1970 JMP 1987 power transformer question

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6adam7
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1970 JMP 1987 power transformer question

Post by 6adam7 » Wed Oct 21, 2020 12:03 am

Hi guys,

I have a 70’ spec’ed JMP 50 clone with Marstran 784 139 OT and c1999 choke...the PT is this guy:

https://www.mercurymagnetics.com/mmpn/ftbp-58-240m/

I’d be interested to hear your opinions on how this PT might affect the feel of the amp, If at all, vs what should be in there? ( is it the 1202 118?)

Regards,

Steve

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Re: 1970 JMP 1987 power transformer question

Post by danman » Wed Oct 21, 2020 7:16 pm

Unfortunately, MM does not offer specs on there PT's so I can't say what the voltage rating or current capacity is. If it's in the same ballpark as your Marshall, it won't affect the sound much at all.

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Re: 1970 JMP 1987 power transformer question

Post by 6adam7 » Thu Oct 22, 2020 4:03 am

Thanks Danman,
I’ve just had filter caps all replaced and it’s all spec’ed according to the 1970 schem but I still feel the bass is a little soft... I was hoping news caps woulda tightened it up a tad.... So now Its got me wondering if the PT might be the culprit....

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neikeel
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Re: 1970 JMP 1987 power transformer question

Post by neikeel » Thu Oct 22, 2020 4:59 pm

Typical specs for a 1202-118 would be 420v on the plates of the EL34s assuming solid state rectifier. Presume you used decent filter caps ie F&T and used 50+50 (=100uF) on mains, 50 screens, 50 PI and a dual 50 in the preamp. They are usually reasonably tight in the low end, it is definitely a 1987 unmodded?
Neil

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Re: 1970 JMP 1987 power transformer question

Post by 6adam7 » Thu Oct 22, 2020 9:58 pm

Yes, my tech used mainly F&T’s - a couple of JJ’s .. I got him to go over the whole amp checking it’s health, using that Unicord 70’ schem floating around on the net... the only component not marshall spec is the PT...

Strong Mullard xf3’s biased at around 37ma, B+ is around 450, which is pretty ‘normal’ for a 50 watter yeah?
Running a pair of Creamback M65’s...

This ‘issue’ has been pissing me off for ages now.... I’m trying to use it as a pedal platform, but Ive noticed if I up the volume to say 7, it’s like the bottom just browns out
(not in a good way!) losing definition and it sounds soft....thump it does not🙄

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neikeel
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Re: 1970 JMP 1987 power transformer question

Post by neikeel » Fri Oct 23, 2020 2:27 pm

That is odd as 450v is higher than usual for one of these, 420-430 normal

Can you post a pic of your board?
Neil

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Re: 1970 JMP 1987 power transformer question

Post by glpg80 » Sun Oct 25, 2020 11:13 am

How is V1 connected to the grid stopper? The “hot shield” input method Marshall was known for will kill treble and can also make for loose bass. It runs from the high input jack to a 68k grid resistor on the board, and then into a shielded cable to the grid on v1 with the shield to the plate.

I’m not sure how your 70 clone is connected, but the changes you do in that area of the circuit are super sensitive to the response of the overall circuit. I added a resonance mod to my 74 to make up for the lost bass. I wanted the shielded input (to ground, not the plate) for a lower noise floor. As always, everything is a trade.

Also worth noting that if you’re wanting thump at high volume you may have to increase the filtering capacitor size and also go with a higher Henry choke. You’ll also need to bias on the colder side (50-60%) max rather than on the hotter side (70%) to keep things tight. Lastly, you’ll want to pay attention to the types of guitar cables you’re using on the front end - it matters a lot to the response to the front end of the amp when talking about bass and treble. I highly doubt it’s your PT.

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Re: 1970 JMP 1987 power transformer question

Post by 6adam7 » Fri Oct 30, 2020 5:58 am

Thanks for replies guys, I really wanted to upload some photos but I’m getting the message “ board attachment quota full”

Neil, I quized my tech - B+ is actually about 430... the Mullards have started to drift from each other a bit over the years, so one is at around 37ma the other 40...

Could I be hearing a bit of blurriness from too hot bias?
The amp definitely sounds better than previously - more ‘perceived’ gain and sounds more together...just not as tight as I woulda expected..

I might do some pre tube swapping, see if I can put something tighter in V1..... Ive been using an Australian made Mullard in there..( apparently the story goes that Blackburn were sending the internals to the Hendon factory here in Aus.).. I’ve never heard a ‘real’ Blackburn Mullard but these seem to meet every description I’ve ever heard of a Mullard... they’ve got this beautiful mid range swirl going on but, they are pretty soft sounding, especially In the bottom end....sound familiar?🙂

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Re: 1970 JMP 1987 power transformer question

Post by neikeel » Wed Nov 04, 2020 4:29 pm

Sounds like your bias is pretty good - set hottest tube to 40mA and having 3mA difference is good IMO.

Mullard pres are very smooth (IMO a good thing) but you will get suggestions for Tungsram or Amperex, RFT and others.

I will use what I have available but I quite like Mullard, Brimar etc. Sometimes simple JJs are ok too!
Neil

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Re: 1970 JMP 1987 power transformer question

Post by 6adam7 » Mon Nov 09, 2020 3:17 am

Thanks Neil, good to know about bias, my tech wasn’t worried about it either...
I had a blast with the amp today and tried a few different pre’s.... actually the amp was sounding Pretty good.. maybe my ears are just adjusting to it?.. have to see how it sounds in a band context...

Thanks again for input, I appreciate it👍

Steve

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