augmented cathode follower?

[smoking-amp]

"though I don't agree with his conclusion about good compensation for linearity."

Anatoliy,
What distortion problem are you concerned about? For non-linear loads, like a speaker maybe? I guess for this case, it would be best to go for absolute lowest output Z, so a bipolar transistor would be called for, since I see your "house warmer amplifier" uses this. Or a N-FDBK loop.

For just driving a grid, I would think class A, P-P, Mosfet should be pretty linear. Serious overkill really, unless driving a class A2 grid, but then that's back to non-linear loads.

Don

[gingertube]

MOSFETs
Don't forget the Zetex ZVN0545A - Its a bit wimpy but if it will do the job then none of the larger devices will match it. I use it as a source follower to drive direct coupled output tubes grids (EL84, EL34, KT77)

I came across this device about 10 years ago in my day job when I was looking for something to switch 300 Volt gating pulses to a photomultiplier tube.

Data:
450 Volts
600mA peak 90mA continuous
600 mW ONLY (this will generally determine if it can be used or not)
BUT
Ciss is only 70pF, thats 1/2 what the best of the bigger devices will have.

It comes in an 'E-Line' package (TO-92 ish).

Cheers,
Ian

[smoking-amp]

There is also a P channel Zetex too:
ZVP0545A 120pF 450V .7W gfs= .04 S TO-92

The Zetex's are a bit expensive though at $2.48/1, or $1.84/10 at Mouser anyway.

There are some surface mount SOT-223 Zetex's: ZVN0545GTA and ZVP0545GTA that are rated 2 Watts and are half the price of the TO-92 versions (Mouser prices again)

Also Fairchild has a TO-92L version
FQNL1N50B 500V 1.5 Watt 115 pF gfs = .55 S priced (at Mouser again) .45/1 .38/10

For driving non-linear class A2 grids, the higher gfs (Gm) of the FQNL1N50B would be a plus for lower output Z.

There are some Fairchild surface mount parts in the FQD... series and tabless heatsink mount parts in the FQU... series.
FQD1N80 FQU1N80 800V 45 Watt 150pF gfs = .75 S

For comparison, the whole bunch:

ZVN0545A (N channel) 450V .7 Watt 70pF gfs .1 S TO-92
ZVP0545A (P channel) 450V .7 Watt 120pF gfs .04 S TO-92
ZVN0545GTA (N channel) 450V 2 Watt 70pF gfs .1 S SOT-223
ZVP0545GTA (P channel) 450V 2 Watt 120pF gfs .04 S SOT-223
FQNL1N50B (N channel) 500V 1.5 Watt 115pF gfs .55 S TO-92L

FQP1N50 (N channel) 500V 40 Watt 115pF gfs 1.04 S TO-220
FQP1P50 (P channel) 500V 63 Watt 270pF gfs 1.26 S TO-220
FQP2P25 (P channel) 250V 52 Watt 190pF gfs 1.2 S TO-220
FQP2N90 (N channel) 900V 85 Watt 390pF gfs 2.0 S TO-220
FQP3P50 (P channel) 500V 85 Watt 510pF gfs 2.35 S TO-220
FQD1N80/FQU (N channel) 800V 45 Watt 150pF gfs .75 S D/I-PAK

Don

[smoking-amp]

Just to be thorough:

Supertex DN2540 (N chan depl.) 400V 15 Watt 200pF gfs .325 S TO-220
The data sheet graph shows Cinput at about 168pF on a graph for drain voltages above 10 volts.

IXYS IXCP10M45S (N chan. depl.) 450V 40 Watt ?pF gfs .2 S (approx. at 100 mA) TO-220
IXTP02N50D (N chan. depl.) 500V 25 Watt 120pF gfs .15 S TO-220

I listed the IXTP02N50D since it is similar to the IXCP10M45S and does give the Cinput of 120pF for that. I would GUESS the IXCP10M45S Cinput to be about 180pF from the ratio of gfs for the two.

also the IXCP10M90S (N chan. depl.) 900V 40 Watt ?pF gfs .03 S (approx. at 100 mA) TO-220

The MUCH smaller gfs here would make me expect a lot smaller Cinput for this part, but no data is available.

A previous thread came to some conclusions on the Coutput (not the Cinput) here:
http://www.diyaudio.com/forums/showt...59#post1220959

With DN2540 at 28 pF
and IXCP10M45S at 125 pF
The similar IXTP02N50D however lists Coutput at 25pF. The discrepency may be due to the low drain voltage used in that measurement since the capacitance generally drops rapidly in the first 20V of drain voltage.

Don

[smoking-amp]

OK, one more and I'm done beating this to death:

Gary Pimm used the IRF820B for CCS's, so I will list that. Actually the IRF820A had better input cap. specs, so I'll include that too.

IRF820A 500V 50W 340pF Cinput, 53pF Coutput, gfs 1.4 S TO-220

IRF820B 500V 49W 470pF Cinput, 45pf Coutput, gfs 2.9 S TO-220

For CCS use, I'll include the Coutputs for the earlier ones too:
ZVN0545 10pF
ZVP0545 20pF
FQNL1N50B 20pF
FQP1N50 20pF
FQP1P50 40pF
FQP2P25 40pF
FQP2N90 45pF
FQP3P50 70pF
FQD1N80/FQU1N80 20pF

Don

[Wavebourn]

Originally posted by smoking-amp
"though I don't agree with his conclusion about good compensation for linearity."

Anatoliy,
What distortion problem are you concerned about? For non-linear loads, like a speaker maybe? I guess for this case, it would be best to go for absolute lowest output Z, so a bipolar transistor would be called for, since I see your "house warmer amplifier" uses this. Or a N-FDBK loop.

For just driving a grid, I would think class A, P-P, Mosfet should be pretty linear. Serious overkill really, unless driving a class A2 grid, but then that's back to non-linear loads.

Don;

I'm concerned about odd harmonics; but I'm going to try your idea.

[smoking-amp]

"going to try your idea"

The class A, P-P I guess? I haven't actually used that for driving tube grids, I just extrapolated from the SS class A amplifiers that have used it. One thing to look out for (I forgot to mention, but is obvious anyway) is the increased input capacitance when using a P channel Mosfet besides the usual N channel one, probably quadruple the input capacitance (and more if two CCS's are used for bias spreading).

But 90% or more of the cap. gets bootstrapped away anyway, and the available parts are pretty good now. I just ordered some FQP2P25 and FQP2N50 Mosfets, so I can try this out too. I may try the P-P buffer on the pentode plate to screen feedback scheme. Then later try putting some On-Semi Thermal-Traks on that to drive a speaker. I would like to see how much of the output circuit I can include in the screen feedback loop before it oscillates uncontrollably.

Guess I better include the FQP2N50 (TO-220) in the data list too:

FQP2N50 500V 55 Watt Cin: 180pF Cout: 30pF gfs: 1.45 S
FQP2P25 250V 52 Watt Cin: 190pF Cout: 40pF gfs: 1.2 S

These are the best match I can find between N and P channel Mosfets.

Don

[tubelab.com]

I have read and reread this paper, and I have come to the conclusion that I must build an "acf-2" or an "acf-3" or both, only using somewhat BIGGER tubes. Then I must "test" it in true Tubelab style. As I am leaving town in a few days my "testing" will be confined to the simulation world for now.

While visiting my very ill mother in law there wasn't too much to do since it was 2 degrees outside, so I did some simulations, designed an amplifier based on the ACF-2 and laid out a PC board. Now that I have returned and almost caught up on the usual stuff, I made one.

The amp used a 6EM7 to develop several hundred volts of clean drive signal, and feeds this into a Macdonald ACF-2 circuit that has been given the Tubelab treatment. The output tube is now the standard octal dual triode so I can use anything from a 6SN7 to a 6336A.

Initial testing with a 6SN7 revealed basic functionality, and several hundred volts (P-P) of clean output with no OPT hooked up. Connecting up an OPT caused red plates on the 6SN7. I popped in a 5998A which made about 2 watts before red plate set in. In true Tubelab style I went for the big guns and installed a 6336A. As soon as the tube warmed up both grid stoppers exploded in flames!

Far more science is needed when I am not in a rush, which means next weekend.

A picture of the PCB is included. Note the crispy resistor just in front of the right output tube. There is another one behind the yellow cap.

[Fuling]

Seems you have built a nice power oscillator...?
Grid stoppers usually don´t go off like that.

[tubelab.com]

Something obviously caused a bunch of grid current in the output tube all at once. It was late so I just turned it off. The DC voltages were not quite right with the smaller tubes in it, but I was in a hurry so I ignored that and cranked things up anyway.

The grids are DC coupled from a 12AX7 which is supplied by a 330K ohm plate load. There should be no way for this to source enough current to vaporize a 330 ohm grid stopper. It was either a serious oscillator or a tube arc. More experiments to follow. Toasted parts are likely!