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Old 28th September 2012, 02:51 PM   #1
dgta is offline dgta  United States
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Default Series connection of regulator tubes

I only see this rarely in vintage gear. A regulator tube in series with B+ to provide a constant drop to Vg2. Basically used instead of a drop resistor. See Altec A333.

While obviously not as good as real regulation, it strikes me as a very decent compromise between cost and function for that era. Certainly much better than a drop resistor.

Why wasn't this used more frequently? What's the downside?
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Old 28th September 2012, 02:57 PM   #2
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I have seen OC2 tubes used in a lot of Decware circuts...mainly in line...not shunt as in this example
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Old 28th September 2012, 03:02 PM   #3
DF96 is online now DF96  England
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Although it can give a lower g2 source impedance than a resistor, it also gives worse voltage regulation when the mains voltage changes (as it often will). To take round figures assume 400V supply rail and 100V regulator drop. 5% mains variation is +-20V on 400V, but then +-20V on 300V is 6.7% variation in g2 voltage. OK with a stabilised supply, but could make things worse with an unstabilised supply.

The circuit shown above throws away some of the impedance advantage with a 1200ohm resistor, although this is presumably present to avoid making a relaxation oscillator with the capacitor. I think the reason why you only see this idea used rarely is that it is not always a good idea!

Last edited by DF96; 28th September 2012 at 03:03 PM. Reason: typo
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Old 28th September 2012, 03:16 PM   #4
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DF96, yes it does poorly on mains regulation, but much, much better on load regulation.

I just measured a 60s amp with a quad of PPP 6L6GC, cathode bias. Uses resistor drop to Vg2 with a voltage divider. From idle to full rated power Va droops 6V and Vg2 droops 31V. That certainly trumps the couple % you give up on mains regulation.
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Old 28th September 2012, 03:27 PM   #5
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The really important thing with pentode outputs is dynamic regulation of g2 voltage, so yes I agree this fixes the more important thing. A pair of 150V regulators in series from ground would be even better, and not much more expensive.
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Old 28th September 2012, 03:52 PM   #6
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They did not have modern MOSFETs and high voltage Zeners then, otherwise they would use routinely parametric stabilizers with source followers. However, stabilizing screen grid voltage you should consider also stabilizing first grid negative bias voltage.
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Old 28th September 2012, 05:33 PM   #7
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WOT'S the sound like? Good? Leave it be, then.
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Old 28th September 2012, 07:03 PM   #8
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Quote:
Originally Posted by Wavebourn View Post
you should consider also stabilizing first grid negative bias voltage.
The Altec in my OP, as well as the amp I was describing, are both cathode bias so this is not an issue.
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Old 28th September 2012, 07:14 PM   #9
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Quote:
Originally Posted by dgta View Post
The Altec in my OP, as well as the amp I was describing, are both cathode bias so this is not an issue.
Right. And I meant stabilized screen grid voltage, not like in Altec that used VR tubes to subtract from B+ a constant value. Cathode bias provides DC servo by current that helps in both cases. But for cathode bias you are limited by 2 extremes: either class A, or class B, otherwise dynamic distortions will be pretty high.
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