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Balancing tubes by adjusting g2 voltage

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gorgon53

Member
Joined 2006
Paid Member
#1
Hi guys,
I would like to ask, has any of you expierence from balancing tubes by adjusting the g2 voltage?
If so, would you please share what you found out?
I searched the forum and the net but was unable to find anything related.
Thanks
Pete
 
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#2
Yes you can do it that way. But most people want a 'stiff' screen supply which would mean using individual voltage regulators for each screen grid, which few people would bother with. Using simple (mismatched) screen dropping resistors would lead to different distortion effects in each tube which would give some people the willies, and you'd have to change the resistors any time you changed the tubes.
 
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#5
I believe it works well, and that the curves of mismatched pentodes can be made to align better by offsetting VG2 than by VG1.

I think it's worth the trouble of making a simple stabiliser: HV N-ch MOSFET source-follower, with adjustable voltage divider for the gate. (ADD heatsink or attach to ALu chassis; add gate→source protection zener 8V2 to 12V). An adjustable current source aimed into a resistor can be used to create a fixed voltage; use a 5V1 or 5V6 zener to program the current source. The variation of FET Vgs makes for some temperature drift, but it is small in the scheme of G2 voltages.

With a follower, the drive impedance to VG2 stays reasonable, especially if you run some standing current in a resistor (try 8-12mA).
 
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#6
Merlinb said:
Yes you can do it that way. But most people want a 'stiff' screen supply which would mean using individual voltage regulators for each screen grid, which few people would bother with.
Click to expand...
Thank you for your reply Merlin.
I can understand why not many will bother with added complexity.
I not only want to balance, but also protect my 4 unpaired EL3010 from g2 overload during the experimental phase of what hopefully someday will be a 50/50 split loaded amp. This necessiates individual feed for each tubes screen grid. My idea is, each tube g2 has its own, ac grounded, adjustable, paralell voltage stab with adjustabl ccs feed. I want to get unpaired tubes gm balanced and g2 fed from a stabilized voltage, fully overload protected, but up to the max average screen dissipation unlimited.
I have not wound the OPT yet, so there is still plenty of time to thinck this over....especially how high is too high and how low is too low for the max-min voltage between cathode and g2 considering balancing and especially the 50/50 splitload, cathodefeedback, "ultralinear", or whatever one migth call it...

Pete.
 
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#7
Rod Coleman said:
I believe it works well, and that the curves of mismatched pentodes can be made to align better by offsetting VG2 than by VG1.

I think it's worth the trouble of making a simple stabiliser: HV N-ch MOSFET source-follower, with adjustable voltage divider for the gate. (ADD heatsink or attach to ALu chassis; add gate→source protection zener 8V2 to 12V). An adjustable current source aimed into a resistor can be used to create a fixed voltage; use a 5V1 or 5V6 zener to program the current source. The variation of FET Vgs makes for some temperature drift, but it is small in the scheme of G2 voltages.

With a follower, the drive impedance to VG2 stays reasonable, especially if you run some standing current in a resistor (try 8-12mA).
Click to expand...
Thank you Rod, i always start out simple, but my simple not always ends that way.
And when i look at it, some years later, it may take quite a while before i am again able to understand how "simple" works.

Pete
 
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