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SE Pentode, regulated screen voltage?
SE Pentode, regulated screen voltage?
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Old 9th November 2017, 07:21 PM   #11
glina is offline glina  Poland
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Thank you all very much. I was able to do some basic measurements and indeed:

Plate current/voltage does not change significantly with signal level. Most variation only due to line voltage variation.

Screen current/voltage changes much less then anticipated, but does vary with signal, measurably higher than plate current.

Zero signal screen current measured at 5mA per tube, 20mA total, so double that of what was declared in datasheet.

I have constructed and installed the Zener-Mosfet voltage regulator from Pete Millett. I'm now seeing a very stable voltage, but not yet listened if the change is audible.
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Old 10th November 2017, 05:47 AM   #12
PRR is offline PRR  United States
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Quote:
Originally Posted by glina View Post
...Zero signal screen current measured at 5mA per tube, 20mA total, so double that of what was declared in datasheet....
6L6 is supposed to be "aligned grid". G2 is wound in the shadow of G1. This is not as easy as it sounds. Ig2 on 6L6 varied some when RCA made them. Now that the machines have been sold around the world, and copied, G2 alignment may only be getting held "close enough".

5mA is not a problem.

The European Power Pentodes were real pigs for G2 current. RCA did it different, and I think they were proud to get it down to "nearly none" (on a good day); but it doesn't have to be real-small to work good.

Note also that under some conditions, a good 6L6's Ig2 can *reverse*. If you have a screen-only supply, it is wise to have a bleeder for 10%-20% of nominal Ig2, because simple supplies won't hold-down when current flows INto them.
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Old 10th November 2017, 06:16 AM   #13
petertub is offline petertub  Sweden
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Quote "My screen supply is a separate transformer tap with a CRC filter (100uf-1k-100uf)."

This is a potential dangerous configuration (for the tubes). If by any reason the main B+
fails the tubes might be destroyed in seconds.
Do make the screen supply as a appendix to the B+, a resistor or resistor + zener is what's
needed.
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Old 10th November 2017, 07:46 AM   #14
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by PRR View Post
under some conditions, a good 6L6's Ig2 can *reverse*.
Care to elaborate? Like what conditions?
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Old 11th November 2017, 06:08 PM   #15
Schmitz77 is offline Schmitz77  Germany
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Klangfilm used in their famous V69 Cinema Amp just a 5K voltage drop resistor and 1uF from g2 to ground. Is that enough for buffering the screen grid, which is pigging out at approx 15-20mA? Why didnt they used a much bigger electrolytic cap for buffering I wonder. Why didnt they used a second resistor to ground to manage for negative current? Power tube is Siemens F2a. Sometimes easy things can look very complex.
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Last edited by Schmitz77; 11th November 2017 at 06:12 PM.
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Old 11th November 2017, 06:11 PM   #16
crazyfrog is offline crazyfrog  Canada
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SE Pentode, regulated screen voltage?
Don't want to "hijack" the thread but since I'm going to put a regulator (Maida type) on G2, the title took my attention...Is this the same thing for el84 types (7189, 6P14P)?
Thanks
Pierre


Quote:
Originally Posted by PRR View Post
6L6 is supposed to be "aligned grid". G2 is wound in the shadow of G1. This is not as easy as it sounds. Ig2 on 6L6 varied some when RCA made them. Now that the machines have been sold around the world, and copied, G2 alignment may only be getting held "close enough".

5mA is not a problem.

The European Power Pentodes were real pigs for G2 current. RCA did it different, and I think they were proud to get it down to "nearly none" (on a good day); but it doesn't have to be real-small to work good.

Note also that under some conditions, a good 6L6's Ig2 can *reverse*. If you have a screen-only supply, it is wise to have a bleeder for 10%-20% of nominal Ig2, because simple supplies won't hold-down when current flows INto them.
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Old 11th November 2017, 06:21 PM   #17
Schmitz77 is offline Schmitz77  Germany
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EL84 datasheet will tell you its exactly the same, just different numbers.
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Old 11th November 2017, 06:25 PM   #18
glina is offline glina  Poland
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I installed the basic Zener-Mosfet and I'm very happy with the results. First, I learned a few important things in the process which I wasn't aware of.

Different sets of my 6L6 tubes pull different screen current which made for a considerable variations of screen voltage. As I also found out, the plate current is very dependent on the screen voltage, which resulted in different operating points. With the regulator now in place, things are more stable.

As for sound improvements, not much at low listening levels, but I get an impression the bass and general imaging is better when played loud.

Crazyfrog: EL84 in Class A operation typically has Va=Vg2. If you intend to use a regulator, you must account for some voltage drop.
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Old 11th November 2017, 08:28 PM   #19
artosalo is offline artosalo  Finland
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I have used this kind of simple power supply for QQE03/12 PP-amplifier.
G2- and G1 voltages are regulated.
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Old 12th November 2017, 12:50 AM   #20
PRR is offline PRR  United States
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Quote:
Originally Posted by glina View Post
...EL84 in Class A operation typically has Va=Vg2.....
In General: common-sense (and low-cost) design results in Vg2 being very near Vp, for fewer parts and less heat.

This does require tubes made for Vg2 ~= Vp. A HIGH-current tube will be low Mu and want a low Vg2. TV sweep tubes, class C radio amps. However we have plenty of choice of "audio" tubes aimed for Vg2 near Vp.
------

Reverse Ig2 is, IIRC (it has been a while), due to secondary electrons from Plate landing on G2, in excess of the primary electrons to G2 (which aligned-grid tends to null). Now that you ask, the last time I saw this was a Metal 6L6.... it is possible that modern builds do not try so hard to eliminate primary G2 current so will not show this effect.
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