Low output one channel

I had a tube redplate a couple weeks back, I am finally getting around to working on the amp.

I replace both tubes on the channel that redplated.
I replaces all four 10ohm resistors on the output tubes.
I cleaned up the wiring a bit.

Powered it up and there is definitely something awry.

Swapped signal source from side to side - issue persists
Swapped output tubes from side to side - issue persists.
Swapped center drive tube with one to the left - issue persists.
I thought it could be C1/C2 - swapped them from side to side - issue persists.

At this point I am not sure what I should check.

So far I have limited my measurements to the driver board starting with Pins 2 and 7 of V2.

I apologize for the size of these images, it is late and I do not have time to resize.


Yellow trace is left channel, white trace is right

Scope01
V2 Pin2 & Pin7


Scope02
V2 Pin1 & Pin6


Scope03
This is the signal after C1 & C2


Scope04
V1 Pin2 & V3 Pin2


Scop05
V1 Pin1 & V3 Pin1


Scope06
V1 Pin7 & V3 Pin7


Scope07
V1 Pin6 & V3 Pin6


Scope08
NFB Left & NFB Right

A first step could be to disconnect the NFB and repeat. If NFB is removed any change
in signal voltage indicates where is happens.

Does this board have the CCS? I have the old, high gain board on my amp but it seems like we have seen several posts with this same issue and a number of times it was the CCS.

peterh wrote:A first step could be to disconnect the NFB and repeat. If NFB is removed any change
in signal voltage indicates where is happens.

Removed L&R NFB.
This did resolve the voltage differences between the drive tubes.
Volume issue persists.

nmchiefsfan wrote:Does this board have the CCS?

I do not know what the CSS is.

I checked all the voltages per the build manual:


Power Transformer

White	500VDC
Red Power	415VAC
Green Power	4.55VDC
Brown Power	2.785VDC
Brown/Yellow	2.8VDC
Green/Yellow	4.56VDC
Red/Black (B+)	55.7VAC

Quad Cap

80	493
40	499
30	383
20	493

Output Tubes

Pin	V2	V3	V6	V7
1	.513	.519	.511	.520
2-7	6.88VAC	6.85VAC	6.87VAC	6.84VAC
3	491	492	491	492
4	492	492	491	492
5	-63.6	-63.2	-58.9	-56.6
8	.514	.515	.515	.521

Driver Tube Heater Voltage

Tube	Pins 4&5
V1	6.82
V2	6.8
V3	6.8

The voltages makes no sense.

What board is this ? Schematics please.

peterh wrote:The voltages makes no sense.

What board is this ? Schematics please.

It is a VTA ST120 kit from Tubes4hifi. Standard stuff.

What voltages specifically.

dalemurray wrote:

peterh wrote:The voltages makes no sense.

What board is this ? Schematics please.

It is a VTA ST120 kit from Tubes4hifi. Standard stuff.

What voltages specifically.

I guess you refer to AC voltages.
So you say that with NFB disconnected the AC voltages are similar between channels ?
This indicates something in the powerstage, transformer or broken powertubes or possibly
an open solder

What is the AC voltages on the powertubes grid ?
( if you have a 500volt probe you could measure AC on the powertube plates)

Removing NFB I removed the disparities noted here (both channels are even):


I've checked continuity on all the P to P wiring.

All the voltages I listed in my recent reply are DC unless otherwise noted.

CCS is constant current source that the newer VTA boards use. It is my understanding that if your VTA board uses 12AU7s then it is the low gain board and has constant current sources to balance the tubes. I have the high gain board so I am just picking this up from reading the forum.

What is the AC voltages at the power tubes ??
And what is the AC voltages at the ot transformer , say 16 ohm tap ?

I have the classic driver board, not the recent ccs.

I've worked on it a bit today.
I swapped the LM334 with each other, no difference.
I swapped C1&C2 with each other, no difference.
I swapped C11&C12 with each other, no difference.
I swapped output transformers with each other, no difference.
I swapped leads from my signal generator, no difference.

I suppose I could try swapping L&R 6550 tubes again.

Signal strength diverges at V2 on the driver board.

This shows signal at Pins 2 & 7 of V2 - They are effectively identical.



This shows signal at Pins 1 & 6 of V2.
Pin1 = 1.16 volts (left channel)
Pin2 = 893.7 millivolts (Right channel)

The kicker is, the left channel is the side with nearly zero output.


I have also taken measurements of everything I could think of within reason using my MM, did not do all of these with a scope.

Excel file can be downloaded here: http://dale-murray.com/ST120/Measurements.xlsx
PDF of excel file can be downloaded here: http://dale-murray.com/ST120/Measurements.pdf

I poked and prodded everything with no result.
I went so far as to touch each solder point with my iron to be sure everything was good, same result.
I had already hit each solder point on all the octals so I was quite certain those were good too.
After all the swapping of parts and soldering, none of this made sense. So I went for broke and replaced the female connections on the left channel octals - I still had the ones that came in the power takeoff.

I replaced female connections 3-6 on V2 & V3 (left channel octals).
I replaced the wires from the pin6 connections on the driver board.
Now everything is working correctly.

I can only imagine one of those female connectors was hogged out and not making a solid connection with the tube itself. I had already checked, rechecked, and checked again, all the solder points.

Anyway, its up and running !

Good instinct , that seems like an almost impossible problem to solve , I'm sure you experience posted here will help the rest of us , should we ever encounter a similar problem . Thank you for posting .