Marshall Valvestate 8100 low heaters and high 15v rails

[Helmholtz]

Measure if the voltage drop across the 180R resistors is larger than 27V.

BTW; what is the AC supply voltage at the PT secondary?

12AX7s heater are intended to be suppied from a constant voltage source. Means the Marshall circuit is no good practice.

[J M Fahey]

Means the Marshall circuit is no good practice.

How come?
Accountants *love* it.

Imagine saving 1 buck per amp, .... in a Million amps it will mean

[Indyryder02]

Seen that problem at DIY Audio and somebody was intrigued enough to do a lot of measuring.

Yours is a real problem, and of course Valvestates left Factory working properly.

What this guy found (by **measuring** none less) is that although all 12A*7 work with 12.6V across pins 4 & 5 THEY DO NOT HAVE THE SAME COLD DCR depending on brand.

Russian/Sovtek ones (what Marshall used) have significantly higher resistance, at least when cold.

IF you feed filaments from a constant voltage source: meaning a transformer winding OR a regulator, say a 7812, no problem, when hot all pull 150mA

IF fed from a constant current source,same thing, fixed 150mA will force them to heat up , in which case they will drop 12.6 Volts.

BUT if designer is cheap, and it´s an SS amp, some avoid the extra 6.3V or 12.6V winding by feeding filament from existing power transformer mid-voltage winding (here 30+30VAC) through dropping resistors.
Cheap ass solution.

Problem is resistors are neither a constant voltage source nor a constant current one, but a voltage divider (resistor + filament resistance which is highly nonlinear and PTC), so it will work properly ONLY with the same type and brand of tube the original designer used, here a Russian 12AX7 ¨Period.

Any other brand/origin tube plugged there will give unpredictable results.

This guy measured many brands, and Russian/Sovtek *always* had a much higher DC resistance than any other.

So others will never reach proper 12.6V needed, filaments will be cold by a significant amount, and dropping resistors will heat up more ... a perfect storm.

I suggest you try a Sovtek/Rusian tube there.

Ok, finally got a chance to look at this amp again. Installed proper Zener diodes. That’s normal again.
installed a Sovtek preamp tube. It works, but no change in heater voltages from one tube to the other.. (I was really hoping for this to prove out too… maybe it was older Sovtek tubes? I have seen a huge fluctuation in these over the last several years. Idk. )

[Indyryder02]

Measure if the voltage drop across the 180R resistors is larger than 27V.

BTW; what is the AC supply voltage at the PT secondary?

12AX7s heater are intended to be suppied from a constant voltage source. Means the Marshall circuit is no good practice.

Following up on this.
no change in heater filaments based on using different tubes thus far.. Chinese to Russian etc.

across 180r resistors the voltage drop is about 25.7. With 61.5v on the secondary. The 15v supplies are normal so I believe it would be isolated to the heater side.

It does work currently. The boost channel has plenty of volume now.. it definitely doesn’t seem right with the low heater voltages. Only thing else I’ve found on this is someone changing the drop resistors to get the 6.3v. Which seems one shouldn’t have to redesign the amp.

[Helmholtz]

across 180r resistors the voltage drop is about

Value missing.

Also what is AC winding voltage?

[Indyryder02]

Value missing.

Also what is AC winding voltage?

Edited last post to add missing info.

[Helmholtz]

Ok, so the heater voltage should be 61.5 - (2x25.7) = 10.1V.
Heater current is 25.7V/180R = 0.143A which looks normal for 10V heater voltage.
"Problem" is that the low winding voltage doesn't allow for 12V heater voltage.
If you think you need 12V , you could change one of the resistors to 150R.

[Indyryder02]

Ok, so the heater voltage should be 61.5 - (2x25.7) = 10.1V.
Heater current is 25.7V/180R = 0.143A which looks normal for 10V heater voltage.
"Problem" is that the low winding voltage doesn't allow for 12V heater voltage.
If you think you need 12V , you could change one of the resistors to 150R.

I don’t particularly think I need it. It’s just unusual. As some stated before it may be perfectly normal for the amps design. I’m just unfamiliar with it and haven’t ran into it before.
I’m going to let it run for a few hours with signal and a load and see how it does. It it’s still good. I’m happy with it.