JRS94 Posted May 27, 2020 Share Posted May 27, 2020 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 Quote Link to comment Share on other sites More sharing options...
kevinmi Posted May 27, 2020 Share Posted May 27, 2020 I have a 10 year old ST-120 that I built. I've added the autobias board last year. Without an autobias board, you manually set the bias for each power tube by turning little pots. Tubes change bias as they get older, so you have to periodically check and re-bias the tubes. The autobias board also offers protection if you have a runaway tube. The biggest problem I've had with my amp is the power tube socket pin sleeves need to be retensioned periodically, especially if you change out power tubes, or do a lot of tube rolling. loose pins will wreak havoc on the bias settings. 1 Quote Link to comment Share on other sites More sharing options...
JRS94 Posted May 27, 2020 Author Share Posted May 27, 2020 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. 1 Quote Link to comment Share on other sites More sharing options...
JRS94 Posted May 27, 2020 Author Share Posted May 27, 2020 Kevin, What tubes have you landed on after 10 years with the amp? Thanks Joe Quote Link to comment Share on other sites More sharing options...
John Warren Posted May 27, 2020 Share Posted May 27, 2020 When you start worrying about line voltage whatever "fun" there is in this adventure, hobby or whatever you want to call this endeavor is basically lost. 1 Quote Link to comment Share on other sites More sharing options...
JRS94 Posted May 28, 2020 Author Share Posted May 28, 2020 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. 5 hours ago, John Warren said: When you start worrying about line voltage whatever "fun" there is in this adventure, hobby or whatever you want to call this endeavor is basically lost. 1 Quote Link to comment Share on other sites More sharing options...
billybob Posted May 28, 2020 Share Posted May 28, 2020 Are those new tubes that came with the unit or other? Quote Link to comment Share on other sites More sharing options...
Could sound better Posted May 28, 2020 Share Posted May 28, 2020 Set the bias after the amp is warmed up. It's small increments, remember that any adjustment on one side will affect the other side. Dude ditch the 5963's and try a balanced Voice of Music branded 12au7 in the center position,Trust me. Outer preamp tubes need to be well balanced and matched. 1 Quote Link to comment Share on other sites More sharing options...
JRS94 Posted May 28, 2020 Author Share Posted May 28, 2020 1 hour ago, billybob said: Are those new tubes that came with the unit or other? Yes new with the unit 1 Quote Link to comment Share on other sites More sharing options...
billybob Posted May 28, 2020 Share Posted May 28, 2020 (edited) 1 hour ago, JRS94 said: Yes new with the unit Have you easy access to others... Suppose you are the troubleshooter, so defer to. others to talk about safety. Also, know very little about tubin'... Lots of experience here. Even the archives. Edit: btw, there are tubes from time to time in the garage sale section. Edited May 28, 2020 by billybob Quote Link to comment Share on other sites More sharing options...
amped Posted May 28, 2020 Share Posted May 28, 2020 (edited) You might try Holger at https://www.erhard-audio.com He's the man in the US and he helped me out a lot with this. Pavel in the Czech Republic designed and makes the board. He is helpful but English isn't his first language. There's also a time delay depending on where you are but, he is responsive. https://www.audioamp.eu/en-detail-901899168-modules-for-to-optimize-bias-with-an-supply-of-bias-ab-q-st70-st120.html Hope you get it sorted out. The ST120 is a fine sounding amp. Edited May 28, 2020 by amped 1 Quote Link to comment Share on other sites More sharing options...
JRS94 Posted May 28, 2020 Author Share Posted May 28, 2020 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 1 Quote Link to comment Share on other sites More sharing options...
kevinmi Posted May 29, 2020 Share Posted May 29, 2020 On 5/27/2020 at 2:22 PM, JRS94 said: Kevin, What tubes have you landed on after 10 years with the amp? Thanks Joe I have used many,many tubes in the last 10 years. I am presently running vintage (expensive) Tung-Sol 6550 power tubes. Rectifier tube is a Mullard GZ-37, also expensive. I have RCA clear top 12AU7's on the 2 outside preamp tubes, and a Amperex Bugle Boy 12AU7 in the middle spot. Here's what I have personally preferred over the years- The Gold Lion KT-88 reissues are great sounding tubes. The Tung-Sol KT-120's are good also. As far as the rectifier tube goes, don't use a cheap one, as it probably won't last. Use either a Mullard GZ-34 or even a Weber copper cap solid state. As far as the 3 small tubes, the one in the center will change the sound the most. The outer ones should be fairly closely matched to each other. I prefer Amperex tubes in the center, but have had good luck with vintage Tung-Sol as well. It's mostly up to your own preference. Make sure you join the forum, there is a lot of knowledgeable people on there that can help you also http://dynacotubeaudio.forumotion.com/forum.htm 2 Quote Link to comment Share on other sites More sharing options...
JRS94 Posted May 30, 2020 Author Share Posted May 30, 2020 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. 1 Quote Link to comment Share on other sites More sharing options...
mark1101 Posted June 9, 2020 Share Posted June 9, 2020 I would give the bias pots some attention. My ST-120 did the same thing. The pots were very touchy, measurements jumped around. Not sure what Bob uses these days but mine were just inexpensive plastic and turned so easily I considered them loose. Had to be careful setting the bias. Still, when things were right, it was a fine sounding amp and had good power. 1 Quote Link to comment Share on other sites More sharing options...
JRS94 Posted June 14, 2020 Author Share Posted June 14, 2020 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? 1 Quote Link to comment Share on other sites More sharing options...
kevinmi Posted June 14, 2020 Share Posted June 14, 2020 11 hours ago, JRS94 said: 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? mine has kept a steady reading. Have you written Bob Latino about this? He is really good at resolving problems promptly. Quote Link to comment Share on other sites More sharing options...
Could sound better Posted June 14, 2020 Share Posted June 14, 2020 10 minutes ago, kevinmi said: mine has kept a steady reading. Have you written Bob Latino about this? He is really good at resolving problems promptly. Yes, contact Bob. I emailed him last week, he promptly responded. Quote Link to comment Share on other sites More sharing options...
JRS94 Posted June 18, 2020 Author Share Posted June 18, 2020 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... Quote Link to comment Share on other sites More sharing options...
Could sound better Posted June 19, 2020 Share Posted June 19, 2020 22 hours ago, JRS94 said: 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... it’ll get fixed Quote Link to comment Share on other sites More sharing options...
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