08-21-2016, 07:32 PM
Hey Ampeg folks.
I picked up a '74 V4 (actually a chopped VT22) a few months ago, and while I loved the sound of the amp at first, it seemed to break up really early, and had quite a bit of hum. I popped it open and saw that at some point the power section had been rebuilt with low-quality, 350V electrolytics, and the main filter caps were hot-glued directly on top of the signal wiring to the power tubes! I was determined to rebuild the power section.
Based on several posts from Hangman over on the V4 forums, I ended up using F&T electrolytics: one side of two 100/100 cans for the main filter with the other side of the bottom can and an axial 100uF for the screen supply.
For C17 I used three radial 50uF F&Ts.
I also changed the screen resistors to 1k/5W, removed a variable bias mod,
and replaced:
D1 thru 6
R49
R52
R53
R54
R56
R57
R60
C11
C13
C16
R55 burnt up at some point in the past, and was replaced with a new 10W Xicon. It looked undamaged, so I left it alone.
So I fired the amp back up after all this, and it works, but all the voltages are slightly off. I'm only getting 535V at node A, where I should be seeing 545V; and a full 534V out of 535V at B!
Here's everything I measured:
Primary: 117VAC
Secondary: 395VAC
A 535V
B 534V
C 426V
D 368V
E 335V
BIAS -59V
Bias currents ~ 17mA
So the tubes are running really cold. It sounds fine, but I don't want to keep running it like this until I figure out what's going on.
I tested and inspected my wiring pretty thoroughly for errors a few times, but didn't find anything.
Any ideas? I may just rebuild it with the Orville Amps boards down the line, because the PCBs are pretty warped, but I'd like to figure out what's going on first. Thanks in advance.
I picked up a '74 V4 (actually a chopped VT22) a few months ago, and while I loved the sound of the amp at first, it seemed to break up really early, and had quite a bit of hum. I popped it open and saw that at some point the power section had been rebuilt with low-quality, 350V electrolytics, and the main filter caps were hot-glued directly on top of the signal wiring to the power tubes! I was determined to rebuild the power section.
Based on several posts from Hangman over on the V4 forums, I ended up using F&T electrolytics: one side of two 100/100 cans for the main filter with the other side of the bottom can and an axial 100uF for the screen supply.
For C17 I used three radial 50uF F&Ts.
I also changed the screen resistors to 1k/5W, removed a variable bias mod,
and replaced:
D1 thru 6
R49
R52
R53
R54
R56
R57
R60
C11
C13
C16
R55 burnt up at some point in the past, and was replaced with a new 10W Xicon. It looked undamaged, so I left it alone.
So I fired the amp back up after all this, and it works, but all the voltages are slightly off. I'm only getting 535V at node A, where I should be seeing 545V; and a full 534V out of 535V at B!
Here's everything I measured:
Primary: 117VAC
Secondary: 395VAC
A 535V
B 534V
C 426V
D 368V
E 335V
BIAS -59V
Bias currents ~ 17mA
So the tubes are running really cold. It sounds fine, but I don't want to keep running it like this until I figure out what's going on.
I tested and inspected my wiring pretty thoroughly for errors a few times, but didn't find anything.
Any ideas? I may just rebuild it with the Orville Amps boards down the line, because the PCBs are pretty warped, but I'd like to figure out what's going on first. Thanks in advance.