Hoffman Plexi 50 power tubes redplating.

[AHeck]

I finished replacing a Bedrock BC75's guts with a Plexi 50 board today.  It immediately started blowing fuses 30 seconds to a minute after switching B+ through standby.  A little observation revealed redplating on V5.  Swapping tubes has not helped, trading V5 & V4 resulted in V4 redplating and V5 same.  Using the E34L tubes that came with the amp.  1 Ohm resisters read 45mA steady, as plate want to melt. Checked all applicable resistors for accidental duh's.  Rebuilt power supply as per Marshall 50 specs from Hoffman world.  I had to mirror image flip turret board, so I scrutinized the build. 
It's hard to take measurements considering time restraints, but I can give it a shot if need be.
Using old transformers, might be a problem?
Anyone's help will be much appreciated.

[tubeswell]

Pull the output tubes out and measure the DC voltage on each of the sockets' signal grid pins. What have you got?

[AHeck]

Thanks, tubeswell

Pin 5, V4/V5 -5.664 VDC

Plate voltage well over 542 VDC which may be a problem...

[tubeswell]

Thanks, tubeswell

Pin 5, V4/V5 -5.664 VDC

Plate voltage well over 542 VDC which may be a problem...

Looks like your bias voltage is the problem. You want around -45V to -55V for EL34s. Something must be draining heaps of current - Check that the filter cap(s) in the bias supply isn't shorted.

[Latole]

tubeswell is right, issue is in the negative voltage bias power supply .

Don't plug output tubes unless issue is fixed with a reading of -40 VDC at V5 and V4 pin 5.

 

[AHeck]

Here's what's happening in the bias circuit.  I can manage between ~-4V and ~-8V by adjusting the bias VR1. 

[sluckey]

The problem is you are only feeding 54VAC into that circuit. You have two choices...

Tape off that 54VAC bias tap on your PT and connect the 220K Bias Range resistor to the PT HT winding as shown in Hoffman's pdf page 2. This is just a simple jumper wire on the board. Jumper the top of the 220K bias range resistor to one of the rectifier diodes on the board. This is what I'd do.

Or... If you want to use the 54VAC bias source then simply replace the 220K Bias Range resistor with something much smaller, 470Ω to 1KΩ. (Experiment with exact value to give the bias adjustment range you want.

On another note... you need two 1N5408s in series where only one is shown on the layout. That's a total of four 1N5408s. A single diode is being operated at a higher voltage than it is rated. Not only are the diodes in danger, if they fail shorted they may take out some high value components as collateral damage.

[Latole]

Here's what's happening in the bias circuit.  I can manage between ~-4V and ~-8V by adjusting the bias VR1.

Second cap ( at left ) are wrong polarized( or circuit are wrong ) , bias pot is connected at wrong place.

[Latole]

Here's what's happening in the bias circuit.  I can manage between ~-4V and ~-8V by adjusting the bias VR1.

Second cap ( at left ) are wrong polarized( or circuit are wrong ) , bias pot is connected at wrong place.
Second picture show right Marshall bias circuit .

[Latole]

Build like this one, use the right resistor values for your voltage.
 

[tubeswell]

Here's what's happening in the bias circuit.  I can manage between ~-4V and ~-8V by adjusting the bias VR1.

Second cap ( at left ) are wrong polarized( or circuit are wrong ) , bias pot is connected at wrong place.

Steve got it. The bias supply is incorrectly designed for the voltage of the PT tap that it is being sourced from. For a 54VAC source, you want a different amount of voltage division. 54VAC rectifies to 76VDC. 8 is about 10% of 76. 10 is 13% of 76. Seeing as how there is 220k in the upper leg, there must be about 22k resistor at the bottom (to get an output that is 10% of input with the bias pot cut) and a 10k pot (to get to 13% with the bias pot dimed). Whereas to get a 55 VDC output from that 54VAC source using the same 22k tail resistor and 10k pot, you want a much smaller resistor than that 220k in the upper leg. You want more like a 12k resistor (to make 44k in the divider overall with the pot dimed). The bias supply caps should be rated for 80V minimum, or 100V to be on the safe side. The single 1N5048 isn't robust enough and should have 2 in series, or replace it with a 1N4007

(However, with a 220k, the supply possibly could be connected to one of the High Tension winding ends and possibly provide enough voltage, but that remains to be seen)

[sluckey]

Second cap ( at left ) are wrong polarized( or circuit are wrong ) , bias pot is connected at wrong place.

THAT'S NOT SO! His circuit is wired correctly except for his use of a 54VAC source. His circuit is exactly the same as the Marshall clip you posted. You really need to look at the Hoffman Plexi 50 docs before giving this kind of advice.

[AHeck]

I am, as always, grateful and humbled.  I am appreciative of y'all's willingness to help and for the opportunity to learn, which is why I do this mess.  All of your advice and knowledge is well understood and will make these bias circuits less nebulous and confusing.  Well, I'm of to fix my mistakes and see how this build sounds.