Power transformer sag ?

jnew · Post by **jnew** » Sat Mar 30, 2013 3:25 pm

So the stock JTM-45 Re-issue Pwr Trannie is probably a piece of shit then. It worked great with the 2204 front end but I'm not getting anywhere real pleasing in the plexi mode. I am using a MM OPT Trannie and it seems to be quite nice.

neikeel · Post by **neikeel** » Sat Mar 30, 2013 5:28 pm

jnew wrote:So the stock JTM-45 Re-issue Pwr Trannie is probably a piece of shit then. It worked great with the 2204 front end but I'm not getting anywhere real pleasing in the plexi mode. I am using a MM OPT Trannie and it seems to be quite nice.

Big jump from asking what PT does a 50watter use to saying the Marshall RI PT is a POS!!!

The re-issue PT gives lower voltages than an original JTM45 PT (RS type 1->RS type 2->Drake 1202-55) all of which give around 450v. They all sag a little under heavy load, assisted by the tube rectifier. The 50watt plexi PT was usually a Drake 1202-118 which could be tube rectified (like my 67 JTM50) or diode rectified this gives around 420-430v depending on the quality of the recifier tube or if you are using diodes. The tube rectifier with this PT sounds very good to my ears and gives enough sag but does not crap out. Nothing wrong with the diode set ups either.
Later amps with standup 1202-169PT (and coincidentally the 45RI have lower voltages and typically run 400v or less) the RI has better quality steel than the old ones is wound on plastic bobbin to higher specs so sags less, which maybe why it sounds better in 2204 modern mode. I ran my RI with the MM Axion 16 OT for a while until I transformed the amp with a Metro Heyboer -103 clone

jnew · Post by **jnew** » Sat Mar 30, 2013 7:47 pm

Well hmm, let me see here. I was asking because I was giving thought to possibly changing out power transformers. The amp used to be a JTM-45 but I threw out the PCB stuff and re did it all to plexi 50 watt 68 ish spec. But since my last post, I'm going to agree with you whole heartedly. I plugged the amp straight to the cab, put everything on 10 and it is fu@#$ng great. Loud as piss but the tone is outstanding so no issues with the power transformer. You are also correct with the naturally low plate voltages. Mine was at 395, straight from the wall. I thing with the variac at 95VAC, it's like 310VDC. And yes, it does sag nicely but holds it together quite well. More experimenting to come but I'm realizing that my problem is somewhere in my slave setup. Not the amp.

jnew · Post by **jnew** » Sun Mar 31, 2013 1:19 am

In fact, I might even try the tube rectifier again just for some comparison. It's been many years but I do recall it wasn't bad at all.

uiovbged332 · Post by **uiovbged332** » Wed Jul 06, 2022 3:03 pm

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Power transformer sag ?

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