JRS94 Posted October 29, 2020 Share Posted October 29, 2020 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 Quote Link to comment Share on other sites More sharing options...
tube fanatic Posted October 29, 2020 Share Posted October 29, 2020 Yes— contact Bob Latino. Maynard 1 Quote Link to comment Share on other sites More sharing options...
MechEngVic Posted October 29, 2020 Share Posted October 29, 2020 https://dynacotubeaudio.forumotion.com/ Quote Link to comment Share on other sites More sharing options...
JRS94 Posted October 31, 2020 Author Share Posted October 31, 2020 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. 1 Quote Link to comment Share on other sites More sharing options...
amped Posted November 8, 2020 Share Posted November 8, 2020 Any luck? My ST-120 has the auto-bias board and a Weber WS-1t (optional thermistor). It's working fine but I had a tough time with the AB install. All fine now. The KT-120's bias 0.49, RCA 5963's on the outsides and a Brimar CV4003 in the middle. I'm also on a variac at 117v. Pavel was very helpful as was Holger at Erhard Audio. Prior to the AB and Weber, I used a TungSol 5AR4 and never had a problem. Amp and variac once blew a fuse but I don't recall why. Have you done the "yellow sheet mod"? It's on the Dynaco forum. Quote Link to comment Share on other sites More sharing options...
kevinmi Posted November 10, 2020 Share Posted November 10, 2020 I've run KT-120's in my ST-120 amp for several years without blowing a rectifier tube. I'm running a vintage Mullard GZ-33 rectifier tube. I also have some Mullard 5ar4 rectifier tubes that I've had good luck with. I've used the weber copper cap ss rectifiers with good luck also. I have found that the new production Rectifier tubes do not last! I have blown a few new production rectifiers until someone tipped me on buying vintage rectifier tubes. Just my experience. Quote Link to comment Share on other sites More sharing options...
Tarheel TJ Posted November 17, 2020 Share Posted November 17, 2020 This may or may not be helpful, but I ran into a similar issue with an ST-70. I bought 4 or 5 rectifier tubes and a whole box of fuses. Eventually, by swapping around power tubes and rectifiers, I was able to find a stable combo. They have been running steady for several months now without issue. I don't know exactly what the issue was, but it seemed like some sets of power tubes and/or rectifiers would blow every time, right away. Others would run for a while and then blow. With enough experimentation, I was able to find a set that worked. I guess I'll just go through the same song and dance every time the power tubes need replacement? Amp works and sounds great other than that. Quote Link to comment Share on other sites More sharing options...
henry4841 Posted November 17, 2020 Share Posted November 17, 2020 A common problem is the surge current on turn on. The voltage can rise a lot above operating point. One way to tell is watch the rectifier tube on start up and see if there is a spark inside the tube. If so you need a soft start of some kind. Quote Link to comment Share on other sites More sharing options...
knucklehead Posted November 17, 2020 Share Posted November 17, 2020 I had similar problems with an ST-120 I put together as a kit from VTA. Either the rectifier tube would blow, taking the fuse with it, or one of the right side 6550 powers tube would red plate. That got expensive fast. I wouldn't have another kit amp for one very good reason, no warranty. If something goes wrong it's always your fault, it is never a problem with parts, etc. Ask me how I know! I sold that amp, gave the buyer full disclosure in case you're wondering, and eventually bought a 'Chi-Fi' Muzishare X7 (KT88 in push-pull) integrated amp. It has a one year warranty, and as we all know, if anything is going to go wrong, it usually manifests itself in the first few weeks of use. It sounds every bit as good as the ST-120, and it works! 1 Quote Link to comment Share on other sites More sharing options...
kevinmi Posted November 17, 2020 Share Posted November 17, 2020 46 minutes ago, knucklehead said: I had similar problems with an ST-120 I put together as a kit from VTA. Either the rectifier tube would blow, taking the fuse with it, or one of the right side 6550 powers tube would red plate. That got expensive fast. I wouldn't have another kit amp for one very good reason, no warranty. If something goes wrong it's always your fault, it is never a problem with parts, etc. Ask me how I know! I sold that amp, gave the buyer full disclosure in case you're wondering, and eventually bought a 'Chi-Fi' Muzishare X7 (KT88 in push-pull) integrated amp. It has a one year warranty, and as we all know, if anything is going to go wrong, it usually manifests itself in the first few weeks of use. It sounds every bit as good as the ST-120, and it works! I've had my ST-120 for 10 years now and still sounding as good as the day I built it. I've had a couple problems with it, but they were all my fault and easily corrected. Quote Link to comment Share on other sites More sharing options...
henry4841 Posted November 22, 2020 Share Posted November 22, 2020 I now remember another cause for rectifier tube failure. The first cap after the rectifier tube being too large. Guys having that problem should try lowering it's value by 5 or 10uf and see if that helps. If the first cap is too large there will too much inrush current on startup. Quote Link to comment Share on other sites More sharing options...
JRS94 Posted January 5, 2021 Author Share Posted January 5, 2021 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. 1 Quote Link to comment Share on other sites More sharing options...
kevinmi Posted January 5, 2021 Share Posted January 5, 2021 2 hours ago, JRS94 said: The ST-120 is great with the khorns. I used to think it was the best with my K-Horns until I tried single ended tube amps. 1 Quote Link to comment Share on other sites More sharing options...
MechEngVic Posted January 6, 2021 Share Posted January 6, 2021 10 hours ago, kevinmi said: I used to think it was the best with my K-Horns until I tried single ended tube amps. I'm this close to getting a single ended, just to have both sides of tube-ampery: A PP and a SE. Quote Link to comment Share on other sites More sharing options...
richieb Posted January 6, 2021 Share Posted January 6, 2021 11 hours ago, kevinmi said: I used to think it was the best with my K-Horns until I tried single ended tube amps. 41 minutes ago, MechEngVic said: I'm this close to getting a single ended, just to have both sides of tube-ampery: A PP and a SE. There is a famous tag line - “Just do it”. nuff’ said. Quote Link to comment Share on other sites More sharing options...
kevinmi Posted January 7, 2021 Share Posted January 7, 2021 23 hours ago, MechEngVic said: I'm this close to getting a single ended, just to have both sides of tube-ampery: A PP and a SE. don't buy a pp amp with switches that switch it to single ended and expect it to sound like a dedicated single ended amp. The Latino amp has this capability, and it doesn't sound anything like my single ended amps. 1 Quote Link to comment Share on other sites More sharing options...
brl0301 Posted January 7, 2021 Share Posted January 7, 2021 2 minutes ago, kevinmi said: don't buy a pp amp with switches that switch it to single ended and expect it to sound like a dedicated single ended amp. The Latino amp has this capability, and it doesn't sound anything like my single ended amps. The latino amps have a switch to run the tubes as triodes not single ended. The amp is still operating as a push pull. Having both a st-120 and a tubelab se running 300b, I can confirm they do sound significantly different. 1 Quote Link to comment Share on other sites More sharing options...
kevinmi Posted January 7, 2021 Share Posted January 7, 2021 2 minutes ago, brl0301 said: The latino amps have a switch to run the tubes as triodes not single ended. The amp is still operating as a push pull. Having both a st-120 and a tubelab se running 300b, I can confirm they do sound significantly different. Whats the difference between a Latino amp in triode and a single ended triode amp? 1 Quote Link to comment Share on other sites More sharing options...
brl0301 Posted January 7, 2021 Share Posted January 7, 2021 25 minutes ago, kevinmi said: Whats the difference between a Latino amp in triode and a single ended triode amp? A latino amp in triode is running each of the power tubes as triodes but in a push-pull configuration. In a push pull amp, each of the power tubes amplifies etther the positive or negative portion of the signal and then the two halves of the signal are combined again at the output transformer. In a single ended amp, the positive and negative portions of the signal are amplified in the same tube, preventing some of the distortion that occurs in a push-pull amp from when the two havles are combined again. 1 Quote Link to comment Share on other sites More sharing options...
MechEngVic Posted January 7, 2021 Share Posted January 7, 2021 47 minutes ago, kevinmi said: don't buy a pp amp with switches that switch it to single ended and expect it to sound like a dedicated single ended amp. The Latino amp has this capability, and it doesn't sound anything like my single ended amps. I'm thinking Decware or one of Tube Fanatics designs. Quote Link to comment Share on other sites More sharing options...
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