individual bias pots for each power tube

[trey85stang]

Would setting up this type of bias supply be good for mixing tubes types and running mismatched tubes?

Or would it be trouble waiting to happen? i.e. running a el34 on one side and a 6l6 on the other?

Or two different brands of el34's?

[skipm45]

Are you looking for an asymetrical output? Just wondering why you'd run a pair of different output tubes?


Skip
www.skipzcircuits.com (http://www.skipzcircuits.com)

[skipm45]

Individual bias pots are a fairly common mod, but I was struggling with the mismatched tubes.


Skip
www.skipzcircuits.com (http://www.skipzcircuits.com)

[Blue Strat]

It couldn't hurt. You could alternately add a bias balance pot in addition to the bias pot.

[trey85stang]

Are you looking for an asymetrical output? Just wondering why you'd run a pair of different output tubes?


Skip
www.skipzcircuits.com (http://www.skipzcircuits.com)

No real reason, I just want to try it out... but I wasnt sure if it would actually work or not. Im not exactly sure what asymetrical output meas? I was just wondering if it would kill a transformer to run say a kt66 on one of the output and 6550 on the other?

Assuming they both get biased out to roughly equal wattage output that is.

[WesKuhnley]

Individual bias supplies and a drive balance pot are a fairly common thing to find in a hi-fi amplifier.

I do not endorse using different pairs of tubes simultaneously, unless specifically endorsed by the manufacturer. What you're suggesting could potentially work, but more specifics on the circuit are necessary to determine for sure.

[trey85stang]

Individual bias supplies and a drive balance pot are a fairly common thing to find in a hi-fi amplifier.

I do not endorse using different pairs of tubes simultaneously, unless specifically endorsed by the manufacturer. What you're suggesting could potentially work, but more specifics on the circuit are necessary to determine for sure.

No specifics yet, it will be my own design. (but I will share freely once I get started on it.. hopeing to make it an "opensource" style circuit so others can make improvements to it)

[doctorx]

In his books KOC advocates using individual bias pots in all builds. It allows for compensation of mismatched tubes, both new and as they wear. Some folks also say having the power tubes slightly mismatched can give richer harmonics.

[Faustine Amps]

No real reason, I just want to try it out... but I wasnt sure if it would actually work or not. Im not exactly sure what asymetrical output meas? I was just wondering if it would kill a transformer to run say a kt66 on one of the output and 6550 on the other?

Assuming they both get biased out to roughly equal wattage output that is.

If the amp is designed to run in class AB / push-pull configuration, ideally the output tubes would be matched in transconductance and biased the same. This would give you a symmetrical output where the positive and negative halves of the waveform at the OT secondary would be roughly the same amplitude, just different polarities. The waveform would also look very similar if not identical (in appearance, not in voltage level) to the waveform at the input jack. This is what a good tube hi-fi amplifier output would look like. The purpose being to accurately recreate the signal at the input with the least amount of distortion.

However, if you use output tubes that are unmatched, the positive and negative halves of the waveform at the OT secondary will be different amplitudes and will be asymmetrical (i.e. not symmetrical). This would be unacceptable with a hi-fi amplifier, but might be quite musical with a guitar amplifier depending on what kind of distortion is produced (assuming you are intending to produce distortion). Even if you bias them the same (with separate bias pots), if they are not matched for transconductance, the output will be asymmetrical.

Now, this is assuming you use output tubes of the same species (if you get my drift). Once you start using output tubes of different species, like a EL34 and a 6550, not only are the tubes going to have different transconductance and different bias voltage (for the same power output), but they may have other mismatched specifications, like different plate resistance. Now, in addition to having tubes that are mismatched, you're going to have a possible impedance mismatch between the plates of the output tubes and the OT primary windings. This could be opening a whole new can of worms.

Have you considered these problems?

Regards,
Tim Gregoire

[Old Tele man]

BIAS ADJUSTMENT = "static" (hence: idle) DC-balance between tubes <--what you can adjust

TRANSCONDUCTANCE = "dynamic" (hence: signal) AC-balance between tubes. <--what you can't (normally) adjust

[WesKuhnley]

BIAS ADJUSTMENT = "static" (hence: idle) DC-balance between tubes <--what you can adjust

TRANSCONDUCTANCE = "dynamic" (hence: signal) AC-balance between tubes. <--what you can't adjust


Placing a pot in series with the plate resistors of the PI will balance (or unbalance...) the AC drive to the power tubes, compensating for transconductance imbalances. This combined with individual DC bias adjustment results in a extremely well balanced output stage.

[Old Tele man]

...replacing the two PI 100K plate resistors with a 200K linear pot (with wiper to B+)?

...yes, it'll work, but "pots" and "high voltages" do not mix well (read: noisey and prone to arcing when being adjusted).

...would be better/safer to "tweek" the PI grid gain(s) rather than the PI plate output(s).

[WesKuhnley]

Naw, leave the 100k's, put a 10k trimmer between em with the wiper to B+ and you're good. This is a pretty common thing to find in older Hi-Fi amps. I've done this in a ton of amps with no trouble whatsoever.