2203 Preamp plate voltage too high

[blackbird]

Hi folks, this JMP 2203 100W head is a bit of a brain teaser. It has these preamp plate voltages without tubes
and with the Variac set to 120VAC:
V1-1 457vdc (schematic says 230v)
V1-6 459v ( " 220v)
V2-1 460v ( " 70v)
V2-6 465v ( " 300v)
V3-1 464v ( " 90v ? - hard to read)
V3-6 463v (schematic says 82v ? " )
These voltages are not reduced by increasing the values of the two dropping resistors in series after the choke.
They were 10K and 10K, and I bumped them up to 51K and 10K respectively.
That changed the above voltages minutely to between 450V to 460vdc.
The rectifier out is 478vdc and B+ is 460vdc.

There are no visible signs of arcing or burning of anything anywhere.

So far, I have tested all PS filter caps, power resistors and preamp plate resistors, and all are within 5% of specified value.

The original problem, whether related or not, was in the power amp. The main fuse (4amp) had blown, so I checked the PT, the OT, the choke, the rectifier diodes,and the bias circuit. They all tested good, and the PT is supplying the correct voltages for heaters, EL34s, and Bias voltage is -58v with -39.9v on pin 5 of the EL34s.
The two middle EL34s were shorted internally (one on each side) so I checked the screen resistors. They were 989ohms and 987 ohms. I replaced them with 1K 5W cement wire-wounds. The other two on the unshorted EL34s were 991ohms and 993ohms each, so I left them in. No signs of arcing or burning ANYWHERE.
It is strange that the HT (1amp) fuse did not blow instead of the 4 amp main fuse.

I hesitate to plug in a set of good tubes as a sacrificial offering just to see how much the voltage drops in the preamp. I doubt it would be anywhere near enough.
I considered the coupling caps in the preamp, but I doubt that they would all go at the same time.
Could the B+ be jumping into the preamp somehow?
What am I missing?
If anyone has an idea of what to do next, I would be grateful.

Blackbird

 

[neikeel]

You will not get correct preamp voltages without the output tubes in.
If you are unsure then use a light bulb limiter for first fire up. It will save you from fires and lots of dead fuses.

 

[Pete Farrington]

It has these preamp plate voltages without tubes
and with the Variac set to 120VAC:
V1-1 457vdc (schematic says 230v)

With valves fitted, current can then be drawn through the HT dropper and anode load resistors, and so voltage will be dropped across them.

 

[Spanngitter]

Again. I am amazed that people do get into an amp that deep and detailed but dunno understand that a resistor doesn‘t drop any voltage if no current is flowing…this discrepancy makes my head hurt…

 

[blackbird]

Thank you Tatzmann and neikeel. I will install the tubes and apply gradual voltage with the variac while reading the preamp plate voltages. I was hoping someone knew if the 4amp fuse blowing was related to the preamp volatges. But one thing at a time. Thanks again!

 

[Pete Farrington]

the 4amp fuse blowing

use a light bulb limiter

Unless you continuously monitor mains current, and your reactions are faster than a lightbulb, winding the supplied mains voltage up with a variac is just giving you a false sense that you’re doing it right.
Like neikeel says, use a lightbulb limiter.

 

[Gene Ballzz]

Light bulb limiter, cheap, easy, peasy, download it, build it, DONE! I believe you can even use it "WITH" your variac!

I would NOT include the inset bypass switch!

Just Limitin',
Gene

 

[AndyD]

Light bulb limiter. Easy to make. 60w will do it.

 

[Jon Snell]

Light bulb limiter, cheap, easy, peasy, download it, build it, DONE! I believe you can even use it "WITH" your variac!

I would NOT include the inset bypass switch!

Just Limitin',
Gene

It works well with a Variac. Probably best on the mains input side of the Variac, otherwise you may run out of current leaving the outout of the Variac.

For those without a clue; a resistor reduces current flow, NOT voltage.

 

[Gene Ballzz]

It works well with a Variac. Probably best on the mains input side of the Variac, otherwise you may run out of current leaving the outout of the Variac.

For those without a clue; a resistor reduces current flow, NOT voltage.

^^^^^^^ Great graphic, right there! ^^^^^^^

Thanx 4 Sharon, She's A Good Girl, In All The Worst Ways!
Gene