In my newest amp which is SE with fixed bias. The power tubes kt88, kt120 and even kt66, the bias starts drifting after the tubes warm up after 5 minutes. I know it is not the tubes because i use the exact same tubes in another fixed bias SE amp with roughly the same design and they were stable after many hours of use and at the same bias currents.
Both amps are designed for kt120 outputs but with fixed bias i can simply dial down the current for the lower power output tubes. They both B+ voltage of around 350vdc. They both use 6sn7 drivers, but one has cathode resistors while the other uses LEDs to set the driver tube bias. The bias voltage is obtained from a reversed 6.3v filament trans of about 1.0A rating. the AC is full wave bridge rectified with Schottky diodes and filtered by a PI filter for 1 amp and a choke input filter for the other. Both circuits have roughly -60vdc feeding the adjustment pots. Weird thing is i had it playing at the shop with 6l6gc for an hour with no bias drift. I brought it home and problem showed up.
Any help or suggestions are appreciated, thanks in advance, Dak
Edits: 01/11/17
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Topic - Fixed bias problem, and no, it is not the tubes. - DAK 18:00:49 01/11/17 (54)
- RE: Fixed bias problem, and no, it is not the tubes. - sony6060 07:12:40 01/15/17 (1)
- RE: Fixed bias problem, and no, it is not the tubes. - Michael Samra 16:03:18 01/15/17 (0)
- RE: Fixed bias problem, and no, it is not the tubes. - sony6060 07:06:48 01/15/17 (0)
- RE: Fixed bias problem, and no, it is not the tubes. - Acoustic-Dimension 00:50:04 01/12/17 (41)
- RE: I changed a 10K 1/2wCC and it seems to be ok. - DAK 16:05:46 01/12/17 (34)
- Now it's back to its bad ways. - DAK 16:41:41 01/12/17 (33)
- Just for a chance.Disconnect the coupling caps - Michael Samra 23:59:45 01/14/17 (0)
- RE: Now it's back to its bad ways. - Tre' 16:52:27 01/12/17 (3)
- Maybe the problem is my first filter caps. - DAK 10:58:35 01/13/17 (2)
- RE: Maybe the problem is my first filter caps. - Tre' 11:20:44 01/13/17 (1)
- the problem is NOT the first filter caps. - DAK 17:52:56 01/13/17 (0)
- enough current to bias 2 x kt150 tubes You don't need current - Chip647 16:50:03 01/12/17 (27)
- RE: enough current to bias 2 x kt150 tubes - DAK 17:08:46 01/12/17 (26)
- RE: enough current to bias 2 x kt150 tubes - 6AS7_6SN7 23:10:22 01/12/17 (1)
- RE: enough current to bias 2 x kt150 tubes - DAK 09:56:50 01/13/17 (0)
- Unstable amp - FenderLover 17:43:26 01/12/17 (23)
- RE: Hi FL. can you contact me offline? - DAK 19:23:34 01/13/17 (0)
- RE: Unstable amp - DAK 19:37:39 01/12/17 (21)
- RE: Unstable amp - PakProtector 04:38:44 01/14/17 (20)
- Powered it up WITHOUT the driver tubes...still misbehaving - DAK 13:09:40 01/14/17 (18)
- RE: Powered it up WITHOUT the driver tubes...still misbehaving - PakProtector 03:34:18 01/15/17 (6)
- RE:What would i be looking for in the meter readings? nt - DAK 23:27:00 01/15/17 (5)
- RE:What would i be looking for in the meter readings? nt - PakProtector 03:48:33 01/16/17 (4)
- RE:What would i be looking for in the meter readings? nt - DAK 08:08:19 01/16/17 (3)
- RE:What would i be looking for in the meter readings? nt - PakProtector 08:38:46 01/16/17 (2)
- I was not being facetious or disrespectful. - DAK 11:44:11 01/16/17 (1)
- RE: I was not being facetious or disrespectful. - PakProtector 16:41:14 01/16/17 (0)
- RE: Powered it up WITHOUT the driver tubes...still misbehaving - Tre' 16:20:34 01/14/17 (10)
- Residual AC in the bias supply is .002v - DAK 15:22:48 01/15/17 (9)
- RE: Residual AC in the bias supply is .002v - Tre' 16:07:07 01/15/17 (5)
- Where would i put the ferrite beads? - DAK 22:52:30 01/15/17 (4)
- RE: Where would i put the ferrite beads? - Tre' 08:27:00 01/16/17 (3)
- RE: Where would i put the ferrite beads? - DAK 11:50:08 01/16/17 (2)
- RE: Where would i put the ferrite beads? - Tre' 19:14:58 01/16/17 (1)
- It helped a little. - DAK 20:47:41 01/16/17 (0)
- RE: Residual AC in the bias supply is .002v - Michael Samra 16:06:40 01/15/17 (2)
- Both coupling caps are new Mundorf Silver/gold and oil - DAK 23:08:58 01/15/17 (1)
- RE: Both coupling caps are new Mundorf Silver/gold and oil - Michael Samra 12:06:23 01/16/17 (0)
- RE:You mean run the amp w/out the driver tube? - DAK 08:47:42 01/14/17 (0)
- I monitored the bias voltage over time - DAK 02:25:27 01/12/17 (5)
- RE: I monitored the bias voltage over time And it slowly declined after about 5 minutes. - Chip647 04:45:31 01/12/17 (0)
- RE: I monitored the bias voltage over time - Acoustic-Dimension 02:30:19 01/12/17 (3)
- RE: I monitored the bias voltage over time - DAK 09:35:23 01/12/17 (0)
- RE: I monitored the bias voltage over time - Uncle Mike 03:23:31 01/12/17 (1)
- RE: I monitored the bias voltage over time - Acoustic-Dimension 04:28:00 01/12/17 (0)
- RE: Fixed bias problem, and no, it is not the tubes. - Jim McShane 19:55:11 01/11/17 (3)
- RE: Fixed bias problem, and no, it is not the tubes. - DAK 20:17:20 01/11/17 (2)
- RE: Fixed bias problem, and no, it is not the tubes. - Jim McShane 05:17:23 01/12/17 (1)
- RE: It is the same circuit as on the other amp - DAK 09:26:57 01/12/17 (0)
- RE: Fixed bias problem, and no, it is not the tubes. - sony6060 19:30:09 01/11/17 (4)
- RE:Here is a photo of the bottom amp - DAK 21:18:48 01/11/17 (2)
- RE:Here is a photo of the bottom amp - sony6060 22:32:44 01/11/17 (1)
- RE:Here is a photo of the bottom amp - DAK 00:44:40 01/12/17 (0)
- I think i rechecked them - DAK 20:19:17 01/11/17 (0)