Amp repair power wiring from Hell

Jewellworks

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ive got my first amp repair job on the bench. it was first brought in because he was complaining bout a god awful Hum. he had fussed with it forever and finally gave up and brought it to me to look at. turns out, that was the easy part. the ground wire had popped off the filter caps from the biasing diode. put it back on, no more hum.
BUT
he wants me to look it over. theres been some hacking in here.

this amp is a Granger (?) KIT of a Marshall 50W Plexi. at first, i assumed it was the 1987 circuit. seems reasonable with the solid state rectifier, 4 input jacks, ect. im still holding that to be "mostly" the case. but the power section is wired very weird...
the schematic shows the Standby switch directly off both legs of the PT 2ndary, on a DPDT Switch. so basically, the heaters come on w the Mains, and the bias has its own tap off the PT, and it continues on to the diode and caps, ect... and feeds the grids of V4 and V5. nothing else is powered unless you close the Standby switch.

this one is wired very differently...

with the standby switch open, Mains ON, you get heaters, the bias, and both legs of the 2ndary continue on to the rectifier. once past the rectifier, it feeds the 1st filter cap (2, 50uf's together for a total of 100uf) then stops. the Standby switch is a SP and its fed 550VDC off the rectifier. once closed, it feeds an HT fuse, the choke, and the rest of the circuit...

1st of all... 550VDC on that switch? that doesnt sound right. if the DP switch was there, each would be getting really high AC voltage. so is it hurting anything to have THAT high DC running through that switch?

this amp also has test points for V4 and V5, off the cathode, which goes to ground through a 1ohm resistor. im not sure what that tells me.
 
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2L man

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this one is wired very differently...

with the standby switch open, Mains ON, you get heaters, the bias, and both legs of the 2ndary continue on to the rectifier. once past the rectifier, it feeds the 1st filter cap (2, 50uf's together for a total of 100uf) then stops. the Standby switch is a SP and its fed 550VDC off the rectifier. once closed, it feeds an HT fuse, the choke, and the rest of the circuit...

1st of all... 550VDC on that switch? that doesnt sound right. if the DP switch was there, each would be getting really high AC voltage. so is it hurting anything to have THAT high DC running through that switch?

this amp also has test points for V4 and V5, off the cathode, which goes to ground through a 1ohm resistor. im not sure what that tells me.
If you read user manual I believe there is instructed to switch Main switch Off before Stby, when powering down the amp and amp operation depletes electrolyts. Obviously it is also accepted that eventually Stby switch fails? When it usually happen after the warranty it bring income to amp techs ;)

DC current wear switch contacts when switching Off because there stay spark even the physical "zero ohm" contact is open. "1mm spark for each 1000V" is a rule what voltage can "jump when circuit builds". Switching Off spark can continue longer, I don't remember was it 300VDC/mm or what?

When switching power On there is not much difference between AC and DC. Slight difference make DC worse here is when voltage is always 500V when in AC case sine voltage averages ~350V.

I have seen schematics and also installed 100k resistor over DC-Stby switch contacts which pre charge voltage to about 100V when mains is On while tube filaments get hot. Obviously this also kill contact spark when switching Off and also make speaker "pop" quieter? This Stby low current should also prevent carhode poisoning?
 

Jewellworks

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So the question about how this power is wired remains. Good ? Bad? Wrong? Doesn't matter much??

Then there's the biasing:
i put my meter across the test points, set it to mA, and i get nothing. i set it to DCV and it reads 17.9mV. ive never had an amp w bias test points, OR a Fixed Bias amp, so I have no idea what this means.

when i measure the old fashioned way, plate to ground, screen to ground, and grid to ground, ive got:
a: 419
g2: 414
g1: 40.3

the vtadiy.com/loadline-calculator shows that to be 24mA.

1) what am i looking at on the test points?
2) i need to raise the mA to about 40, and lower the bias voltage to about 20? but that will lower the plate voltage by quite a lot. ive seen examples where this should be about 460v @ 42v. so is something wrong?
 
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nathan5782

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I've built a couple of kit amps and repaired some black face Fenders and DIY maintenance but I'm not an amp tech. Let me help with one of your questions. The Bias test point in mV can be read in mA because it's coming off a 1 ohm resistor, you should be able to adjust the bias pot to get the desired range. The link below might help.

 

Lowerleftcoast

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when i measure the old fashioned way, plate to ground, screen to ground, and grid to ground, ive got:
a: 419
g2: 414
g1: 40.3 the vtadiy.com/loadline-calculator shows that to be 24mA.
This method may *estimate* what we want to know but it does not *measure* the plate dissipation.
this one is wired very differently...

with the standby switch open, Mains ON, you get heaters, the bias, and both legs of the 2ndary continue on to the rectifier. once past the rectifier, it feeds the 1st filter cap (2, 50uf's together for a total of 100uf) then stops. the Standby switch is a SP and its fed 550VDC off the rectifier. once closed, it feeds an HT fuse, the choke, and the rest of the circuit.
It sounds like you are describing the standby switch placement in a typical Fender from the Blackface era.

Since this is a kit build, I see no need for a standby switch. You will have to talk with the owner to know what his wishes are. The options are to: 1) leave it be, 2) place a 100k to 220k resistor in parallel with the switch (helps prevent *pop* and always has some current flowing, longer switch life), 3) remove the high voltage and use the switch to mute the signal (still able to use the switch to change guitars or leave for break time), 4) remove the switch and do whatever the owner wants with that space.

EDIT: I see this thread is a week old. You may have this solved already.
 

Paul G.

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So the question about how this power is wired remains. Good ? Bad? Wrong? Doesn't matter much??

Then there's the biasing:
i put my meter across the test points, set it to mA, and i get nothing. i set it to DCV and it reads 17.9mV. ive never had an amp w bias test points, OR a Fixed Bias amp, so I have no idea what this means.

when i measure the old fashioned way, plate to ground, screen to ground, and grid to ground, ive got:
a: 419
g2: 414
g1: 40.3

the vtadiy.com/loadline-calculator shows that to be 24mA.

1) what am i looking at on the test points?
2) i need to raise the mA to about 40, and lower the bias voltage to about 20? but that will lower the plate voltage by quite a lot. ive seen examples where this should be about 460v @ 42v. so is something wrong?
You can't measure amps "across test points" as you are parallel to current flow. I'm assuming the test points have a 1 ohm resistor so when you measure the voltage drop across (in mV), ohm's law is telling you the mA across those resistors [I = V/R] where R is 1 ohm so I = V. If there are test points for each tube, Your plate to cathode is 414V, so you want to see something like 40mV for EL34s which equals 40mA across the 1 ohm resistor.
 

Paul G.

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This method may *estimate* what we want to know but it does not *measure* the plate dissipation.

It sounds like you are describing the standby switch placement in a typical Fender from the Blackface era.

Since this is a kit build, I see no need for a standby switch....
Solid state rectifier. Keep the standby switch. I don't see an issue with the way it is wired.
 

Jewellworks

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just to update this thread a bit, i had posted a biasing issue with this amp in another thread, and along the way discovered the center tap for the HT and the Bias tap were reversed. which threw the voltage swing on the PT off by 50v on each side of the CT, for a 100v difference total. once i corrected this, the voltage off the rectifier (feeding the standby switch) dropped from 550 to 490. -still high, but not 550 high...
as of now, im leaving it alone. ive considered adding a dropping resistor to lower the voltage, because im biased at 466pv @ 35mA at a 3.2k reflected load. looking at it on the vtadiy.com/loadline-calculator, the Class B line is shooting pretty well past the Max PD. i understand thats "ok", because its class B, but i was thinking 445 or so would cool it down maybe a little...?
doubling the reflected load seems like an easier option. put an 8 ohm load on the 4ohm tap, and everything tilts to the left and runs cooler, and may increase overall gain. i can even bias up a little more. say 40, 42mA....
but ill probably just leave it alone.
 

wabashslim

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ive got my first amp repair job on the bench.
Since some common guitar amp practices are puzzling to you you might consider taking this to an experienced tech to go through it and then explain to you what's what. Sounds like this amp would be a good candidate for a thorough physical.
 




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