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Old 5th March 2008, 05:12 PM   #1
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Thumbs up Wavebourn Alligator amplifier

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Old 5th March 2008, 05:24 PM   #2
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Couple of obvious errors corrected:

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Old 5th March 2008, 06:15 PM   #3
EC8010 is offline EC8010  United Kingdom
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Default OK, I'll bite the alligator...

I saw a circuit similar to this recently on AA that described itself as PP. You seem to have a conventional input stage with an output valve buffered by a source follower. The op-amp and MOSFET are a bias servo(?) to balance the currents in the output transformer and allow use of a PP transformer? It's hard to tell exactly what's going on without values.

Oh, and I forgot the time machine. May 2008?
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Old 5th March 2008, 06:31 PM   #4
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Default Re: OK, I'll bite the alligator...

Quote:
Originally posted by EC8010
I saw a circuit similar to this recently on AA that described itself as PP. You seem to have a conventional input stage with an output valve buffered by a source follower. The op-amp and MOSFET are a bias servo(?) to balance the currents in the output transformer and allow use of a PP transformer? It's hard to tell exactly what's going on without values.
It's a PP amp with triode SE signature where one shoulder is a triode, another shoulder is a controlled current source. Yes, it allows to use better transformer (no need for massive iron with gap so no need for an extra copper) and twice more of power from one triode.

As you know a dynamic resistance of a current source is very high so transfer function is determined by a triode only that works in a privileged more, i.e. less plate current variations.

V102 -- power triode,
A1 и Q103 -- controlled current source,
V101 -- driver for a power triode, also a phase splitter. Triode control signal goes from anode, VCCS control signal goes from cathode, to the same cathode goes positive feedback by current and negative feedback by voltage.

Q101 -- source follower to drive a grid positively,

Q102 -- bias regulator for the output triode.

Resistors in cathode of V102 and in source of Q103 are equal so DC currents through them are equal. Because of time constant of R113 and C103 currents are balanced by average value. However, it introduces some transients, but anyway SE amp is prone to them.
Plus some positive feedback by current will go through C103 to cathode of the 1/st tube, but it may be made very small if to use an opamp with small input currents and increase R113 value in respect to R103.


This idea is result of my previous success with "Tower" amplifiers where I used SS output stage (a voltage follower loaded on a variable current source). They sound very pristine. Alligator is the way to implement similar trick in order to get a SE triode signature.
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Old 5th March 2008, 06:34 PM   #5
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Default You did not bite; you pretended to bite...

Quote:
Originally posted by EC8010


Oh, and I forgot the time machine. May 2008?
Flush you cache please; I've corrected couple of errors already including this one.

Edit: a simplified version, especially for you:

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Old 5th March 2008, 08:26 PM   #6
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Ah, so you have a concertina phase splitter. I wondered about that earlier, but it could just have been a feedback path, depending on values. Have you considered the balance of the capacitive loading on each output?
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Old 5th March 2008, 08:57 PM   #7
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Quote:
Originally posted by EC8010
Ah, so you have a concertina phase splitter. I wondered about that earlier, but it could just have been a feedback path, depending on values. Have you considered the balance of the capacitive loading on each output?
Yes. But disbalance is not a big deal since CCS has huge output dynamic resistance. Also, the Concertina is already very disbalanced (VCCS needs much less voltage swing than a triode).

Meanwhile, I'm working on a 2'nd version that will be much easier to tune. I'm going to avoid a feedback path to cathode of a phase splitter that would add a dynamic disbalance.

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Old 5th March 2008, 09:41 PM   #8
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Here you go, version 2.0

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Old 5th March 2008, 10:40 PM   #9
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Actually, the amp of a version 2 consists of 2 parallel parts: tube SE amp and a SS VCCS, input signal goes to both of them simultaneously.

The tube amp has self-balanced by a P-MOSFET bias.
DC bias for a VCCS is taken from output tube's cathode so idle currents of both shoulders are equal and cancel bias of output transformer.

Tube amp is inverting, VCCS is not inverting, so AC currents they supply equal and counter-phased (a tube part has a negative feedback by voltage, a SS part has a negative feedback by current). Since dynamic resistance of a CCS is very high a transfer function is determined by a tube part that is single ended with triode output.
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Old 6th March 2008, 01:22 AM   #10
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Default gain matching?

The FET in the lower "shoulder" is not running as anti-current
mirror slaved (in reverse) to the tube "shoulder". It is servo'd
to balance DC well enough, but the AC gate drive drive through
the op-amp is a unity gain (because of your NFB capacitor.)
Gate tracks the cathode of your Concertina splitter, but not the
anti-current of the other shoulder.

Nothing here forces your FET to have a similar voltage gain to
the tube, nor any sort of anti-tube SEPP curvature... The way
you got it driven is sort of Concertina driven Pentode-ish FET.
Which would probably cancel a lot of SE in the real Pentode of
the other shoulder...

I do like your A2 superdiven grid for the output tube...
I am uncertain about automatic baising. I don't think
there is anything wrong with it, just don't understand
your circuit yet. I probably need to stare at it for awhile...

This might be a fun one to tie grid #1 to the cathode,
and drive screen grid #2, just to see what happens...
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