100w plexi twin bridge rectifiers in power supply

[Alex R]

Afternoon all. I have in an early 100W Marshall 'plexi'. The power supply is a bit odd and I can't find a schematic that fits it, yet it appears original. There are twin secondaries each producing about 190vAC, whilst the B+ was over 500vDC, though I suspect one of the rectifiers was breaking down as the B+ intermittently was dropping about 100vDC then coming back again, making crackles through the amp.

Each of the twin secondaries has a bridge rectifier of its own (as I realised when i took them off to replace them, they were rectangular flat types bolted to the board upside down). The +ve output of one bridge and the -ve output of the other are connected to the centre nodes of two pairs of series 22uF 350v filter caps, with the nodes connected together, as in the attached diagram.

There is then a choke, and two more pairs of series caps, this time with bleed resistors.

Does this look ok to you? Anything to watch out for? Was a single bridge perhaps not meaty enough for the amp back then? Should I just connect the primaries together and use a modern bridge? Would the B+ be the same?

thanks for thinking about it and sorry if I got the schemo wrong...



Rats it is wrong I will go and change it... polarities on the bridges... the damn phone kept ringing!

[Alex R]

both worse and better. Hope you can see what I'm on about...

[pdf64]

Yes I think that your reasoning as to why it was done like that, and the proposed way forward, seems good. I can't think that the B+ should be any different; maybe ~1V higher due to fewer series diode junctions.
Need to ensure that the phasing of the secondaries is correct.
The secondary link node can also act as the 1/2 B+ balance node for the reservoir caps.
Pete.

[jrfrond]

I'm all for leaving things original, but we now have bridge rectifiers that routinely handle 35A at 1kV, and could easily work ALONE in that circuit. (MB3510/MP3510). Components have become more hardy since those days.

[Alex R]

Thanks Pete and John. I'll try a single modern bridge, with the secondary centre tap to balance that first set of caps, and get back to you!

[Alex R]

- worked a treat, thanks guys.

[pdf64]

Excellent - what bridge did you end up using?

[Alex R]

Oh sorry not to get back to you on this - it was an 8 amp 1000v volt bridge with leads rather than solder tags, fitted down on the pinboard nicely and worked just fine - thanks for all the help.