Approximating voltages for homebrew Marshall SV20 clone

[luthierwnc]

Hi all, I plan to repurpose an amp that didn't quite measure up into a little Marshall SV20H.  I won't bother with the 5-watt version.  It all seems pretty straight forward except for the B+.  The schematic shows the higher-voltage PT leads at 141v and the lower voltage at 108v.  In reading this we've got two bridge rectifiers with a common ground.  The power selection switch is on the standby.  To keep the preamp voltages stable those are pulled off the high-voltage rail.
I'll definitely need a different power transformer -- even for the 20-watt only version.  In running the Duncan simulator, 141 VAC on the ends gets me up near 370VCD on the plates and that's with a 3k load.  The actual amp shows 295.  The PSUD2 gets me to the right B+ at around 225-0 VAC on either end.  The power transform in these amps is not robust -- neither is the OT. 
Do you think it will really take 282-0 VAC on the wall-side to get me 300 VDC on the plates?  FWIW, I'll probably use an Antek toroidal transformer which can be had cheap in a variety of voltages.
Thanks, Skip

[pdf64]

Although it’s been photoshopped well, beware of schematics by Hugh Jorgan.
These are more reliable https://www.marshallforum.com/threads/heres-the-sv20-schematics-from-marshall.131978/

Also maybe some ideas here from a guy that’s built what you’re planning https://www.marshallforum.com/threads/marshall-plexi-cathode-biased.134490/

[luthierwnc]

I'm taking the plate and rail volts from a YouTube mod vid. 

After gutting all the 5-watt alterations it's the same circuit we all know pretty well -- although I might still decouple the pre-amp from the power amp.  Even at 20-watts, less than 300 VDC on the plates doesn't call for a stock after-market transformer.  In this video the presenter also shows the current draw on the cold side.  If I do this project I might use separate K-resistors.  The amp already has a choke and that would stay.
Antek sells toroidal transformers that are good and reasonable.  Not always mentioned is they show the secondary voltages but usually have taps 10-ish percent below that.  A 240-220 transformer probably gets me there one way or the other. 
Cheers, sh 

[pdf64]

Here’s a post by a chap that I think used to be an EE or something at Marshall https://www.marshallforum.com/threads/cloning-an-sv20.131774/page-2#post-2364273

[luthierwnc]

Fooled me!  Thanks again pdf64.  Not sure why someone goes to elaborate pains to make an inaccurate forgery.  I mean; it's not like a Manet.
The legit schematic, stripped of the 5-watt stuff, looks pretty straightforward.  I probably won't bother with four-holes.  And the effects loop might get skipped too.  If I read this right it runs off a 9-volt supply from the 12-volt heater leads to drive a pair of op-amps.  That's bypassed by a conventional DPDT relay.  Not being a traditional Marshall player I don't fully understand how those are used in a dirty amp.  More study is needed!
Cheers, sh