JTM 50 questions. (Transformer power)

Get support and show off your MetroAmp 50 Watt kit builds.

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dan_tramble
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JTM 50 questions. (Transformer power)

Post by dan_tramble » Mon Feb 13, 2017 2:57 pm

Hi all. I sure enjoy the forum and I have a specific question

I am building another JTM 50. The first one I used a 1202-55 style PT. I made it for GZ34 rectifier. Sometimes I will install a SSR plug in for more attack. I initially wanted a AC/DC amp but the JTM 45 PT was just too soft and I couldn't get the amp to compress enough

I am building another one however I am wondering if I can use a 1202-118 PT with a tube rectifier and install a SSR if I want the more crisp attack of a SSR

here is my question. The red primaries on a 1202-55 PT make 690 v @ 150 mA. The 1202-118 makes 625v @ 150 mA.

I believe the 118 transformer will give me a lower B+ resulting in a more compressed sound This is what I am after. My intention is not to create a clean amp at all

The screen drop on a 55 is a single 8.2k and the screen drop on a 118 is a 8.2k and a 10k in series.

Using a 118 on predominately a tube rectifier setup will the 8.2k suffice as proper drop for the screens?

Please
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Opinions to thread

Thanks for your time

Regards
Dan
MetroAmp addict!
JTM 50/45,1987's & 1986
1959 and 2203/2204 Amps
1974 amp

Can't stop building, very addictive!

danman
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Re: JTM 50 questions. (Transformer power)

Post by danman » Mon Feb 13, 2017 4:49 pm

If you notice on some 50 watters there is a single 10k dropping resistor and on others, there is a 10k and 8.2k. The amps that ran the higher B+ around 450v usually came with both resistors to get the voltage down to the preamp. In the late 70's Marshall dropped the B+ down to the high 300's. These amps normally have the single 10k dropper because using both would bring the preamp voltages down to far. You can do it either way but if you are going for a more compressed brown sound, the lower voltage preamp would help you achieve that. Going to far down does sacrifice some sound clarity though. If you do decide to use the single 10k dropper, don't forget to install the jumper between turrets for the missing resistor. Don't be afraid to play around with some different values either. You may hit on something you really like!

dan_tramble
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Posts: 37
Joined: Fri Dec 19, 2014 11:34 am
Just the numbers in order: 13492

Re: JTM 50 questions. (Transformer power)

Post by dan_tramble » Mon Feb 13, 2017 5:17 pm

Yes. I think the 118 vs the 55 transformer with a GZ34 will already get me nice and low.

I should measure my b+ on my 45 (55) and 50 plexi (118). Basically what I want to do is put a latter 118 into a GZ34 tube rectifier amp. I'd like to safely install a switch to switch between SSR and tube rectifier like the Germino club 40 (basically a JTM 50) I'm trying to find a clean diagram as to how to wire it up

I think I am going to install a split/shared cathode on V1 as well. I am hoping this will be a fun amp!

Thanks for the opinions DanMan
MetroAmp addict!
JTM 50/45,1987's & 1986
1959 and 2203/2204 Amps
1974 amp

Can't stop building, very addictive!

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