1959T transformer question

[roylfuchs]

I was looking at an old schem for a 1959T and I noticed the voltage readings on this baby are 560V B+ voltages. My 1959 Metro build has a MM tranny that puts out 460v B+. What would have to change on my current build to hook up a tranny that pumps out 560V (besides the tranny itself)? Will the 500v rated F&T caps handle it? Diodes? Tubes?

[paulster]

You'll want to use the caps in series for B+ like that. Otherwise you'll have 560V on a 500V cap (already 12% over rated voltage) plus the possibility of a high wall voltage taking it higher, so definitely a no-go. Of course, two 50uF 500V caps in series becomes an effective 25uF 1000V capacitor, so you end up having to change your cap values to suit as well.

Take a look at the Marshall 1959 reissue schematic on drtube.com for a good example of how to hook up the series cap arrangement, and a nice, clear schematic.

It'll also be tough on new power tubes since the screen voltages will be really high. You may need to increase your screen resistors to get the voltages down depending on the tubes you use. Larry has posted some good advice on this before.

[roylfuchs]

Maybe someone can help me figure out the wiring that I have circled on this schematic? The caps are in series 32/32. And the diodes seem to be configured a bit differently.

[paulster]

That's got a centre-tapped transformer designed for a full wave rectifier (like a 50W has) rather than the usual 100W transformer that's designed for a bridge rectifier (and doesn't require the centre tap). They've used three diodes per leg to get the voltage rating up.

It'll have something like a 400-0-400V secondary rather than the usual 185-0-185V secondary that you'd get in a 100W amp.

You don't want to use a bridge rectifier with this tranny or you'll have sky-high B+, and conversely you don't want to use this rectification type with a regular 100W tranny or you'll have half the B+ you are expecting.

That's a Marshall oddity for sure, probably when they got a run on some cheap transformers or the spec was misinterpreted. My 1959T schematic dated 1970 shows a conventional transformer.

[roylfuchs]

Paulster, I'm sending my 1959 Metro build out to Tim Caswell this week. I want to get as close to that #39 tone as possible. My circuit is currently set-up like the conventional 100 watt superlead, with 460v B+. Would there be a huge difference in tone between 460v and 560v PT's? Just your opinion.

[paulster]

With a 560V B+ you should have more headroom and it'll remain clean longer, but when it goes...!

I've just dug through my schematic archives and I can find several 1959T schematics, but all of them show the conventional bridge-rectifier type transformer and none have any voltages on, so it's difficult so say whether that kind of B+ is the norm or the exception on a 1959T. I do remember hearing about high B+ on Super Trems before but I don't know whether that was from real-world experience or from people also having read the same schematic.

Since #39 was a 1959T and since it was Tim that did the mod (although we now that that #36 is the one on the record) perhaps he'll remember the kind of B+ it was running before you feel the need to swap out your transformer.

Are you getting Tim to put his drop-in PCB in that adds the FX loop as well, or are you going for the handwired solution he did originally? That's the one I'd be looking for.

[roylfuchs]

I'm sure he's just going to use the PCB. I don't believe he would make an exception to hand wire it for me. He'd probably charge me an arm and a leg for that. I've heard some fishy things about his mods. Some say his effects loop isn't very good. Others say that the mod altogether is crap. Any opinions? I'm also pondering Fortin or Voodoo amps.

[paulster]

I think everyone who's had the mod done has seemingly been happy with the sound of the amp.

The trouble is, with the loop added, it has affected the tone to a degree.

SDM makes some good observations about it in this thread:
http://forum.metroamp.com/viewtopic.php?f=4&t=16712" onclick="window.open(this.href);return false;

The extra triode in use for the loop will bring the preamp voltages down a bit.

At the end of the day though, once you've had it done you can always reverse-engineer it and put in a small board or tag-strip if you want to get a more authentic version, since the mod itself is the same. It's a shame to have to go through the process first, but someone has to be the guinea pig I guess!