High plate voltages and screen voltages in my Express.

[beasleybodyshop]

Almost finished with my Express build. Here are my preamp voltages

all measurements taken with standby off, power on:

V1 - 292vDC, 0, 3.7vDC,3.5vAC,3.5vAC,197vDC,SIGNAL, 1.7v, 3.5vAC

v2 - 288vDC, 0, 3.4vDC, 3.5vAC, 3.5vAC, na, na, na, 3.5vAC

v3 - 212vDC, 27vdc,42vDC,3.5vAC,3.5vAC,223vDC,26vDC,42vDC,3.5vAC

Voltage after the bridge rectifier is 430v. after the big 1K 25W resistor is 423v.

The plate voltages seem super high. Also my screen grids for the power tubes (tubes not installed) are 423 - once again super high. Thoughts?

[beasleybodyshop]

UPDATE -

Installed known working power tubes. here are there voltages:

V4 -gnd, 3.5vAC, 413vDC, 404vDC,-34v, 406vDC, 3.5vAC, gnd

v5 gnd, 3.5vAC, 413vDC, 404vDC, -34v, 405vdc, 3.5vAC, gnd

also, my secondary voltage is 305vAC on each node.

After installation of power tubes, plate voltages of all my preamp tubes dropped about 10 volts, still too high.

Im stumped.

[Bob S]

Difficult to tell from the pics.
The 1k5 on the 1st stage cathode might need a little more solder.
Are the cathode bypass caps polarized ? Might be backwards.
The second stage cathode resistor might be 27k.
The PI - V3 - common cathode resistor looks like it might be 47 ohms.
Might be me though - my eyes ain't great.
Good Luck - you're close.

[beasleybodyshop]

Difficult to tell from the pics.
The 1k5 on the 1st stage cathode might need a little more solder.
Are the cathode bypass caps polarized ? Might be backwards.
The second stage cathode resistor might be 27k.
The PI - V3 - common cathode resistor looks like it might be 47 ohms.
Might be me though - my eyes ain't great.
Good Luck - you're close.

added some more solder to the 1K5

Bypass caps are oriented properly - black band on the end indicates positive end, check them - positive end is facing the preamp tube section.

Good eye - where the 2K7 resistor should be was a 27K resistor - would this cause the high plate voltages?

further more on that point, is the cathode resistor on the preamp stages what determines how much voltage the plate has?

I need to order a 2k7 resistor. ugh. this is killing me.

[beasleybodyshop]

Also noticed a steady hum when the standby switch was flipped off. Sounds like the A note of the low E string on my guitar.

[M Fowler]

I have a stupid question: You have voltage so you must have wired your dropping string under the board, right?

[sepulchre]

The bypass caps, are they 22uF? Are they paper? If the band is on the positive side it would be the first time I've seen it that way. But I could be wrong. However, I can't find anywhere that says the band indicates positive. Here are a few quotes I found from some reputable sites:

"Capacitors will have a white or a black stripe on the negative side."

"Although non-polar, old paper capacitors had black bands at one end. The black band indicated which end of the paper capacitor had some metal foil (which acted as a shield). The end with the metal foil was connected to the ground (or lowest voltage). The main purpose of the foil shield was to make the paper capacitor last longer."

"Electrolytic capacitors are often marked with a stripe. That stripe indicates the NEGATIVE lead."

Just sayin. Even so, that would not account for the high voltages.

[M Fowler]

Is the B+4 wiring under the board?

[M Fowler]

I have about 20 of those white caps and they are 22uf/16v and it is not very well defined but they appear to have a - mark on white side and black band on opposite side. I never used them because I was unsure of how good they were.

You can't do much about the high HT you must have over 120vac mains.

[beasleybodyshop]

Dropping resistors are wired under the boards, yes. And no I completely forgot about the B+4 jumper to the coupling cap - d'oh!

I saw that little negative symbol on one side of the cap and assumed it meant that side was negative - maybe I should just order some different ones.

[geetarpicker]

My wall voltage here is about 123-125v and even my two original Express amps typically run about 412-417v B+. Yours does seem a little high but not by that much, where are you running your bias?

[beasleybodyshop]

My wall voltage here is about 123-125v and even my two original Express amps typically run about 412-417v B+. Yours does seem a little high but not by that much, where are you running your bias?

What do you mean by where? I removed the VVR, so the bias signal is running from the 15K resistor to the bias input on my preamp board.

Bias for both of my power tubes seems close to spec, -34V

[beasleybodyshop]

Attached is some revised photos. I had a 2.2K resistor on hand, so I swapped that in until my 2K7 resistors come in. Swapped in some nichicon bypass caps just to rule out the funky ones i had.

Whenever I connected the jumper at B+4, I got crazy squealing noises when I flipped off the standby switch. This is where Im at right now.

also I redressed my heaters and removed the VVR until I could get everything sorted out. Also my indicator light isnt connected.

[M Fowler]

I got crazy squealing noises when I flipped off the standby switch.

Hmm might have to switch the OT plate wires around.

[Bob S]

What Mark said - sounds like positive feedback.
Still think the phase inverter catthode resistor between the 2 x 1 Megs is 47 ohm instead of 470 ohms. This would screw up the PI drive big time.
Almost there !