Trouble shooting an original Model T

[brennansrevenge]

Gtr_tech - the bias cap is good.

i think you're right about the fizzle and caps.  i've isolated the C17 & C18 caps (power caps), and the fizzle stops.

the schematic shows these with negative to ground.  the schematic also calls out for 20 uf, 600v caps, but i could only get 40uf, 500v, when measuring the voltage, it's only 120v, so i don't think i'm maxing out under no load.

my next test is to make sure the caps are not leaking.  i do this by measuring resistance to see it go up and hold.  if you know a better way to test these i would appreciate thought on that.

[brennansrevenge]

well, if Joe or anyone is still interested in my resurection of the T, here's where i'm at.  if anything, this may help others in the future.  i've purchased Morgan Jones Third Addition Valve Amplifiers book.  although the valve and amp info is based on the UK equipment, he has approached the amp design from engineering theory and offers great detail on the reasoning behind the design.

With everything wired together after fixing the bias loop (replaced the rectifier diode), going live produces a "sizzle" sound after 10-20 seconds.

after some isolating, i determined that the power caps C17 & C18 (original malory's replaced with Sprague Atom TVA 1908 40uF, 500vDC axial lead aluminum electrolytic capacitors) where the noise culprits.

although these had been replaced, once out of the circuit there was no sizzle.

Disconnecting downstream of terminals A,B,C and C17&18, and standby "on" or closed position, i measure:
A=100 vDC
B=60 v
C=50 v

At this time with the stanby switch "off" or open, measuring from the bias test point i get -20v min and -25v max by adjusting the pot.

my question is:

are the voltage readings at A,B,C too low?  Relative to the schematic i should be seeing something 519, 419, and 330.

[JoeArthur]

Ok... I tried real hard not to post this.  I tried for several hours, but I couldn't in good conscience let this go.

Understand this:  "It's your amp and it's your life, and you are free to do what you want with both."  So please feel free to ignore this at the risk of both.

I strongly suggest that if you have any interest in living, hopefully with it being THE primary interest and only a secondary interest in getting this amp fixed - you leave the amp alone and take it to a competent repair facility.

Consider these things which you already know but apparently haven't connected together:

EDIT.  Sorry, I said the leakage resistance would increase where I should have said decrease.
#1 You should expect to find 519 or so volts at point A in the power supply, yet you measure 100 volts DC.
#2 Your measurements of the B+ side of the power transformer supplying via the two rectifiers was 440 volts AC each.
#3 You replaced the primary power supply electrolytics with what you could find - 40mfd @500 volt, not with what is specified.
#4 You have a sizzle sound that disappears only when those primary electrolytics are disconnected.

Put them together.  

You have intentionally placed caps in an over-voltage condition.  Now while a cap would be able to sustain a short term and temporary over-voltage, you have them in a permanent over-voltage situation - and they will break down.  

When they break down, their ability to hold a charge will be greatly reduced - no longer able to charge up to the peak voltage provided by the transformer.   This accounts for your below normal voltage readings.

Their leakage resistance will also increase, passing ever more current through generating heat inside those caps.  The heat will increase until you have the electrolyte boiling which is probably accounting for the sizzle sound. The boiling will continue until enough pressure is built up to explode the cap.

You are lucky so far... and I implore you not to push it.  A face full of exploding electrolytic capacitor will not be a fun experience.