RE: I had a Fisher melt all of it output tube because the bias was not held right by the 12AX7 heaters

"Done well the heater on a 12AX7 can be very quiet with AC"

AC on a 12AX7 is absolutely NOT quiet enough for a high-end phono preamp. Perhaps the Fisher wasn't properly designed. HK used a circuit that combined resistors with the filament drive circuit, and they also used output tubes operating well under their ratings. These two techniques pretty well ensure that the choice of preamp tubes won't be critical. I can't say it's impossible to damage the outputs, but judging by the schematics, it seems extremely unlikely.

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Follow Ups Full Thread

Topic - using cathode bias voltage for preamp tube heaters - DAK 09:36:49 07/20/14 (17)

RE: using cathode bias voltage for preamp tube heaters - Thermionic27609 12:41:43 07/24/14 (0)

RE: using cathode bias voltage for preamp tube heaters - FenderLover 08:54:33 07/21/14 (7)

RE: using cathode bias voltage for preamp tube heaters - shane. 22:10:26 07/21/14 (0)

RE: using cathode bias voltage for preamp tube heaters - Thomas Mayer 09:02:39 07/21/14 (5)

And a great new failure mode of having a shorted input tube heater take out a pair of 6550s (nr) - Chip647 13:59:08 07/21/14 (4)

Not really. - DAK 10:53:28 07/22/14 (3)

I had a Fisher melt all of it output tube because the bias was not held right by the 12AX7 heaters - Chip647 06:37:17 07/23/14 (1)

RE: I had a Fisher melt all of it output tube because the bias was not held right by the 12AX7 heaters - Triode_Kingdom 08:26:59 07/23/14 (0)

RE: Not really. - Triode_Kingdom 12:04:38 07/22/14 (0)

RE: using cathode bias voltage for preamp tube heaters - Chip647 16:59:28 07/20/14 (7)

RE: using cathode bias voltage for preamp tube heaters - DAK 23:10:45 07/20/14 (6)

Here you go. - Chip647 05:12:58 07/21/14 (5)

RE: Not the one. - DAK 08:14:06 07/21/14 (3)

RE: Not the one. - Chip647 12:20:14 07/21/14 (1)

RE: Not the one. - jazbo8 22:10:59 07/25/14 (0)

RE: Not the one. - drsx 12:11:25 07/21/14 (0)

RE: Here you go. - Thomas Mayer 07:24:33 07/21/14 (0)

Follow Ups

RE: I had a Fisher melt all of it output tube because the bias was not held right by the 12AX7 heaters - Triode_Kingdom 08:26:59 07/23/14 (0)

RE: I had a Fisher melt all of it output tube because the bias was not held right by the 12AX7 heaters

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