Alligator: PP amp with SE sound

[Guest]

Questions/suggestions/objections?

It's a PP amp with triode SE signature where one shoulder is a triode, another shoulder is a controlled current source. 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 mode, i.e. less plate current variations.

V102 -- power triode,
A1 & 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.


Here is a very simplified version, just to show the concept:

 

[Guest]

I like this version more:

I'm going to get up to 80W of triode sound from it.

 

[emrr]

Thanks for sharing. I'll have to study that several times, at minimum.

 

[Guest]

[quote author="emrr"]Thanks for sharing. I'll have to study that several times, at minimum.[/quote]

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.

 

[Guest]

Gm6 A7 Dm, G7 C!
G#, CCCC!

 

[Guest]

Before I started drilling and punching, any suggestions about layout?

 

[Guest]

CJ, it is a layout, it is not a construction yet. Relax. I'll have appropriate hardness ribs where needed.
And I don't care who made transformers, the end result matters.

 

[Guest]

Sorry CJ, I thought you were stoned. :roll:

What free software to download to draw PCBs with tube sockets, to get files to order small production run?

 

[burdij]

Very neat! I suppose you are going to need an output transformer with a fairly high ratio since, as I see it, you are really only using one side of the PP for signal. It might be a good use for all of those great PP transformers laying around from amps that used 7591s and other obsolete high plate resistance tubes (Fisher, Harmon Kardon, Sansui, etc.).

 

[Guest]

[quote author="burdij"]Very neat!
[/quote]

Thank you!

I suppose you are going to need an output transformer with a fairly high ratio since, as I see it, you are really only using one side of the PP for signal. It might be a good use for all of those great PP transformers laying around from amps that used 7591s and other obsolete high plate resistance tubes (Fisher, Harmon Kardon, Sansui, etc.).

Why? It is triode strapped, so 4K P-P only will be what a doctor prescribed for 50W output. When used as pentodes I loaded pair of GU-50 on 10K P-P, with 800V plate voltage.

What can you recommend instead of 2SK727? Something with less capacitances, 1 KV of a voltage, current is not critical since it will be 200 mA peak, 40W to dissipate constantly.

 

[Guest]

Thanks God I did not make PCBs yet... Some errors found, including too high anode voltage for 6F12P, and not enough of a negative swing for GU-50 so a driver has to be redesigned.

 

[Guest]

A new driver:

 

[Guest]

CJ, do you like this layout better?

 

[Guest]

Improved bias regulator:

 

[PRR]

Where do you get the nice GU50 sockets and holders?

 

[Guest]

[quote author="PRR"]Where do you get the nice GU50 sockets and holders?[/quote]

Sockets (with holders) from Ukraine, tubes from Russia and Bulgaria.

I have some better holders (casted aluminium, with latching lids), but very few of them only, so keep them for "naked" designs...

 

[Guest]

First block finished: tube drivers, B+ voltage regulator, magic eye indicators.

 

[Guest]

While soldering changed schematics significantly.
Now pentodes are loaded on CCSs with servos, G2 voltage for them taken from 150V gas discharge lamp through a source follower (one more gas discharge lamp in series gives 105 more, so plate voltage for preamps and drivers is +250V now -- more than enough).