Non-matched pair?

[Leevi]

I'm wondering a weird behaviour of a pair of 6550s.
If I measure the tubes with a measuremt instrument I get about similar cathode current for both tubes.
When I measure the cathode current during the operation (when idle) in the power amp over 1.2 Ohm resistors
the first tube shows 71 mA and second one 64 mA cathode current which means that the deviation is about 10%!
Bias voltage is the same, UL tap voltages show same value as the plate voltage (450V). The cathode resistor values have been
measured and they are same. What could be the reason?
/Leevi

[kagliostro]

What happen exchanging tubes each other  ?

Kagliostro

[Leevi]

I have not yet tested that. It is the next thing I'll do
/Leevi

[kagliostro]

May be a stupid question

the 1.2 Ohm resistors (you use as to read the voltage to deduce the current) are perfectly matched ?

Kagliostro

[Leevi]

I switched the tubes and it seems that they behave in different way.
So they are not really matched. To find a real matched pair is the solution.
Thanks
/Leevi

[kagliostro]

You can double the bias circuit and give each tube exact drive

Kagliostro

[jjasilli]

10% deviation is within normal. 

[plexi50]

Question:

If you have two bias pots to independantly adjust each tube for evenly matched ma, wont each tube have a different plate voltage on it?

One tube drawing more current than the other? Does that really matter?

[kagliostro]

I forgot what jjasilli told

10% deviation is within normal.

---

If you have two bias pots to independantly adjust for evenly matched tube in ma, wont each tube have a different plate voltage on it?

One tube drawing more current than the other?

With not matched tubes the intention is to avoid different prerformances (of the tubes)

Kagliostro

[Leevi]

If you have two bias pots to independantly adjust each tube for evenly matched ma, wont each tube have a different plate voltage on it?

Yes, that is a solution but since I have installed the trimmer pot in such a way that it is adjustable outside this would cause too much extra work.
Btw. VOX AC50 is a good example of that kind of bias setting.

http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/voxamps/ac50_2.pdf

Thanks
Leevi

[HotBluePlates]

Question:

If you have two bias pots to independantly adjust each tube for evenly matched ma, wont each tube have a different plate voltage on it?

One tube drawing more current than the other? Does that really matter?

What is the d.c. resistance of 1/2 your OT's primary? Could we say 100-200 ohms is typical?

What amount of current difference would be reasonable to balance? Could it be as much as 20-30mA?

If each of these seems okay, then 30mA through a 200 ohm 1/2-primary results in 6v of IR drop. I wouldn't be concerned with 6v of difference in plate voltage.

Most of the impedance that is likely to result in B+ drop is more likely in the B+ line itself, especially the rectifier (if tube) and the PT.

[plexi50]

Thanks HBP. Good explaining. I could live with a B+ or tube plate variance better than a 13ma mismatch

All these years i have had really good tube matches. Now for the last 6 months all i have been getting when i buy the used tubes off Ebay is wide variances between -33ma & -48ma