6V6GT's in push pull @ 250V

[stevehoover]

I have a 269EX Hammond transformer and a 8K OT laying about.
I have some preamp ideas that I have wanted to try out.  This build doesn't have to have a lot of output wattage.
I want to stay in the 250V range on the plates of my finals.



Looking at the data sheets for 6V6GT's, can anyone tell me why the recommend load for 6V6 types jumps up at 250V (10K load) on the data sheets.  For other output tubes I have looked at (EL84's for example) the recommend load drops or remains close to the higher voltage value when plate voltage is lowered to 250V.


Is this an anomaly of 6V6 types, and what would one anticipate sonically if using an 8K OT @ 250V with 6V6 finals?


Just curious, Thanks for you reply...
Steve

[jjasilli]

It's  a "feature" of the tube.  One could ask why about every trait of the tube; but this particular trait coincides with OT you have on hand.  A 50% mismatch is OK.  10K : 8K is well within acceptable tolerance for a guitar amp.

[printer2]

Let's see, the datasheet I have has,

250V - 10k - 10W  -  5%
285V -  8k - 14W  - 3.5%
250V - 12k - 13W  - 

so what gives?

I am no expert at tubes but have read a little. One thing that I have come across is the distortion curves of the odd tube or though. Can't remember which but it is either the second harmonic that rises with increase in output impedance while the third starts higher in the lower impedances and then is reduced as you go higher (or maybe it is the other way around, too lazy to go searching for it). So the composite distortion curve is higher on the ends with a lower point at some middle impedance. This also varies with voltage so there is no one ideal operating point. Because even harmonic distortion cancels out in the transformer (theoretically with everything being matched) you might want to run the amp with more second if it reduces the third. Mind you it was engineers that figured out this stuff and they were shooting for the most output for the lowest distortion.

We in the guitar world are not that fussy about a couple, even up to 100%. So recomended  values really depends on what the people that wrote them were aiming for. How much you are in Class AB may also have a hand in deciding the impedance. 8K has been known to work well for us, I would not worry about it.

[printer2]

Came across one with the separate distortion curves.

[PRR]

> the distortion curves .... it is either the second harmonic that rises with increase in output impedance

2nd harmonic cancels in Push-Pull.

> why the recommend load for 6V6 types jumps up at 250V (10K load) on the data sheets.

For marketing and historical reasons.

The 6V6 replaced the 6F6 and some earlier tubes.

B+ in home radios is traditionally 250V. (450V peak AC off the PT so 450V electrolytics don't blow at turn-on, 50V lost in rectifier, 150V lost in speaker field coil.) The two-6V6 radio is a big step up from the one-6V6 radio. In power and in cleanliness. The two-6V6 model does not have to be WAY better. In particular it is nice to have a plan which draws little more current than the one 6V6 (80mA vs 50mA, but a radio also has another 10mA-20mA in tuner tubes so 100mA vs 70mA), and/or can use the same iron as a two-6F6 model.

In a more liberal design, you raise the B+ and lower the load impedance to get a higher Output number for the specs.

And 8K or 10K are the same for any practical purpose (other than verifying that Ken-Rad got the test numbers right).

Optimum load for screen-limited push-pull tubes *does* go up as voltage goes down. But slowly. Peak current rises faster than G2 voltage, and vice-versa. Half the G2 voltage allows 0.35 of the peak current. Optimum load runs about inverse of square-root of voltage.

> For other output tubes .. recommend load drops or remains close to the higher voltage value when plate voltage is lowered

Tube amp design is about many factors. Sometimes a tube offers more current than we need. Then we may make the load the same or maybe lower when working at lower B+.

[stevehoover]

Thank you gentlemen, your input is appreciated.
I have been at this for a while with some decent sounding builds, but being "self taught" and with funds usually an issue...it's great to get feedback here to supplement my lack of knowledge and experience.
 
[size=78%]Steve[/size]