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About JRS94

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    Advanced Member

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  • My System
    1979 Khorns with Bob Crites Type A Crossovers, AG 55 Mids and CT 120 Tweeters

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  1. Update, swapped to a Weber WS-1 with thermistor, amp has been running well since then. Bias still moves up a bit for no reason with the AB board, but at least the amp turns on and runs. As stated earlier I'm on a variac, keep voltage to 115v as much as possible, House voltage swings from 112v to 124v and back quite often. Bias is set at .450v and swings to .600 at which point I flip the amp over and readjust the AB board. Not optimal but not enough to want to change amps. The ST-120 is great with the khorns.
  2. Yup thanks, Using both Bob and the dynaco forum. Gonna try a Weber ws1. Bob thinks it may be auto bias board related. In contact with Pavel as well.
  3. My VTA ST 120 blows the rectifier tube. Last one was 5ar4 tung sol. Happens Every 3 months or so. Usually takes the fuse too. Amp is running off a variac kept at 115v. Power tubes are kt120, Drivers are 5963 hp branded RCA. I keep bias at 450ma via an internal auto bias board. Speakers are khorns. Any ideas? Thanks
  4. Well, it never held a constant value. Bob knows all about it, it blew the fuse now and is on it's way back to Arizona where it was built. More to come...
  5. My St120 has the auto bias board. Supposed to be set it once and forget it. Only 1 trim pot on the inside. This week it drifted from 0.500 to 0.700vdc. All 4 tubes read the same but 40% increase in a week. Anyone else have this experience?
  6. Thanks , currently using KT 120's, TAD gz34, 3 5963's all supplied by Bob. This bias thing is getting crazy. Amp ran up to 0.900vdc today. More to come.
  7. Amped, Thanks for the contacts, I’ll reach out if it acts up again. Since I opened it up and readjusted the autobias board pot, bias has been steady. Keeping a close eye on it since I have no idea why it ran up. I do like the amp a lot, but this one has been problematic so far. Take care, Joe
  8. Yes new with the unit
  9. Well that's true. I didn't expect to have to do the variac thing. Still , not ready to give up on the st 120.
  10. Kevin, What tubes have you landed on after 10 years with the amp? Thanks Joe
  11. Thanks , I re-seated all the tubes, not doing any rolling yet . Bias was still high. Had to open it up and adjust the pot on the board. Its been solid since doing that, (last night) just no idea how it went from .500 to .900 on its own. This amp has been a challenge, 2 hard failures so far. Now this bias thing. It was built in February this year. Bob's tech has been great to work with.
  12. I have a new ST 120 with upgraded caps and autobias. I did not build it. I've had a few problems. After 30 days the Quad cap failed - replaced by Bob's tech After 3 weeks it fried the 4.7 kohm step down resistor which supplies B+ on the VTA Board. - I replaced this one with a 4.7 kohm 5w resistor, - ok now, B+ is 340. Yesterday I found bias at 0.900vdc, adjusted the board and now sitting at 0.500vdc with no or low input. Questions for you experienced Tube guys; With the way my wall outlet levels swing (114v right now ,could be 126v anytime today). I have to pick the voltage boundary because obviously I’m not sitting here watching wall voltage all day. With that said, I set my variac for a high amp voltage of 120v. That way, the variac protects from high voltage. But it definitely drops to the 110 range when the wall voltage hits 114v. Thoughts? Bias is holding at 0.500 vdc now. This board shifted bias on its own, the amp was closed up the entire time. As I turn the volume up the bias increases. 0.500 becomes 0.520 then slowly drifts down and oscillates. I guess that’s normal ops depending on how hard the amp is working? I realize this is a small drift, just wondering what “normal” band is. What does an amp without the autobias board do? Who is the expert on this autobias board? I know lots of questions, sorry. Thanks System is; ST 120 driving 79 KHorns with Crites type a Xover, Volti mids with A55G, T120's and 1526c Crites woofs. Inputs are CDs, Vinyl, MP3, and NVIDIA box for net inputs, through a Luxman SS preamp (T117). ST 120 has KT 120 power tubes. 5963's , and either a 5AR4 or a 5R4GYA rectifier
  13. I bought the VTA ST 120 with the cap upgrades, auto-bias board, and Tung sol KT 120's. So far the thing is great, fits my space, and really sounds good with my Khorns. Happy camper. Thanks for all the input. Just thought I would close out this topic. Take care all.
  14. Saturday Audio has a nice Sansui 8080 for $699. Looked pretty good in person. They are on Belmont, north side. http://www.saturdayaudio.com/gear/sansui-8080
  15. I'm going to have it built with AB. Just had to research a little more. Bob says 3 weeks from receipt of my payment. Thanks Kevinmi and Amped! Joe
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