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Single Ended: the pentode retaliation
Single Ended: the pentode retaliation
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Old 10th June 2021, 04:38 AM   #61
Chris Hornbeck is offline Chris Hornbeck  United States
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Much thanks. So CED is the series differential/ Rush cascode part, and series feedback here means the Crazy Drive part?


Thanks again,
Chris
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Old 10th June 2021, 06:48 AM   #62
zintolo is offline zintolo  Italy
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Quote:
Originally Posted by Tubelab_com View Post
Unknown until about 10 years ago, mosfets also have a secondary breakdown limit that is not well defined, or even stated in some data sheets. Most mosfets available today are designed for switch mode operation. Long term operation in the linear region can result in random failures. ON Semi / Fairchild does not show this at all in their SOA graphs in the data sheet.
Thanks, I wasn't aware of that and I was only basing the choice on datasheet. Thanks for the info!

Quote:
Originally Posted by Tubelab_com View Post
I have not yet blown a Fairchild FQPF9P25 mosfet, but I keep them under 10 watts and 75 degrees C. The chassis plate itself will not be a sufficient heat sink for two mosfets each dissipating 35 watts.
So you stay safely below 200V across it. Well noted. Have you also tested bigger brothers of it? In this case I would need somenthing like a PQPFxP50 to withstand the 400V swing of the preamp.

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Originally Posted by Tubelab_com View Post
If you experiment with this operation put a fast blow fuse of 1/2 to 1 amp in the cathode lead of the tube. It may save the tube and OPT. Do not try to fuse the plate with a 250 volt AC fuse, it may explode on DC.
Thanks again to save me from fireworks and injuries.
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Old 10th June 2021, 06:51 AM   #63
zintolo is offline zintolo  Italy
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Quote:
Originally Posted by SpreadSpectrum View Post
The surprise to me was that the screen voltage modulation in CED with a ground-referenced screen supply seems to make this even more pronounced and increase gain. I think the screen voltage modulation is responsible for more of the improvement to the curves than the series feedback is, but there does appear to be an improvement for that as well.
Very interesting! May I ask you to share the LTSpice file you used? I would like to perform some tests and compare results.

Thanks!
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Old 10th June 2021, 08:11 AM   #64
Bandersnatch is offline Bandersnatch  United States
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Have to ask, why do y'all use the P-channel under the cathode?
cheers,
Douglas
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Old 10th June 2021, 11:36 AM   #65
zintolo is offline zintolo  Italy
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Basically what you have there is a source follower where the drain is grounded and the source copies the signal at the gate. In this configuration a pmosfet does a source follower, whilst a nmosfet would contribute with its gm, that is what we don't want.
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Old 10th June 2021, 11:52 AM   #66
Bandersnatch is offline Bandersnatch  United States
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Originally Posted by zintolo View Post
Basically what you have there is a source follower where the drain is grounded and the source copies the signal at the gate. In this configuration a pmosfet does a source follower, whilst a nmosfet would contribute with its gm, that is what we don't want.
OK, that makes perfect sense.

In the schematic with two 100k R-fbk and two 68k to ground, B+ is not listed. Given how high the grid is I am going to assume it is fairly high( as the tube will still need a respectable B+ anode-cathode, yes?).

Also given the 68k:100k we have something excess to the usual suggested by Schade of 10% fbk, right?

I'd be quite tempted to try this leaving the input stage un-Schaded at first. Likely a try with a slightly larger pentode( EL84-6AV5 window, 6V6 maybe? ). Un-Schading the input would also probably let me get away with a resistive plate load too...

cheers,
Douglas
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Old 10th June 2021, 12:05 PM   #67
zintolo is offline zintolo  Italy
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You can see the Va label, that is 500V, scale it down with the voltage divider and you'll find it will be around 202V whilst the cathode is above that (as it has to be).

The feedback here is 40%, even if based on George's feedback it could be too much stress for the pMosfet, and could be better to reduce it.

How do you want to connect the grid then?
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Old 10th June 2021, 01:17 PM   #68
Bandersnatch is offline Bandersnatch  United States
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Ultimately it could be that I will take a very mild Schade applied to a medium sized transmitting triode I want to use. Its plate resistance is at high end of marginal, and the MOSFET source follower at the 100mA of idle would likely do it. From zero bias on the triode I need about +/- 35V of swing...and the FET drivers put an end to the hunt for a matching3k:600 line OPT. Back to two stages... and pentode voltage amplifiers.

So given the partial thoughts I am assembling in the aether, 5% Schading( 100k dropping from anode to grid, and 5k grid to ground with a 600V B+ leaves the grid at ~28V...and so I'll need a bit of MOSFET headroom. Is there any issue tying the drain to a solid -20V rail?
cheers,
Douglas
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Last edited by Bandersnatch; 10th June 2021 at 01:25 PM.
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Old 10th June 2021, 09:18 PM   #69
smoking-amp is offline smoking-amp  United States
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I did some curve tracing for a 31LZ6 Sweep tube using UnSET. Both pics are 50 mA/div Vert, 50V/div Horiz. and 100V supply for grid 2. 31LZ6 doesn't have as sharp of pentode knees as the 6HJ5, not sure that makes any difference here.

1st pic has the Vg2 supply grounded, so Vg2 to K is varying with Cathode drive V. Using 4V steps.

2nd pic has the Vg2 supply floated on the cathode, so Vg2 to K is constant, using 5V steps here. (no assisting Vg2 drive this way)

I was going to do a normal shunt "Schade" curve trace too, but the tracer crapped out. When I get it fixed I'll do that. I also wanted to do an internal triode trace for the 31LZ6, but that will have to wait too. Previous 31LZ6 triode tracing looked typical for a Sweep tube in triode mode, with some Mu variation (roll-over in curves)

I see the same variation in the bottom of the Unset curves bends at low current here as in the simulations by Spreadpectrum. Looks like one wants to keep Vg2 to K more close to constant for better triode emulation.

There could be some advantage in having some Vg2 drive though, since it seems to remove the roll-over effect, but fully grounded Vg2 may be putting too much correction in, causing the curve bending at the low current ends to actually tighten up a little at the HV side.
Attached Images
File Type: jpg 31LZ6_Unset_gndVg2 100V, 4Vsteps_50mA_50V.jpg (631.8 KB, 71 views)
File Type: jpg 31LZ6_Unset_FLTVg2_100V_5Vsteps_50mA_50V.jpg (620.2 KB, 63 views)

Last edited by smoking-amp; 10th June 2021 at 09:44 PM.
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Old 11th June 2021, 02:16 AM   #70
SpreadSpectrum is offline SpreadSpectrum  United States
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Quote:
Originally Posted by zintolo View Post
Very interesting! May I ask you to share the LTSpice file you used? I would like to perform some tests and compare results.

Thanks!
See attached. Don't laugh at my ugly hacking of your schematic! I was trying to be quick.

I thought it was very interesting how the normal CED curves without the floating supply had less rollover at high voltages than at lower voltages.
Attached Files
File Type: asc Curves CED ConstVg2-k.asc (2.2 KB, 6 views)
File Type: asc Curves CED.asc (2.1 KB, 4 views)
File Type: asc Curves ShuntFB.asc (2.5 KB, 4 views)
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