Assistance troubleshooting vacuum tube amplification stage...

Thread Starter

Externet

Joined Nov 29, 2005
1,847
Hi. since a kid, have not worked enough on vacuum tubes, perhaps someone can guide me :

The amplification section does not amplify and the tiny amplitude seen is distorted. I want to troubleshoot by measuring, not by blindly replacing parts in guess mode when possible.

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V1A plate is supposed to be 70V, it is 45V
V1B plate is supposed to be 80V, it is 35V
V1C plate is supposed to be 100V, it is 88V
Signal input is good at V1A grid pin 6, about 4Vpp
Signal is good at V1B grid pin 3 about 4Vpp
No signal at V1C grid pin 11
B+a is 132V
V2 pins 2 and 7 grids show a very weak distorted signal.
I prefer not to probe with oscilloscope the plates with my shiny new cheap Fnirsi.
If C10 is open or R14 is shorted; that would be the symptom, right ?
I do not have nor want to order replacement tubes to just swap in guess mode.
 
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nsaspook

Joined Aug 27, 2009
9,741
A cathode follower is used when you need a relatively low output impedance in tube circuits. This allows for lower resistor values in the V amp calibration/sensitivity circuit increasing V amp bandwidth.

I think I first would check C8 (DC current path on one that only should be AC) and C10 (DC leak into grid bias) for DC high leakage currents causing bias point shifts.
 

DickCappels

Joined Aug 21, 2008
8,710
Early in my electronics education I was mentored by a technician who had been in the television repair business. He used to tell me, often, "The first thing you do it check the power supply voltages". Incorrect power supply voltage can be the cause of low plate voltages. How is you B+?
 
I see two problems, they could be related or separate.
V1A,B,C plate voltages are all too low, I would suspect B+α is low in voltage either from too much load on that rail, or a power supply problem like low value filter capacitor sections C33A, C33C or leaky C33D.

It could also be one tube stage that is fed B+α is drawing too much current. It can even be C34B, the sync section drawing too much current and loading down B+α for everyone.

The second problem of V1C having no gain could be leaky or shorted C10. They're using space charge to bias V1B and V1C. So any tiny leakage current in C10 or R14 low value would upset V1C. If V1C is biased on too much, it will draw a lot of current from B+α. Reading voltages at those grids will affect the stage's bias due to multimeter loading, tube gear can have high impedances.

You could also check DCV across R56, R55, R13 for figuring out why B+α is low and if B+b, B+c are also low.
 

Thread Starter

Externet

Joined Nov 29, 2005
1,847
How is you B+?
B+a is 132V
C33 a,b,c,d and C34 a,b,c,d where replaced with new before starting any repair attempt. B+a, B+b and B+c voltages are not specified on the manual. :(

R6 drops 50V for a correct plate of 70V at V1A
R12 drops 11V for a low plate of 48V - should be 80V
R16 drops 48V for a low plate of 71V - should be 100V
 
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Thread Starter

Externet

Joined Nov 29, 2005
1,847
Last edited:

debe

Joined Sep 21, 2010
1,294
Had the same problem in a audio amp. R30 5W 220 ohm resistor in the power supply had gone high in value & lowered the voltage.KRIESLER STEREO CIRC.2.jpg
 

MisterBill2

Joined Jan 23, 2018
11,913
The first thing to do is, with power off, unplug V1, then apply power and check the voltages at the V1 socket. Also check the power supply voltages. With that information we can provide valid suggestions.
 

Thread Starter

Externet

Joined Nov 29, 2005
1,847
Awakening the thread for a moment... Got back onto troubleshooting the problem. Fixed. Ceramic disc C10 found bad, being resistive. Replaced C9 also just because being same of same age. Now they are rated 400V just in case need to work a few decades more:)
 

Audioguru again

Joined Oct 21, 2019
4,693
Why did every convenience store in town 70 years ago have a tube tester and sell replacement tooobs?
The sold as many replacement tubes as they sold cigarettes. Tubes had a short lifetime, the smokers too..
 

nsaspook

Joined Aug 27, 2009
9,741
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