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Tube DIY Asylum: Amplifier Bias by Triode_Kingdom Do It Yourself (DIY) paradise for tube and SET project builders. |
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In Reply to: I have one somewhere in storage... posted by Ivan303 on June 22, 2021 at 09:10:33:
If these amplifiers are biased AB1, none should be rebuilt with self bias. As I wrote recently in another thread, DC voltage across the self-bias network increases at higher amplifier outputs and anode currents. That causes bias voltage to increase, moving the output stage closer to Class B. Unless the bias is set very high at idle, this movement creates crossover distortion that is easily observed on a scope. This distortion will persist into quiet musical passages due to the time constant of the bias circuit, muddying nuance and degrading fidelity.
The only reason manufacturers ever got away with this is that printed specifications minimize the impact. At higher amplitudes, the crossover distortion represents a numerically small percentage that simply adds to the naturally increasing distortion levels. In addition, the loudness of the system at higher levels tends to hide what's happening from the listener.
These older designs can be greatly improved with either the "Enhanced Fixed Bias" cathode bias technique described at the tronola.com site, or with the addition of fixed grid bias. This should not be neglected if you want your restoration work to be fully worthwhile.
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Topic - Building clone amp - question about grid resistor and coupling cap - PeterI 09:25:55 06/21/21 ( 38)
- phase shift vs OPT saturation - Ralph 10:35:42 06/23/21 ( 3)
- RE: phase shift vs OPT saturation Thx. Amp pics - PeterI 12:24:42 06/23/21 ( 2)
- RE: I built something similar. Amp pics - Ralph 13:04:14 06/23/21 ( 1)
- Thumbs up NT - PeterI 06:16:04 06/25/21 ( 0)
High G2 voltage - Paul Joppa 14:15:30 06/22/21 ( 13)
- Not sure that's right... - Ivan303 15:21:50 06/22/21 ( 11)
- The HF-81 looks much better, but ... - Paul Joppa 17:03:23 06/22/21 ( 10)
- Not sure it's an EICO HF-14 schematic... - Ivan303 18:33:39 06/22/21 ( 9)
- RE: Not sure it's an EICO HF-14 schematic... - Paul Joppa 09:41:04 06/23/21 ( 0)
- RE: Not sure it's an EICO HF-14 schematic... - Tre' 18:55:05 06/22/21 ( 7)
- RE: Not sure it's an EICO HF-14 schematic... - Ivan303 20:10:30 06/22/21 ( 6)
- RE: Not sure it's an EICO HF-14 schematic... - Tre' 07:06:14 06/23/21 ( 5)
- RE: Not sure it's an EICO HF-14 schematic... - PeterI 08:02:28 06/23/21 ( 4)
- Watch Out for SAM! - Triode_Kingdom 08:47:38 06/23/21 ( 3)
- Strangely, here's the Eico manual schematic: - PeterI 08:55:44 06/23/21 ( 2)
- YES! The Self-Powered Output Transformer! - Triode_Kingdom 10:28:12 06/23/21 ( 1)
- A long time ago! nt - Tre' 16:12:21 06/23/21 ( 0)
RE: High G2 voltage - Tre' 14:34:33 06/22/21 ( 0)
If you are looking for better bass response... - Ivan303 10:04:43 06/21/21 ( 19)
- RE: If you are looking for better bass response... - PeterI 15:55:29 06/21/21 ( 18)
- I have one somewhere in storage... - Ivan303 09:10:33 06/22/21 ( 17)
- Or you could just sell it to me ... :) - PeterI 08:25:00 06/23/21 ( 0)
- Amplifier Bias - Triode_Kingdom 06/22/21 12:18:56 06/22/21 ( 15)
- EFB - 1973shovel 04:44:07 06/24/21 ( 9)
- RE: EFB - Triode_Kingdom 09:42:19 06/24/21 ( 8)
- "should be all that's required" - 1973shovel 08:50:21 06/25/21 ( 4)
- RE: "should be all that's required" - Triode_Kingdom 16:17:17 06/25/21 ( 3)
- The Hipocratic Oath - 1973shovel 08:42:50 06/26/21 ( 0)
- RE: "should be all that's required" - Paul Joppa 22:31:54 06/25/21 ( 1)
- RE: "should be all that's required" - Triode_Kingdom 06:19:26 06/26/21 ( 0)
- RE: EFB - 1973shovel 16:09:03 06/24/21 ( 2)
- RE: EFB - Triode_Kingdom 16:31:01 06/24/21 ( 1)
- I'm glad you saw the humor in that - 1973shovel 08:39:03 06/25/21 ( 0)
RE: Amplifier Bias -Thanks - PeterI 08:21:02 06/23/21 ( 4)
- RE: Amplifier Bias -Thanks - Triode_Kingdom 08:35:32 06/23/21 ( 3)
- RE: Amplifier Bias -Thanks - PeterI 09:08:21 06/23/21 ( 2)
- RE: Amplifier Bias -Thanks - Triode_Kingdom 13:47:20 06/23/21 ( 1)
- Thanks NT - PeterI 15:18:33 06/23/21 ( 0)