RSA PA Amp Resurrection - Power Supply Questions

[Jennings]

Just picked up a mid-40s RSA PA head I'm looking to get up and running again.  Couple of questions I'd love some advice with before I start though...


The amp has no mains/isolation transformer, just a big mains dropper supplying the dual rectifiers and pilot lamp.  What way would folks recommend I go with updating that setup?  1:1 isolation transformer and leave the dropper (assuming it still functions)?  Eliminate the dropper and install a modern mains transformer (supplying both B+ and filaments...in which case any suggestions there?)?  And finally, what are the thoughts on the series filament supply?  Leave as-is?  Those tubes are designed for use in series supply, I believe.


Schematic attached (sorry, not had a chance to re-draw any clearer).  Tubes are dual U31 rectifiers, dual KT33C output valves and three KTZ63 (6J7G).  Filament requirements are 26V 0.3A, 25 or 13V 0.3 or 0.6A and 6.3V 0.3A respectively for those tube types. 

[Latole]

I leave it original with only a wall outlet isolation transformer.
Do you live in Europe ? This amp need 250 vAC input power.

[Jennings]

I like the idea of not hacking it about too much and keeping it original...Hopefully that dropper will be functional when I check it to allow that as an option.  I'm in the UK, and measured wall voltage is typically 230 - 238V, so lower often than old equipment was originally designed for.  I do have a variac though.  There's a two-option mains selector switch on the front panel.  Top marked "200/220" and bottom marker "230/250", so looks like it was expecting to see a bit of a range at the two operation points.

[Latole]

UK !  That is why 250 volts AC  input power

Never use this amp without isolating transformer, you may have serious injury.

[Jennings]

Totally agree...especially considering it also has a metal case, and for guitar use I'd be connected to it by the guitar strings and cable ground connection.  That's the reason for my thoughts on the power supply     I'm tempted to try and fit an isolation transformer either in the case, or hardwired to the mains cord.  So it can't be accidentally fired up without it connected.  And disconnect the front panel selector switch...hardwiring it to the "230/250" option.


Any suggestions on makes/models of isolation transformer?  I think I have a couple of multi-tap Block made units kicking about somewhere...can't remember what they're specs are though off hand.

[PRR]

Never use this amp without isolating transformer, you may have serious injury.

IF the input(!) and output transformers were fresh and healthy, this equipment is nominally "safe".

I would not trust it today. Uncertain original quality, unknown abuse, unknown storage (flood?). I might trust large power and speaker transformers, but small signal transformers use thin paper and fine wire.

There is also L1, the phase split autotransformer. Get some Ohms numbers on that before you go too far. These very often burn open and there's no new-made replacement, except another tube or transistor.

Very nice box even if you have to gut it and build a  euro-Champ in it.

[Latole]

There is not Power ( input) Transformer in this amp , we these amps are called ; widow maker.

https://robrobinette.com/Widowmakers.htm

[Jennings]

Cheers guys…I’ll open it up and start checking it over soon so thanks for drawing my attention to the phase inverter. Luckily the standard UK plugs prevent you reversing the live/neutral, but I’m definitely going to fit a suitable mains 1:1 autotransformer. I’ve got a few old valve radios that don’t have a mains transformer, and are direct coupled via a big dropper…luckily they’re all enclosed in plastic or wooden cases, with Bakelite knobs, and I don’t have to plug myself into those 😂

[sluckey]

I’m definitely going to fit a suitable mains 1:1 autotransformer.

An autotransformer does not provide isolation from the mains. Did you really mean to say isolation transformer?

[Latole]

I’m definitely going to fit a suitable mains 1:1 autotransformer.

An autotransformer does not provide isolation from the mains. 

Right, this is a auto xfr; no isolation