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Old 17th September 2008, 08:54 AM   #31
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Quote:
Originally posted by Wavebourn
I am going to try a double-drive such a way:
1. Differential phase splitter,
2. 2 asymmetrical Concertinas (higher value of resistors in anodes) coupled to each anode of a diff stage directly;
3. Source followers from their anodes to screen grids of output pentodes,
4. Capacitive coupled cathodes of Concertinas to control grids of output pentodes.
The result should be kind of opposite to UL mode.
Ok, let's draw the schemo in imagination.
First tube: a pentode 6J53P, 120V on screen grid, 27K from anode to 250V, cathode resistor sets 90V on anode, current is 10 mA (it's happy).
Second tube: 6N1P, 56K in both anodes to 250V, 175V on anodes, 3 mA ccs in tail,
One grid of diffstage connected to anode of 6J53P through 100K grid current limiter shunted by 200 picofarad, second grid connected to the same anode through 2 megaUhm, and grounded through 0.47 microFarad,
Now, concertinas.
IRF740, directly connected gates to anodes of a differential phase splitter, sources connected directly to screen grids of GU-50 tubes.
What current draw screen grids? We have 172V there idle.
Depending on the current and swing needed for control grids now we may choose resistors from drains to +400V B+ supply...

0.33 microfarad capacitors go from drains to first grids of Gu50.

As the result, we have almost directly coupled thingy that drives both grids of GU-50 so can get a good damping factor on low end.
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Old 17th September 2008, 02:28 PM   #32
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You have made the mistake of posting an actual professional
schematic on more than one occasion. Good ones at that.

What now, is your mouse broken? Or maybe you gone full-
Prof Hawking on us, and would have to draw with your chin?
Sorry to hear about that... Get well soon.

Pics please.
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Old 17th September 2008, 02:45 PM   #33
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hey-Hey!!!,
The 172V idle is the deal killer. At that g2, there will have to be negative at g1, and it sure looks to me like that one should be tied to either g2 or better still the cathode. We'd be running some serious idle current I think.

Also, what is the attraction to run the input stage's plate lower than its g2? Adjust the load, and then the g2 so it comes out lower than the anode I think. The Citation II did this in spades, and I suspect that it would work better at a more likeable OP.
cheers,
Douglas
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Old 17th September 2008, 02:51 PM   #34
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"0.33 microfarad capacitors go from drains to first grids of Gu50."

I take it that the IRF740's are acting as both source followers for the screen grids and as concertinas to derive g1 drive off their drain resistors. One problem I see is that the varying screen current (ie, non-linear varying screen current) will affect the signal gain to the g1s.

I would suggest using a source follower off the diffl. splitter plates for the g2 drives and connect their drains to the tube plates. That way the screen current gets returned to the output, making for very linear outputs. The g1 drives --could-- come directly (well, cap coupled and resistive divided) off the differential splitter plates if they are low enough Z to drive both g1 and the Mosfet gate.

Or just go with George's approach and resistively divide down the g2 voltage (from the Mosfet source) to get the g1 drive (probably don't need another Mosfet follower for g1 since the divider will be low Z there).

You will need some provision to adjust the DC idle condition on the g2s (at the MOsfet gates) for non flaming system operation.

Don
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Old 17th September 2008, 03:19 PM   #35
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Quote:
for non flaming system operation.
I thought I smelled smoke.


Quote:
Or just go with George's approach and resistively divide down the g2 voltage (from the Mosfet source) to get the g1 drive (probably don't need another Mosfet follower for g1 since the divider will be low Z there).
When I don't know what I am doing, I like to have as many knobs to turn as possible. The two mosfet approach allows the DC and AC voltage to be adjusted individually for each grid. This could be done with only R's and C's for G1 on many tubes where grid current is low, but why limit yourself to wimpy tubes. Who knows I may find myself wiring an 813 into the circuit
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Old 17th September 2008, 04:20 PM   #36
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I think the mosfet concertinas are a good idea. In his application,
Plate to Plate (or Drain) feedback might be used for UL. Triode
coupling down to the cathodes and then to the opposing grids,
would be an unintended positive feedback loop. Mosfets won't
couple through in that manner, so no such problem can arise.

I still want him to draw it.
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Old 17th September 2008, 04:38 PM   #37
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Anatolyi:
"IRF740, directly connected gates to anodes of a differential phase splitter, sources connected directly to screen grids of GU-50 tubes."

"0.33 microfarad capacitors go from drains to first grids of Gu50."

Ken:
"Plate to Plate (or Drain) feedback might be used for UL. "


My take on the first quote is that the Mosfet concertina sources are driving the g2s and the Mosfet drain resistors are driving the g1s. I think you are picturing the two drives swapped. (A diagram would be helpfull here of course)

Could still use plate to gate neg. feedback (like UL) like you say, but that would affect both g2 and g1 drives in this setup.

Don
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Old 17th September 2008, 05:19 PM   #38
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Why would he put the higher value resistor at the anode if it
was his intent to drive G1 from there? I don't think he meant
that it should be connected that way, but without some sort
of vague chicken scratchings on the back of a pizza coupon in
broken crayon, we can never be sure.

Translation: Gonna look pretty silly if we have to get Don to
draw it for you.
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Old 17th September 2008, 05:21 PM   #39
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Sorry Gentlemen;
last night I suffered from a tooth pain, so smocked in garage reading the forum. I have a mouse there on server, but no drawing software.
Yes, control grids need a negative bias.
Yes, they will be driven non-linearly depending on screen grid currents, but my tooth felt like this weird idea may lead to good end results.
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Old 17th September 2008, 06:13 PM   #40
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"vague chicken scratchings on the back of a pizza coupon"

You mean like this?
I couldn't find a crayon handy though.

Don

OOPs, I just noticed that the GU50 g1 drive signals need to be crossed over between tubes due to the Mosfet's inversion at its drain. Hmmm, so the non-linear screen current only effects the "OFF" tube's g1 drive. Hmmmm, but then the Mosfet driving the "ON" tube's g1 has no current thru it's drain resistor. I think some fixup is called for here.

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