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

[Cameron]

Just to add to bias ideas.....you can also get the bias from the filament supply by using a 6.3 volt transformer backwards. Run the 6.3 volt filament from the amp to the 6.3 volt side of the new transformer....out comes 120 volts AC from the primary when you use a 120/6.3 volt transformer.

[donzoid]

Thanks Cameron...almost identical suggestion from Firestorm. That's really sounding like the way to go until I chip for a replacement PT. Or, Martin had a suggestion also for some tweaks to make the bias more like they did on the 50w JMP. This think is SO kludged, I have pretty much made up my mind to bench it, remove the PCB and check EVERYTHING, cause I have no intention of getting rid of her, and want it to be right. That should be good prep for my first amp build.

So, why would I want to put a diode bridge rectifier after the transformer as Firestorm suggested? Would that be a safety feature? Just curious since as Cameron says its AC coming out (and the original bias tap is also AC)?

[Cameron]

Thanks Cameron...almost identical suggestion from Firestorm. That's really sounding like the way to go until I chip for a replacement PT. Or, Martin had a suggestion also for some tweaks to make the bias more like they did on the 50w JMP. This think is SO kludged, I have pretty much made up my mind to bench it, remove the PCB and check EVERYTHING, cause I have no intention of getting rid of her, and want it to be right. That should be good prep for my first amp build.

So, why would I want to put a diode bridge rectifier after the transformer as Firestorm suggested? Would that be a safety feature? Just curious since as Cameron says its AC coming out (and the original bias tap is also AC)?

The usual fix for this is the voltage doubler..... you can look up "Hiwatt bias" on Google and read about your transformer and bias circuits.

I didn't see his post .....but...with the transformer you could do it either way..with the bridge rectifier.... as he was saying .... or just wire it up like the stock circuit and adjust the 27k resistor. This transformer way also has lower impedance too....which is good! Getting it from the B+ .....will be higher impedance.

[Firestorm]

Just one more thought to confuse things: to the extent that Partridge transformers had bias taps at all, it was usually a separate winding. I found one reference to this PT showing up in a SoundCity 120, which schematic shows this. Are there two purple wires, one grounded, one going to the bias supply? If so, you could rectify with a FWB and get a larger bias voltage. It's only the difference between ACx1.1 and ACx1.4,so it may not be enough anyway.

[donzoid]

Hmmm...most of the secondary wires coming out of the PT are "basic black" as if they had been re-sleeved...but I will investigate this today and see if I can suss that out.

Yeah these transformers are supposed to be pretty high-grade ones for their day, re-purposed from Hi-Fi applications. So I'm not concerned about the quality of the thing...after all its lasted 30 years and two installations! Nyuk Nyuk... would I check that using AC +/- volts ? I would think so.

[martin manning]

Just one more thought to confuse things: to the extent that Partridge transformers had bias taps at all, it was usually a separate winding. I found one reference to this PT showing up in a SoundCity 120, which schematic shows this. Are there two purple wires, one grounded, one going to the bias supply? If so, you could rectify with a FWB and get a larger bias voltage. It's only the difference between ACx1.1 and ACx1.4,so it may not be enough anyway.

Separate winding or not you could use either HW or FWB. The only difference is going to be in the ripple for a given level of filtering with the FWB, which will be offset by an additional diode drop. The net effect will be maybe a couple tenths of a volt.

[Firestorm]

Ah yes, ignore what i said. Martin is right. The different charging cycle times don't matter unless there is a lot of current drawn, which there isn't. Back to the previous fixes. I think I'd lean toward a doubler as easiest to implement.

[donzoid]

OK...that works!

[donzoid]

Over the weekend I "couldn't help myself" on Sunday...took the PCB board and front pots down so I could flip it over and inspect. Frightening! I've found two obviously bad solder joints (oh what a surprise) a lifted corner of a trace, and almost all of these and four other highly suspect spots almost all in the bias circuitry. I can see why some of you guys are netting $1.38 an hour...it just gets deeper and deeper!

Anyway now that its flipped over and I am going to re-flow all the joints on this end of the board... I thought for the range resistor (the one right next to the bias trim pot) I would either solder in some bus bar wire or other good grade wire to allow me to play with this resistor value or install a pot without further risk to burning, lifting traces, or having to flip it. Would this be a fair idea, or would I just be better off putting in the el34 resistor? With the "wire" option I could also put in a pot... My Gosh I know this doesn't even address the bias feed voltage issue/PT yet, but hey I may as well get experienced input and do this thang right. If I had the tools I think I would put turrets there. Or perhaps I haven't thought of the best option because of my lack of experience. Anyway later this afternoon at the very least I will have the thing re-flowed, and I'm going to see if I have a 56K resistor lying about. This darn thing has now become my defacto test bed.

[renshen1957]

Over the weekend I "couldn't help myself" on Sunday...took the PCB board and front pots down so I could flip it over and inspect. Frightening! I've found two obviously bad solder joints (oh what a surprise) a lifted corner of a trace, and almost all of these and four other highly suspect spots almost all in the bias circuitry. I can see why some of you guys are netting $1.38 an hour...it just gets deeper and deeper!

Anyway now that its flipped over and I am going to re-flow all the joints on this end of the board... I thought for the range resistor (the one right next to the bias trim pot) I would either solder in some bus bar wire or other good grade wire to allow me to play with this resistor value or install a pot without further risk to burning, lifting traces, or having to flip it. Would this be a fair idea, or would I just be better off putting in the el34 resistor? With the "wire" option I could also put in a pot... My Gosh I know this doesn't even address the bias feed voltage issue/PT yet, but hey I may as well get experienced input and do this thang right. If I had the tools I think I would put turrets there. Or perhaps I haven't thought of the best option because of my lack of experience. Anyway later this afternoon at the very least I will have the thing re-flowed, and I'm going to see if I have a 56K resistor lying about. This darn thing has now become my defacto test bed.

Hi Don,

I'd vote for the bias pot. Tubes age and the bias needs to be adjusted with the next set tubes, even if these tubes are the same type. Marshall used a compromise resistor because that's cheaper than a pot. Garnet amps favoured the resistors from a techs viewpoint how many people know how to bias an amp or have an O scope etc.

I don't deal with PCBs anymore unless to replace them, not worth the frustration and it's economic suicide for the time as others noted.

Best regards

Steve

[donzoid]

Steve, thanks and yes it seems to make sense to have it adjustable. So would I replace that function in the current trimmer, or in place of the range resistor (which if I am reading right from other threads is the one inline with the trimmer, just before it)? It seems the right thing would be replace the resistor, with a 60K pot and drill a hole in the top of the chassis and mount it there.

Of course being as this is negative voltage, and I am feeling dyslexic about it when I am ready someone is going to have to treat me like a 5 year old and tell me "where" to place the three connections...

[Cameron]

Over the weekend I "couldn't help myself" on Sunday...took the PCB board and front pots down so I could flip it over and inspect. Frightening! I've found two obviously bad solder joints (oh what a surprise) a lifted corner of a trace, and almost all of these and four other highly suspect spots almost all in the bias circuitry. I can see why some of you guys are netting $1.38 an hour...it just gets deeper and deeper!

Anyway now that its flipped over and I am going to re-flow all the joints on this end of the board... I thought for the range resistor (the one right next to the bias trim pot) I would either solder in some bus bar wire or other good grade wire to allow me to play with this resistor value or install a pot without further risk to burning, lifting traces, or having to flip it. Would this be a fair idea, or would I just be better off putting in the el34 resistor? With the "wire" option I could also put in a pot... My Gosh I know this doesn't even address the bias feed voltage issue/PT yet, but hey I may as well get experienced input and do this thang right. If I had the tools I think I would put turrets there. Or perhaps I haven't thought of the best option because of my lack of experience. Anyway later this afternoon at the very least I will have the thing re-flowed, and I'm going to see if I have a 56K resistor lying about. This darn thing has now become my defacto test bed.

Take a beep breath and calm down.....First.... fix the amp!! You may add to the issues even if you are trying to fix other things.

Just fix the bias.....in this amount of time you could have done the voltage doubler. At least try one of the ideas here or this tread is a waste of time.
Don't resolder if it doesn't need it .........if it does...... use a solder sucker and don't heat the circuit board too much. If you use too much heat...the traces will come up.....the circuit board is not a problem you just need to know how to work with them....I have fixed and modified allot of circuit board amps and never had to put in a PTP board unless it was bad or someone that did know what they where doing and killed it.
If you are in LA I could have that amp up and running right in an hour.

[Cameron]

also...

I'm going to see if I have a 56K resistor lying about.

Why...you don't have too much voltage........bypass the first resistor going to the bias diode....see what bias you get...it should be higher....if its still not enough....bypass the 15k resistor above the bias trim....see what voltage you have and let us know.

[Cameron]

Over the weekend I "couldn't help myself" on Sunday...took the PCB board and front pots down so I could flip it over and inspect. Frightening! I've found two obviously bad solder joints (oh what a surprise) a lifted corner of a trace, and almost all of these and four other highly suspect spots almost all in the bias circuitry. I can see why some of you guys are netting $1.38 an hour...it just gets deeper and deeper!

Anyway now that its flipped over and I am going to re-flow all the joints on this end of the board... I thought for the range resistor (the one right next to the bias trim pot) I would either solder in some bus bar wire or other good grade wire to allow me to play with this resistor value or install a pot without further risk to burning, lifting traces, or having to flip it. Would this be a fair idea, or would I just be better off putting in the el34 resistor? With the "wire" option I could also put in a pot... My Gosh I know this doesn't even address the bias feed voltage issue/PT yet, but hey I may as well get experienced input and do this thang right. If I had the tools I think I would put turrets there. Or perhaps I haven't thought of the best option because of my lack of experience. Anyway later this afternoon at the very least I will have the thing re-flowed, and I'm going to see if I have a 56K resistor lying about. This darn thing has now become my defacto test bed.

Hi Don,

I'd vote for the bias pot. Tubes age and the bias needs to be adjusted with the next set tubes, even if these tubes are the same type. Marshall used a compromise resistor because that's cheaper than a pot. Garnet amps favoured the resistors from a techs viewpoint how many people know how to bias an amp or have an O scope etc.

I don't deal with PCBs anymore unless to replace them, not worth the frustration and it's economic suicide for the time as others noted.

Best regards

Steve

???? WTF Marshall amps have a bias trim pot in the amp...not just a resistor. Boogie and Peavey and others have just a resistor.

[donzoid]

Thanks Cameron...at the very least, for the "take a deep breath" part. OK I understand your questions and the return to focus on the bias. What I am seeing though, may affect the readings I was getting because:
-One end of the bias diode is clearly a cold solder joint and broken.
-The bias range resistor (right before the trimmer) is highly questionable on one end.
-Granted I thought lowering the resistance would get me closer to the range I need as you say -- I was using the 56K value called out on the schematic...and I see your point that would take me in the wrong direction. I have little experience breadboarding, but think I may be able to do the filament supply/transformer "in reverse" thing...but I have to get one first. I just wanted to correct the cold solder joints, and re-measure it to see if it shifted the values before moving back to the bias voltage feed. Then I will do what you suggest and bypass another resistor to see where it gets me. This will take a while but I appreciate it.