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

[Cameron]

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.

What is the voltage coming from the AC bias wire without being connected to the board?
Working on these PCB is not hard...the board comes up enough without unsoldering much.... most of the time its the yellow wire from the screens filter to the board and sometimes the bias ground . If you need more space just take out the pots...you don't need to unsolder them..and you should be able to get to everything.

[donzoid]

hi Cameron, and thanks for that info...I have never pulled one (I have gotten one tilted enough to stick an angled tip in there for one solder joint) and didn't even realize I could do it that way...whew! THAT is good news!

If I have to disconnect the AC bias wire to get an accurate reading, I will do that this afternoon and report.

[Firestorm]

The reason I hate PCBs is because I can't bring myself to do hack fixes on them. I unsolder/disconnect enough leads to flip them over, desolder with a low-watt iron, clean the pads and holes with good braid and reassemble so it looks close to factory. Why I wind up making $1.38/hour on them probably. But it can be done.

[donzoid]

I have certainly learned a lot from this adventure. And, after seeing this thing with at least slightly better educated eyes, my goal may not be to restore it fully to factory, but I want to UN-do much of the hacking not "add" to it. These things are gigging amps and beat up, but they have to be reliable.

I just feel fortunate that even after having this amp "repaired" 20 years ago, it hasn't seen many hours...I would bet about 20 in all that time. With the bias running that "cold" I can hardly believe I didn't smoke it again with self-welding tubes. The big question for me is if they are voltage starved because the bias AC is so much lower than a stock PT but today I will disconnect the wire and measure whats coming out of the PT. A turret type board would certainly have made all of this easier and cleaner.

[donzoid]

Cameron, the AC voltage is 35.7 direct off the PT for this tap.

Note: I was just checking the un-used leads and one of them, a smaller diameter wire thats grouped with what I am assuming is the 220 or 240 leads (because of their larger diameters) is 250 volts. I suppose thats too high since stock calls for 100 (or so).

Since we keep coming back to the bastardized kluge PT, I thought for kicks I would post up some pix of it... perhaps the thing is traceable from the numbers? Or some of you experienced TAG folks have seen similar or identical ones?

The numbers printed on the top are:
T69743
and
H1928

The partially torn off label has
PAR...
Transf...
Mad(e)...

Now, 20 years ago I saw it in its original box, before he put it in. But my own behaviors back then make recall a bit "fuzzy"...no elaboration necessary yeah? I "think" it was a Sound City head. But it could have been Hiwatt, or some other brit type with block letter logo.

[martin manning]

That is a Partridge transformer. The problem is that 35VAC is on the ragged edge for generating the DC bias voltage you need.

[donzoid]

Hmmm...OK thanks Martin. Now that I know that (and that they were used in both Hiwatt and Sound City) perhaps this darn thing has some value to those owners/collectors. I guess I'd better start researching some options for a replacement. I've seen a lot of threads in here for it so that shouldn't be too difficult!

In the meantime, I have been looking at tube specs. While I expected the opposite, if I am reading the specs right there are other tubes that spec out lower bias voltages, and though -34 volts may seem to be "just enough" (or the ragged edge) its interesting to note that this PT can be found documented in other Sound City B100 models -- which ran 4 el34's. Not sure why they did it with such a tight tolerance but may try to figure that out. None of the SC schematics I just looked at show their intended bias volts...but a different quad of say kt66 or kt77 might work as-is...

[JazzGuitarGimp]

That is a Partridge transformer. The problem is that 35VAC is on the ragged edge for generating the DC bias voltage you need.

How about a voltage doubler? Bias circuit is a good candidate for a doubler since it has a very low current requirement.

[Cameron]

That is a Partridge transformer. The problem is that 35VAC is on the ragged edge for generating the DC bias voltage you need.

How about a voltage doubler? Bias circuit is a good candidate for a doubler since it has a very low current requirement.

Yes...a voltage doubler should do it.

http://hiwatt.org/Schematics/BiasCkts2.gif

[martin manning]

Sure a doubler would work fine, but that wouldn't be my choice. It would have to be kludged in there somewhere, and the bias circuit on the board would be abandoned, IMO adding to the mess. A Mag Components PT could be had for about $100, no? Sell the Partridge and recoup some or most of your cost.

[donzoid]

Well I can see there are several options. You guys are awesome. This is certainly proof I should never try to fix stuff just by cost cutting. I have looked at the MC site and even though it is not in the budget for the moment, these transformers seems to get good reviews on the site and are surprisingly affordable (sure I'd like to have a Merren, there's just no way...I have kids and last I checked they are still eating)



So if it IS possible to double voltage, would it also be possible to take the 250 volt (AC) un-used tap from this trans and step it down? Or would that also be another Kludge ?

[martin manning]

Using that 250 VAC tap is a good option since the circuit on the board can remain intact. Here's what I'd do:
Remove the jumper and piggy-backed resistor to return the circuit to its original values (15k, 15k, 47k, and 22k trimmer). Connect the 250VAC winding to the 15k resistor where the old bias tap was connected (the end oposite from the diode) but run it through an 86k 1W resistor first. That should get you close. Measure the bias range by rotating the trimmer stop-to-stop with no power tubes installed.

[renshen1957]

Well I can see there are several options. You guys are awesome. This is certainly proof I should never try to fix stuff just by cost cutting. I have looked at the MC site and even though it is not in the budget for the moment, these transformers seems to get good reviews on the site and are surprisingly affordable (sure I'd like to have a Merren, there's just no way...I have kids and last I checked they are still eating)



So if it IS possible to double voltage, would it also be possible to take the 250 volt (AC) un-used tap from this trans and step it down? Or would that also be another Kludge ?

Hi,

Yes you could do it, but is it worth the effort?

If this was my amp, I would take the bias from both sides of the B+, but I'd get rid of the PCB and replace it with a hand wired board. I hate PCB to work on as built by the major manufacturers, no secret. Less grief in the long run.

Edcorusa.com has some excellent transformers built to UL (underwriters lab) specs, I recommend these now that Hammond has become more expensive expensive. The prices aren't bad, too.

Some of Hammond's filament transformers and other step down xformers carry a made in China label. I have nothing against Chinese made parts but I don't want to pay Canadian prices for these especially when Hammond doesn't disclose the origin beforehand.

You might check out tedweber.com. Yes the weber brand is made in China, but if money is a consideration it's another alternative. I would spend the extra for MC or Edcor as Mercury Magnetics isn't an option.

Best regards,

Steve

[donzoid]

OK...so it looks like I have both a short-term and a more permanent fix. Especially if I go back to 6550/KT88 or a larger bottle tube. Even if I go with a kt66 or 77, e34L or 6CA7 for that matter.

Short-term I see the advantage of adding the one 86K resistor (if needed) that Martin suggests to tweak the bias value into "just the right range" after removing all that kludge piggybacking and bypass crap. Or, I could do the B+ thing.

Long term, Ren is right I won't want to be keeping this PT in there as it's likely affecting the tone, and would be far better with a "stock type" replacement from any one of several vendors such as the M-6 upgraded one from MC or Weber, etc.

[Firestorm]

I don't think Martin is saying to "change" a resistor, just put it in series with the unused tap, right on the wire, heat shrinked, etc. From there you should be able to leave the board stock (remove the piggyback and the short on the first R).