marshall jtm 45 doesn't see bias right

[stukidboy]

The bias is correctly connected, but it isn't showing the right information
I've done this a LOT and this is my first bias problem, and it is weird.

first question....caps facing the right direction Positive to ground.....yes
all resistor values check on my Fluke meter



1) won't adjust
50k pot checked (adjusts)
caps are 8Uf 150v

2) using tap from gz34 220k resistor to diode (1n4007)
diode to resistor and caps, pot, etc

3) changed resistor values and nothing makes a difference, pot does not adjust anything


I keep wanting to think ground issue
But I have been over and over it.

may need another pair of eyes, mine may be seeing things by now.

voltage from rec tube is 334vac
voltage on the other side of the 220k is 75 vac
voltage on the amp side of the diode is 294
6 vdc
equating to -294vdc on the bias supply, going to the 220k resistors (divider)

gz34
pin 8 to b+

pin 6 to bias tap

pins 8/2 b+ or 325 0 325 taps
pins 6/4 0 and 5v taps

what would keep the voltage from adjusting


again, my mind want to drift to a grounding issue

[stukidboy]

please ignore the 6vdc after the 394 vdc
thanks

couldn't find "edit post"



sorry

[sluckey]

Schematic? Layout? Hi-rez pics?

[wpaulvogel]

Sounds like there’s no ground connection at the positive ends of the bias capacitors (8uF caps).

[pompeiisneaks]

couldn't find "edit post"
sorry

It's the pencil icon to the top right...

~Phil

[stukidboy]

I checked that ground, but I may tend to agree.

I will recheck the solder joint on the ring lug on the pt.

schematic is actually 5F6A with dual caps, not singles

except 5F6A has bias tap from the PT

This marshall PT does not, so I tapped the gz34 and even tried the incoming ac (lower voltage)

nothing changed

like I said, I suspect a bad ground 'somewhere', but have called it a night at 9:45
had ribs on the grill
My 4 legged Rocky, loves his rib bones......lol


Thanks for your insight

now...if I don't SAY thanks, I always mean to.....
don't think I don't appreciate all the help I get.
There is always someone smarter (better, etc)
But I am one that actually appreciates people's help in remembering what I have already forgotten, or never knew

[wpaulvogel]

An easy way to determine if the ground is present at the positive end of both bias caps is to measure voltage from a known good ground point on the chassis to the positive ends of the capacitors. If you have no voltage drop (0 volts), the ground is present. If this is the case, work back to the pot, then the 47k. Voltage will be dropping through the voltage dividers because current is moving through the resistances.

[stukidboy]

I thought I would post the solution, which was ,as I said, or suspected, a grounding issue.

took my eyes a minute or 2 to see it

I could see something odd,.....but.......

I had the negative bias and ground backwards at the adjustment pot (50k pot)
[yeah....go figure that one out...sadly he says....]

"haoow did I do Thait?

Only time I can remember doing this

must have been some good stuff, but hopefully it was after 5 p.m.....!

lol!

[stukidboy]

on a worse note, I was building a marshall 200 watt and fried the 1st power tranny.


cause:?

shorted capacitor, in the first stage of the bias supply.
sprague 8uf 150v dead shorted



luckily, and really luckily I found 'that' issue before I fried tranny 2

at 600 bucks a pop, I can't afford to fry too many.

only transformer I ever fried in 40+ years

only time I ever got the bias screwed up, either.

never take things for granted.

[Stevem]

Fusing every secondary supply feed off of a power transformer of that size/ cost is very cheap fast acting protection!

[TUBEDUDE]

I usually make those efforts after a spectacular event. In hindsight it's always obvious protection was prudent.