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lukas

Well-known member
Joined
Sep 29, 2010
Messages
317
Location
Poland
Fonia Cenrit is Polish manufacture( i think this is very worth attention pre...i don't have, i listen old polish records i believe they from this preamp  ). I redrawn circuit from  what i found on web, but oryginal schematic is very bad quality,  if someone have information about(or even Fonia tubes Prea) or have this stuff, and can check correctly value, and tracking in my schematics, i will be happy.
 

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Correct version schematics of the Fonia WAB31a ... run and checked. Works ok.
 

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first switch is doing a few things; altering bias of v1 would require MBB , also feedback from tertiary winding is varied with this same switch.

curious what the merits are of the v2 topology
 
shabtek said:
first switch is doing a few things; altering bias of v1 would require MBB
I don't think this switch has anything on bias.

, also feedback from tertiary winding is varied with this same switch.
That's what it is here for.
I was concerned with the grid becoming floating between positions if the switch is BBM.

curious what the merits are of the v2 topology
Just a way to get (almost) balanced drive without a PI. Works well enough for a class A push-pull.
 
The gain switch is interesting. It appears to alter the open loop gain in step with the closed loop gain. This means the amount of feedback is kept more or less constant which considerably simplifies maintaining stability over a wide gain range.

Cheers

Ian
 
Ecc81 its not the best tube on the first stage, but this is communist time... less everything...if i good remember Polish tube manufecture(...Philips Licence) made the tubes like ecc82-83 but in very small quantities. Ecc81(Ecc82, and Ecc83 too) taken form RFT, and Tesla companies, but Fonia decide to Ecc81...something...
All amplification stages, including the output transformer,
are subject to negative feedback with a value independent of rotary adjustable gain switch...Quotation from Fonia Manual.
I gave the original Fonia TK-15-005 output transformer (very good fraquency response...permalloy EI60 24mm stack), the input Lundahl 7903 ratio1: 8.
Grayhill switch without MBB i think - AJN (N - non shorting if i correctly reading)...my friend testing on the studio, and i don't tell me enything about  the pop's on them...
 
as drawn the transformer 'cold' end and electrostatic shield will are switched , the grid would me momentarily open or unbiased (I think is the word) when switching  BBM/ non-shorting...
 
The output tubes grid resistors May be wrong. 270k, 500k, and 2k4. seems it would make the signal swing asymmetrical. But yes. Interesting gain management with tertiary winding feedback that disposes of caps like an old Langevin 5116b arrangement Had.

good chance both grid resistors are 500k
 
I do exactly copy of WAB51.
The output tube grid resistors are ok...
 

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I have these 4 pres left to rack, and 6 more EQ's to rack. Love to chat more about your 51 PCB! Each of the EQ's has a unused transformer in it, if my memory serves me correctly today.
 

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