Plexi clone power and bias problems

[jewellworks]

ive got a Granger M50 (Marshall 50W Plexi) on the bench.  i assume its the 1987 circuit.  but the power section is wired very different.

the schematic shows the Standby switch directly off both legs of the PT 2ndary, on a DPDT Switch.  so the heaters come on w the Mains. nothing else is powered unless you close the Standby switch.  thats the schematic.

this one is wired very differently...

with the standby switch open, Mains ON, you get heaters, the bias, (the bias has its own 50v tap off the PT, and it continues on to the diode and caps, ect... and feeds grids V4 and V5), and both legs of the 2ndary continue on to the rectifier.  once past the rectifier, it feeds the 1st filter cap (2, 50uf's together for a total of 100uf) then stops.
the Standby switch is a SP and its fed nearly 550VDC off the rectifier.  once closed, it feeds an HT fuse, the choke, and the rest of the circuit...

1st of all... 550VDC on that switch?  if the DP switch was there, each pole would be getting really high AC voltage.  so is it hurting anything to have THAT high DC running through that switch?

2nd...   im having a biasing problem.

i put my meter across the test points, set it to DCV and it reads 17.9mV. /17.9mA.
when i measure the old fashioned way, plate to ground, screen to ground, and grid to ground, ive got:
Pv: 419
g2: 414
g1: 40.3

the vtadiy.com/loadline-calculator shows that to be 24mA. (3600 plate load)

(I don't believe those 1ohm resistors on the cathodes are precision resistors, but whatever...)

Seems like i need to raise the mA to about 40, and lower the bias voltage to about 20? (It's exactly -50 off the PT tap).
but that will lower the plate voltage by quite a lot. ive seen examples where this should be about 460v @ 42v. /40mA, or thereabouts...

However, my bias adjustment pot is already turned down to 0, and I need to lower the voltage more. So I changed the 1.5k resistor coming off the PT bias tap, (before the diode) and replaced it w/a 5k. Now my current limiter is glowing fairy bright, and I'm still not at the right voltage, AND my Pv is now at 387 or so.

i then put it back to the 1.5k (before the diode) and lowered the 47k dropping resistor (before the trim pot) to 33k.  i dont have the glowing current limiter light anymore, but it still didnt get me close enough.  im sitting at 386Pv and 25mA.  still really cold, and not much different than the 24mA sited above, but with a much lower pV.

what is the Pv and bias mA "ideal" for a pair of EL34s?  im thinking alot higher than this. 

and im a little lost as to WHY this amp has a dropping resistor before the diode anyway?  its got its own tap of the PT, so why would we want to lower the voltage?  even tho it seems like what im looking for is about -20...?

either I'm doing this all wrong, OR something else is drawing too much current.


Thoughts?

[SILVERGUN]

Thoughts...
1st - The fact that you're asking some of these questions leads me to believe that you should take it to a repair person.

550 volts is a lotta volts.

[acheld]

That is a lot of volts on that switch, but we've seen that quite often.  I don't know how safe it is -- the switches are usually rated for amps at 120 and 250 VAC, at least in my meager experience.  I don't think I've seen a max voltage for arcing (not even sure what the correct term is) published for this type of switch. 

As a hobbyist, I would want the circuit to conform to what I think is best for me.   If I saw a standby circuit like that, I'd re-wire it.  Not that it has anything to do with the problem at hand.

As a pro (which I am not), I'd focus on safety and liability first, and customer desires second.

Good questions, and I'll be interested in what you actually do.

[jewellworks]

@silvergun
yes, its a LOT of volts, which is why i ask. 
and thanks for your "opinion".  ill be sure to ignore it


@acheld
i AM tempted to rewire it, and have the meager 350VAC on each pole, but id have to get a different switch.  this SP switch is what it came with.  im "guessing" its been fine all this time, but it also seems like quite a lot.  i can measure again to be sure

[uki]

Welcome to the forum!

WARNING: Tube amps have lethal voltages, even when turned off!

If you are new to electronics/tube amps and want to learn, some reading is a good start.

Here some links:

https://el34world.com/Forum/index.php?topic=23286.0
http://www.valvewizard.co.uk/index.html
https://www.aikenamps.com/index.php/white-papers
https://robrobinette.com/How_Amps_Work.htm
http://paulrubyamplifiers.com/info.html#FirstPowerUp

Posting the schematic of the amp in question and some high resolution pictures helps a lot.