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Klimon 1st May 2007 11:39 AM

Please help amend 6AS7PP circuit

I would like to build my first pp; a 6as7 circuit to suit the tubes and opts I already have (opts are from vintage Philips el84 parallel PP recording studio amps, 4k to 8ohms). I have been looking around for schematics and found a few candidates but each poses a problem to my limited technical know-how:

--> looks promising; my plan would be to leave out the PS and desing one using PSUDII to suit my old stock power trafo; is the part of the circuit on the top right meant to delay B+ for the 6as7's? Additional problem would be to achieve the high 550V B- (my trafo is about 375V@ 200mA, I never designed a PS with B+ and B- rails) + the circuit is not so simple for a beginner like me

--> Biggest downside is the class A operation (to suit the toroids I guess); I would prefer the 10watts output of class A/B. However, amending this circuit with simple class a/b output stage cathode bias would, if the preceding stages can drive it, seem like an easy 'beginners'-circuit. I'ld also design a different PS with the additional filtering these amendments would necessitate

--> I can't identify the phase-splitter used, looks weird to me + the output stage and PS have a few elements I don't understand (the connection starting from the power trafo's centre tap)..

I would be very happy if some of the members with more technical luggage could comment on these circuits or do some propositions; for now the second schematic with new output stage and PS looks like the best option...

Thanks a lot!!

Simon :D

Yvesm 1st May 2007 05:05 PM


I can at least tell something about the last design in your list.

The phase splitter, even if curiously drawn, is nothing else than a paraphase built around two SRPP.

The connection at the center tap of the PT provides a +200V B+ for the power output stage while the driver is powered by +400V.
A must for driving the lazy 6AS7 with an SRPP based driver.


Klimon 1st May 2007 06:46 PM

Hello Yves,

Can you say a few words about the performance of the amp; e.g. how does it compare to the other PPs on your site / are you still happy with the design even after a few years of probably increasing know-how?

PS: When using a transformer without centre tap, could the "0"-point created by the "backside" of the diode bridge be used as centre tap?

I find the design very attractive but am more inclined towards LTP with ccs in the tail as I've read so much good things about this topology; unless ofcourse paraphase SRPP is an overachieving underdog.

Mille mercis!

Simon :D

Yvesm 2nd May 2007 09:34 AM

Hi Simon,

This was my very first design 8 years ago for my "tube come back".
It's a very good sounding amp, delivering some 8 Watts per channel at vy low THD from 15Hz to 50Khz, thanks to he OPTs I've bought from a French winder (ACEA near Toulouse, I can find exact ccordinates if you need).

You cannot go without a true center tapped PST or you must design another power supply able to deliver 150mA at 200V per each channel (for the power stage) plus some 400V, 5 to 10mA for the phase splitter/driver.
(This is probably the key design of this amp: use low voltage for the 6AS7 because that's what they like, but high for the driver because that's what they need, voltage gain begin around 1.5 !)

LTP PI won't give gain enough and will force you to use an additional gain stage with the innevitable link caps and so on.
Could be done of course but it will be a completly different design while I've found the paraphase SRPP perfectly matched the 6AS7 drive requirements.

Indeed, the design philosophy was highly influenced by years of industrial job where you should obtain the best possible results using the simplest, the cheapest and the lowest number of components.
For exemple, no cathode by pass caps ! !

Not an high efficiency design anyway due to the huge appetite of the 6AS7 (at least for heather) and the very simple (but safe) cathode bias.
But who cares about efficiency should not build tubed amplifiers !

Try and listen, that's all I can add.


Klimon 3rd May 2007 10:36 PM

Thanks for the info!

Regards -- Simon

Shoog 4th May 2007 10:54 AM

I would go with the triodeguy amp. I have built something very similar but with interstage transformers. It will work with the output transformers you have. It address the key issue with the 6AS7 - which is current balance.

The only thing I don't like is the operating point as I think these tubes will perform better at lower voltage and higher current.

My experience with these tubes is that they sound sweet and powerful if driven hard.


Klimon 4th May 2007 03:19 PM

Thanks for the advice Shoog. I was planning to take that route and copy the whole signal circuitry including current balanced output stage. Would you know by any chance how I can calculate the necessary value of the cap between the 6as7 cathodes? I'll have to recalculate to suit the primary inductance of my opts

Cheers -- Simon

Shoog 4th May 2007 03:31 PM

My technical knowledge is sadly limited. I'am a suck it and see man myself.
In my case I had 1000uf cap bypassed by 10uf caps which where originally referenced to ground. At Brian Becks advise I tied their "-" together and applied a 1mg resistor from the tie to ground to turn them fully differential. This will give an effective cross coupling of 500uf. This seems adequate, but on further thought the bass isn't quite as strong as it was originally. This could be because I have effectively halved the bypassing cap. It wouldn't be to difficult to add another 1000uf to each cathode to bring it up to 2000uf per cathode (though its getting very tight down there) to see if it brought the bass back up a little.
Because the output impedance is so very low with the 6AS7 the cathode bypassing has to be much larger than with more usual output valves.

Thats a bit rambling - but I hope it gives a few pointers.


edit: I just looked at his schematic again. I can't quite believe that he is only using 10uf between the cathodes - but thats what he seems to be doing. My understanding is either very poor or he has compromised his design. Unfortunately its probably the former. If I knew the theory behind how to calculate that cap then I would be able to offer a more useful answer.

Klimon 4th May 2007 05:46 PM

Well, thanks for trying! Much like you I'm a technical dummy although we might call ourselves 'artists' contrary to the lower class nerds who actually understand what they're doing :o

I'll send triodeguy a mail and then resume being an artist.


sorenj07 5th May 2007 07:15 AM


Originally posted by Yvesm
The connection at the center tap of the PT provides a +200V B+ for the power output stage while the driver is powered by +400V.
A must for driving the lazy 6AS7 with an SRPP based driver.

Hi, I'm also interested in the 6AS7, but driven by a CCS LTP 12AT7 with your 400V, with a regular grounded-cathode 12AT7 input stage. All the gain anyone could need, I hope. My question is this: With your power supply, aren't you unbalancing the power transformer by only using half a winding to feed the 150mA to each 6AS7 pair? Would a transformer with completely separate 200V windings work better? Maybe I'm just reading the schematic wrong...

One more general question - is there any preference between 6080's and 6AS7's? Or the Russian 6N5S or 6N13S?

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