Sirshredalo

Hello,

New guy here, My name is wes and I thought my screen name was Sirshredalot, but it shows up as Sirshredalo...hmm.

Either way I have a 1978 100w jmp master volume, with the dual vertical inputs.
The amplifier has been modified by Uncle Albert out of indianapolis IN.

IM still not too sure what was done, but the low input is crystal clean untill 11, and the high input is dirty until 1.
There was an effects loop added also.

My question is, Recently I have noticed (ever since I had a tube go microphonic) that the treble is just absolutely piercing. It has always been a bright amp, but now it just seems so much more shrill.

It turns my Wolfgang into a tele.

IM playing it through a peavey 412 cab with un known speakers, but theyre not the original peaveys.

Is there a cap that can be installed to bleed off some high end? Maybe some alternate power tubes or preamp tubes to cut some of it down.

Its forcing me to play with the mids and bass full tilt, with the treble and presence all the way down.
Ive had two sets of EL34's in there and they both sounded the same.
they where EH's and Groove tubes...number 5's.

Also, the back of the chassis has a sticker that says "EL34's" ONLY. Is there a mod that can be done to make an amp only useable with a certain tube?

IM not an amp tech so I couldnt tell you the plate voltages or anyting fancy, I just know it used to be a killer amp, now its still killer, just much less flexible.

God bless
-Shred

Brett Blackmore

Hi Shred,

you did not say if the blown tube was a main power tube or not - still, you have a tube issue. Do not go making mods and changing things before doing this:-

If it is a main tube then you need to re-bias the amp (and hope the new tube is not a crappy one) or else that won't help either.

If it was a pre-amp tube then rotate it with the others to see if the problems goes away. If it does and you are happy that there is no problem elsewhere then ok (it might not be crap - just wrong sound type) - otherwise buy another tube.

Cheers Brett.

Sirshredalo

After I had a tube go microphonic, I replaced them with an old set of tubes that where in the amp before.
The amp had sat unplayed for about 2 1/2 years, and when I came back to it, the tube was bad.

I swapped out a set of (4) microphonic EH EL34's for the old set of Groove Tubes. Same result...except the amp didnt keep blowing fuses.

So it was a main power tube that went.

The amp functions just fine, only it picks up radio stations....and is bright.

The amp was modded before I got it...I believe in the early 80's.

God bless
-Shred

thegaindeli

First - I would have it put back to stock.Then I would hunt-down and purchase a Soldano "Hot Mod" tube,and an S.I.B. tube driven Varidrive... That's all the "mods" you'll ever need with a Super Lead 100!
If really want a great (and most importantly) reliable "modded Marshall" sound - buy an 80 watt Bogner "Shiva".You can pick up a mint 2008 model on ebay for around $1,600.00 USD.

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