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

[martin manning]

very little current draw = very little voltage drop? I say add power tubes.

[donzoid]

6.65v at the heaters, 369v at the secondary with preamp tubes installed and glowing (but still on standby).

[donzoid]

OK at first I thought I had a problem...power tubes in, switched on and nothing... but looking back I saw that the bulb was glowing, and dimming. THEN I saw that the heaters very slowly starting to glow.

Now what? Take the bulb limiter out of the equation?

And, should I attach a speaker load at this point?

[martin manning]

Yes, definitely connect a load. Since you don't have a bright glow the current draw is reasonable, i.e. no shorts, so you can try full voltage.

[donzoid]

OK...load attached. Full voltage gives me a nice (heater) glow. Standby switch on, not blowing any fuses. Check bias?

[martin manning]

Yes, power string voltages and bias.

[donzoid]

Bias set to 38 mA...going to let her sit for abt a half hr. and re-check but so far, so good! The only smoke I see is coming from the incense burning at the far end of the room.

...Could this actually be "it"?

[martin manning]

Voltages, man! Current is only half of the bias story.

[donzoid]

B+ is 486 volts
Pin 5 output tubes 475

Bias - volts at swamp resistors is -45 while mA are 36

What else do I need to measure?

[martin manning]

Pin 5? That's the grid... What's on pin 3 (the plate)? If that is 475, and you have 36 mA then you have 68.4% of Pa max... Pretty good. Got sound? Preamp and master volumes work?

[donzoid]

Pin 3 = 467v

I can hear the hiss as I turn up the volumes. OK light a match and stand back.

[donzoid]

Guitar sounds come out!

[Reeltarded]

great job!! (martin)

[martin manning]

So what's the verdict? Does it sound good? Different? Better?

Does the Lar-Mar work as well or better than the previous MV?

How about the dual pot preamp volume? Any ill effects there? Does it work well with the channels jumpered?

You don't really have 475V on pin 5 do you? Screen voltage should be a few volts lower than the plate, so maybe the 475 and 467 are pin 3 and pin 4? What value are your screen resistors?

[donzoid]

Yes, great job and big thanks Martin!

Its sounds "different" and a bit better (certainly more depth, better low end), but its going to take some futzing around with it today. Some better pre tubes, the ones I had in for test are military Chinese, a fairly good and robust tube but not the best for tone to get a feel for it.

No more squealing at the top of the preamp and yes the LarMar is working. It was too late last night to run it up without waking the kids next door nyuk nyuk but its getting the expected volume. And, no red plates.

Also I have not yet tried the channel patch, and the normal channel so I will do that over the day of course at some point I'll have to set up a decent condenser and fire up sonar so you can hear a clip.

1st things first though in about an hour I'll re-check my voltages on those pins and report values.