OK

I am glad I have a cold as I do not mind having spent such a nice day inside
working on a radio.

Fixing an AK-55 for a friend of mine. Not the first restoration I have done
and not my first AK-55. But this one is driving me nuts.

I have replaced all of the filter caps and am trying to tame the AF output.
The rest of the radio seems fine. I have a LOUD AC hum that I cannot get rid
of.. The HV drops substantially and the hum kicks in as soon as I put any
45s in the sockets. I have tried some 45s that work in my other AK 55 so
that rules out the tubes. I can see the AC ripple on B+ go way up on my
scope once the tubes are plugged in

The only thing that seems at all out of line to me is that the CT for
neither the output primary or the secondary to the grids is an exact CT.
They are both slightly higher on one side. It is more like 60/40 then CT.
However there are colors called out for and the high side for the grids and
output is consistent on one tube so I am assuming that it may be a design?

All voltages look otherwise Ok, and I am using the correct speaker with the
field coil. Resistance in the coil looks fine too.

How much could go wrong with a simple PP triode amp??????

Anybody have any thoughts on this?

--
Carl
WA1KPD
Visit My Boatanchor Collection at
http://home.comcast.net/~chnord/wa1kpd.html

Reply

I wouldn't suspect the p-p audio xfmr, although anything is possible. When you measure the unbalance betweet center-taps, are you going by winding resistance? That can be misleading -- though the two half windings will have equal turns, they are different lengths.

Are you saying that the hum only appears when you plug in ghe 45s? But keep in mind, without the 45s, the speaker isn't even in the circuit.

I suspect the power supply here, either that or a short somewhere. Have you checked the various resistors in the power supply?

But you might try this: pull the 45s out of the ckt, and temporarily connect a high-impedance speaker across the secondary of the xfmr after the first audio tube. Is there hum?

:OK
:
:I am glad I have a cold as I do not mind having spent such a nice day inside
:working on a radio.
:
:Fixing an AK-55 for a friend of mine. Not the first restoration I have done
:and not my first AK-55. But this one is driving me nuts.
:
:I have replaced all of the filter caps and am trying to tame the AF output.
:The rest of the radio seems fine. I have a LOUD AC hum that I cannot get rid
:of.. The HV drops substantially and the hum kicks in as soon as I put any
:45s in the sockets. I have tried some 45s that work in my other AK 55 so
:that rules out the tubes. I can see the AC ripple on B+ go way up on my
:scope once the tubes are plugged in
:
:The only thing that seems at all out of line to me is that the CT for
:neither the output primary or the secondary to the grids is an exact CT.
:They are both slightly higher on one side. It is more like 60/40 then CT.
:However there are colors called out for and the high side for the grids and
:output is consistent on one tube so I am assuming that it may be a design?
:
:All voltages look otherwise Ok, and I am using the correct speaker with the
:field coil. Resistance in the coil looks fine too.
:
:How much could go wrong with a simple PP triode amp??????
:
:Anybody have any thoughts on this?
:
:--
:Carl
:WA1KPD
:Visit My Boatanchor Collection at
:http://home.comcast.net/~chnord/wa1kpd.html
:
:Reply

:I wouldn't suspect the p-p audio xfmr,

Hi Doug. Solved. I grunded the B+ filter caps when they should have been above ground with a resistor network. Old PS habbits die hard.
Thanks for the info

Thanks for the help