On May 29, 8:23*am, JIMMIE wrote:
I want to replace the vr 0A3 tube used in the bias circuit of a 6146B
amp with a 75V zener string. Can I just directly replace the tube with
the diodes or *will I need to make other modifications to the circuit?

Jimmie

Thanks all, problem turned out not to be the tube. still not
absolutely sure but it was either the bias pot or a cap. I'm fairly
certain I changed out the cap about a year ago so best guess is the
pot. What I am thinking was happening is the pot was opening up
sending its share of the current through the tube overloading it with
about 90 ma of current causing the tube to arc. Should have just
shotgunned the circuit from the beginning.

Jimmie

On Jun 4, 4:11*pm, JIMMIE wrote:
On May 29, 8:23*am, JIMMIE wrote:

I want to replace the vr 0A3 tube used in the bias circuit of a 6146B
amp with a 75V zener string. Can I just directly replace the tube with
the diodes or *will I need to make other modifications to the circuit?

Jimmie

Thanks all, problem turned out not to be the tube. still not
absolutely sure but *it was either the bias pot or a cap. I'm fairly
certain I changed out the cap about a year ago so best guess is the
pot. What I am thinking was happening is the pot was opening up
sending its share of the current through the tube overloading it with
about 90 ma of current causing the tube to arc. Should have just
shotgunned the circuit from the beginning.

Jimmie


hey OM

The dead give away should have been the series resistor to that OA3
heating up big time since P=I^2R the current doubled, the power
quadrupled that resistor would have could have let its' smoke out.

73 OT
de n8zu

On 06/04/2011 10:07 PM, raypsi wrote:

hey OM

The dead give away should have been the series resistor to that OA3
heating up big time since P=I^2R the current doubled, the power
quadrupled that resistor would have could have let its' smoke out.

73 OT
de n8zu
Unless the original designer was paranoid enough to use an over sized
resistor in the first place. Back when I first started out in ham radio
building with tubes I was guilty of using a 50W resistor in a 5-10 watt
circuit. (half because I got the 50W part cheap and half because I knew
that I didn't quite know what I was doing and wanted to be safe).

On Jun 5, 12:16*pm, Kenneth Scharf wrote:
On 06/04/2011 10:07 PM, raypsi wrote:

hey OM

The dead give away should have been *the series resistor to that OA3
heating up big time since P=I^2R the current doubled, the power
quadrupled that resistor would have could have let its' smoke *out.

73 OT
de n8zu

Unless the original designer was paranoid enough to use an over sized
resistor in the first place. *Back when I first started out in ham radio
building with tubes I was guilty of using a 50W resistor in a 5-10 watt
circuit. *(half because I got the 50W part cheap and half because I knew
that I didn't quite know what I was doing and wanted to be safe).

In my case problem was only momentary, maybe a second sometimes going
days without doing it sometimes 3 or 4 times in a couple of minutes.
For a long time I was convinced it was in the amp instead of the power
supply because I once disconnected the power supply from the amp and
it ran continuously for a few weeks without failing. This turned out
to be just bad luck. Fortunately I am on the air about as sporadically
as the amp was failing. There is no telling how many times it failed
while I was using it and I just didn't notice.


Jimmie