| Audio Tweaks - Tweak Ref.
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| Compound Bias for tube power amplifiers. Not just fixed OR auto Bias, but both |
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| 211 |
Mart |
Excellent |
$5.00 |
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| Techies at 'Heart Holland' amplifier production plant |
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| This is what this tweak is about: Part of the Bias is generated through a cathode resistor, while a small negative power supply takes care of the rest of the Bias.
Example: Say we want to improve the sound of a typical stereo EL34 PP amplifier which runs on a fixed bias of approx. minus 45 V. Cathodes are grounded, while only one Bias-potmeter is provided. This urges to use a matched quad, unfortunately. Plug in your
iron, here we go: Unsolder all cathodes (Pin 1&8 combined) from ground and solder four combinations of a 150 ohm / 5 watt resistor paralleled with a 150 uF/ 40 V elyt.cap in place. That's 8 parts, 2 parts for every tube. Now turn the back on, and you'll be able to
measure the idle currents! For Free! (multimeter to 10 Vscale. The measured voltage divided by 150 ohm is the idle current, in Amperes) If you had the possibility to do so before the tweak, you could have been able to notice that by adding the four resistors,
the idle Bias has increased and thus the idle current has dropped to maybe 30 mA.
Readjust the original Biascontrol pot to a point that you measure approx. 50 mA.
Whether these 20 mA are class A or not, I'm not sure, but they sure sound like it.
Doing this tweak on a class A machine involves replacing the cathode resistors by ones with half the original value AND building a new Bias voltage supply circuit into the amp.
But first unsolder the four 'grid resistors', typically 100k or 220k between the coupling cap's (leading to grid 1) and GND. I do NOT mean an approx. 1k resistor leading FROM the coupling cap to grid 1 (Pin 5); this one should stay in place. Buy a very small 24V
PCB-mountable transformer, glue it into a corner of the amp and use the small soldering pins to built a 4 diode (Negative!) rect. bridge, a 1000uF/ 63V over it (Polarity!) and also a 25k lin pot in series with an 8k2 resistor over it. The center tap of the pot goes to the four grid-resistors, formerly attached to GND. Fit a second 1000uF / 63V to this point for hum-free operation. Or copy some Bias schematics form a book or from the web into your amp. |
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| Open your amp, do one of the two versions of the tweak, do some other essential tweaks and close the amp. |
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| Advantages:
amps sound better on all volume levels, mostly play louder and offer maximum control over operating conditions and tube life. By better is meant: Improved dynamics, better imaging, less stress, the High End blah blah in general. |
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| Tube amp output stages are run either in Fixed or Auto Bias. I never understood this black & white approach. Fixed works better, Auto sounds better - a dilemma.
Now get the best from both-if not all- worlds: 'Compound Bias'. Obtain good sound from class A (auto Bias) AND obtain maximum control over the settings of the power stage and it's idle currents.
If in doubt, ask for advice: martbak@dds.nl (non-commercial) |
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