Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: luthierwnc on August 26, 2023, 10:20:09 am
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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
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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/
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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
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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
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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