What is an acceptable replacement 12ax7 for a Grundig 4090? There are
many choices and price ranges. Does the choice of tube make a big
difference in this radio? I'm new at this and appreciate any help.

Also, when this radio has been warmed up after about 1.5 hours, I start
to get a low buzzing (hum) sound from the set. It's higher in
frequency than 60hz and is not affected by the volume. It was
suggested this could be a problem from the 12ax7, but I'm not sure.

Thanks.

It shouldn't matter which brand of tube. No need to pay audiophile
prices, any working tube should do.

gar wrote:

Also, when this radio has been warmed up after about 1.5 hours, I start
to get a low buzzing (hum) sound from the set. It's higher in
frequency than 60hz and is not affected by the volume. It was
suggested this could be a problem from the 12ax7, but I'm not sure.

no, it is not the 12AX7. it sounds to me that the filter capacitors in the
power supply are starting to fail.
most grundigs of that age require a full re-capping to bring them back up to
specs.
--
"The philosophy behind Free, Open Source Software has parallels in several
cultures which practice ethical cooperaton. Perhaps that's why it threatens
unethical business practices which feed on the unknowing."
-- Bassam A. Hassan. CoFounder of the ILUG,
Vice president of the former IOSO,

fatboy wrote:

gar wrote:

Also, when this radio has been warmed up after about 1.5 hours, I start
to get a low buzzing (hum) sound from the set. It's higher in
frequency than 60hz and is not affected by the volume. It was
suggested this could be a problem from the 12ax7, but I'm not sure.

no, it is not the 12AX7. it sounds to me that the filter capacitors in the
power supply are starting to fail.
most grundigs of that age require a full re-capping to bring them back up to
specs.

You could be right there.

But anything from the 1950s/60s should have all the electros and
coupling caps replaced as a routine part of a full service.

A failed coupling cap between an output tube and the driver tube can cause a
high anode current
in the output tube hence more ripple voltage and hence more hum.

Every single old radio I have come across needs all its caps replaced,
and some if not many resistors which have drifted more than 20% away from their
design value.

Patrick Turner.


--
"The philosophy behind Free, Open Source Software has parallels in several
cultures which practice ethical cooperaton. Perhaps that's why it threatens
unethical business practices which feed on the unknowing."
-- Bassam A. Hassan. CoFounder of the ILUG,
Vice president of the former IOSO,

And for a giggle, try a 5751 or a 7025. Both are low-noise 12AX7 subs,
and might make an audible & positive difference. Both are also _cheap_.


Peter Wieck
Wyncote, PA

The buzz.... after 1/2 hour, you say?

This sounds like a cap failing due to heat. My guess is that it is
120hz, or chopped DC not being filtered properly by the power-supply
caps. That it happens after 1/2 hour or so suggests that it is
heat-related.

FIX IT. A failure could eat your power transformer, output transfomer
or any of several components in between.

Obvious Cautions:
1. Do not simply bridge in the new cap. Remove the old cap (at least
electrically) from the circuit. If it is leaky, adding new capacitance
does no good. If it is open or opens after heating, that too could be
a problem with too much capacitance.
2. Replace all the electrolytics if possible. Use equal/better voltage
caps, and try not to exceed 200% of OEM capacitance.
3. The other low-value caps are a moving target. Most will be good. In
some cases, all will be good. ~1965 or so is the cusp before which one
should shotgun replace, after which technology changed sufficiently in
cap manufacture (for those makers on the cutting edge) that reliability
shot up.. especially in Europe, most especially in Germany.

Peter Wieck
Wyncote, PA