ampex 351 psu

Looking at this version 351 schematic they're showing 330V rectified DC.  Whatever PT that's closest to either should work fine.  The Hammond you originally mentioned would be too high though. More like a 117-0-117 or 230ish no CT iron.

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EDIT on those numbers - probably more like 270ish no CT or 135-0-135.


630V will be too much.  You would be looking at going about 80V higher than 300 or 310V for an unloaded voltage figure used for calculating your PT numbers.

IDEA - To save any miscalculation and hunting for odd voltage PTs, consider using the diodes in a standard full wave configuration.  I can't see any advantage to using a bridge rectifier here and the full wave will roughly halve the rectified DC value.  Assuming your 630V unloaded figure came from the shown bridge rectifier that's about a 225-0-225 PT you're currently using - sound right?  Going to a full wave should give you an unloaded DC of ~ 315V.  That will likely end up a little lower than 310 when loaded, but - you can change the 6.8K resistor to a lower value.  300V to the output section is the goal.

To figure what the new resistor value will be - reconnect rectifier to full wave, fire up the circuit and measure the drop across the 6.8K.  This will give you a real current draw number for calculating the new resistor.

 

Thanks a million for all the help.

lassoharp said:

Looking at this version 351 schematic they're showing 330V rectified DC.  Whatever PT that's closest to either should work fine.  The Hammond you originally mentioned would be too high though. More like a 117-0-117 or 230ish no CT iron.

Click to expand...

Assuming your 630V unloaded figure came from the shown bridge rectifier that's about a 225-0-225 PT you're currently using - sound right?  

Click to expand...

Actually the number came from the transformer connected to nothing. The 630 is an AC measurement leg to leg and the number goes to @322AC if I measure leg to C.T.

I just want to avoid getting a transformer which will be instantly bogged down to an unusable voltage by the circuit.
Really Im not looking to paint by numbers I just dont understand these transformer specs.
And I'll be dammed if I haven't looked for 3 days to figure out if a transformer marked 6.3vac CT is 6.3vac leg to leg or leg to tap.
Also, is there anything wrong with just not connecting the center tap?

And I'll be dammed if I haven't looked for 3 days to figure out if a transformer marked 6.3vac CT is 6.3vac leg to leg or leg to tap.

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That means leg to leg.  1/2 that to tap.

With a 322V winding you will still be a bit high using a full wave rectifier(way high for bridge).  You will have to drop about 140V.  You could do it with resistors or a choke input.  Probably better to get a lower voltage PT.  250-0-250 would work well for a full wave.  Hammond or Edcor should have several to choose from.  Anything over 40ma should be fine.

I just want to avoid getting a transformer which will be instantly bogged down to an unusable voltage by the circuit.

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Unless you use a PT that only puts out about 50V of rectified DC that won't happen.  Assume a 50% leeway .  Just make sure filament current rating is adequate for the tubes in use.  The B+ current draw for the amp is well below pretty much anything off the shelf I've seen from Hammond or Edcor so that's why I say anything 40ma or over.  Most will be higher.

The 351 is drawing roughly 20mA B+.  You're gonna be hard pressed to find a plug -n- play replacement transformer.  Even finding a transformer with a close HT, 12.6V filament, AND a phantom source is gonna be tough w/o having it custom wound.  I agree with Lassoharp and think you ought to consider a plain ordinary full wave rectifier.  Attached is part of a schematic I came up with for my mic pre project.  It's in the ballpark of what you're needing.  Phantom source is a separate 50V bull shit transformer I had laying around.  I used a Hammond 270DX for HT & filaments.  To get to 12V I strapped the 6.3V and 5V winding in series (polarity is important), bridge rectified it, then used a 12V regulator.  This supply puts out 350VDC B+.    Add a 1K 1W resistor between C 26 & C28 and you should have 330ish volts.  Food for thought.

-Richard

Yep just did it. The amp is alive and running cool as a cucumber. Now to track down the noise its freaking everywhere.

So what did you end up using?

its a toroidal xformer with
310v 100ma 6.3@3a  I sourced locally from a boutique amp shop.
spits out what I want without much change. Unfortunately it does not have cts so Im running into the freaking grounding issue.