can anyone explain this to me?

[iknowjohnny]

The links below are to PDF files showing the specs for two PT's. Both are magnetic components marshall PT's, one a 50 watt and the other the 100. You guys told me to use the 100 watter next time because my 50 watter is only 150 ma. But i don't understand the specs because the 50 says 690V CT and the 100 says 350V CT.I figure the 100 is showing the voltage on one side while the 50 is showing the combined sides. But i don't get why they'd spec one with a different method then the other. So the questions are....

1-am i correct in my assumption?

2- if i am, then the 100 watter would be 10v more....what would that amount to at my plates compared to the 50 watter with 690v which ends up at 475 on my plates?

3-whatever the final plate voltage is on the 100, assuming it's notably more than the 50's 475v, will it be too much for 2 el34's?

[Andy Le Blanc]

It all depends on the rectifier circuit, full wave or bridge.

[iknowjohnny]

Full wav? It's two diodes in series on each leg. A bridge is the 4 diodes all in series in a circle with the DC tapped off two corners?

[John_P_WI]

Not answering your question directly, the Hammond 272JX / 372JX are excellent transformers paired with the 50 watt OT.

Note on the 272JX, the line voltage is rated at 115v, therefore you will have higher plate and heater voltage if your line voltage is higher. The 372 has multiple input primary taps and a bias tap. At 117v input, I get 425v out with SS rect on the 272 jx.

Hammand makes excellent power transformers IMHO.

I should add I have two amps in the works with the 272JX and MC 50 watt OT.

[iknowjohnny]

I HAVE a 272JX in my last amp b4 this one. But the MC is cheaper so i'd prefer to go that way as long as theres no difference. Plus the 272 puts considerably less on the plates. I get 425 with that and 475 with the MC. i fear that would change the sound i now have.

[Andy Le Blanc]

At some point you have to realize that it is what it is, and appreciative the tone you do get.

I like to go on the low side for B+, and I like to be cheap too.
Each design choice you make is going to have a generalized character,

irregardless of the voltages.

Voice all you want want your not gonna change the character of your choices,
build it, give it a listen, then be critical, you might even build it again.

[David Root]

The 100W PT must be designed to use a bridge rectifier. Here you use it as 350-0 ie the CT is not grounded or normally connected to anything.

So you get 350x1.4=490VDC

Here's where it gets complicated. Some Marshall 100W designs that use a bridge rectifier also connect the PT CT to the middle of two power caps in series, eg JMP and JCM 900 100 watters. JCM800 100 Watters did not connect the CT but did use bridge rectifier.

Music Man used the same design to take off half the power tube plate voltage for the screens (after a voltage doubler, which further complicates that design).

They all work. I would suggest reviewing some old Marshall schematics at schematicheaven.

In general a bridge rectifier is a better (more efficient) way of getting high voltage DC and the standard full wave w/grounded CT is a better (more effective) way of getting high DC current, I think.

[stoo]

IKJ. Can't see the PDFs you're refering to.
You get full wave rectification with the 2 diodes (4 if 2 in series) if you ground the center tap. You need a bridge (4 Diodes) if you don't have a CT or don't use it.
http://www.hammondmfg.com/pdf/5c007.pdf

[iknowjohnny]

Ahh, i see now, thanks stoo. As for the pdf's, doh ! here ya go...

100 watt...
http://classictone.net/40-18024.pdf

50 watt...
http://classictone.net/40-18023.pdf

[rbryerton]

Just to reiterate a few posts (I've used both of these MC trannys), the 50W version is designed to use a full wave rectifier, and the 100w is designed to use a bridge rectifier.

Look at a couple of MetroAmps 100w schematics to see how the bridge is wired up in a 100w Marshall...

[desu boi]

Like stoo sais:

With a PT center tap you can get full wave rectification with only 2 diodes.
Without a center tap you need four diodes in a bridge for full wave rectification.

If you connect the diodes in a "circle" instead of a bridge you will create blue smoke.

In general a bridge rectifier is a better (more efficient) way of getting high voltage DC and the standard full wave w/grounded CT is a better (more effective) way of getting high DC current, I think.

No.

[iknowjohnny]

So then tell me this....if i used the 100 watter and use a bridge, can anyone tell me what my plate voltage would be?

[dynaman]

That does kinda stink about the Marshall HV windings having such low current ratings. I've used a few 50W Marshall PTs for builds and they all ran hot. In fact, I've a shelf loaded down with fried Marshall PTs. All have bad HV windings. I save them for making layouts on DIY builds and occasionally rob their endbells to convert laydown trannys into standup units.

Oddly, the 50W trannys are huge. I've had PTs from other mfrs. that had higher current ratings but were physically much smaller. Not sure why the repro units don't up the current ratings. I guess it's part of the Marshall sound.

[iknowjohnny]

Oddly, the 50W trannys are huge. I've had PTs from other mfrs. that had higher current ratings but were physically much smaller. Not sure why the repro units don't up the current ratings. I guess it's part of the Marshall sound.

i wonder if they are super conservatively rated? I haven't used mine loud for any period of time really, but it never gets hot at all.

[dukeamps]

all my trans are hot after a gig. you can still touch them but are hot