Marshall 1992 preamp power supply problem

[smokey]

Hi, I've just built a marshall 1992 clone with a modified power supply because I had a spare 350-0-350 3.15-0-3.15 PT with no bias tap. Here's my quick drawn schematic :




The plate voltage on V1 V2 and V3 is way too high around 500vdc in fact, there's no voltage drop and I'm getting the screen grid voltage. I've checked the lead dress several times and everything's ok.
Please Help

Thanks

[Alex R]

Are you measuring the voltages with no tubes plugged in? If so, the plate connections will remain at high potential.

[smokey]

All tubes were in but I'm not operating at full voltage (1/10 th) with a variac and a light bulb current limiter. The 500vdc reading is the result of my reading x 10.

[Enzo]

Um, you can't do that. Putting 12 volts on the mains and then assuming that all the operating voltages will be 10 times what they read that way is not valid. If it is not blowing fuses, there is no reason not to apply full mains voltage and then start making meaningful measurements.

If your scaling is as it seems, then the tube heaters will be trying to run on 0.6 volts. That is hardly enough to get the cathodes emitting and so the tubes won't conduct. So as far as the circuit is concerned then, the tubes might as well not be there. And that is just the heater issues.

[smokey]

Thanks, I finally raised the voltage to 170vac (I live in europe) and then plate voltages were within the operating range. When I raised the voltage again, the mains fuse popped. I used a lower value fuse, 1.6A instead of 2A. How much current does a 4 el34 + 3 12ax7 kind of amp with a 420 B+ take from the wall?

[Andrei_T]

Check the bias voltage. 420VDC on the plates is a bit low for 350VAC in the PS. Maybe the bias current is too high and the power tubes are drawing too much current. A 1.6A fuse should not blow if the amp is at idle.

[Alex R]

A 2A fuse is normal for such an amp on 240v AC. Some 100W Marshalls use a 2A slo-blo. You will get a bit of an initial current surge as the caps charge, then it settles down. With a variac at a lower than design voltage and the heaters glowing the amp will sometimes try to draw more current to compensate for the lower volts; V=IR and all that. Put those two things together, the surge and the variac, and you have a potential explanation for a 1.6A fuse blowing.

Personally I don't use variacs as they only confuse things - a light bulb limiter is enough. Some people believe in using them to form caps. Not something I have found necessary in day to day repairs. So I say, if the light bulb settles down to a glow after the initial brightness as the caps charge, put full mains voltage on through a 2A fuse and see what happens.

[smokey]

Check the bias voltage. 420VDC on the plates is a bit low for 350VAC in the PS. Maybe the bias current is too high and the power tubes are drawing too much current. A 1.6A fuse should not blow if the amp is at idle.

Max bias voltage I can get with the bias pot turned all the way up is -32v which is pretty low I guess. I may have to lower the value of the 56k resistor.

[Andrei_T]

smokey, you will need to increase the value of that 56k so that the bias voltage will go more negative, lowering the tubes current.

[Alex R]

It is now off the variac? - Just checking!

[smokey]

The mains' fuse pops at full power with variac off and a -55v bias voltage while the HT fuse remains intact. Maybe a short in the heaters circuit ?

[Enzo]

Have we now tried powering up the amp without the power tubes? When fuses blow, the FIRST thing I look for is a bad power tube. The bias would have to be WAAAAAYYY off to make fuses blow. Even a lack of bias usually doesn't blow fuses, it just melts tubes.

[Alex R]

...put that variac aside! - and put it on a light bulb limiter only. Does it shine bright with no tubes in? (just clarifying matters)