Heavily modded Marshall JMP - PT heater rating

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Phil_S
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Re: Heavily modded Marshall JMP - PT heater rating

Post by Phil_S »

I've unwound a PT. The heater winding is really as simple as you might expect. Depending on when it was wound the primary voltage would have determined the exact number of turns, but for 120V mains, it's about 19 turns of fairly fat gauge wire. It is unlikely there is a shorted turn and it's still working. If you were fortunate enough to short the first or last turn only, that would cause the voltage to go up by nearly 5% and the current capacity to go down. If there was a short in the middle somewhere, it would surely blow the fuse. I'm no expert on this topic, but it seems this PT is simply either tired or there is an extra turn or two in the winding. I want to suggest it's wound to a bad spec.
pdf64
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Re: Heavily modded Marshall JMP - PT heater rating

Post by pdf64 »

I don't see how a transformer could develop a fault that resulted in this; rather it seems a lot more likely that it was always like that, due to a mistake in the design / manufacture.
pompeiisneaks wrote: Thu Jul 20, 2017 6:50 pm... if you've got it on the 110 winding, and you're at 120, that may be why you're getting lower voltages. ... It seems logically to me that if you put 120v into a 110v you'd RAISE voltages, not lower them, otoh if you're in the 120v and your house tends to be closer to 110 it 'would' lower the other side voltage...
Yes, I think that a lower than intended voltage on the primary would result in a lower than intended secondary voltage, and visa versa.
So 120 applied to a 110 primary would increase the secondary voltages.

I guess that the other primary was for 240V mains.
Did using it tap result in the secondary voltages about halving?
Rockwell666
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Re: Heavily modded Marshall JMP - PT heater rating

Post by Rockwell666 »

hooking up the second primary lug blew a fuse (thinking it was a diff tap but I guess not haha). With the heaters at 5.6v the el34 plates were at 490VDC. I am alittle nervous about unwinding a vintage marshall dagnall PT!!
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Phil_S
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Re: Heavily modded Marshall JMP - PT heater rating

Post by Phil_S »

I've been away on vacation and just returned. Don't unwind it. It may never work again. Sell it.

Once you take it apart, you might not get it back together. The procedure requires splitting off the lams one at a time, slicing through the varnish with a razor blade. You have to keep track of the lams, and set them down on some screws so that the order and orientation remains unchanged. This is the only way to free the core. After you reassemble, you'll have to varnish it again. Before you do this, you'll want to read up on transformer assembly. The one I took apart was clearly toast. I wanted to find out what was involved. I learned a few things.
Rockwell666
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Re: Heavily modded Marshall JMP - PT heater rating

Post by Rockwell666 »

Phil_S wrote: Mon Jul 31, 2017 1:36 am I've been away on vacation and just returned. Don't unwind it. It may never work again. Sell it.

Once you take it apart, you might not get it back together. The procedure requires splitting off the lams one at a time, slicing through the varnish with a razor blade. You have to keep track of the lams, and set them down on some screws so that the order and orientation remains unchanged. This is the only way to free the core. After you reassemble, you'll have to varnish it again. Before you do this, you'll want to read up on transformer assembly. The one I took apart was clearly toast. I wanted to find out what was involved. I learned a few things.
Oh Ya, I am not taking it apart haha
tubeswell
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Re: Heavily modded Marshall JMP - PT heater rating

Post by tubeswell »

The PT may have been wound for 125VAC input in the US, or 250VAC in the UK.If you hook it up to a Variac and put 125VAC on that primary, what do the voltages look like?
He who dies with the most tubes... wins
Rockwell666
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Joined: Mon Nov 21, 2016 4:56 pm

Re: Heavily modded Marshall JMP - PT heater rating

Post by Rockwell666 »

tubeswell wrote: Tue Aug 01, 2017 4:43 pm The PT may have been wound for 125VAC input in the US, or 250VAC in the UK.If you hook it up to a Variac and put 125VAC on that primary, what do the voltages look like?
most of my test equipment is in storage at the moment (gotta love moving) but im getting 490v on the plates of the el34's. it is a Canadian import model with no voltage selector switch. I sent the amp home with my bud to play with. I biased up the 6l6's and it sounds like a mesa with more presence now haha. he will drop the amp back off on the weekend for some final tweaks. he hasn't blown it up yet :-)
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