Volume Pot issue Harmony Transformerless Tube Amplifier

[devilsindetails]

Ok gang...have an issue that's driving me a little nuts. I have a Harmony H303. Tube complements consists of 35Z5 Rectifier 50L6 Output and 12SQ7 pre-amp tube.

For the first 5-10 minutes everything is tickety-boo but then the volume pot cuts out completely whenever I attempt to take it past 8....I can hear the pot start to get noisy "crackly" and get worse just prior to this.

All resistors have been checked for tolerances. Ditto: Caps. Also checked for loose wiring, connections etc.

All tubes also replaced with new stock.

The problem pot was a .5 Meg volume pot with on/off AC switch.

I have substituted pots, albeit of different (lower) resistances but the issue maintains itself.

I have also removed the A/C switching function from the pot and installed a separate on-off switch thinking that perhaps this was a problem but no change in the symptom.

The current volume pot is rated at 100K. Is this too low given the original 500k rating?...is the lower rated pot being overwhelmed by voltage/current?

[Cygnus X1]

Are you going to update this widowmaker?

[jjman]

H303A model shows a transformer despite suspicious tube filament voltages.

http://www.el34world.com/charts/Schemat ... _h303a.pdf

[devilsindetails]

H303A model shows a transformer despite suspicious tube filament voltages.

http://www.el34world.com/charts/Schemat ... _h303a.pdf

Yes, but this is not the H303A....it is an H303...most definitely no transformer the tube complement is almost a dead give-away for transformerless design.

In any event still no hints or ideas about my volume pot?

Thks Leon

[devilsindetails]

Are you going to update this widowmaker?

Yes...including a proper chassis grounding scheme to circumvent this classic example of the circuit of death

But that aside, any hints about what may be going on with my volume pot?

[Cygnus X1]

It may not be a volume pot issue at all.
I'd go through the updates and then see what happens.

[devilsindetails]

H303A model shows a transformer despite suspicious tube filament voltages.

http://www.el34world.com/charts/Schemat ... _h303a.pdf

Hey jjman....some mea culpa here on my part...you know the saying "sometimes you can't see the forest for the trees?" Well I must be totally blind (too many transformerless designs on my bench of late) because as I probed and picked and dutifully measured voltages with the chassis turned over imagine my surprise to find a small trafo had suddenly materialized on the top of the chassis If I had posted the pictures first you would have had a great time asking me what that thing was sitting there on top of the chassis or wondering if I was completely out of it...

Anyways I updated the whole shibang...caps, resistors, pots etc and still the same ole volume pot sag after a few minutes. Then I thought to double check continuity to ground from the pot to chassis and voila a cold solder joint.

With an independent on/off toggle and separate volume pot now it's trouble free.

Now it's tickety boo again. Just thought I'd fess up.

[Structo]

Right on.

Great looking Harmony.

Although I probably would have put the switch on the chassis.

[NickC]

..... I probably would have put the switch on the chassis.

+1 Abso-freekin-lutely! Exposed mains voltage is completely nuts.

[jjman]

The pot is also the grid leak for the output tube so a 100k is going to reduce the available signal to the power tube's grid and load down the 12au6 more than the intended 500k pot.