machting output tubes

[francesco]

Sorry something went wrong uploading the files, so I try again.

[Robert M. Martinelli]

Hi again Francesco,
This is the way I would do it,
the series 68K between the bias pot and GND is there to keep the voltage from going "too positive" ( actually less negative ) which could make the tubes "redplating", with the values shown the variable voltage divider should allow you to go from ( 68/(68+47) )*(-58)=-34.3 VDC to -58 VDC.
( actual values might be slightly different due to components' tolerance ).
Hope this helps

Best regards

Bob

[francesco]

bias

Hi again Francesco,
This is the way I would do it,
the series 68K between the bias pot and GND is there to keep the voltage from going "too positive" ( actually less negative ) which could make the tubes "redplating", with the values shown the variable voltage divider should allow you to go from ( 68/(68+47) )*(-58)=-34.3 VDC to -58 VDC.
( actual values might be slightly different due to components' tolerance ).
Hope this helps

Best regards

Bob

Well, in that way I think I'll be able to only lower the negative voltage.
What if I want to be be higher? say -65?
I think it's not possible,that's why I thought to replace the 22k with a 4,7k resistor. am i wrong?.
What do you think about adding that mod on my super reverb? that is, making a balance bias control on it?
I'd like you have a look at it and let me know your opinion.
Thanks. Ciao. Francesco

[Robert M. Martinelli]

Yep, it's a variable voltage divider, so it can only divide whatever voltage appears at its input, it can't elevate it.

If you need a "more negative" voltage, you need a different tap on the transformer, or a separate transformer - I think this is not the case, as setting the bias voltage so negative would bias the tubes waaaaaaaay tooooooo cold and you would have cross-over distortion caused by tubes switching on and off.

Bias is there to keep cross-over distortion from occurring, and -58V is already "cold", I doubt you will ever need to go lower than that.
OTOH going towards "less negative" voltages increases the amount of bias current, at the point that, if the control grid approaches 0V electrons would no longer be rejected by the control grid and the tube would merely act as a diode.

To keep this from happening I added the 68K resistor that narrows the range from the maximum ( -58VDC ) to about -35VDC.

As I told you, I would do it exactly the way I shown you.

Hope this makes it clearer.

Best regards

Bob

[Robert M. Martinelli]

Oh, one last thing,
reducing the series resistor's value as you would do would have no effect, because you get a voltage drop across a resistor only if you have a current flowing through it ( Ohm's law ). This does not happen with control grids which are voltage-operated and don't draw any current in A, B and AB1 designs.

Regards

Bob

[Gtr_tech]

Hi , I have a twin reverb (ultralinear) and some doubt
I replaced them recently but they are not completely machted, in fact their reading is;
V 7: 23ma
V 8; 29 ma
V 9: 26 ma
V 10 : 27 ma
so, what position should i put them to be the "most machted" possible?
I mean should I place, side by side, ( for example)the one with 23 ma with the one with 27 ma. or what?
Thanks:Francesco.
P.S.the tubes are 6L6

I wouldn't lose any sleep over a couple mA.....they're close enough to play nice.