Marshall Lead 12 Japanese 100 Volt Import Model

[DrGonz78]

So I acquired this Marshall Lead 12 Japanese imported model amp and I am in the process of cleaning it up. My question has to do with line voltage rated for 100v(Japan) and what complications could occur running the amp on 120v(U.S.)? I have left the amp open on the bench as I might want to just change out the transformer, but first want some feedback. I have a Marshall 10CD power transformer and I am contemplating switching it. I realize that the Marshall 10cd amp would be 120vac stepped down 1:10 will be at  -/+12vac center tapped. I am thinking this is the way to go as the 100vac version might be over powered by the 120vac line. What is the best solution or do I even need to change out the transformer?

While searching for some info about the Marshall 10cd I came across a website and just have to link a page here for others to see. I know this amp is very simple but to mere beginners this web page here is very well put together. It really takes a simple amp and explains the whole thing in great detail. Of course the example illustrated it European 220vac.

http://www.electrosmash.com/marshall-mg10

[g1]

  I would not recommend plugging the 100V amp into 120V.  If any of the caps operate near their voltage rating, the extra voltage may be too much for them.
If you could use a variac and check them, you may get away with it, but the amp will now be capable of producing more power which could strain the transformer.
As far as the transformer swap, the lead 12 runs on +/-19V, where the MG10 runs on +/-15V, so  the transformers are not the same.

[DrGonz78]

I just recall this old thread on MEF where Enzo points out that a 28vac center tap could possibly be replace by a 24vac center tap. It was on this thread here....

http://music-electronics-forum.com/t33915/

Same amp too. So as far as replacing the transformer it might work, but still I could order a new one too.

[Enzo]

yes, in the absence of a 28v transformer, your circuits would probably work just fine on 24v transformer.   But that is exactly the opposite of your situation.  Your transformer would be operating at 20% higher than normal voltage, not lower.

[DrGonz78]

So I can't really figure out the model on the transformer to really track down what it is exactly. The 100vac model transformer is labeled DET-5102J. Looking over schematic in comparison to my board I don't see anything different component wise. The only component that appears different is the transformer. I guess it is a 1:7 transformer then and that would get the 14.3v before rectification when run on 100vac lines. So yes plugging in the amp at 120vac line I would expect to see 17.14vac (20% increase) before rectification. At the rails then I would be seeing around 22.8vDC and with 25v 2200uf caps that is really cutting it close. I am merely guessing that the they use a 1:7 step down, but it would make sense right? Of course this is not including the amperage changes and that part leaves me confused a bit.

So if I have a 120vac stepped down to 24vac center tapped transformer then wouldn't I see -/+12.5vac rails before rectification? Assuming that is correct then I would expect to 16.6vdc for the B+ rails after rectification. The preamp specifies 16vdc rails(refer to schematic) and the byproduct of 16.6vdc would then become 13.94vdc. Hopefully my math is right and I make sense :duh.

Perhaps the amperage changes in relation to my thought process messes everything up? If I were to just buy this hammond transformer shouldn't it be perfect for this modification?
http://www.mouser.com/search/ProductDetail.aspx?qs=KAyCPn5KmeqMmzQg87sC%252bQ%3d%3d&utm_source=findchips&utm_medium=aggregator&utm_campaign=546-166J28&utm_term=166J28

Did I make any sense just now?    :lmao:

[Enzo]

I never gave turns ratios a thought.  If a 100v primary makes 28vCT, then plugging that into 120v would make 20% more, around 17v.

I get 24vDC when I rectify and filter 17vAC.

I don;t follow part of that.  24vCT is 12v either side.  rectify and filter for 17v.   What do you mean a byproduct is 13v?   What is a byproduct?   I see no regulation, the IC is trusting the main rails to be within the ICs limits.  If you start out with 24v instead of 28v, then everything will be a little less, so the IC should not be stressed.   SO the thing runs on a couple less volts.  All you lose is a little clipping headroom in the preamp.


Hammond 28vCT?  Probably work fine.

[g1]

For byproduct I think he means the preamp IC rails after the 1K droppers.  In the stock Lead 12, main rails are +/-19, IC's +/-16V.
  So with 24V rather than 28V transformer, main rails approx. +/-17, IC's +/-14.
Am I close Dr.Gonz?

[DrGonz78]

You guys are quick posting and G1 got what I meant while I was starting my reply to Enzo. Yeah I am speaking my own language there... :loco Yeah I meant the preamp voltage(IC's etc) after the drop down resistors. You got it G1 that was exactly what I should have typed. I always take words like byproduct and make some new definition.  :lmao: I should start my own alternative dictionary! lol

[Enzo]

Oh...


Weell then, make sure to properly calibrate the vyssonyw;xoo.   if you don;t, the griinen might blarnt the frollic.

[DrGonz78]

"blarnt the frollic" Did that a couple times at some party or another... xP

:lmao:

[J M Fahey]

The Math is fine, as far as turns ratio and resulting secondary voltages, but that's not the main point here.

Lead 12 , like any SS amp, will run fine with 20% lower voltages, and even with the MG10 transformer, so if you want to salvage it for a poor friend  or as, say, a bench or bedroom amp, cool.

The Lead 12 in particular, which is way overengineered with its TO3 metallic power transistors, will probably stand 20% higher ... but that's not the main point either.

Fact is, the 100V transformer *primary* will be way overstressed with 120VAC (which by the way might easily be 125 to 127 VAC, the way Power trends are running these days).

So best cheap solution is to use the MG10 tranny and best overall is to use some Hammond or any other proper 120V primary one.

Fact is

[DrGonz78]

Well I went with the best cheap solution and it is now all wired up, sounding pretty good. I guess I learned some more about power transformers and such. Always appreciate all your help along the way too, great sources of knowledge. Thanks!  :tu: