Hello all,
This is spooky-weird.
I have been following this thread from the beginning only because I am
the original owner of a 425 amp. Just general interest.
A few days ago, I started trouble shooting an older amp that was given
to me. It used the same 3cx800 tubes and was not showing any output for
10 watts input. Wound up swapping out the tubes for the ones in my 425
to make sure they were not the issue. It was used for contesting and
could have just been beat to death.
Tuned up my 425 with it's original tubes and got several hundred watts
output with very little drive, maybe 20 watts, just to make sure
everything was OK with the test amp.
Swapped tubes and made good output, so I swapped them back out again.
Today, I was back at it finishing things up, as I had to purchase some
fish paper to put between tube anodes and silicone chimneys. During the
tube swap, the old stuff was brittle and crumbled, so I replaced it.
Anyway, after replacing one meter bulb, I fired things up again. While
waiting for the timer to expire, I reached over the amp for some reason
and found the air exhaust was HOT. Looked at the plate current meter
and it was reading 300 ma.
So, it looks like I lost my cut-off bias as well. How co-incidental is
that!
On top of the high plate current, I also now notice no discernible grid
current indicated. I am assuming that is consistent with the loss of bias.
Now, at least, I don't have to ask the same question. It seems you all
have addressed the cause/fix for TWO of us.
Thanks.
(Btw, the other amp was suffering from a bad vacuum relay stuck in the
N/C position. On order.)
73 de Steve, NR4M
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