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

[martin manning]

You could take it one step at a time and do the PT replacement with relatively little effort, likewise for the Lar-Mar. If you want to switch to 6V6's then I think you would be looking at a different PT to bring the plate voltage down to around 400, and you'd want ~3k4 primary impedance. That would mean a set of 50W Marshall JMP transformers, or at least a new PT, and miss-matching the 100W 1k7 OT you have by one click. Personally, I think I'd keep the EL34's and the MV.

Thanks for the kind words. If you've browsed around the site a bit I'm sure you've found that there are lots of sharp guys here who do some very fine work.

[donzoid]

Yes one step at a time IS the plan. Sounds like a 6v6 setup would be more trouble than its worth...HOWEVER, you bring up a really great point and have me thinking I might want to swap out both the PT and the OT and make this thing into a 50 watt instead of 100. I think I will go poke around the garage threads and see if its been done before.

I have a theory (and being a newb I will expect the smart folks here to shoot it full of holes but thats all part of the learning and so a good thing):

"What if" the bias current in the Partridge PT gets "starved" as the volume was raised to saturation (and thus at higher volumes, kept the thing from redplating by virtue of that downward supply curve)? Then by actually engineering a fix (with Martin's half-wave almost doubler I installed) it just can't handle the extra plate voltage of 512? Too far-fetched?

Anyway even if I keep it at the stock 100 watts the next thing I do will be order one of the (several) suggested proper transformers and at least get that back to stock so I can work from a known good point to move forward in the troubleshooting process.

[martin manning]

"What if" the bias current in the Partridge PT gets "starved" as the volume was raised to saturation (and thus at higher volumes, kept the thing from redplating by virtue of that downward supply curve)?

That would be going in the wrong direction, since the bias voltage would be reduced, i.e. less negative, making it run hotter. Can you miss-match the speaker by one click (impedance set to one lower than the cab)? If it doesn't red-plate that would support my theory that the combination of voltage and load is too much. Watch for glowing screen grids if you try this and crank it up loud.

[donzoid]

Ah, OK its the negative voltage and virtual ground thing again -- thanks Martin. I might try that experiment, but have gigs Sat. and Sun. so I am probably just going to order up a PT and then go from there when it arrives.

[donzoid]

Yep...he's at it again. OK so I have checked the PCB to a great extent and found very few deviations from the "Marshall norm (if there is such a thing) I DID find several problems with the PCB. The diode bridges on this thing are (were) fried. What I mean is, there are no conductive traces left on the diode portions of the PCB. Only a guess, that they fried the diode chain when the original PT melted down. So, what I did was to rebuild the actual stock circuit --hardwire the diode chain and the bridge to ground, using a 27K resisitor and the result was about what I expected...immediate red plate of the tubes. So I adjusted the 22K variable resisitor (bias) to the most - voltage and now it blows the mains fuse. Several times. I have ordered a new PT from Weber, since Edcor has a lead time of 6 weeks + this time of year. I would have preferred the Edcor but will save that for the trainwreck build I plan after this. In the meantime I have the parts for a LarMar MV so I will yank what's in there, and go to that (the only diversion from a stock setup, I have checked every single resistor and cap value) and once the new PT arrives, get that in to the chassis and see what happens. My hope is, once I can actually get this thing functional again, is to go to two 6550 or KT88's on the grid (not 4) and use this thing as a functional 80 to 100 watt with those. But I will report when the new PT is installed what happens. Right now I just want to get the stock PT installed (with the stock bias values) and see what happens with the 4 el34's that are in it.

[donzoid]

Ummm... I think I should be using the 700volt B+ not the 640v, correct? But I am wondering if 640 might be less headroom and easier saturation.

[martin manning]

The transformer diagram you show is for a full-wave (FW) rectifier, not a full-wave-bridge (FWB). You need something with ~350V across the outer leads of the secondary, like this: http://www.classictone.net/40-18024.pdf or the upgraded version with M6 steel and flux band http://www.classictone.net/40-18053.pdf The Marshall part number you want to replace is T4145.

[donzoid]

Well...that's an expensive mistake on my part....thanks Martin.

[donzoid]

I guess that gives me time to install the LarMar after all...lol.

[JazzGuitarGimp]

The transformer diagram you show is for a full-wave (FW) rectifier, not a full-wave-bridge (FWB). You need something with ~350V across the outer leads of the secondary, like this: http://www.classictone.net/40-18024.pdf or the upgraded version with M6 steel and flux band http://www.classictone.net/40-18053.pdf The Marshall part number you want to replace is T4145.

Martin, why not just ground the center tap, and use two diodes on each of the 350V taps?

[martin manning]

Could do, but the PCB would have to be reconfigured as a FW. It'd be best to find some way to put two diodes in series on each leg, and something might have to be done with the two 0.22uF caps on the PT secondary, since they would see twice the voltage. Balance resistors would be needed on the reservoir caps, too.

[donzoid]

Well I guess hindsight is 20/20....should have asked the question before I dropped the order. Unfortunately I took their description as being useable for this, as its the same exact description for the Heyboer they offered.

"100 watt Marshall power transformer, standup type. Fits Weber 6M100 and many Marshall 100 watt chasses."

I'll order the Classic tone and eat the restocking fees.

[donzoid]

OK my ClassicTone is in transit (What the heck, I figured since I wasted some money on the re-stock, I'd "waste" some more so I got the upgraded one knowing that my lust for large bottle tubes might come into play and it would keep the heat down).

Can someone take a quick look at these pics to make sure I am correctly re-installing? I have my TAG bulb limiter built and ready to go.

And one technical question: On the large multi-caps (where the CT for the secondary connects for example) double their value because of the way they are wired. In other words does connecting both hot leads of the cap represent paralleling them, to make the 50/50 into 100uF? I ask because the Ceriatone (and the ClassicTone) layout shows them as 100. These stock old Daly's are 50/50.

Red box on the caps photo is where the original CT of the secondary was connected...so that's where it goes, correct?

[martin manning]

On the large multi-caps (where the CT for the secondary connects for example) double their value because of the way they are wired. In other words does connecting both hot leads of the cap represent paralleling them, to make the 50/50 into 100uF? I ask because the Ceriatone (and the ClassicTone) layout shows them as 100. These stock old Daly's are 50/50.

Yes, paralleling two 50's makes a 100, and then stacking those two paralleled pairs makes it a total of 50 from the diodes to ground. Connecting the CT in the middle insures that the voltages are balanced.

Red box on the caps photo is where the original CT of the secondary was connected...so that's where it goes, correct?

You need to rewire it to look like the picture below. Can you find the two 0.22 uF caps that are shown on the schematic, across each half of the PT secondary?

And BTW, your next project will be replacing all those old cans. :^)

[donzoid]

LOL...yes Martin, not the first time I have heard that (replace those cans!) Are the AI cans decent quality or is there a better brand? I think I saw F&T at AES. Those, CE (supposedly made on the old Mallory equipment) and JJ's.

OK so I "did" see the .22 caps on the schematic, but I don't think they are in mine. From the standby switch, the secondary leads go straight to the diodes.

I guess since that was a follow-up question and since you are asking, I will need to install them. Makes me wonder if they were ever on here. What voltage do I need for them? 500?

I can re-wire the caps though, the way you have pictured.