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Old 14th April 2012, 04:30 AM   #21
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Join Date: Nov 2007
Location: Dallas
Oddly enough, I just did this same amp and saw the same thing...
The problem was the grounding switch, when grounded to black.
Old wiring had no preference of polarization, and was dangerous.

The cord needed replacement anyways, so I opted to ground on
third green wire. I used 2 diodes in series, and another two in
parallel going the opposite way, so there would be 1.4V of lift.
I made sure to use big enough diodes that the fuse could blow.
2A fuse means you are using at least 3A diodes or bigger...
You can use a modern diode bridge, if you wire + and - together.
Put one AC to green ground, and the other AC to the chassis.
I do mean only AC marks on the bridge, not any actual AC....

I disconnected the option to ground on black. Can now ground
only on white, or the default open ended option is now diode lift.

That stopped the crazy high voltages I was measuring.

There is a 50uF/50V on the diode bridge board (actually piece
of strangely treated cardboard, maybe asbestos? I don't know).
That one is the negative bias. Should measure -57V at the cap,
and -45V at the tube (or the center tap of the balance pot)...
Let me clarify this is not the same diode bridge as the one
I mentioned earlier for grounding, entirely different things...

Since they got a 50V cap doing a 57V job, and this one was
clearly leaky (measured only -22V at the cap). I put a pair
of 200V 33uF panasonic cap (105C rated) in its place... This
was 66uF, closest I could make with parts I had on hand...

As for re-capping the topside, there is some sort of cover
for the caps, so any new ones need the same shape and size.
I pushed the old (Mallory) caps out of the paper tubes and
put 105C Nichicon's in place. Inside the original paper tubes.

I used two in parallel, but physically bottom to bottom with
a solid core wire hanging out each end of the paper tube.
I insulated enough to get past the metal can of the caps.
I siliconed the caps into the paper tubes, which is easier
undone than it sounds, cause this paper don't want to stick.
And the tops of both new caps in each tube are exposed
in case they need to burst for any reason...

Oh what else? I think I might have rewired the fuse to
make sure it was on the hot (black) side. And but spliced
two of the original wires to make longer (remember one
less was needed for the grounding switch). I re-used all
the original wire without trimming anything, just incase
someone ever needs to put back like original...

If you test this upside down and use gator clips for the
speaker load. Remember the first speaker jack has to
be propped open, else it shorts out the transformer...
This is safer than transformer open, but not as safe as
having the right load attached. The 6L6 tube nearer the
transformer will get crazy hot if the jack isn't propped.
I don't know why the overheating isn't symetrical...
If you plug only into the "external" jack, it doesn't do
anything to remove the safety short.. I don't know
what is the label on the first jack, cause it had been
scratched out by some idiot...

Last edited by kenpeter; 14th April 2012 at 04:59 AM.
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Old 14th April 2012, 03:20 PM   #22
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i'm just a novice at this, if i can't find a simple way, i may have to take it to a pro. [DARN]
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Old 14th April 2012, 06:27 PM   #23
MelB is offline MelB  Canada
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Cowboy I'm grasping at straws. Is it wired transformer to resistor to diode to (negative terminal of capacitor, pot and 10K resistor)? This should be simple???

Unsolder from the pot and see what you get. So it's just resistor-diode- cap in your little circuit. Should be -60v or more. (Standby switch OFF for SURE!)
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Old 14th April 2012, 06:59 PM   #24
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mel: yes you are right on the circuit, with pot out of circuit i have -68 vdc, with pot in circuit it drops to -29.9 vdc.
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Old 14th April 2012, 07:45 PM   #25
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That potless measurement might be RMS DC, like
with no cap at all (if the cap died open somehow)...
It does not mean it is steady at that voltage, unless
you measured on a scope or something fast like that.
I got -22VDC here, till I recapped, then got -57VDC.
This was measured with pot connected in both cases.

The cap has to hold down the negative volts through
90% or more of the power cycle. Cap only charges
briefly at the most negative peak. The circuit might be
dragging your cap down too easily during the rest of
the cycle. If the cap is not holding a charge, or open,
or high internal resistance (dried out inside).

And since you see -68V, and I saw -57V. We have
to ask why this cap is only rated 50V? Should we
be replacing with something rated slightly better
in the voltage department? I used 200V Panasonic.

------

You still need to make sure your grounding switch is
NOT somehow in the position that grounds black to
the chassis through a small value capacitor hidden
underneath the switch. Especially if you have white
or green safety ground hard wired (or even with a
diode lift) to the chassis.

Black ground caused all the crazy volts I measured.
I saw unexplainable things over 800V, and beyond...
Might also have contributed to electrolytic cap death.
The cap under the ground switch could be leaking?
Just cause its not elko, doesn't mean lasts forever.

Since I got rid of black as a grounding option and
started using a polarized three prong plug, it has
ceased to be a problem. Even if the grounding cap
leaks a little, it will no longer cause any harm.

Last edited by kenpeter; 14th April 2012 at 08:10 PM.
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Old 14th April 2012, 08:15 PM   #26
MelB is offline MelB  Canada
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I think cowboy said he just put in a new cap. Just his luck it's maybe leaky?? I'd try a new cap, yes with a higher voltage rating. That surely won't hurt. Ken I agree with your ground theory. Cowboy do you have a three prong plug on your amp?
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Old 14th April 2012, 08:24 PM   #27
MelB is offline MelB  Canada
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I can not find a value for the bias cap on any schematics. It's value should not be changed as that will change the bias voltage. What is it's official value?
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Old 14th April 2012, 10:20 PM   #28
MelB is offline MelB  Canada
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Here's something to try. With amp off and unplugged how many ohms do you measure from the pot side of the diode to ground?
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Old 15th April 2012, 02:00 AM   #29
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Original was 50uF/50V. I replaced with 2x33uF/200V/105C,
cause those were the best and closest match I had on hand.
Obviously two caps in parallel don't look very authentic...

With 66uF in total, I measured -47V at the point on the
schematic that indicates -45V. I didn't sweat the 2V diff,
since was slightly over on the safe side, and sounded great.
The voltage on the cap itself was -57V.

Last edited by kenpeter; 15th April 2012 at 02:21 AM.
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Old 15th April 2012, 04:30 AM   #30
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to answer your questions, yes i put a new cap in, the cap i took out was a 50uf- 50v, schematics value is 50uf-50v, pot side of diode to ground 21ohms, 65 bassman has 2 prong plug. looks like i bought a bad cap. i guess i will try another 50uf-50v. i am using blue atoms
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