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

[martin manning]

I agree it's a better solution, but the problem with increasing the value of the 47k is that you have to get under the board to do it, and you risk damage to the traces. I don't like the shorted resistor ahead of the diode either. If the plan is to go back to 6550's, it might be worth considering a replacement PT with the correct specs.

[donzoid]

The bias trim is functional...it's just that it only "goes up from here".

What is the voltage value "supposed" to be from the PT Bias feed?

Yes I thought they would have been redplating all along and yet, they don't. I even keep the backs off these things just to be able to see such things. I had better pop them out, test them and check pin 5 as you suggest to see what range I am "actually" in.

Thanks!

[donzoid]

Oh and if we are talking about changing the 47K resistor, its grafted on, on the top side of the pcb...I could change that pretty easy. And yes I never liked the el34's and (long term) plan to switch back, likely to KT88 (also a 35w tube like 6550). If I had a quad of them lying around...I'd do it now.

[donzoid]

Interesting side note: I met Tom Mitchell in '94 at a namm show. He was a very nice fellow.

[martin manning]

There are two 47k's- the one that's grafted on, which I suggested reducing to 15-20k, and the one closer to the pot, which you would have to increase in order to make the bias range more negative.

[donzoid]

Ah...thus the reason one would have to pull the pcb to change...OK, thanks. I'm going to go find a value chart for the PT and see how this weirdo one compares to the bias feed value. And yes, it really ought to have a spec PT oddly enough I have a spare OT on the shelf, and no PT (I guess thats usually how it works).

[donzoid]

The bias feed voltage reads 35. Since the schematic "looks like" 63, and (for example) MC specs say 98volts, would this be the root of this issue? It still seems like the resistor would have to be changed, but I am not far enough in my theory to know. Great point though Martin...it IS quite a bit different than stock.

[martin manning]

Do you mean the bias feed is -35VDC at this point, i.e. on pins 5 of the power tubes? What about the AC voltage on the resistor with the jumper wire over it?

No doubt the tech who put the PT in wanted to avoid lifting the board, hence the scabbed-on parts.

[donzoid]

I meant the AC supply voltage before it hits the diodes--right at the turret point from the PT lead.

...same point as the resistor with jumper, pretty much.

The DC volts at pin5 of the tube socket is -34

[donzoid]

Here's a follow-up... I went and got a few resistor values to play around with. What I did was to solder a resistor to alligator clips and create a third piggyback on top of the one already piggybacked (current config is a 15K stock on the PCB, with the original mod 47K = 11.37K at that point, then I added a 33K to bring that resistance point down further (it calculates to 8.46K, as an easy way to see which way it was going to go without de-soldering anything. This results in a reading of 61mA (vs. the original 74.5).

So I think Martin was onto this when he "guesstimated" replacing with a 15k to 20k...using my woeful math (aka a piece of software to do it for me) this would get me 7.5K (with the 15K + 15K on-pcb). Just not sure that will be enough to bring it down into range. I have a 13K on-hand so might try that here in a bit...that would get me to 6.9K.

Just for curiosity's sake I might pull my other 100w out of the box to measure what the (original, stock) PT bias voltage reads for comparison.


It also "occurred to me" that if I lifted the bypass wire on that first resistor before the diodes, it might just be fine.

[martin manning]

I wondered if my guess wasn't a little high :^) You could clip a pot on there and dial it in... There might just be enough muscle to get the current down to a reasonable level. I'd shoot for (25W x 0.7)/469V = 37 mA. The plate voltage is likely to come up a bit when you lower the current draw, so remeasure it when you get close.

If you were to remove the jumper wire on the 15k that will take it in the wrong direction, making the bias voltage less negative and increasing the current.

[donzoid]

Alas...Martin you have been a very patient fellow and for that you have my eternal gratitude.


I put in the 13K in place of the 47K piggyback this morning hoping it would get me in range. But this amp will just have to wait and not go to todays gig...sniff. Its down, but only to 58mA. I will have to go with a lower value, and since I leave for the gig in 25 min....just don't have time. I will do it early next week though and report on results. I'll take my Mark 2c as a backup instead. Love 'em or hate 'em...they are bulletproof.

Cheers!

[Firestorm]

No easy fix it seems. Looks like new PT with an appropriate bias tap... Or tapping the bias off the HT winding, which would entail messing with the PCB most likely. Further proof that PCBs are the work of the devil.

[renshen1957]

Oh and if we are talking about changing the 47K resistor, its grafted on, on the top side of the pcb...I could change that pretty easy. And yes I never liked the el34's and (long term) plan to switch back, likely to KT88 (also a 35w tube like 6550). If I had a quad of them lying around...I'd do it now.

Hi,

Just a suggestion, you might try the KT77 tube as a replacement for the EL34 first. I know a Country artist who used an EL34 amp (Matchless DC); I turned him on to the KT77 after his amp went to eating up EL34. Even the drummer noticed the difference and approved. This might be a stop gap solution until you switch the amp back to 6550/KT88.

Best regards,

Steve

[donzoid]

Thanks all! Yes Fire, I agree (now) PCB's are evil. But I am still going to look into the possibility of taking HT voltage to supply the bias....I belive that is how its done on my 1987 50watt. And also thanks Ren for the KT77 suggestion...since funds are limited (non-existent at the moment really) I can't afford to do the right thing even though replacing the PT is the obvious answer. I can't in good conscience run this amp like it is, either. I may just have to pull that evil PCB and swap resistors (or I am liking the idea of an adjustable pot in place of one), but it would have to have a wide enough range to accommodate a 6550 too ... too much research from here so I think I've beaten this one to death for now.