RE: Harman Kardon TA5000X output transformers

Posted by Triode_Kingdom (A) on May 6, 2016 at 05:55:42

In Reply to: RE: Harman Kardon TA5000X output transformers posted by Tubecase on May 6, 2016 at 05:22:09:

Most tube audio amplifiers don't regulate the screens. However, it's necessary to do so if lowest distortion is to be assured. I'm planning to add a string of VR tubes for this purpose when I rebuild the A500. The rebuilt amplifier will not include the original preamp/tone sections, so there should be a little extra HV current available for the regulators.

Eli is also correct about the need for NFB with pentodes. I found that the original HK configuration and values worked well when I substituted 6L6s. I should probably mention that this wasn't a simple "drop-in" replacement. The 6L6s exhibit considerably less gain than the original 7355s, so they won't provide the original factory output power (25W/ch) as-is. I raised B+ in my prototype by using a bench bias supply. That allowed me to ground the output cathodes and recapture the voltage previously lost in the bias resistors. The subsequent increase in cathode-anode voltage was just enough to allow driving the 6L6s to 25W output. The final design will include a fixed bias supply using a separate bias transformer and filter.

I'll just add that the HK transformers that were paired with 7355s back in the day are really very good. I wouldn't hesitate to use any of them as a basis for a high-end pentode amplfier.

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Follow Ups Full Thread

Topic - Harman Kardon TA5000X output transformers - Tubecase 06:21:21 05/05/16 (23)

RE: Harman Kardon TA5000X output transformers - Tom Bavis 07:05:22 05/09/16 (2)

RE: Harman Kardon TA5000X output transformers - Triode_Kingdom 08:03:39 05/10/16 (0)

RE: Harman Kardon TA5000X output transformers - Tubecase 14:20:01 05/09/16 (0)

RE: Harman Kardon TA5000X output transformers - Triode_Kingdom 07:59:39 05/05/16 (19)

RE: Harman Kardon TA5000X output transformers - Tubecase 05:11:27 05/06/16 (0)

RE: Harman Kardon TA5000X output transformers - Eli Duttman 14:03:52 05/05/16 (17)

RE: Harman Kardon TA5000X output transformers - Tubecase 05:22:09 05/06/16 (16)

RE: You can go old school with VR tubes - DAK 11:36:01 05/06/16 (1)

Plus: They Glow! - Triode_Kingdom 12:32:41 05/06/16 (0)

RE: Harman Kardon TA5000X output transformers - Eli Duttman 07:52:33 05/06/16 (8)

Is This Documented Somewhere I Don't Have to Join Their Crummy Forum? - Triode_Kingdom 10:07:43 05/06/16 (3)

RE: Is This Documented Somewhere I Don't Have to Join Their Crummy Forum? - Eli Duttman 10:20:55 05/06/16 (2)

RE: Is This Documented Somewhere I Don't Have to Join Their Crummy Forum? - Triode_Kingdom 10:30:21 05/06/16 (1)

RE: Is This Documented Somewhere I Don't Have to Join Their Crummy Forum? - Eli Duttman 10:50:00 05/06/16 (0)

Okay, Eli, I admit... No WAY can a.choke match that set-up, hmmm-nT - Cleantimestream 08:31:51 05/06/16 (3)

Maybe, Maybe Not - Triode_Kingdom 10:18:35 05/06/16 (2)

RE: Maybe, Maybe Not - Eli Duttman 10:25:20 05/06/16 (1)

RE: Maybe, Maybe Not - Triode_Kingdom 10:38:07 05/06/16 (0)

RE: Harman Kardon TA5000X output transformers - Triode_Kingdom 05:55:42 05/06/16 (4)

PP 6L6 Small Signal Circuitry - Eli Duttman 11:21:27 05/06/16 (3)

HK Amps - Triode_Kingdom 19:25:15 05/06/16 (2)

RE: HK Amps - Eli Duttman 20:04:30 05/06/16 (1)

RE: HK Amps - Triode_Kingdom 22:12:29 05/06/16 (0)

Follow Ups

RE: Harman Kardon TA5000X output transformers - Triode_Kingdom 05:55:42 05/06/16 (4)

PP 6L6 Small Signal Circuitry - Eli Duttman 11:21:27 05/06/16 (3)

HK Amps - Triode_Kingdom 19:25:15 05/06/16 (2)

RE: HK Amps - Eli Duttman 20:04:30 05/06/16 (1)

RE: HK Amps - Triode_Kingdom 22:12:29 05/06/16 (0)

RE: Harman Kardon TA5000X output transformers

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