I have a couple of old tube amps I am rebuilding that each have a pair of 6L6's for output devices. the cathodes of both are tied together and then to ground through a single 225 ohm resistor. this offers no way to adjust individual or overall bias.
I would like to covert the amp to fixed bias by removing the 225 ohm resistor and tying both cathodes to ground. then adding a negative bias supply and individual bias pots. this in theory sounds easy enough, but is it just that simple? I understand HOW to do it, just wondering if there are any reasons I am unaware of as to why I couldn't do it?
it has a 5600 ohm CT output transformer if that makes any difference.
Zc
I would like to covert the amp to fixed bias by removing the 225 ohm resistor and tying both cathodes to ground. then adding a negative bias supply and individual bias pots. this in theory sounds easy enough, but is it just that simple? I understand HOW to do it, just wondering if there are any reasons I am unaware of as to why I couldn't do it?
it has a 5600 ohm CT output transformer if that makes any difference.
Zc
It should work, however you will probably have to lower the B+ voltage in some way as there will be an extra 35V or so across the tube.
Otherwise you could give each cathode it's own 430 resistor bypassed with a 330uf 100V cap. While you still can't adjust it it helps a lot with equalising the current.
Cheers.
Yes, but keep a small cathode resistor (1R-10R) for each tube to measure the current.
Use 100k or less for the grid resistors.
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Also understand that grid circuit resistance has to be much lower for fixed bias, i.e. 100K max vs 500K max, which can affect drive circuit design.
Zero Cool, You will OK without question if you are tubes are 6L6GB or 6L6GC variants. If the tubes are older plain jane 6L6's you will be pushing it with a HV of 395 volts. Fundamentally, your thinking is correct by simply grounding the cathodes and then supplying the negative bias via the grid leak resistors. Just keep the resistor values withing limits of what is spec'd. I would insert a 1 ohm resistor in each tubes cathode to ground connection for an easy way to set bias.
Regards, Mickeystan
Regards, Mickeystan
we are going in opposite directions....😀
me, i convert fixed bias amps to cathode resistor bias.
why? durability and safety of output tubes....
why not try cathode resistors on each cathodes instead?
this is what i do in my amps...
i recently built a JJ6L6GC pp amp using 500 ohm cathode resistors,
it turned out that at a plate voltage of about 340 volts, bias was 60mA each,
mind you, the JJ's were well matched to a decimal point figure,,,
my observation, sound quality was fantastic...🙂
me, i convert fixed bias amps to cathode resistor bias.
why? durability and safety of output tubes....
why not try cathode resistors on each cathodes instead?
this is what i do in my amps...
i recently built a JJ6L6GC pp amp using 500 ohm cathode resistors,
it turned out that at a plate voltage of about 340 volts, bias was 60mA each,
mind you, the JJ's were well matched to a decimal point figure,,,
my observation, sound quality was fantastic...🙂
I suggest a "compromise". Use combination bias and enjoy most of the benefits of both "fixed" and self. 😉
"Stand" each pair of O/P tubes on a shared 100 Ω/470 μF. network. A single "idle" current trim pot. serves both tubes. The 100 Ω resistor provides a convenient test point and stabilizes the operating conditions set. While tight matching of gm is always needed in a PP pair, minor differences in cathode current will self correct. Modest liberties can safely be taken with the "fixed" bias grid to ground resistance limit.
"Stand" each pair of O/P tubes on a shared 100 Ω/470 μF. network. A single "idle" current trim pot. serves both tubes. The 100 Ω resistor provides a convenient test point and stabilizes the operating conditions set. While tight matching of gm is always needed in a PP pair, minor differences in cathode current will self correct. Modest liberties can safely be taken with the "fixed" bias grid to ground resistance limit.
That's interesting! I was going to put 1 ohm resistors in the cathodes to measure bias but I could certainly use 100 ohms.
> cathode resistor biasing has less murphy built in...
+1.
Radiotron Designer Handbook agrees with you: "use cathode bias whenever possible."
> a single 225 ohm resistor. this offers no way to adjust individual or overall bias.
So split to two 470r resistors (with bypass caps), and tack few-K resistors across each to trim bias. However a self-biased stage is very self-correcting, and a few-Volt difference is no real difference at all.
+1.
Radiotron Designer Handbook agrees with you: "use cathode bias whenever possible."
> a single 225 ohm resistor. this offers no way to adjust individual or overall bias.
So split to two 470r resistors (with bypass caps), and tack few-K resistors across each to trim bias. However a self-biased stage is very self-correcting, and a few-Volt difference is no real difference at all.
tried that too....for the 6l6 with a 100 ohm cathode bias,
at 60mA cathode and 35 volts negative grid, means that the negative bias of -29 volts...
if you make the cathode resistor 250 ohms then negative bias is -17.5 volts..
the thing that i like with resitors in the cathodes is when the output tube
fails, then there is 250 ohms + opt primary dcs limiting current,
opt is safer this way and will probably survive such failures...
at 60mA cathode and 35 volts negative grid, means that the negative bias of -29 volts...
if you make the cathode resistor 250 ohms then negative bias is -17.5 volts..
the thing that i like with resitors in the cathodes is when the output tube
fails, then there is 250 ohms + opt primary dcs limiting current,
opt is safer this way and will probably survive such failures...
thanks PRR, i remember you said as well, the extra power in a fixed bias mode,
does not really matter in terms of sound power/quality perception....
in all of my tube amps i sold, i converted to cathode resistor bias, no back jobs
because of that...
does not really matter in terms of sound power/quality perception....
in all of my tube amps i sold, i converted to cathode resistor bias, no back jobs
because of that...
another plus for the self bias method, you can use higher value
grid leak resistors, that means higher gains for the driver stage
and possibly larger swings.....
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