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What driver tube for 200Vrms?

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I am looking at getting linear swing in the 200Vrms level for amps with heavy local feedback in the output stage.

Basically my approach will be to load this driver only with a source follower biased with 1M resistor and having 5.5pF reverse transfer capacitance, so AC load will be relatively light on this driver. I am just concerned with linear voltage swing under these conditions.

I have achieved the following results with a 6SN7 loaded with 10M90S cascode, biased at 6mA and 380V on plate, and followed by FQPF2N90 source follower:

1.2% THD @ 200Vrms, output taken from source follower source.

Spectrum is attached.

My question is, has anyone done better and if so with which tube?
 

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I guess I neglected to mention that these tests are with about 800V B+. Remember everybody 200Vrms is almost 570Vpk-pk.

Eli,

Unless I am mistaken, no choke will give me a high enough impedance to get 200Vrms @ 20Hz starting at 6mA. Inductance would have to be unrealistically high. AC load line needs to stay very flat across all frequencies to avoid clipping on the anode upswing.

I have wondered if I would get better performance with this circuit and an ECC99 at higher currents. It is hard to tell how a tube will perform when you are swinging almost twice as high as the published curves go. Is the 5W limit for ECC99 per half or for the sum of both? I imagine it is the sum. What would be really cool is maybe an octal ECC99 with a limit of 8W per half.

Wavebourn,

Gyrator might be a better design due to the stable anode voltage, but I still have to make sure the impedance stays very high all the way down to 20Hz or I will clip at low frequencies. I have noticed that there is about a 20V variation in where the anodes end up when I plug in different tubes.
 
How 'bout 6BL7 or 6BX7? Rated 500V continous and 2000V during peaks.

Transmitter tubes? Like 809? Perhaps a bit fancy and expencive as driver.

You could also use a current mirror and swing the voltage into a negative rail. That way you can use just about any tube you like. Actually, come to think of it, I am not sure you'd find PNPs or P-MOSFETs with high enough voltage ratings.
 
How 'bout 6BL7 or 6BX7? Rated 500V continous and 2000V during peaks.

Transmitter tubes? Like 809? Perhaps a bit fancy and expencive as driver.

You could also use a current mirror and swing the voltage into a negative rail. That way you can use just about any tube you like. Actually, come to think of it, I am not sure you'd find PNPs or P-MOSFETs with high enough voltage ratings.

Those look like some good options. I don't think I want to use large tubes for a driver. The cool thing about this approach is that you don't need to burn a bunch of power in the driver as it is driving a very light load. The source follower does the heavy lifting on the power tube grid. I AC couple driver to source follower and bias the gate of the fet, which is DC coupled to the power tube.

I have had difficulty with the lack of high-voltage p-fets in the past as well. 500V is about as good as it gets.
 
I think that's 5 W. per triode, for the ECC99. Remember the '99 is similar to the 5687 and the 6Н30П (6n30p).

60 H. should be enough as the load for ECC99 sections, as that yields a XL at 20 Hz., which is > 3X RP.

5W per triode is reasonable. I will definitely have to get some and experiment.

I guess I'm still not on board with the choke idea. Maybe I am missing something. Being able to swing these large voltages is only possible with these low power tubes with light AC loads, hundreds of k or megaohms. I just don't see how low frequency performance can be adequate with the relatively very steep load line that will exist only at low frequencies. Am I missing something? The low frequency response gets trashed enough in the OPT, I don't want to make it worse.
 
3X RP is quite sufficient for good linearity and gain retention. It gets better as the freq. rises. Ask a real expert, like Doc. B. or Paul Joppa about choke loading. They've done quite a few parafeed designs.

A big plus to choke loading is the ability to swing voltages above the "idle" anode potential.
 
What kind of amp are you building? I know you followed my 6L6 MOSFET follower driven amp, as you gave me plenty of helpful advice. I was getting about 130v P-P from the driver stage with two 6SN7 diff amps DC coupled with very low distortion. Tubelab stated that higher P-P was possible with capacitive coupling and/or 6SL7 in first stage (his driver stage is used and it was developed for high P-P output for his sweep tube experiments). I have not posted much on my amps since completion, too busy listening!

Cheers,

Chris
 
I am looking at getting linear swing in the 200Vrms level for amps with heavy local feedback in the output stage.......Tubelab stated that higher P-P was possible.......What kind of amp are you building?

570 volts P-P, no I haven't tried to squeeze that much out of it. Heavy local feedback? The only thing that I can think of that uses a LOT of feedback and needs that much drive is a cathode follower.

Need a butt load of drive voltage with very low distortion, look up the "augmented cathode follower". It is an expired patent from 1957. The circuit can be set up to have gain, although the best I could get without instability is a gain of 2. I made a big one using a 6336A and used it as an output stage. Power output was limited by my power supply and output transformer. Now that I have a bigger power supply I need to drag it out and try it again.
 
570 volts P-P, no I haven't tried to squeeze that much out of it. Heavy local feedback? The only thing that I can think of that uses a LOT of feedback and needs that much drive is a cathode follower.

Need a butt load of drive voltage with very low distortion, look up the "augmented cathode follower". It is an expired patent from 1957. The circuit can be set up to have gain, although the best I could get without instability is a gain of 2. I made a big one using a 6336A and used it as an output stage. Power output was limited by my power supply and output transformer. Now that I have a bigger power supply I need to drag it out and try it again.

Thanks for the tip, I was thinking of doing a circlotron or something. 50% of plate swing fed back to grid circuit. I think I would still come up a little short on a cathode follower output on this. I don't even want to think of what it would take to drive those.

A while back I built a +-500V power supply and have been doing experiments with it. That way I can have up to 1000V but am only 500V from ground potential. I don't know how much safety that buys me, though. Whether it is 500 or 1000V, neither will feel good.

I think I may have to look at some 6BL7 and 6BX7.

My main goal was to make a good tube choice so that distortion will be as low as possible before I apply any feedback. The output stage may be low enough distortion that not much is needed, so I'd like to start with a linear tube and was just wondering if anyone had done similar experiments with other tubes. I am young and don't have a huge tube stash like some of you older guys.
 
Here's a preliminary design that required solving that particular problem. For this, I needed a pretty good output with linearity. The 6DR7 answered that pretty well with active plate loading. The 6DR7 includes a high gain, small signal section that also features excellent linearity. The large signal section can handle unusually high voltages, just like the other vertical deflection types mentioned here.
 

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Thanks for the tip, I was thinking of doing a circlotron or something. 50% of plate swing fed back to grid circuit. I think I would still come up a little short on a cathode follower output on this. I don't even want to think of what it would take to drive those.

A while back I built a +-500V power supply and have been doing experiments with it. That way I can have up to 1000V but am only 500V from ground potential. I don't know how much safety that buys me, though. Whether it is 500 or 1000V, neither will feel good.

I think I may have to look at some 6BL7 and 6BX7.

My main goal was to make a good tube choice so that distortion will be as low as possible before I apply any feedback. The output stage may be low enough distortion that not much is needed, so I'd like to start with a linear tube and was just wondering if anyone had done similar experiments with other tubes. I am young and don't have a huge tube stash like some of you older guys.

It sounds like it will be a push-pull topology. Is the 200V RMS the total drive swing from grid-to-grid or is it each grid (400V RMS grid-grid)?

200V RMS grid-to-grid can be accomplished using a differential pair of high-gm triode-wired pentodes like 12GN7, 12HL7, D3A...

Getting a very linear 200V RMS out of a single tube seems like it would require something like an 801A, unless you use a choke or transformer. With an inductive load a lower Rp would be indicated and maybe something like a 2A3 would do the job.

400V RMS grid-to-grid would probably require a pair of high voltage tubes or at least a push-pull transformer coupled driver with low Rp tubes. Then you need a CT plate choke or IT that can handle the 400V RMS. That's pretty ambitious. Lundahl makes the LL1660S IT good for 440V RMS end-to-end at 30 Hz. THe LL1667 anode choke is rated for 390V RMS at 30 Hz.

This reminds me of the "Thor" circuit I was considering that had loads of voltage mode plate freedback driving transmitter tubes. I was thinking of using a pair of 801A in push-pull, IT coupled, for a driver.

Cheers,

Michael
 
It sounds like it will be a push-pull topology. Is the 200V RMS the total drive swing from grid-to-grid or is it each grid (400V RMS grid-grid)?

200V RMS grid-to-grid can be accomplished using a differential pair of high-gm triode-wired pentodes like 12GN7, 12HL7, D3A...

Getting a very linear 200V RMS out of a single tube seems like it would require something like an 801A, unless you use a choke or transformer. With an inductive load a lower Rp would be indicated and maybe something like a 2A3 would do the job.

400V RMS grid-to-grid would probably require a pair of high voltage tubes or at least a push-pull transformer coupled driver with low Rp tubes. Then you need a CT plate choke or IT that can handle the 400V RMS. That's pretty ambitious. Lundahl makes the LL1660S IT good for 440V RMS end-to-end at 30 Hz. THe LL1667 anode choke is rated for 390V RMS at 30 Hz.

This reminds me of the "Thor" circuit I was considering that had loads of voltage mode plate freedback driving transmitter tubes. I was thinking of using a pair of 801A in push-pull, IT coupled, for a driver.

Cheers,

Michael

Thanks for the response, Michael. It is 200Vrms per grid. The output stage is 50% feedback planned to run at 450V. Thus, peak drive voltage for one tube comes out to 200V or so plus Vgk to saturate. 280V is safe but not much margin. 10dB extra headroom isn't really feasible on these kinds of drive levels.

I was considering this for a conversion of an existing amp (which uses an octal dual-triode driver), but maybe I can considering building from scratch. The 801A looks really linear from 25mA on up with a flat load line. It would need a little power to drive the grid to get the plate reasonably low but what's another mosfet?

I also looked through my tube stash and found two 6BL7s which look like beefed up 6SN7s. I suspect that the 6SN7s would perform better (they already aren't bad) at 8-10mA but they would be running at around max dissipation. The 6BL7s will be more comfortable there.
 
I don't want to apply much if any global feedback. Indeed, I won't need any to have a good damping factor. The only reason I would have to apply global feedback is to reduce distortion. My first stage is very low distortion, my output stage should be very low distortion. That leaves the last voltage amplifier. I am not some nut who thinks a little feedback will ruin the amp, I just want to start with the best tube for the job.

The output stage is a half-cathode follower, so even if you apply a lot of global feedback, your final voltage amplifier still has to swing the necessary volts to drive it.

Feedback can straighten the transfer function, but can't fix a driver that clips too early.
 
The 6DR7 answered that pretty well with active plate loading.

Yes, I used a 6EM7 in one itteration of my cathode follower amp. The little section provides most of the gain. The big section provides some more gain and is capable of big output with enough B+. I used a CCS and a 600 volt supply to make about 500 V P-P.

The 6EM7's have a wide tube to tube variation, especially gain in the first stage. I had to try about 5 to find two that were the same.
 

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