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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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In Reply to: Re: How do I modify my ST70 to individually bias the EL34's? posted by hifitom on December 11, 2000 at 02:35:16:
I guess Uncle Ned is busy processing those Christmas tube orders, so I'm going to fill in here.
HifiTom is essentially correct, but don't you DARE set the bias to 1.00V with a 10 ohm resistor for each tube! Using yer Ohm's law, this gives 100mA, which is the stock bias for a PAIR!!!
So either use a 20 ohm resistor on each cathode, or use a 10 ohm but set the voltage drop to only 0.50V, or 50mA of current.
The other thing is that if you just put another pair of 10K potentiometers in parallel with the existing ones, you upset the voltage distribution in the bias supply. What will happen now is that the RANGE of voltage change that each bias pot creates will be halved, since instead of forming a 5K resistance (10K divided by 2), the pots will now form a 2.5K resistance (10K divided by 4).
I would replace all the pots with 20K linear taper ones, or 25K if you can't find the 20K value.
Separate the 270K grid resistors as HiFiTom suggests. I do this to all my ST70's that I modify, and locate the pots in between the output tubes on the chassis. I then run additional wires from the new cathode resistors to an unused pin on the octal socket on the front panel so I can measure the bias as before.
A tube that doesn't bias the same as the others is not necessarily weak or "bad". Of course, it COULD be! But you should test all your tubes anyways, this is just a good idea period. A tube that draws more or less current than others just doesn't match the others very well, that's all. The tube tester isn't going to show you how the tube biases in an actual amplifier, although if it's weak enough relative to the others, odds are it will draw somewhat less current than the others, as well.
I think this mod improves the sound, and it definitely improves reliability.
One more suggestion. Since the ST70 is fixed-bias, it won't hurt to run it with one tube removed from one channel for the purposes of testing. If you want to see how each tube is biasing in an amp that has only one cathode resistor & bias pot, just yank one tube at a time out of a given channel & measure the voltage across that 15.6 ohm resistor. Remember that half the current should flow across that 15.6 ohm cathode resistor, so the stock bias voltage is now to be read as 0.78V NOT the full 1.56V!!! Remember to allow the tubes to warm up a good 15 minutes first, which can make this procedure a tad tedious. And don't forget to short or at least close the input & put a speaker load on the amplifier, just for safety's sake. Any input signal, if large enough, will cause the bias reading to "dance" when you are doing this measurement.
If the cathode voltages are within 10-20% of each other or better, you're doing pretty good.
I would use a high-accuracy DVM and measure the resistor value first (with the amp off, of course), so that I knew it was at least close to the 15.6 ohm value (or else how do you know what the heck you're REALLY measuring?).
Hope you try adding those pots, it's well worth it!
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Follow Ups
- Re: How do I modify my ST70 to individually bias the EL34's? - Joe Rosen 16:45:22 12/12/00 (1)
- Thanks very much! - MitchN 11:53:17 12/13/00 (0)
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