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Solid state phase splitter
Solid state phase splitter
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Old 7th February 2019, 02:03 PM   #1
Metalgarri is offline Metalgarri  Netherlands
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Default Solid state phase splitter

Hi all,

I was thinking to build a tube guitar amplifier, and to save a bit of power and space to use a solid state phase splitter to drive a push-pull EL84 power stage.

Any experience with that? Suggestion on a good schematic?

Thank you very much!

Carmine
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Old 7th February 2019, 04:50 PM   #2
Tubelab_com is offline Tubelab_com  United States
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Mosfet or other semiconductor substitute for cathodyne?
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Old 7th February 2019, 04:58 PM   #3
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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The problem is that EL84's need about 16Vpk for full driving, so you will need a power supply of about 20V minimum. Also, in case of tube problem, the SS drive will blow together.
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Old 7th February 2019, 08:06 PM   #4
Gnobuddy is online now Gnobuddy  Canada
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Quote:
Originally Posted by Metalgarri View Post
Any experience with that? Suggestion on a good schematic?
I have a "source-o-dyne" phase splitter made with an LND150 MOSFET in a small push-pull 6AK6 amp I built a few years ago. It works very well, exactly like a valve-based cathodyne, but even better.

(A true valve cathodyne has some nasty characteristics when overdriven unless you take several precautions. There is a section on this in Merlin Blencowe's valve preamps for guitar book.)

IMO, a real valve is wasted in a cathodyne. With so much negative feedback, the cathodyne produces too little harmonic distortion to hear. It sounds sterile-clean, just like a transistor. So why not just use a transistor (MOSFET) instead, and save the triode for somewhere else in the circuit, where it can actually contribute to the sound?

A generic schematic and LTSpice simulation is attached. The exact MOSFET you use doesn't matter much, as long as it's rated for a sufficiently high voltage. I just picked something built into LTSpice. The back-to-back 10 volt zener diodes are necessary to protect the MOSFET gate.

Incidentally, minimum B+ is at least four times the largest peak signal you want to drive your output tubes with. Put another way, if you use, say, 300 volts B+ on the MOSFET, it can spit out two phase-reversed signals of up to almost 75 volts peak (150 volts peak-to-peak) each.

That's without any load on the source-o-dyne outputs. Once you hook them up to the EL84 control grids, there will be some loading from the grid bias resistor, and heavier loading from grid current once the EL84s start to overdrive, so the maximum available signal swing will decrease.


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Old 7th February 2019, 10:12 PM   #5
PRR is offline PRR  United States
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Quote:
Originally Posted by Osvaldo de Banfield View Post
...EL84's need about 16Vpk for full driving, so you will need a power supply of about 20V minimum.....
We can now easily find 500V MOSFETs, so we can drop-in a MOSFET and run it on the same 300V rail as the tube we are replacing.
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Old 8th February 2019, 06:45 AM   #6
Metalgarri is offline Metalgarri  Netherlands
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Quote:
Originally Posted by Gnobuddy View Post

IMO, a real valve is wasted in a cathodyne. With so much negative feedback, the cathodyne produces too little harmonic distortion to hear. It sounds sterile-clean, just like a transistor. So why not just use a transistor (MOSFET) instead, and save the triode for somewhere else in the circuit, where it can actually contribute to the sound?

-Gnobuddy
That's exactly what I am thinking!

Especially in the case of projects that are small in terms of component number and output power, it will be useful to save space in the chassis (or use a smaller one), and also require less power from the power transformer which has to power one filament less.

And what about using the case of a phase splitter based on a differential pair cathode biased? Using an equivalent mosfet circuit will it influence the sound?
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Old 8th February 2019, 07:44 AM   #7
klingo is offline klingo
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For the sourc-o-dyne I will make sure to not feed them with more than a 1/2 B+ peak to peak signal if you want to avoid SS clipping, the preceeding AX7 might swing 2/3 B+ and a tone stack could have no attenuation above 3-4khz with treble dimed...also put a 100k grid stopper (EL84 will stand if cathode bias) on power valve, especialy the cathode driven one to avoid the "nipple" effect.
the differential one has been done by R.G.Keen. High Gain Phase Inverter - MOSFET Follies 8-)
hard to avoid clipping this one!
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Old 8th February 2019, 10:54 AM   #8
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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Quote:
Originally Posted by PRR View Post
We can now easily find 500V MOSFETs, so we can drop-in a MOSFET and run it on the same 300V rail as the tube we are replacing.
This is undoubtedly true, but in such a case, then go for a full SS amplifier. Personal taste only: I dislike mixing SS and tubes. Different technologies, different behavior, different requirements. Not for me.

In the other hand, a MOSFET with 300V per se doesn't warranty that you will get clean 16Vpp from them.
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Old 8th February 2019, 09:43 PM   #9
Gnobuddy is online now Gnobuddy  Canada
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Quote:
Originally Posted by Metalgarri View Post
That's exactly what I am thinking!
I'm with you. If I ever build a tremolo oscillator into an amp, I'll use a MOSFET for that, too. And I think MOSFETs make complete sense for the send and receive electronics in an FX loop as well.

Quote:
Originally Posted by Metalgarri View Post
And what about using the case of a phase splitter based on a differential pair cathode biased? Using an equivalent mosfet circuit will it influence the sound?
A MOSFET differential-pair PI is a no-go, unfortunately. The reasons are not immediately obvious, so I'll start at the beginning:

A proper (triode valve) diff. pair, unlike a cathodyne, does have voltage gain (say 25x - 30x to each output). It also overdrives gradually as the signal level goes up, and this provides enough distortion to be quite audible, at least if the amp is designed so the output stage hasn't gone into hard clipping long before the phase splitter starts to overdrive.

To me, the long-tailed-pair valve phase splitter has a sort of "growl" to it, probably from significant amounts of third-harmonic distortion. It is a push-pull amplifier, after all: that means even harmonic distortion is cancelled out, and only odd harmonics are generated (assuming perfectly balanced MOSFETs.)

So, to replace a proper valve diff amp PI, our hypothetical MOSFET differential amp phase-splitter has to do two things: it has to provide voltage gain like a valve diff amp PI, and it has to overdrive and "growl" like a valve diff amp PI does.

It's easy enough to wire up a pair of MOSFETs in the same differential amp configuration (though self-bias is a no-no). But we immediately have a problem: the voltage gain will be way higher, maybe ten times higher, than a valve diff amp. This is because modern MOSFETs have enormous transconductance compared to a half-12AX7 or similar triode.

Okay, we can fix the too-high voltage gain by putting an unbypassed source resistor betwen each MOSFET and the shared "tail" resistor. But the fix introduces another problem - now you have a sterile-clean PI that provides no audible distortion at all. It will sound like a cathodyne, not like a long-tailed pair.

Maybe there is some clever way around these issues. But at the least, it's not a case where a simple substitution (MOSFET for triode valve) will work well.

Incidentally: FETs and self-bias are a very bad idea. It doesn't work at all for enhancement-type FETs, and it doesn't work reliably for depletion-mode FETS either, because of the wide parameter spreads all semiconductor devices suffer from.

This is why, for instance, the circuit I posted for a MOSFET "source-o-dyne" doesn't use self-bias. Instead, a pair of resistors across the power supply biases the MOSFET gate to one-quarter B+. The source of the FET will settle to within a few volts of that, and with a few hundred volts B+, we can just ignore those few volts, and say the source will be at one-quarter B+. Now chuck in equal value source and drain resistors chosen for reasonable current flow and reasonable power dissipation in the MOSFET, and you're done.


-Gnobuddy
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Old 8th February 2019, 09:47 PM   #10
Gnobuddy is online now Gnobuddy  Canada
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Quote:
Originally Posted by klingo View Post
<snip>

...the differential one has been done by R.G.Keen. High Gain Phase Inverter - MOSFET Follies 8-)
Thanks for the link! I note that RG Keen experienced exactly the issues I just described in my last post. He didn't mention that adding local NFB to lower the MOSFET PI gain also linearizes away any trace of audible distortion, but perhaps he was okay with an audibly transparent PI, rather than one that sounds like one made with a pair of triodes.


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