply the superior GE 6550A matched and tested for $400 for all 8, and Telefunken <> bottom selects run $75 or so; that's under a $1000 for tubes that will knock the Russian items on their ass!! VTV or tubeworld, not known for low prices (though their tubes are great) could get him into matched, tested Tung Sols for $1200 the Octet!! Oh well. Just my opinion. Chris
To continue with Chris' thinking about AA sponsors, Kevin Deal at Upscale Audio stocks NOS Russian 6H23EBs (highest military grade) that sound BETTER than current production Sovtek 6922s, while being "Iron tough". The Russian NOS costs $20 per tube.Triode Electronics stocks the JJ E88CC, that is arguably the best sounding current production 6922, for $21.90 per matched pair. Single JJs cost $8.95 per tube.
An option for O/P tubes is the EI KT90 Type 3. AA sponsors Upscale Audio and Jim McShane stock the KT90 for about $150-160 a matched quad. The price isn't high to begin with. When you factor the LONG service lives KT90s have in, the value is outstanding. Bias the KT90s a tad "hotter" than 6550s, run them in for 500 hours, and you will get good service for several years.
BTW, Jim McShane's site shows 276 GE JAN 6550As in stock. The price is a LOW $189.50 per matched quad.
Eli D.
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