Bias????

[IIIBOOMERIII]

How is it that there are amps out there that can take tubes
like 6v6,5881, 6l6, EL34, or 6550 tubes without the need for
rebiasing?

There has to be a trade off for having the ability to do this?

[Blue Strat]

How is it that there are amps out there that can take tubes
like 6v6,5881, 6l6, EL34, or 6550 tubes without the need for
rebiasing?

There has to be a trade off for having the ability to do this?

There's definitely a trade off UNLESS there's some sort of bias switch.

6V6s and EL34s bias similarly. 6L6s, 5881s and 6550s bias similarly. When trying to use tubes from BOTH groups in the same amp one group or the other will not bias optimally, or the amp can be adjusted so that NONE of them bias optimally, but all somewhat safely.

[GearHeadFred]

Unless they are self-biasing... Like the THD univalve, for example.

[eleanor296]

From what I understand the Univalve isn't self-biasing, it rather has a set bias point that will accept most any tube SAFELY, yet not OPTIMALLY. Shoot me down if I'm wrong, my memory's not all the way here right now.

[Blue Strat]

Unless they are self-biasing... Like the THD univalve, for example.

My statement includes self biasing amps. It's impossible to have an amp self bias OPTIMALLY for both groups that I mentioned.

[Blue Strat]

From what I understand the Univalve isn't self-biasing, it rather has a set bias point that will accept most any tube SAFELY, yet not OPTIMALLY. Shoot me down if I'm wrong, my memory's not all the way here right now.

I'm pretty sure that it IS self biasing.

[MrChet]

The term "self bias" gets construde to meaning that an amp can automatically set the correct bias condition for the tubes. However, that is not what it actually means. I just means that w/ a cathode resistor to ground the tubes grid is negative with respect to the cathode so the tube biases itself because of that condition.

[Blue Strat]

The term "self bias" gets construde to meaning that an amp can automatically set the correct bias condition for the tubes. However, that is not what it actually means. I just means that w/ a cathode resistor to ground the tubes grid is negative with respect to the cathode so the tube biases itself because of that condition.

...and it doesn't mean that every type of tube will bias CORRECTLY in the circuit.

[RedMan]

There is a big tradeoff. Almost every tube that you put in a Univalve comes up cold and the impedance is usually not matched very well.

[drbob1]

"Self-biasing" is really just another name for cathode biased. The resistor (usually bridged with a cap) keeps the grid negative compared with the cathode. As BlueStrat has said, you can set it up so almost all tubes will bias in a safe range, but you can't set it up so any given tube will get exactly the amount of bias that it needs. You can get closer with a 6L6/EL34 switch (why don't any single-ended cathode-biased amps have one?) but it's still unlikely the bias will be optimal. Amps like the AC30, which are cathode biased but set up for just one tube type, get closer but obviously can't be optimized for any particular set of tubes. So, why don't cathode biased amps have biase adjustments to get it exactly right?

[IIIBOOMERIII]

I was going to ask the same thing Bob. I was going to use Mesa/Boogie as an example.

[donnyjaguar]

There are some circuits that perform an auto-bias function. This is a much more complicated circuit than kathode biasing. I do believe the new Traynors are so equipped. Whether these auto-bias circuits have enough voltage swing to accept going from say a 6L6 to a 6CA7 would be interesting.

There are hi-fi amplifiers that will accept all different types of output tubes. These generally use a combination of kathode resistors, with various switchable parallel kathode resistors to set up the nearest appropriate idle current step through the output stage. Whether this is optimized is a point of conjecture as I would argue that any tube amp that will take any output tube cannot be optimized for any one type.

YMMV.