Nice mess in a 78 JMP Superlead...need advice!

[donzoid]

Well...I HAD to plug into it now didn't I???

HUGE thanks Martin (and Steve, Cameron, Firestorm, Lou, and Tom...and everyone else who chimed in).

She is working pretty good! Certainly less brittle sounding than before. My test cab is a bit flat (its an old 65w 800 series cab). Next step will be to unpack my "usual" angle (with 2 H30's, and two 65w) and set up a mic.



OK, some cleanup work, and perhaps some "other" work before she goes back in the box (this thing sure could use the presonance control, and the LarMar), but it's definitely better than before (tone wise and for peace of mind). One thing I WILL keep in there for now is the el34's to see if, over time I had not formed a wrong opinion about them (based on this amp I might add).

I'll have to set up a decent condenser and grab a sound byte for my wrap-up.

(in my best three stooges voice)... Success!

[JazzGuitarGimp]

Excellent job!

[renshen1957]

(in my best three stooges voice)... Success!

Great job!

""What is this you're doing?" "The elevator dance." "Elevator dance?" "Yeah, there's no steps to it."" Ted Healy and Larry Fine, Soup to Nuts. (First Stooge appearance in a film).

[martin manning]

Yea great; glad it worked out! Lots of good ideas in this thread for future reference.

Here's a layout that you can use to make it look a little better. I believe there is an isolated and unused pad, to which I moved the connection to the 3k9 resistor from the PT, and the new diode is intended to be down on the board, below (partly under) the new 10uF cap.

[donzoid]

Apologies for not getting a sound clip up yet...its gonna be a while unfortunately...

[martin manning]

Can't quite figure out what happened there. Is that a cast on your arm too? What does the other guy look like?

[Structo]

Ouch!

I hate finger injuries!

[Firestorm]

Ow!

[renshen1957]

Apologies for not getting a sound clip up yet...its gonna be a while unfortunately...

That's a double "ooch!"

[donzoid]

Yeah...sometimes you just wish you could have a "do over".

Well Martin, I was already nursing along a strained wrist, so what you see is the wrist brace one of those velcro-it-on types. That was getting better...

The finger didn't fare so well last weekend in an unintentional bout with an electric hedge trimmer. It kicked up when I was trying to pull it loose off some large blackberry vines, and my hand slipped off the safety handle and I will say, that as ugly as it is, there is no nerve damage (cause yeah I can still feel EVERY bump) and with a tool like that, it could have been much worse. Heck, if I had cut the tip of it OFF (chopped the other direction) it would have had to be sewed back on and would have taken months, and probably with some nerve damage. As it is, I may have to find a sub for next saturday's gig, but the way its coming along, I think I can do it with the super-glue and vellum surgical tape routine (this ain't my first rodeo).

Yeah man...you SHOULD see the other guy! Anyway its a bummer that I can't really demo that amp and chaps me every time I look at it...but I'll be back at it shortly and it gives me a lot of time (uh oh) to think over where I want to go in the next adventure with it. I think after a good run with the el34's in it, I am going to install a presonance control, and 4 6V6. Then the sound clip differences will be an interesting compare along with live ears.

Oh and did I mention that the next day I went back to work on this project, stepped on a wasp nest and got stung 11 times in about 20 seconds? LOL. Glad all my weekends are not "that" exciting. I haven't run that fast in more than ten years.

[martin manning]

Man, and here we are encouraging you to mess with high voltage ;^) Here's to a quick recovery!

[donzoid]

Yeah the head of IT ops where I work told me to stay off the mainframe...same idea. At least I still have my sense of humor(?)

[donzoid]

Enough flesh has re-grown onto my finger to actually put on a guitar and play it (outside the already-committed performance dates, which required a fair amount of super glue gel and paper surgical tape, plus having two dear friends sit in on six+ songs through the last two sets of the gigs...God bless them).

I set up the 100watt on top of my trusty angle front (the one with 2 very old H30's, and two early 80's G12-65's). I was just going to run through a few chords and pump it up a bit keeping in mind that it's been two months since I got sidetracked with that damn finger cut and I wanted to record that sound clip.

Well, I fired her up and was surprised. Preamp was working seemingly well at 2/3 up (a good amount of dirt) but when I started turning up the master I was confounded. Where is the volume? This a hundred watter, and should be tearing my little head off at about 2. No such luck. Slowly turned it up to half (12 o'clock)...almost no change. Clearly something must be wrong here.

Checked the bias and its at 28 mA. I had left it at 38. Ohhhhh....back out of the head box and back onto the bench. Re-checked the bias still at 28. Re-bias to 38. Pull the bias probe and turned it on for a half hour (idle) to fully warm up...still at 38. OK, plug it in and see if I can see what's up. Start playing some big "E" chords and started turning it up on the master. Kept checking the PT and OT to see how hot they are getting...not bad but PT is "hot" but I can still put my fingers right on it without even thinking I'm going to get burned. Check the tubes--they are redplating. ShOOt!

So, now what? Pull the tubes and re-measure the PV and Bias values? I would have thought if the OT is failing (by my messing with bias, OR pure coincidence) that it would be hot enough to fry eggs. I have a new DVM and a temp probe if that would be a good measurement to take...but it wasn't even what I'd call hot. Only the PT. Oh and for what its worth I also have a dB meter and with pre and master at 3/4 she is only getting about 85 dB. Cab measures proper at 13.6 ohms (checked it just to be sure it hadn't popped a greenback on the last gig because a shorted speaker coil can be hell on OT and tubes...been there). What measurements would help me (and any of you) suss out where the problem is? It didn't do this in test 8 weeks ago...master on 2 was about the volume I expected. And, after putting back into the cab, it's (literally) sat there in the same spot since.

[martin manning]

I have no idea why the bias would shift like that, but my first thought on the red plating is that there is an oscillation happening, maybe related to the MV lead placement. Is the bias holding steady now?

[vibratoking]

...Heck, if I had cut the tip of it OFF (chopped the other direction) it would have had to be sewed back on and would have taken months, and probably with some nerve damage. ...

You're right, cutting the tip off can be bad. I was using a kitchen knife to open a sealed plastic package a few years ago. Thi knife slipped and I something bounce down to the other end of the kitchen counter...my middle finger tip left hand. Big bummer...nerve damage, but could have been worse. Didn't significantly affect my playing, although that finger is a bit shorter. Best of luck healing. Finger tips have ALOT of nerves and can be very painful. Work gloves, work gloves, work gloves.