Marshall Valvestate 8040 Quiet Boost Channel

[x-pro]

Earlier, in post 3, your autotransformer produced a voltage of 80V. Where did they go?
It is likely that the copper winding wire does not have reliable contact with the transformer terminal.
I'll say it again. There are no miracles in electronics.

[chilidawg]

Earlier, in post 3, your autotransformer produced a voltage of 80V. Where did they go?
It is likely that the copper winding wire does not have reliable contact with the transformer terminal.
I'll say it again. There are no miracles in electronics.

The auto tx still does that. Here I supplied 10VAC at the input, and it outputs 72VAC. In post #3, it output 80VAC, and I had 40V DC at HT pins, now it doesn't. 72VAC is as high as it goes.

And yes I know there are no miracles in electronics. I repair complex electronics for a living, mostly computer mainboards, graphics cards and SMPS. Occassionally I also repair audio stuff from amplifiers to digital audio consoles.

[x-pro]

This is where there may be a bad solder contact. Something will be induced during the measurement. As soon as you connect the filter capacitor, it charges. A charging current is created. The voltage drops.

[g1]

From pin that connects to GROUND and to the pin that goes to the diode, resistance can't be measured (meter reads OL).

The autoformer is bad. Somehow when you connect your meter you are supplying an alternate charging path but the the winding is open. Sometimes you might get lucky and find a broken wire right at the connecting lug, but otherwise you will have to replace it or do without the tube section.

[chilidawg]

The autoformer is bad. Somehow when you connect your meter you are supplying an alternate charging path but the the winding is open. Sometimes you might get lucky and find a broken wire right at the connecting lug, but otherwise you will have to replace it or do without the tube section.

Ah, now that makes sense! The meter is providing a path to ground. Maybe something as silly as a digital AC voltmeter display in between these two pins can have the same result?

What is the expected resistance between these two pins if the winding is not open? When I pulled out the auto tx, I did not see broken wires so maybe I can still save it.

In the case that I can't save it, will it be possible to replace its function by using a TRIAC to lower the main AC voltage (230V here) to 125V and feed this to the diode?

[Helmholtz]

Problem with measuring the DCR of a transformer is large inductance.
Many DMMs just read OL when the inductance is too large.
An analog (needle) meter would work.

Using a triac to lower AC voltage would introduce lots of switching noise.

[g1]

In the case that I can't save it, will it be possible to replace its function by using a TRIAC to lower the main AC voltage (230V here) to 125V and feed this to the diode?

Agree with x-pro that triac will create switch noise issues.
You think 125VAC would be enough there? I was thinking maybe double that as they are using a 350V cap after R64 so I figured probably around 300V there.
Very little current needed, a low current 1:1 isolation transformer off the line AC would probably do the trick?

[x-pro]

Any transformer with 110/10(12)V or 220/10(12)V windings can be used, with the 110V(220V) winding matching the current..
Backwards, of course.

[chilidawg]

Agree with x-pro that triac will create switch noise issues.
You think 125VAC would be enough there? I was thinking maybe double that as they are using a 350V cap after R64 so I figured probably around 300V there.
Very little current needed, a low current 1:1 isolation transformer off the line AC would probably do the trick?

It seems the auto tx boosts the incoming voltage by 8 times. In post #3 when the winding was not open, with 10V AC in, it gave 80V AC out, and 40V DC at HT pins.

With 17V AC in, which are the secondary taps of the amp's main tx, I get from 125V AC to 136V AC out. Not sure about the maximum DC voltage as I have only touched the auto tx output pin with the meter probe for no longer than a minute or two. I guess I will try that for 10 minutes and report back.

EDIT: Maybe using a small 0.5A 220VAC to 18VAC transformer in reverse?

[chilidawg]

Gave it a try with a 300mA 240V AC to 24V AC step down transformer that I salvaged from an old power supply adapter. Wired it in reverse. With 10V AC in, I got 100V AC out and about 136V DC at HT pins! so yeah, this confirmed that the auto tx is bad

Thanks for the help guys! Really appreciate it.

[g1]

With 17V AC in, which are the secondary taps of the amp's main tx, I get from 125V AC to 136V AC out.

Those sound about right. The voltage readings I found elsewhere were 163VDC at pin1 of the tube, and 96V at pin6, which would mean about 180VDC coming off the rectifier diode.
https://www.ssguitar.com/index.php?msg=25556

[chilidawg]

Those sound about right. The voltage readings I found elsewhere were 163VDC at pin1 of the tube, and 96V at pin6, which would mean about 180VDC coming off the rectifier diode.
https://www.ssguitar.com/index.php?msg=25556

Ok, thanks! After a bit of calculation, I think a 240V/220V to 30V step down tx is more suitable. This gives an 8 and 7.33 multiplier when wired in reverse, so with 17V AC in I can choose either 136V AC or 125V AC out

[g1]

so with 17V AC in I can choose either 136V AC or 125V AC out

Just be very sure about that 17V. If you were measuring anything to ground, instead measure across half the PT secondary winding, with tube installed (for heater loading).

[chilidawg]

Just be very sure about that 17V. If you were measuring anything to ground, instead measure across half the PT secondary winding, with tube installed (for heater loading).

Got it. At the moment I only have that 300mA 1:10 step down transformer that I wired in reverse, which gives me 165V AC at D3 anode and 208V DC at the HT pins without the tube installed.

I'm tempted to install the tube. I know that 12AX7/ECC83 nominal HT supply is 250V, and it can be run at a higher voltage as well but what I'm not sure if it's ok with it at 200V in this circuit, though I'm guessing it will drop with the tube loading it.

[chilidawg]

Ok, I put the tube in.

Here are the results.
11.83V AC across heater pins, didn't take a picture.
161V AC at D3 anode.
184V DC at HT pins.

I think it should work fine now.

UPDATE: Added a short video. Tube is alive!!!

https://we.tl/t-EesqrcbfOb

Thank you all!