Partridge OT/PT in a JTM50 build?

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Partridge OT/PT in a JTM50 build?

Postby MarcD on Mon Mar 03, 2014 3:52 am

I'm planning to build a Triode JTM50 kit (love that EL34 sound) and have a Partridge OT and PT from a '70's Sound City Fifty Plus on hand; any thoughts on if they'd work and, if so, how they'd perform?

Thanks
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Re: Partridge OT/PT in a JTM50 build?

Postby Papa Dog on Mon Mar 03, 2014 6:14 am

that has the potential to be loud for 50 watts. big, full with lots of headroom.
rich, clear and clean.
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Re: Partridge OT/PT in a JTM50 build?

Postby pdf64 on Mon Mar 03, 2014 12:56 pm

The VB+ can be at the higher end of normal, resulting in shorter tube life if you intend to crank it.
However, the PT in some 50+ had a lower voltage tap on the B+ winding, so if you've got that consider using it or having it switchable.
The bias winding voltage is insufficiant to accommodate the normal range of tubes, a voltage doubler will likely be necessary for the bias supply.
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Re: Partridge OT/PT in a JTM50 build?

Postby Papa Dog on Mon Mar 03, 2014 3:36 pm

pdf64 wrote:The VB+ can be at the higher end of normal, resulting in shorter tube life if you intend to crank it.
However, the PT in some 50+ had a lower voltage tap on the B+ winding, so if you've got that consider using it or having it switchable.

That is some good info...never saw anything about a Sound City having different B+ windings like that.
The bias winding voltage is insufficiant to accommodate the normal range of tubes, a voltage doubler will likely be necessary for the bias supply.
Pete

Another intersting piece of info. Didn't 50+s come stock with El34s? If the amp would handle EL34s it should handle 6L6GCs just fine,and maybe some others.
Might be that the 50+ was designed to run "colder" and thus run "cleaner" or it could be just poor design on the bias circuit.
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Re: Partridge OT/PT in a JTM50 build?

Postby pdf64 on Mon Mar 03, 2014 5:37 pm

Didn't 50+s come stock with El34s? If the amp would handle EL34s it should handle 6L6GCs just fine

Yes, I think they fitted Mullards and ran them hot.
The stock bias supply voltage can't be turned up enough (ie sufficiently negative) to run many current EL34 at a reasonable dissipation, and is rather too low for any 6L6GC.
However, as the bias winding is independent, ie not a tap, a voltage doubler is fairly easy to implement on the existing tagstrip, and that fixes the problem.

the PT in some 50+ had a lower voltage tap on the B+ winding

Well, a caveat is that is based on a sample size of 2, in that one of mine has the tap, the other doesn't!

I'm afraid that I've neglected to investigate any others in enough detail.
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Re: Partridge OT/PT in a JTM50 build?

Postby DaveW on Tue Mar 11, 2014 7:38 pm

I built a JCM800 2204 with the transformers from a SC Concord. I didn't use the bias tap, I just used the HT winding, see this in the schematic here.

http://mhuss.com/MyJCM/

This is a picture of the completed amp, the only issue with the Concord OT is that it only supports 8 ohm speakers, a problem you won't have.

Image
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