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

[martin manning]

Sounds like you've got it pretty well squared away. Probably there isn't anything wrong with the voltages, but it'd be nice to know where they are. Be very careful poking around the power tube sockets- you don't want to slip and short something. You will be close enough if you measure at the top of the reservoir caps for the plate voltage (OT CT, HT fuse), and just touch the pin 4 side of the screen resistors to get screen voltage.

[donzoid]

478v pin 3

471v pin FOUR (yep...I misreported...sorry)

Screen grid resistors are stock 1K 5watters

Yes, I am well aware of the voltages here. I caught 510 off the old PT a few weeks ago, and that was a "nice" wakeup call. Still nothing compared to the 440 AC I took 20 years ago (that literally burned a hole in the end of one finger). A great reminder though WHY everyone should stay aware of the volts inside these things!

It still sounds a bit dry and "brittle" but I still have to swap out the pre tubes for mullard/tungsram and then I will get a sound clip out for suggestions from "the mob". I am thinking the "fizzy" cap might need a change. So far though, I would say its going to this Friday's gig!

[martin manning]

Great! Ya know, new PS filters would help fill out the bottom end some more...

[donzoid]

[donzoid]

OK I put a Blackburn Mullard in V1 and tried the channel patch.

Thumbs up on the Mullard -- much better already with just that one change for tone. I'll bet right now that swapping V2 and V3 will also help.

Thumbs sideways for the dual volume thing. Not sure yet but I may try a resistor on vol. 2 channel (maybe the one I took out) to bring it down "just a tad" as a way to mix it. Its much fatter and much less fizzy with channels patched. But its almost "too" fat because it loses the mid-high bite I love. Jury is still out on that one. But, if I were to put in a push-pull pot with separated level controls -- that would be the shit!

PLENTY of gain from this beast with the LarMar...so far the sweet spot is abt 12 o'clock to 3 o'clock on the preamp volume it gets pretty ratty after that...but not really bad until the last bit then it just turns to mush.

As for caps yes it needs a swap...but before I drop that order, I'm going to play this thing! And take a good look at my 50w since it probably needs them too.

[martin manning]

Do you have a 0.005 bright cap on one preamp volume pot? I don't see one... But I like the pull pot idea! That does it all.

[donzoid]

I don't have one...but I might have a cap around here I will check. Is that the one they used to put right on the pot (or between that and the jack), or would it be on the pcb?

[martin manning]

It would be on the preamp volume pot, from the input (CW terminal) to the wiper.

[donzoid]

OK would that trim brightness off? If so maybe that's why its sounds brittle to me.

But if it ADDs brightness, that may be good with the channel patch too.

So far I have found a 100pF cap, but I am sure I can come up with a 500 if I keep digging.

[martin manning]

It will add brightness when the volume is turned down, no effect at 100%. The schematic shows 0.005uF (which is 5,000pF) on Ch 1, the bright channel.

[donzoid]

Ah, I missed a decimal...must be that nasty head cold. Sorry. I can get one this afternoon...that would be worthwhile to try.

Would putting the 100K resistor back on vol. 2 between the CW and wiper leads knock off a little, or a lot of the channel 2 volume?

[martin manning]

A lot of people clip that bright cap out. The lead channel has the 500pF//470k treble peaking network on it too, so I guess that is enough for most people when they are plugged into it. But, if you can jump and blend the channels I think it might be nice to have. You can easily give it a try with the dual volume pot- a 50-50 mix of the two might sound pretty good.

I wouldn't put that 100k back in. That was a way to fix a preamp volume setting for that channel since the MV went into its hole.

[donzoid]

Yeah I really am liking that idea. So my excuse for running back down to the local parts place this afternoon (ahem) will be to look for one of those! But knowing them, I'll have to order it from Mouser or similar place that carries a better selection... Going offline for a while but for now, HAPPY THANKSGIVING!

I for one am VERY thankful for this forum, and for your help Martin! Yay! I got my hundred watt back!

...."you want earplugs with those fries?"...

[martin manning]

Same to you, and nice work on this, too!