Bias balance pot for matched tubes ???

[lespaulebony]

Hi Everybody,
This my first thread here. So first, please let me thank you for your help and I apologize in advance for my English. It's not my native language, and believe me, I try to do my best.

Here is my question :
I recently built two Vibrolux AA964 and I have the same issue with these two amps. I tried to match the two grid resistors, the screen resistors and the two bias 220K resistors.
But despite this, with matched tubes (and I know they are matched because they are new and it dit it each time with several different pairs) I always get a difference of 10 ma between the tubes.
Of course I reversed the tubes to be sure ...

But it appears that I can only have a matched pair in the amp when the tubes are mismatched.

I am thinking at a bias balance pot but first I would like to know if there is something special to do that I forgot ...

Thanks for your help

[DonMoose]

Unless you specifically buy a 'matched pair' of tubes, you cannot assume that any two tubes of the same brand/model/batch are anywhere near matching.

If I misread your statement, then I apologize for sounding ... impatient. Such is not my intent.

Then there are some other things to consider - you matched Rg, Rg2 and bias resistors (to what percent?) but that assumes the OT is balanced between push and pull. There will be a DCresistance difference between the two, but don't know if that would account for a 10mA imbalance.

Hope this helps!

[lespaulebony]

Unless you specifically buy a 'matched pair' of tubes, you cannot assume that any two tubes of the same brand/model/batch are anywhere near matching.

If I misread your statement, then I apologize for sounding ... impatient. Such is not my intent.

Then there are some other things to consider - you matched Rg, Rg2 and bias resistors (to what percent?) but that assumes the OT is balanced between push and pull. There will be a DCresistance difference between the two, but don't know if that would account for a 10mA imbalance.

Hope this helps!

These are matched pairs new production of the same brand -model and sometimes batch. In any case sold as this.
I tried to match the resistors very close. In fact the resistors difference is inferior at 0,1%.

[martin manning]

...I always get a difference of 10 ma between the tubes.
Of course I reversed the tubes to be sure ...

Bonjour! Just to be clear on the problem, the higher current follows the tube or stays with the socket when you did this?

[lespaulebony]

...I always get a difference of 10 ma between the tubes.
Of course I reversed the tubes to be sure ...

Bonjour! Just to be clear on the problem, the higher current follows the tube or stays with the socket when you did this?

bonjour

When the tubes are supposed to be matched, it stays on the same socket.

When there is a mismatch between the tubes, I can play with it and decrease or increase the difference.

[martin manning]

Measure the volatage at the junction of the PI coupling cap, 220k and 1k5 grid stopper resistor to see if it is the same on both sides.

I think it is likely that there is a high voltage leaking into one grid circuit from somewhere. Since you are observing this on two amps, is the problem on the same side in both cases? This could be caused by a leaking coupling cap, but it would be a strange coincidence if the same cap was bad on both amps.

Is it possible that your circuit board material is conductive? If you followed the historic Fender layout, one of the grid leads is closer to the high voltage eyelets than the other. Does that lead go to the tube socket that has high current?

[Racing]

Measure the volatage at the junction of the PI coupling cap, 220k and 1k5 grid stopper resistor to see if it is the same on both sides.

I think it is likely that there is a high voltage leaking into one grid circuit from somewhere. Since you are observing this on two amps, is the problem on the same side in both cases? This could be caused by a leaking coupling cap, but it would be a strange coincidence if the same cap was bad on both amps.

Is it possible that your circuit board material is conductive? If you followed the historic Fender layout, one of the grid leads is closer to the high voltage eyelets than the other. Does that lead go to the tube socket that has high current?

+1. Find the actual source of the problem.

To in turn set a balance circuit up to be able to use any mismatch basicaly is easy enough.

[img:300:400]http://www.102983.org/gallery/albums/us ... C00354.jpg[/img]

In this case an old Roland Bolt 30 running 7591 powertubes that has been completely remodeled into all tube but the same basic principles apply.
What you see in the pic is the dedicated PI card. Negative bias voltage is led to the viper of a 1k trim pot. The OEM 220k resistors leading for the powertubes are just rerouted slightly to meet up with respective side of the trimpot legs. At that same junction a pair of 56k resistors are added as controlled leaks vs ground.
By fooling around with these you get control over hysteresis of the trimpot. Easy and reliable.

[lespaulebony]

Measure the volatage at the junction of the PI coupling cap, 220k and 1k5 grid stopper resistor to see if it is the same on both sides.

I think it is likely that there is a high voltage leaking into one grid circuit from somewhere. Since you are observing this on two amps, is the problem on the same side in both cases? This could be caused by a leaking coupling cap, but it would be a strange coincidence if the same cap was bad on both amps.

Is it possible that your circuit board material is conductive? If you followed the historic Fender layout, one of the grid leads is closer to the high voltage eyelets than the other. Does that lead go to the tube socket that has high current?

Thanks for all these tracks ... I will check asap ...

[tubeswell]

Have you tried swapping the tubes to see how the plate current measures up the other way around?

It is not unusual for the DC resistance on one side of the OT primary in a push pull amp, to be different from the DC resistance in the other side of the OT primary, because one side of the primary is a longer wire because its wound over the top of the other side. This can account for differences in plate idle currents.

[martin manning]

The high current stays with the socket (see above). There is a 10mA difference in idle current here, which is likely 25-30%. It's worth a a check of the resistances of the two sides of the primary to be sure, but it would take much more than the usual difference in DC resistance to cause that much difference in current.