[tym]

And today I noticed two instances of volume drop within ~30 mins of turning the set on. I'm going to try swapping in my spare 36 tube to see if that helps.

Anything else I might be missing?

[tym]

Welp, subbing the other 36 tube resulted in motorboating on the broadcast band, so I put the original 36 back in. I *thoroughly* cleaned tube pins, grid caps, and grid cap clips for the 36, 44s, and 75. The clips had a fair amount of oxidation on the contact surfaces, so knowing how notoriously picky the oscillator in these sets is, wish me luck.

[Brad Winder]

I experience volume drop on pretty much all my sets now and again; I've narrowed it down to RF interference from switching power supplies in various electronics around our house. Our rural power supply isn't that stable.

[tym]

Very interesting. I live in a condo complex so there are plenty of SMPS around. What's odd is I haven't noticed this phenomenon in my other sets - but I also haven't play-tested them as much as this one, either.

[David]

Something I have started doing, supplying power from a Jackery 240 to a set in Question and turning off the mains power.

[morzh]

Short of overloading somehow the AVC circuit (how?), how would SMPS interference lower the volume?

[tym]

Listening to the set last night and this morning after cleaning the grid clips, etc., there have been no further volume drops. Phew!

This weekend I'll also double check that the tube shields have good continuity to chassis.

[Ed Locker]

Something else you might try for the intermittent volume problem is reflowing the solder on the grid cap of the 75 tube.

[tym]

Good thought, Ed. I thought I had done that already but will double check my notes and reflow it again this weekend if necessary.

[tym]

Ed, I followed up on your suggestion. I had reflowed the solder on the grid cap previously, so today I used a solder sucker to remove the solder for a better view. I found that the grid wire was broken off flush with the hole in the cap.

I used a trick that has worked for me before and doesn't require removing the grid cap. I tinned a strand of hookup wire, bent it 90 degrees, and poked it into the hole. I then thoroughly heated with the gun and flowed a generous amount of solder in.

Separately, I verified that all tube shields have good continuity to chassis when installed.

I'll test everything out later today and report back.

[tym]

Well, my shortcut for fixing the broken grid wire did not prevent the volume drops, as one occurred in less than 30 minutes of operation.

So, I removed the grid cap, cleaned the grid wire stub of oxidation, crimped on a short extension wire, and soldered it. Following a suggestion I read a while ago from Marcc on ARF, I then added a gentle bend to the extended grid wire for strain relief, reattached the grid cap, and soldered.

I'm playing the set now and will see how it performs. If the problem persists, I'll get and swap in another 75 tube...it's the only tube in the set for which I don't have a spare for troubleshooting.

EDIT:

Issue persists.

I then decided to pull all the other tubes with grid caps (36, two 39/44s). Two of them had the same issue as above (tiny grid wire stub barely poking through the hole on the grid cap). The RF 39/44 was good in that it had an 1/8" of grid wire bent over and soldered to the grid cap.

I repaired the 36 and subbed a good tested Philco globe tube in the meantime...issue persists.

I then attempted to repair the IF 39/44, but the evacuation tip came off with the top cap...oops. Oddly, the getter didn't turn white. I subbed in a replacement (milspec VT-49) and play tested for an hour (all the time I had today). So far, so good.

Fingers crossed.

[tym]

Here's a picture of the tiny lead stub - less than 1 mm - protruding through the top cap hole from the 39/44 tube that fell victim to my repair efforts. Three tubes were like this - even with reflowing the solder, the chances of a "dry" joint seem high if no extension wire is added.

[tym]

I think I'm zeroing in on the (very) intermittent volume fluctuation being a heat-related tube issue.

Replacing the IF 39/44 tube reduced how often the volume fluctuates and the degree of volume change. The volume change is now pretty small, so I could live with it as-is.

The 75's grid cap and tube pins have all been resoldered, and swapping it with another tested 75 makes no difference. Same with swapping the 42 output tube.

I had not yet reflowed the solder on the grid cap of my replacement 39/44 tube, so I did that today. I'll see if that improves things.