| Author | Message | ||
| Mark Passaro |
..in a 210 HD130. Just re-capped the power supply (originals, never done before) and retubed with a matched quad of JJ EL34's. Bias at .049 each pair (read on a 20 volt scale) across the 10 ohm resistors. Any suggestions as to where to look next, as there are no preamp tubes to swap out? TIA- | ||
| michael kaus |
Did it distort before? Is that why you re-capped or was it a maintenance thing? If it didn't do it before, I'd start looking at the solder on the caps and check your plate voltage.If the plates don't show around 700v, I'm back to the caps and maybe one's in backwards. Also, the electrolytics on the circuit board as well as the ones on the bias board should be changed too. Is this the SS drive model? | ||
| Mark passaro |
Hi Michael- 12AX7 phase inverter; the caps needed changed- they were ruptured; one of the GT 6CA7's had a getter rolling around inside it, a 1.5k resistor and a .047 cap on the center two tube sockets had been cooked. Replaced them and the the two 100 mfd @400 vdc, but not the ones on the board. I can't speak to how it sounded before, as it came to me in the above described condition. Do you think a bad e-cap on the board would cause such a low volume/distortion? Additionally, since posting, I've found two pots that need replaced as they're not responding to contact cleaner, and are working intermittantly. Thanks for your thoughts, and I'd appreciate any additional ones. | ||
| michael kaus |
Saw your post also on weber. Says you have two dif bias VOLTAGE readings in the two pairs. I would find out why you have 40mA's of idle current on one pair first. You had better look at the tubes separately as far as current draw to see if you have one runaway in the bunch. New and matched are not always a sure thing. The bias feed is mono so you should be getting the exact same control voltage going to the grid on all four tubes. Unless, for some weird reason the 10 ohm resistor is vastly out of spec. Just a thought. | ||
| Mark Passaro |
Michael- thanks again. Maybe I didn't have a tube firmly seated at the initial bias reading- I swapped the pair's locations, and now read consistant .049 across either 10 ohm resistor. Left the amp on "Play" for nearly an hour, and no appreciable fluctuation, so...as for the caps on the board- do they really have that drastic an effect? I thought, to look at the schem, that the LM307's were used (more or less) rather than 12A type tubes- wouldn't THEY be more suspect? Anyway, I have several- I think maybe I'll start swapping them out. Thoughts? Thanks again- | ||
| michael kaus |
The caps will have an effect on the on the voltage readings FOR the IC's, causing them to overdrive or underdrive. Swapping the IC's is very easy and prbably would be my first choice IF you have them. Then I would start looking at the voltages in and around the IC's. I still don't like the taking of a voltage reading per pair of tubes. Too easy to get a hot one and a cool one. Mike. | ||
| Mark Passaro |
Hi Michael. Well...the switching of tube sets got the "balance" of bias settled. Now I just can't get the voltage to even the .5 MM recommended. 445 (+/-) vdc on pin 3 of each EL 34. With the bias pot maxed, on the "2 volt" range the best I can muster here is about .253. You mentioned the caps on the bias board- any way to test, or would you just assume with age it's time to swap? Haven't looked at the IC's yet, 'tho I DO have them. Thanks again for your time- | ||
| michael kaus |
The electrolytic caps on the bias board should be replace after this length of time. They will affect the amount of neg voltage available to the control grid. Just replace them and see what your ned vootage is and see what your idle current is. | ||
| Mark Passaro |
OK, OK, I PROMISE I won't bother you any more, just ONE more question. Bias/ plate issues resolved; new op-amps in hand. The leads are at least twice as long as the ones I'm pulling- just cut to fit, no length issues otherwise? Thanks again all- | ||
| Mark Passaro |
Done- amp sounds great, and my son is VERY anxious to gig with it. Michael, I want to thank you very much for your time and help, and if you're ever near the Pittsburgh Pa area, let me buy you a beer. Take care. | ||
| michael kaus |
Not a problem-didn't really do anything. I'm always glad to help if my tiny hard drive works(brain!). Mike. |
Tue, 03/25/2014 - 09:26
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