| Author | Message | ||
     Greg Williams (gwmn) Username: gwmn Registered: 08-2013 |
I'm really up in arms about this. Before I changed any caps I had one tube that would have run away bias. (same tube in either socket) I then replaced all the caps under the hood and the five 150/50's inside. Same bias problem. Put in new JJ 6CA7 tubes. Same problem. Thought maybe it was a faulty new tube. Tried another, same thing. They come up evenly at warm up and then one will climb to about 37+++ while the other stays around 22. But then it will start to drop and the other one will climb. (By the way it will stabilize on the low power setting) I also have the bias adjustment so low that if I drop it any more the amp starts to hum, so I can't get them back in the 20's. I also have tried 2 different bias meters. Any help would be great! | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
First off, I assume by the fact that you say "bias" that this means that you have the 12ax7 version with the normal style bias supply. If so,, first thing is when it goes to runaway, what does your actual control grid voltage do? Measure at pin 5 on both tubes and see if the control grid voltage is changing WHEN it goes wacko. Can you remove the tubes and let it cook and measure the control grid voltage after a while. Does it change without the tubes in it? | ||
| Greg Williams (gwmn) Username: gwmn Registered: 08-2013 |
Measured that just now. Showed -33.68v to ground on both sockets and when it acts up it drops to -26.8? on both. Should I be measuring to ground like I am on these amps? It shows a negative reading. | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
OK-that answers a lot. It seems to me that the bias circuit is failing. Did you change the caps on the filter board? You MAY need to measure the voltage going INTO and OUT of the pot to see if IT'S failing. They are pretty cheesy little pots. The control grid voltage dropping is the dead giveaway. The control voltage enters the main circuit board on the third point from the end. See if THAT styas contant. If it does, maybe you just have a crappy pot. If it doesn't, you have to work backwards to the supply at the filter board and see which component is failing. You are measuring correctly. SHOULD be negative voltage. | ||
| Greg Williams (gwmn) Username: gwmn Registered: 08-2013 |
Before I powered up to check the voltages you suggested I took a couple of measurements. I measured across the two 150k 2 watt resisters on the bias board and they read 36.90 and 35.88 across, coming up at a pretty slow pace , amp unplugged and drained of course. I had to check some others just to make sure my MM was working and it's fine. I even cleaned up the leads on the 2 resistors and got the same reading. Shouldn't these read at least close to 150? | ||
| Greg Williams (gwmn) Username: gwmn Registered: 08-2013 |
Ok, I just pulled one side of those resistors and they read correct. Does that make any sense? | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
ALWAYS pull one end of resistors in a circuit to measure them. You are reading resistance from through that circuit all the way around and back again. Same with checking caps. Always pull one end UNLESS you have ESR meter. | ||
| Greg Williams (gwmn) Username: gwmn Registered: 08-2013 |
I just finished the control voltage test you suggested. I will first say that I found and re-soldered what could have been a cold joint that some prior owner/tech left on the bias board on the red/white to filter board and red to OT. The measurements were as follows: After 15 minutes: Voltage at main board -45.3 at bias pot in -45.3 at bias pot out -28.5 at pin five's -34.2 After 45 minutes: at main board -44.9 at bias pot in -44.9 at bias pot out -28.1 pin five's -34.11 Thanks for the tip on pulling the resistors and caps. I just started this about a year ago, so I'm learning. (Tougher in your 50's though!) | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
Sounds like you may have the problem licked. Now I suggest that you bias a tad cool-somewhere in the 22-23ma PER tube range and try it there. Hey-50's is the new 25 (I'm 59). Mike. | ||
| Greg Williams (gwmn) Username: gwmn Registered: 08-2013 |
Everything seems pretty stable until I use the bias meter. The grid voltage on the tube sockets drops to 27 something. After 15 minutes the bias on one goes high. I'll try to bias it with my mm. I know it's not as accurate, so I've heard, but will at least confirm. | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
SOME of those bias meters get flaky if you leave them on too long and they get hot. If you still get high current on only ONE tube, and it's always the same socket, you might try re-soldering the contact on pin 5 there too and cleaning and re-tensioning the sockets. Maybe it's loosing control grid voltage. | ||
| Greg Williams (gwmn) Username: gwmn Registered: 08-2013 |
I must have been posting my last message as you were so I didn't see yours until I was done. Ya, I just wish my body was even 35. Kinda shot. Eye's going, hands shake, perfect for working on small intricate parts with high voltage. I've built some tweeds and they are a tad more simple than these. Thanks for all your help Mike. What's the best way to bias this without a bias meter? Thanks again } | ||
| Greg Williams (gwmn) Username: gwmn Registered: 08-2013 |
I noticed there's only a 10K resistor on one of the tube sockets, pin 8, and none on the other tube socket. | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
I usually put 1 ohm precision resistors from cathode TO the big resistor. Those two are tied together at the socket. Then you can measure the cathode current per tube and get an accurate measurement of each tube seperately. Then you measure ACROSS the 1 ohm resistors. NOT to ground. Mike. | ||
| Greg Williams (gwmn) Username: gwmn Registered: 08-2013 |
I just tested each probe on my bias meter on a single ended 6V6 amp and got 44.2 after 10 min. and 47.4 and unstable after 10 min. Get what you pay for I guess. Also, the 1 ohm add on resistor you mentioned goes to the one 10K resistor (There isn't one on the other tube socket) Thanks | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
You disconnect the 10 ohm from the pin socket on the one tube. Put 1 ohm resistors from EACH pin 8 to the free end of the ten ohm. I usually put a stand off if I am going to leave them in there. Mike. | ||
| Greg Williams (gwmn) Username: gwmn Registered: 08-2013 |
One thing I just noticed and I will check again. The out reading on the bias pot stays in the 28 range no matter where I turn it. 28.80 or so when I adjust up and 28.40 or so when I adjust down. I'm going almost a 1/4 turn each way. Bad pot all along? You'll be pissed at all of your time I wasted if that's it. Think I'll get a new bias meter also. Any suggestions? Thanks Mike, for your time. Pretty scarce now days to find people who will help for nothing in return other than a thank you. | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
Sounds like the pot's crappy(THAT sounds like the 70's). I might suggest maybe getting a cermat multi turn in te same value. Much less touchy and easier to adust. I actually don't use a regular bias meter. I do have one of the adapters somewhere but I rarely use it. I just keep 1 ohm resistors in stock and instal them. That way I don't have to keep taking tubes in and out. By the way, glad to help. Mike. | ||
| Greg Williams (gwmn) Username: gwmn Registered: 08-2013 |
Thanks Mike, I'll get a cermat. Radio Shack has them. The trim pot actually grinds and squeaks now, while the trem pot is smooth. Do you mount them with the screw pointing up, wired of course, and epoxy them in? Also, the precision 1 ohm, will any with 1% tolerance work? I hope this cures it. It distorts easy, about 5 and 5, on both the high or low power setting and I don't see a whole lot of volume difference. The jewel flashes once in a while on the low setting. Hope it's not a tranny. | ||
| Greg Williams (gwmn) Username: gwmn Registered: 08-2013 |
I suppose this one I got from radio shack won't work. It's a cermet 15 turn 10K but only 0.75 watt. | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
That's plenty big. There is practically no current going through them I would lay it sideways on the board and epoxy it on. I have even drilled a couple holes in the board where they weren't hitting the traces, if you can, and drizzle some of the epoxy into the holes and wrap it around so it kind of ropes it in. By the way, 5 and 5 is usually pretty damned loud! Mine breaks up too and I have always thought about going in and doing something about it but I never did-it works the way I use it for recording and the occasional out gig. | ||
| Greg Williams (gwmn) Username: gwmn Registered: 08-2013 |
Just wanted to check on these precision resistors. The Mouser ones are 3/4 watt, 0.1%. Just want to make sure cause they're $8 each. (71-RS1/2-B-1) Also my jewel took a dump on the low setting. Know of any replacements that are close to the same? Thanks again | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
The 1 percent ones are fine. I think I paid like .30 cents each. Jewel light doesn't work on low? Common problem from what I remember. Nobody else even replaces them! | ||
| Greg Williams (gwmn) Username: gwmn Registered: 08-2013 |
Well my 1% resistors will be here tomorrow so i can finally bias this amp correctly. Like i said, I'm kind of new at this. Can I bias other amps this way? Sounds like a better and more accurate way. I'd also like to put a post about some speakers I've tried from Chris at Jupiter Condenser. Amazing. Is it okay to post about products? thanks | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
OK with me! | ||
| Greg Williams (gwmn) Username: gwmn Registered: 08-2013 |
Sorry to pester again. I'm guessing once I install the 1 ohm resistors as you described, I would eliminate the wire that ties the two pin 8's together? | ||
| Greg Williams (gwmn) Username: gwmn Registered: 08-2013 |
Da. Use the wire for one of the resistors. Mind goes once in awhile. Damn pain meds. | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
Yes-the path to ground has to go through the 1 ohm resistors to ground. If they are tied together, untie them so that the only path to ground is THROUGH the resistors to the 10 ohm. | ||
| Greg Williams (gwmn) Username: gwmn Registered: 08-2013 |
The bias I set and measured stayed at 22 on both tubes for 30 minutes. (reg. DC setting on multi-meter so it read .022) What an accurate way of biasing! I know this is a little high bias, but let's see how good these new JJ 6CA7's are. Mike, I just want to say thank you again for all of your help. I couldn't have done this without your willingness to help and more importantly, your time. I just hope in the future I can help someone else by my good/bad endeavors, but this amp may outlive me now. | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
That's not high at all. Just about right. You bought the JJ 6CA7's? I'll be curious how you like them and their tone. We've all been using EL's for years in these thing because 6CA7's were gone. I haven't bought any because I just don't have any hole to fill right now! Mike. | ||
| Steve Kennedy (admin) Username: admin Registered: 03-2002 |
Thanks for jumping in and helping solve this problem, Mike! Steve |