High Gm Amp

[ANSA]

As a consequence of the following thread: http://www.basaudio.net/gm70/index.php/topic,32.msg81.html#new
...I was thinking that (even though I have a preference for the SE) there would be many valid reasons to build a PP with the 6528 valves.


http://ansaht.altervista.org/uv638jed4/Amp1/6528PP_1.jpg


First of all it would be a high transconductance amplifier (much higher than any other valve amp), with all the advantages that this characteristic involves.
Besides the high damping factor, it could have a topology with only two active stages in Class A, therefore extremely suitable to musical reproduction.
The PP in Class A is the only configuration where the power supply is not on the path of the audio signal; therefore we could have a power supply without transformer.
Furthermore, by connecting the two triodes contained in the 6528 on the opposite sides of the PP, the sensitivity to vibration would be reduced.
Also, it would be simple and quick to vary the configuration of the amplifier using only one 6528 valve in the output, or two, three or four, depending on how much power you need... or to verify the musical performance by varying the number of triodes in parallel.

[ANSA]

Here is the datasheet of a suitable transformer.


http://ansaht.altervista.org/uv638jed4/Amp1/XE-65-1.jpg


Has the correct primary impedance and, using the outputs, the impedance can be optimized for one or more tubes 6528.

[Bas Horneman]

Hi ANSA,

I've attached the jpg's to the posts. Since it seems embedding to that site does not work for some reason.

Kind regards,
Bas

[ANSA]

Hi Bas,
thank you for JPG.

Continuing the discussion ... here is the interstage transformer, to drive a quartet of 6528:

http://ansaht.altervista.org/uv638jed4/Amp1/NC-22.jpg


And here's a picture of how result the aesthetic of the output block:

http://ansaht.altervista.org/uv638jed4/Amp1/DSCN3363-1.jpg

[ANSA]

Design update of audio section.





8233 triode connection:

Gm= 50 mA/V
Ri= 600 Ohm
Pa= 10 W

http://www.mif.pg.gda.pl/homepages/frank/sheets/009/e/E55L.pdf






Tango NN-6 datasheet:

[Bas Horneman]

Hi ANSA,

What does it sound like? It's refreshing to see someone build with tubes you don't see every day.

By the way the Tango transformers are quite expensive are they not?

Kind regards,
Bas

[kheper]

Tango went out of business in August 2000. The XE-65-1 transformers must be worth a mint now. I would kill for a pair of them.

[Bas Horneman]

I would kill for a pair of them.

Phew.. glad I dont have any.

[ANSA]

In a general way, I can say that from the musical point of view the high transconductance tubes are not only endowed with great ability in dealing the smaller details but also with a lively temperament, able to adapte themselves instantaneously to sudden shifting of the musical peaks, with an outstanding efficiency and otherwise difficulty attainable “sense of rhytm”.
 
 
I consider the Tango between the best transformers of all the times, moreover they are equipped of a documentation parallel to their real performance; this remarkably helps in the conception of a design. This is also a significant assistance in ideating a scheme and makes easier to interpret the requirements for those who read the diagrams, in order to orient oneself to alternative transformers.
The Hirata Tango has been taken over by ISO and currently proposes a catalogue of ISO-Tango transformers, furthermore as far as I know ISO is disposed to realize transformers upon request (Michimori, ETF 2008).

 
I take the opportunity to propose the diagram of the HGA1.

[ANSA]

The following is the MARK3 of the HGA1:





The input step-up transformer optimized sensitivity and enables the use of a power supply "combat rock", ie without power transformer.

I tested for a long time the power supply "combat rock" (in a power amplifier HGA1.3 with tubes EC8020) and I must say that (in the PP Class A) I liked very much.

[ANSA]

 

[ANSA]

A project similar to the HGA1.3, is Pete Miller’s push-pull amplifier with EL802 tubes: http://www.pmillett.com/el802_spud.htm
We must highlight that this is also a high transconductance amplifier, since the tubes used have a Gm equal to 40mA/V.
The EL802 and PL802 tubes, in a triode configuration, represent a further alternative to the list of valves shown in the HGA1 and HGA1.3 diagrams.

I am going to add the HGA2 version to the previous diagrams.





This version has intermediate power and it stands out for the superior input sensitivity, typical of an integrated amplifier.
Furthermore it has mixed connotations, since it uses a single-ended configuration in the input stage and a push-pull one in the output stage. The same is true for the power supply, with supply transformer and tube rectifier for the input stage, whilst for the output stage (since the signal doesn’t involve the power supply and the push-pull topology has excellent elision qualities) the rectification is in solid state and without a supply transformer.

The tube used in the input stage belongs to the mythical Western Electric and its characteristics are fantastic:

mu = 250
Gm = 65 mA/V
Ri = ~ 4KOhm