PT HT secondary winding miscount?

[rfgordon]

Guys,

I'm working on a 100 watter for a guy, and I went with an American made Marshall-style standup PT that's supposed to have an HT secondary of 350-0-350, yielding about 460 VDC on my plates.

OK, so I got the amp mostly built--to the point of progressively firing up the voltages to check for missed wires, etc.

The heater voltage is perfect. The bias voltage is perfect. I check my plate voltages and they're all waaaaaaaay down. I check the 6L6 plates: 254 VDC!! What?

I pulled up the HT leads, so they're in the air with no load: 185-0-185 VAC. Double checked that I was, indeed, using the 120 V primary leads.

I got a replacement unit today--same deal!! They behave as if they're missing half the secondary windings.

So I ask y'all: how common is a miscount on a winding like that? Anyone else had that issue?

[RB]

Marshall used a voltage doubler in some of the 100 wat amps.


Check this discussion at metroamp forum:

http://forum.metroamp.com/viewtopic.php ... 46d5f3784f


Regards
Randy

[rfgordon]

Randy,

Yes, that is true. However, the PT in question is sold as a 350-0-350 machine.

A voltage double could be used in this case, but if the secondary IS mis-wound, I'd be making a fire bomb, not an amp....

[paulster]

There's nothing wrong with it. A 100W PT for a Marshall guitar amp (can't be certain about their bass range) expects you to use a full-wave bridge rectifier on the output. The secondary will still be centre-tapped because Marshall use this to balance the voltage across the totem-pole caps on the mains filters in a 1959.

All that's wrong is the description of what you've got, probably because of the centre-tap causing confusion. It's actually a 0-350V transformer, centre-tapped.

Only the 50W PTs are used with full-wave (non-bridge) rectifiers, which also helps to confuse matters.

[rfgordon]

paulster,

What you say may, in fact, be true. Unfortunately, for liability reasons, I can't use a part like a PT in a manner different from the manufacturer's supplied specs. This unit is specified as a 350-0-350, so I'm kinda stuck not using it.

[paulster]

Can you not get the manufacturer to re-specify the tranny, since it's clearly wrong for its intended audience? Anyone who is looking for a 100W replacement tranny for a Marshall and actually checks the specs will look at this and think it's wrong, so they're risking sales as well.

That would sort out your liability risk issue as well.

[Phil_S]

Rich,
I'm sure you know as much as anyone on this topic, but I'd encourage you to check the obvious and please excuse me if I'm telling you what you already know.

Indulge me here for just thinking out loud, as maybe it will be helpful.

If there are 120 and 240 primaries, I would take a very close look at the appropriate way to wire them.

If there are four wires on the primary, on some trannies there are actually two equal primary windings. One ties the two "middle" wires together (windings in series) for 240 service. For 120v service you tie the two windings in parallel. This doubles the VA capacity for 120v service, which you will need, or at least want to have.

For example, making for easy math here, if the input is 240v and output is 360-0-360 (720), you need turns of 1:3. For 120v input, you'll need turns of 1:6 for the same output. Check for the turns ratio on the various combinations.

Put a meter on the primaries. I'm assuming you've got two. Ohm them. If you find them to be the same and not connected to each other, you've got two primaries and it shouldn't matter which one you picked (though, like I said, you should parallel them making sure they are in-phase). If you are getting 185-0-185 (I feel certain you know the CT from the outer legs and metered the right leads), then you have probably reached the right conclusion.

If you find that the PT has one primary that is tapped for 120v service, i.e., 240-120-0-120-240, then maybe, even though you are "sure", you've simply picked the 240 legs.

Maybe it's time to just punt that PT and get something else?

I can appreciate that you don't want to mention the maker, but maybe you can draw us a picture that shows all of the wires. Use color, as someone might see something you don't.

Phil

[paulster]

Phil

Assuming this is a regular old Marshall 100W Superlead or JCM800 replacement tranny then the primary will be a tapped winding rather than a pair of windings.

They're usually 0-120-220-240V with a secondary of 175-0-175 to give you 350V across the bridge.

This sounds just like this tranny and therefore I think they've simply described it wrong. Personally I prefer to have a pair of 120V windings that you can put in series or parallel but that's another story.

To my knowledge Marshall have never used anything other than a bridge rectifier in the 100W amps from the Superleads onwards with the exception of the JTM45/100 which was a 375-0-375 secondary, whereas the 50W amps have always used just old-school full-wave rectification.

Paul

[Phil_S]

They're usually 0-120-220-240V with a secondary of 175-0-175 to give you 350V across the bridge.

This sounds just like this tranny...l

Yuk! IMO, a single primary just isn't the way to build it. It is a waste of VA.

Rich: Probably for about $80-100 (just guessing) these guys will roll you one exactly like you want.
http://www.edcorusa.com/home.html

[paulster]

Yuk! IMO, a single primary just isn't the way to build it. It is a waste of VA.

That's why I like dual 120V primaries, and why Marshalls sound better in the UK than they do in the US (unless you use a step-up transformer) because they're under spec-ed for 120V operation. I don't imagine Marshall ever bothered testing with a 120V supply back in the 60s, or even 220V. I think they just pulled the transformers off the shelf, wired them for 240V, and went for it.

Metroamp has a nice selection of 100W PTs from Heyboer, including a JTM45/100 transformer if you don't want to use a bridge rectifier.

[skyboltone]

Guys,

The heater voltage is perfect. The bias voltage is perfect. I check my plate voltages and they're all waaaaaaaay down. I check the 6L6 plates: 254 VDC!! What?

This answers the primary winding question.

So the likely answer is that it's a 175-0-175 and requires a bridge. When you say the bias is right, what do you mean? Is it wired one HT lead, R, diode, voltage divider, ground? Then it ain't right. Right? So the tranny is mislabled and you need, as builder, to re-label it, or seek another correct tranny and return the ones you have.

Dan

[paulster]

When you say the bias is right, what do you mean? Is it wired one HT lead, R, diode, voltage divider, ground?

Dan

The 100W trannies have a separate bias winding since you can't use the centre tap with a bridge rectifier (with the capacitor coupling caveat) which is also what leads me to believe this is a legit Marshall 100W replacement tranny that's been incorrectly specified by the manufacturer.

Paul

[Normster]

Is it this one? I've got one of these on the shelf that I was going to use in my next build. Looks like you need a voltage doubler to get high enough HT voltage.

[rhinson]

Is it this one? I've got one of these on the shelf that I was going to use in my next build. Looks like you need a voltage doubler to get high enough HT voltage.

you don't have to voltage double it----just ignore the ct and use a bridge---it works out the same and (depending on how high the n.l. dc voltage) can save you a cap over the marshall style of doing things. rh

[paulster]

Is it this one? I've got one of these on the shelf that I was going to use in my next build. Looks like you need a voltage doubler to get high enough HT voltage.

you don't have to voltage double it----just ignore the ct and use a bridge---it works out the same and (depending on how high the n.l. dc voltage) can save you a cap over the marshall style of doing things. rh

I think Normster is using the term voltage doubler to mean bridge rectifier, which lots of people seem to do.

I don't consider it a voltage doubler at all, just a different way of wiring your rectifier (which happens to make more efficient use of your transformer secondary as well). A voltage doubler to me would be something like a charge pump.

Marshall don't use a cap either; they just use a bridge rectifier. The centre-tap exists only to balance the voltage between the first pair of
filter caps which are placed in series to get a high enough voltage rating. They could have omitted it though, and used balancing resistors across the caps instead.