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widowmaker 1st July 2006 11:26 AM

6A3/6B4G/6S4S(2A3/2S4S) project
2 Attachment(s)
Schematic, (standart Loftin - White schematic):
Amplifier - early version, input stage with double triode 6N3P in paralel (without power supply module):
Gallery of Bulgarian Audiophile Society

Input stage - russian single triode 6S3P-EV
Output triode - 6A3/6B4G/6S4S; 2A3/2S4S (with 2.5V heating)
Kenotron - russian 5C4S(EZ81 with 6.3V h.v.)
bypass el. capacitors - russian silver-tantalum, military production.
Geometry of used core in attachment.

widowmaker 3rd July 2006 07:23 AM

2 Attachment(s)
Need two (or more) 2A3, sovtek production.
Summer months (VI-VII-VIII) production is preferable.
Also need 6S4S(or 2S4S;-186) - svetlana, early production - (1950-1969).
Picture of -186:

widowmaker 20th September 2006 05:37 PM

The project was "slightly" modified :)
Here is the last version:

AndreasS 20th September 2006 06:32 PM


the 6B4G has a maximum plate voltage of 300V and plate dissipation of 15W; 340V x 50mA gives a plate dissipation of 17 Watts.

Better connect the cathode resistor and bypass cap to the negative pole of the

regards Andreas

widowmaker 21st September 2006 12:05 PM

Yes, 17 watts is more than the absolute maximum... Ua max=360V (for 6S4S) not 300
It is a little more than recommended power dissipation, but all is OK, Anode become reddish when power dissipation is more than 21-22 watts :D :D :D
The sound is much "bigger" than early wersion - 250V a-c Ia=60mA
Also I changed the transformer ratio (29.63:1) and anode load (more than 3k)

James D. 22nd September 2006 08:02 AM

I find the 6S4S likes lots of volts too, I run them at 350V and 50mA so very close to your point. I'm using a 5K load aand it is super. There is more delicacy in the amp at this loading than at 2.5K.

I prefer the 6S4S run like this to the 6B4Gs and 2A3 that I have tried at their recommended points.

AGree on the getting the pre 1969 6S4S too:)


AndreasS 22nd September 2006 09:13 AM

Hello James,

you're increasing the plate dissipation and degreasing the output power..., what are the benefits?

regards Andreas

planet IX 22nd September 2006 10:53 AM

The benefit seems to be a better sounding amp - but surely on cost of the lifespan of the tube

James D. 22nd September 2006 04:47 PM

Yes, exactly that - a better sounding amp.

As for reducing the output power - No it doesn't as I can still swing the full current range but with greater voltage swing than a 3K load... thats a secondary benefit of the lifted Va - more voltage swing for the same current swing and a higher load with better sound...

So I get an honest 4Wrms output. Price is higher dissapation and shorter tube life - propbably only 2-3,000hours... small price for a better sounding 2A3 amp with tubes less than 20% the price of premium tubes that don't sound as good...

I should bequiet - or the price will go up!


AndreasS 22nd September 2006 06:24 PM


Originally posted by James D.
....a better sounding amp.
...output power - No it doesn't

Hello James,

please look at Fig. 13.14 (RDH4, p. 558), showing the power output and distorsions of a 45 against load:
at a load resistance of 6 x Ra the output power ist reduced to 1/2, but there is no further reduction of distorsions.

regards Andreas

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