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

[donzoid]

Hi Marshall Geeks (and I mean that in the most endearing way...really!),

Since the noob (that's me) learned a LOT with my last question on my 50watt Mk2 JMP...and it sounds SO much better (I have had great luck with my 50watt since making a fix and re-bias plus changing preamp tubes) and plan to re-tube outputs soon...that one is all good (thanks to all of those helping on this forum, that is...)

Not to "leave well enough alone" I decided I wanted to take one of my 100watters (my favorite one in fact) out and check its bias. I wanted to use the same method I used on my 50 (measure OT center tap to each side for Ohms, then turn amp on and measure voltage across the same, divide the voltage by the resistance measured earlier, to get idle current dissipation of the tubes.) So far trying this method on my '78 JMP 100 watt, I have blown 4 HT fuses. I think that meets the def. of insanity, yeah?

Long story a lot longer, I've had this amp through a lot of thick and thin...about 30 years. During that time (early on...25 years ago) after I blew up the PT, I had it replaced with a Sound City (I think, or perhaps a HiWatt) because the tech had one, and it tested good. (Yeah...I know. What do you expect from a 20-something guitar player who just wants his baby back, and has little money?). 6550's were just stopped for US production, and VERY hard to get. The advice of the tech was to go to EL34 and he installed German Siemens (even then I was loathe to ANY Chinese tube). I was convinced at the time that My Altair PW-5 was responsible for blowing it up. So I also had a MV installed. More about that later...

Taking it out of the box, I can see that its quite a hodgepodge (read: mess) as far as wiring (& quality). Every time I go to measure the voltage across the OT center tap to either side (pin 3 of an output tube) I blow the dang high tension fuse. I just want to make sure (first) the HT fuse is wired correctly to protect the OT, then ...

After that point what I REALLY want to know is "what exactly do I have here" in terms of the MV I had installed about 25 years back. As with most of you professional amp repair guys I guess half of it is "reverse engineering". From what I have learned off AG so far, its a PPIMV. But it also has some mod on V1. I will attach the obligatory gut shots.

So...Is the HT fuse wired incorrectly? Or is the fuse blowing because of the MV mod (wild guess there because the dual pot is grounded)? Perhaps because of the oddball PT? Or ... none of the above? Let me know if you smart guys need to know values, because this camera may not cooperate. Finally I will say that the amp has blown VERY FEW fuses in 20+ years. Always worked, but I never did really care for the EL34's. Plus the tone is quite "brittle/dry" at lower volumes compared to its old non-master state (using a power attenuator), yet it sounds pretty good cranked.

Anyway now that I know "what I have learned so far" I plan to do several things (like clean UP the wiring...my gosh...). Observations by the attending Doctors please? How bad is this mess? Wires crossed? I have limited theory but am a fairly quick study...Hit me with your best shot! I DO know that any violation of Ohm's Law = smoke. So far (thank goodness) there hasn't been any of that.


Key to the comments/numbers on the images:

"Nice..." -- Yes, the wires are soldered underneath that 'lectrikle tape'.

"OT/Choke" -- This is the point where I "tried" to measure voltage from OT center tap to pin 3 of tubes, blowing HT fuse. Wrong place?

1 -- Jumper wire across this resistor...no idea why.

2 -- Piggybacked resistor from the EL34 conversion. That will go away when I replace tubes with 6550 or KT88.

3 -- As you can see the original resistors were lifted on one end, run to the (dual) pot, and the output side has .022 K Nicholson's heading back to the PCB...thus the reason I "think" this is a PPIMV.

4 -- V1, shielded wires and quite strange in that the shield goes to a separate pin on each side of the valve, yet is not connected at the PCB end (thus the ???).

[martin manning]

"OT/Choke" -- This is the point where I "tried" to measure voltage from OT center tap to pin 3 of tubes, blowing HT fuse. Wrong place?

Measuring a voltage shouldn't blow fuses. My only thought is that when you put the probes on the amp goes into an oscillation, and that blows the fuse. You could try clipping the black probe to ground and measuring the voltage at the center and either end of the OT primary to ground, then do the arithmetic to get the voltage drop across the windings.

1 -- Jumper wire across this resistor...no idea why.

Looks like this is a mod to adjust the bias voltage for the tube swap.

2 -- Piggybacked resistor from the EL34 conversion. That will go away when I replace tubes with 6550 or KT88.

Same as above.

3 -- As you can see the original resistors were lifted on one end, run to the (dual) pot, and the output side has .022 K Nicholson's heading back to the PCB...thus the reason I "think" this is a PPIMV.

Looks like it. If you want a PPIMV, I would change this to a Lar-Mar type. What value are the pots?

4 -- V1, shielded wires and quite strange in that the shield goes to a separate pin on each side of the valve, yet is not connected at the PCB end (thus the ???).

The shields are connected to the tube plates, This is one way to shunt noise voltage off the shields, but it rolls off a bit more HF that way since it puts a small capacitance between the grid and plate. You could leave it, get rid of it, or redo it with the shields connected to ground.

[donzoid]

Thanks Martin... I will measure the pots and get a value...had not done that yet.

Of course that makes me wanna ask more questions...

What function do the caps coming off the master vol pot serve? (is it to cut noise). And yes I thought to convert this to LarMar (one of a few I have studied) but I like a lot of gain coming from the front end of the amp (vs a tube screamer for example) I like to then use a buffer type of boost to drive the front end into mayhem for solos. Lately that has been in the form of a voodoo labs Sparkle drive -- love the way I can mix just a bit of dist. in with clean boost on that pedal!

I suspected the V1 shield may have been for isolation but since this amp doesn't seem to have much "problem" with treble I think grounding them would only make it more "shrill". It prob. doesn't need it (unless it changes the treble frequency response, that may be useful). I was curious why the original tech patched them over from those resistors?

Yeah, on the 50watt the HT fuse goes from the diode bridge out, to the fuse, then on to the center tap of the OT (which makes a lot more sense to me). This one is way different. Granted its four years later design-wise and has been modded all to heck. With that odd PT in there, and all the secondary wires out of it being "basic black" I wasn't sure about how all that got wired in or how much was original. I looked at several Metro schematics that are closer to the way the HT fuse is wired, but since the OT is still original didn't want to start letting the smoke out until I asked. OK one more point on this one...measuring the Ohms on the OT from center tap to the sides is about 16 Ohms per side. My 50 watt is about 44 per side. Is that normal?

[donzoid]

My (super crappy) meter says the pots are 860K.

[martin manning]

What function do the caps coming off the master vol pot serve?

The MV pots are connected to ground, yes? The extra caps are there to isolate the grids from that path to ground. The Lar-Mar doesn't have that problem since the pots are "grounded" to the bias supply.

I suspected the V1 shield may have been for isolation but since this amp doesn't seem to have much "problem" with treble I think grounding them would only make it more "shrill". It prob. doesn't need it (unless it changes the treble frequency response, that may be useful). I was curious why the original tech patched them over from those resistors?

Just to have the entire length of the wire shielded.

Yeah, on the 50watt the HT fuse goes from the diode bridge out, to the fuse, then on to the center tap of the OT (which makes a lot more sense to me). This one is way different. Granted its four years later design-wise and has been modded all to heck. With that odd PT in there, and all the secondary wires out of it being "basic black" I wasn't sure about how all that got wired in or how much was original. I looked at several Metro schematics that are closer to the way the HT fuse is wired, but since the OT is still original didn't want to start letting the smoke out until I asked. OK one more point on this one...measuring the Ohms on the OT from center tap to the sides is about 16 Ohms per side. My 50 watt is about 44 per side. Is that normal?

This one should be very similar (electrically) to what you describe for the 50W. Do you know if it is a model 1959? I'm not surprised that the primary DCR is lower since the primary/sec impedance ratio is half of what the 50W has, and the wire has to be heavier for 2x the current.

My (super crappy) meter says the pots are 860K.

So the MV pots are probably 1M nominal.

[donzoid]

Yes the MV pots are grounded to the bus wire.

I traced the HT fuse back, and you are right it is similar to my 50watt except that it comes off one of the large (I think these are coupling) caps before routing through the fuse block and over to the CT of OT. I saw something almost (if not) identical this morning on a splawn diagram.

Interesting to note now, after playing with various old stock tubes in V1 that the preamp when set all the way up squeals like a stuck pig with either the treble or presence up more than half. Back it off 3 numbers and it stops. So the beast must be right at the verge when pre is full up (and these are good tubes...mullard blackburn and tungsram). That supports your statement that its oscillating and blowing that fuse. I put in a gold pin (meh) tube and was able to get it to 8.5 without all that squeal. But of course the tone wasn't quite as good...good enough for tonight's gig though (I use the 50, but always have this one there as a backup). Unfortunately...I still don't know what the bias is. Guess I'm gonna have to do your measurement on the voltages of the OT to see.

Thanks again!

[martin manning]

You're welcome!

I traced the HT fuse back, and you are right it is similar to my 50watt except that it comes off one of the large (I think these are coupling) caps before routing through the fuse block and over to the CT of OT.

Those large caps are the power supply reservoir. Coupling caps are used to block DC and pass signal from one stage to the next.

[donzoid]

AAAAHHHHGH!!! Or...

UH OH! I just got my bias tool yesterday and I'm shocked that my el34's are at 75mA bias! I don't really know why they aren't red-plating. BOY am I glad I have not put hardly any hours on this amp in the last several years. Since the gut shots are already there-- I wonder if I removed the wire bypass on the upstream bias resistor, if that would bring the current down because the trim pot is all the way back, and I can't get it lower (or run it in good conscience knowing its this far off.) It should be @ 40mA by my figure of 75% of max.

Should I do that or try piggybacking a different resistor instead of the one that's piggybacked now (the downstream side of bias). Let me know if either is better--or neither. Thanks...I was hoping to use this head at a Saturday outdoor show. Wow. I sure am glad I'm checking out all this stuff.

[Firestorm]

75mA per tube or 75mA per side? At what B+?

[martin manning]

You have two things going on: a replacement PT with (presumably) a different bias winding voltage, and a tube swap. The values of the resistors on the board appear to be consistent with the USA variant with 6550's, and the jumper and piggy-back resistor are trying to adjust for the both the PT and the tube swap. Since you need more negative voltage, and the first resistor is already at 0 ohms, you will need to reduce the piggy-back resistor or increase the trimmer/47k combination. The easiest route would be to lower the piggy-back resistor, to say 15-20k (SWAG).

If the idle current is 75mA per side, as Firestorm hints, then you might be ok now.

[donzoid]

Firestorm...would I measure the B+ from what, center tap of OT primary to pin 3 of output tube? In other words if its plate voltage, it was measured at 469. If something else I can measure it.

This is for one tube my bias probe is a Eurotubes type that plugs into one socket and then into my DVM. Set to 200mA range setting (as it instructs).

[donzoid]

I picked up the bias tool when I went and picked up the tubes 'cause I live very near the Eurotubes shop...it worked really well for biasing the new pair of KT88's in my fifty watt so I don't think IT has problem...

[donzoid]

OK I get what you're asking...if the reading is 75 per side, I would divide by two then for current to each of the pair?

[martin manning]

Correct, but if you are reading current through one socket, then that is current per tube. Doing it the way you did previously, measuring the current through one half of the OT, which carries the current for both tubes on that side, you would need to divide by two to get current per tube.

[Firestorm]

Something is funny. 75mA per tube at that plate voltage is 35 watts and the tube plates should have a nice red glow. As Martin said, the 15K dropping resistor (the one shorted out) is stock value for 6550-equipped 1959 Mk IIs. That's also the one people mess with in tube conversions, thanks to Tom Mitchell. Value for EL34 models is higher (27K or 22K). The "piggyback" resistor (I think) parallels the 15K between the bias caps. All of which should mean plenty of negative voltage. But...

Maybe get a negative voltage reading on Pin 5 of an output tube socket (tubes out for now probably) and also see how much range you get from the bias adjustment pot.

If the bias tap on the PT is dramatically off, I think the best bet is to increase the 47K in series with the pot. I suppose it's possible the pot shorted (kind of uncommon) and turning it does next to nothing.