Using Solid State devices in support of Vacuum Tubes

[R.G.]

Thank you Bob. You prodded me to go do some research.

It is *possible* to make depletion mode MOSFETs, it's just that no one does.

Upon looking, I'll have to amend that to no one DID. IXYS released a line of depletion mode MOSFETs of suitable characteristics back in 2002 (just yesterday to me) that will fill the bill and I bet that's what your device is.

More later as I figure it out. It is possible that the schemo is correct. Think what happens if you mentally put a pentode in, plate to drain, grid to gate, cathode to source and ignore how the screen works for the moment.

[R.G.]

Rats. Looks like no one stocks them. They may be available only from IXYS. Too bad. That device is a direct replacement for a lot of tube positions.

[R.G.]

OK, I found available devices. Supertex makes the LND150 which is rated at 500V, up to 30ma, Idss=1-3ma and 740mW in a TO-92 package. They're available from Mouser for $0.55 each, and in stock. These would make good 12AX7 section replacements because they have Ciss of only 10pF.

I bet your current regulator device is a DN3535N3. This is a TO-92 device rated at 450V, 740mW, but with Idss of 200ma, so it would do the current regulation at anything up to that value if the voltage was low enough not to exceed the power rating. Their Ciss is 350pF, which makes them slow for amplification use, but they'd do the regulation just fine.

And I told you wrong. The schematic symbol is correct. That's the symbol for a depletion mode MOSFET. The little isolated bar in the middle of the channel indicates that. I missed that along the way. Now I know.

This pretty much ices it - using the Supertex devices, it IS possible to do a very much stock-looking phase inverter out of depletion mode MOSFETs that will happily be linear outside the range of triode sections they replace. They will even bias with the same methods.

It is possible, using these devices, to replace every single tube in a tube power amp with a MOSFET.

That does not say you'll like the tone, only that the MOSFET will fit in the place and work. They'll probably be too linear, too clean if all the tubes are replaced. But selective replacement might point out the positions where tubes really contribute to the sound, and let us answer the question of what part of the amp sound is the output transformer.

ACK!!! I have to go model some circuits.

[bob p]

OK, I found available devices. Supertex makes the LND150 which is rated at 500V, up to 30ma, Idss=1-3ma and 740mW in a TO-92 package. They're available from Mouser for $0.55 each, and in stock. These would make good 12AX7 section replacements because they have Ciss of only 10pF.

I bet your current regulator device is a DN3535N3. This is a TO-92 device rated at 450V, 740mW, but with Idss of 200ma, so it would do the current regulation at anything up to that value if the voltage was low enough not to exceed the power rating. Their Ciss is 350pF, which makes them slow for amplification use, but they'd do the regulation just fine.

I've looked around for the DN3535N3 -- couldn't find it. I went to the Supertex site and found their Depletion Mode MOSFETs. The closest device I found listed was the DN3535, which comes in a TO-234AA package. This device is only rated at 300V, Idss 200mA. The part number for ordering is DN3535N8 and DN3535N8-G. Notably, the specs are different from those you've listed, so I'm thinking I might not have the right part, or maybe there's a typo somewhere. If you could provide a link to your data I'd appreciate it.

Now that we're homing in on the identity of the part, I wanted to ask about its role as a current regulator for the cathodes of the phase inverter. I'm still trying to determine just why the audio section of the amp was designed this way, as the audio section of this amp is otherwise a plain-jane circuit; it uses a 6N1P triode at the input, going to a pair of 6N1P that act as the phase inverter, and the PI drives a pair of fixed-bias 6550EH running in pentode mode with a plate voltage of about 420 VDC.

What escapes me is why an otherwise conventional amp is using a MOSFET there in the first place. I seem to be missing the problem that they're trying to solve.

[bob p]

FWIW my schematic diagram for the amp has a drawing date of 11-07-01, which suggests that somebody was making depletion mode MOSFETs earlier than we had thought. Does that help to narrow down the manufacturer and/or the identity of the part?

[R.G.]

I've looked around for the DN3535N3 -- couldn't find it. I went to the Supertex site and found their Depletion Mode MOSFETs. ... Notably, the specs are different from those you've listed, so I'm thinking I might not have the right part, or maybe there's a typo somewhere. If you could provide a link to your data I'd appreciate it.

Sorry - I had cross-eye-itis when I typed that in, flipping from one window to another. The device is the DN3545N3-G. See:the Supertex datasheet.

Now that we're homing in on the identity of the part, I wanted to ask about its role as a current regulator for the cathodes of the phase inverter. I'm still trying to determine just why the audio section of the amp was designed this way, as the audio section of this amp is otherwise a plain-jane circuit; it uses a 6N1P triode at the input, going to a pair of 6N1P that act as the phase inverter, and the PI drives a pair of fixed-bias 6550EH running in pentode mode with a plate voltage of about 420 VDC.

What escapes me is why an otherwise conventional amp is using a MOSFET there in the first place. I seem to be missing the problem that they're trying to solve.

The big reason to use a CCS in the tail of a diffamp (which is what this is) is to force a high impedance between the cathodes, which forces the balance of the diffamp to be much, much better. The diffamp-derived PIs depend on the transfer of signal from the input cathode to the inactive-side cathode. The second side is effectively a common-grid stage. Anything less than infinite impedance to ground at the joined cathodes leaks signal from the sources and the second tube gets a smaller input signal. The diffamp puts out a bigger signal on the second side than the first. That's why a lot of guitar amps use unbalanced plate resistors in the PI - to try to rebalance the unbalanced output signal from not having a good, high tail CCS.

A CCS in the diffamp tail is the high accuracy way to do it. Using a MOSFET for that CCS is a good application of the parts.

As to why they did it? No way to tell. It's a forward-looking way to do things. Tubes can't really get to a low enough saturation voltage to do that job. A depletion mode MOSFET is perfect.

[bob p]

The big reason to use a CCS in the tail of a diffamp (which is what this is) is to force a high impedance between the cathodes, which forces the balance of the diffamp to be much, much better. The diffamp-derived PIs depend on the transfer of signal from the input cathode to the inactive-side cathode. The second side is effectively a common-grid stage. Anything less than infinite impedance to ground at the joined cathodes leaks signal from the sources and the second tube gets a smaller input signal. The diffamp puts out a bigger signal on the second side than the first. That's why a lot of guitar amps use unbalanced plate resistors in the PI - to try to rebalance the unbalanced output signal from not having a good, high tail CCS.

Thanks. I appreciate that with a diffamp, the balance is improved by constant-current supply to the cathode. Traditionally (in the tube era), a high value Rk would be connected from a highly negative supply, or a constant-current pentode could be used.

On a related note, I have an old HP 739AR Frequency Response Test Set that uses a diffamp that's configured as I described above. Essentially, the 739AR is a variable-frequency constant-voltage signal source that's used for calibrating O-scopes, VTVMs, etc. Interestingly, it uses a diffamp that has a high value cathode resistor (220k) that's tied to -250V that's provided by stacked OA2 and OB2 voltage regulator tubes. (Alternatively, I guess that a constant current pentode could have been used.)

I guess that using the MOSFET in the audio amplifier circuit we've been talking about essentially amounts to the modern equivalent of the 1950s approach that used voltage regulator tubes to balance the diffamp.

Thanks for helping to put this into perspective.

A CCS in the diffamp tail is the high accuracy way to do it. Using a MOSFET for that CCS is a good application of the parts.

As to why they did it? No way to tell. It's a forward-looking way to do things. Tubes can't really get to a low enough saturation voltage to do that job. A depletion mode MOSFET is perfect.

Well, it looks like we've answered the question that prompted me to start this thread! Thanks!

The MOSFET example in the audio amp circuit looks like it definitely fits the bill in terms of using SS devices to help tubes to be better audio devices. Now I just can't resist asking: Is this application going to make it into the MOSFET Follies at GEO?

[R.G.]

Yeah, I guess I'm going to do the next installment there. New devices to play with!!!

[billings]

My interest along these lines isn't based so much on the desire to eliminate tubes with SS devices as a cost-saving measure. To some extent, that's what led us all down the path to SS amps in the first place. What I'm more interested in personally is using SS devices to enhance the performance of tubes in performing their intended duties. To that extent, it would be very helpful if I could wrap my mind better around what's really going on with Q1 in that schematic snippet that I posted up at the top of this page. Admittedly, my knowledge of SS components is the weakest link in this discussion, and any help that anyone could offer would be greatly appreciated.

I'm no expert at any of this, but maybe I can be of some small assistance...

As R.G. said, that's got to be a current source in that schematic. Effectively, it's increasing the resistance in the tail of a long tailed pair so that no extra current over the bias current can be sourced or sunk from ground by either tube, it must be stolen from or lent to the other tube's bias current. Thus any signal swing on one tube is more precisely the opposite on the other tube than it would be if there were a current signal to ground.

As far as how the current source works, well, you can actually put a triode or pentode in the exact same position as Q1 in that schematic, just with a few different resistor values. I'm sure there's a better explanation than this, but it might be understood as an amplifier that uses that resistor between cathode/source and ground as a current sensor - if the load tries to draw more current, Vgs will go more negative and plate voltage will rise very quickly before current increases appreciably.

Just looking at the plate curves you can tell that a pentode makes a much better current source than a triode. R.G. has said in articles that in a certain sense a power MOSFET is like a high powered pentode, and the same applies in this case - it's even more apt for a depletion-mode MOSFET, like in the schematic posted.

There is a whole slew of different current sources you can make with transistors. If a hi-fier were resorting to them, I'm surprised they would do something as basic as the one in that schematic.