Marshall JMP clone no sound issue (with scope photos)

[beasleybodyshop]

So I finished rewiring my JMP clone I built, but added a slope and resonance control. Afterwards I fired it up - sounds ok, but when i fiddle with the slope control it doesnt really change much tone wise. I assumed i wired it in incorrectly, so i rewire the slope control (slope input going to outer lug of 50K pot, slope output attached to wiper and the other lug) and fire it up a day later - no output, but when i crank up the master i some high end oscillation. So i check plate voltages - everything is within spec, except pin 6 of the PI is about 30 volts higher than spec (i dont know if this really means anything) so i put my scope that i just acquired on the output and this is what im getting with volume control at 2, TMB set to noon, presence 0 slope 0 and master at around 2:

What does this mean? When i do the pop test im getting nothing, nada. All my voltages check out on both power tubes as well. Any thoughts guys?

[beasleybodyshop]

sorry, forgot to mention i have a signal generator on it set to 1Khz

[John_P_WI]

Do you still have a slope resistor "on top" of the pot? It would be: one junction of the slope resistor is tied to treble cap and tone stack feed, the other end of the slope resistor would be tied to the "top" of the slope pot as you described. The wiper and bottom leg would then be tied to the mid and bass cap.

You should still have sound unless you grounded out the signal somewhere. Use your scope starting at V1 and look at the waveform after each coupling cap to see where you are losing the signal.

You can disconnect the feedback as well to check that there is no wiring error there.

[beasleybodyshop]

Yes I have the slope control wired precisely as you have described. Here are some more pics with annotations:

[Meat&Beer]

If the last thing you did between when it worked and when it stopped working was the variable slope, i'd be looking there. With chopsticks, etc.

I've found that the variable slope doesn't have a huge impact also, you probably had it right to begin with. Like I've read a lot from other users of this pot, it's kind of a set it where you like it "more" and never end up messing with it again.

Mine is gonna be magically reincarnated as a fixed R.

[Reeltarded]

The slope is a 50kl with a 33k off either outer lug. The connections are center to the treble cap/CF junction, 33k terminates at the bass and mid caps..

THERE IS NO GROUND THOUGH the other lug floats.

In my opinion, slope is more useful than any other control. Face change!! Shape of a Bandmaster!! -whaaaaaaaa....- super amp powers activate!

[beasleybodyshop]

The slope is a 50kl with a 33k off either outer lug. The connections are center to the treble cap/CF junction, 33k terminates at the bass and mid caps..

THERE IS NO GROUND THOUGH the other lug floats.

In my opinion, slope is more useful than any other control. Face change!! Shape of a Bandmaster!! -whaaaaaaaa....- super amp powers activate!

So i changed it to what you described. I am getting super weak output now - and when i cranked up the master all the way I watched a spark jump from one power tube socket to the other. lol...

Eh....

[Meat&Beer]

Reel- would you recommend this in more of a low gain amp? Mines in a dang high gain amp... either that or I dinked mine up somehow also...

[Reeltarded]

I have it in raging three stage things and it works when they are set clean, so..

The Marshalls sound like bandmasters at 83k, death metal at 50k, and rawkin roll at 33k. Midrange throat shift that makes an amp work with nearly any guitar with a couple twists. All my tone controls are basically straight up the middle with slope doing the work from there. You can dial out the clouded mids and stay off a bass player and the vocalist with a bunch more punch.

[beasleybodyshop]

After an enlightening conversation with Reel i discovered the problem - the shielded wire from the resonance control to the impedance switch was the culprit. Reel explained to me that low frequency oscillations were possibly the problem. I disconnected it and viola! Sounds like a kick ass amp again! And now that the slope control is hooked up right, I really dig it! Hell yea! Reel you are the man as always!

I noticed that the waveform on my scope got super dirty when i checked AC signal where the resonance connects to the output - That was the first clue. Never gonna use shielded wire there again!

[Reeltarded]

Rockin' doody!!

[beasleybodyshop]

Yep, just hooked up the resonance with unshielded wire. Definitely it. Now this thing sounds sweet! I also finished tolexing a headcab for it:

[sepulchre]

Sweet Cab!

[beasleybodyshop]

Thanks. I made a matching dumble style 2X12 cab for it too:

[NickC]

Nice!

I like the faceplate, how'd you do that?