I've always worked on solid state stuff until recently when I picked up a lot of 5 tube AM radios for $20. I've recapped and rewired(insulation was cracked and gone) An RCA AA5 (RC1064)and it works but audio is very muted and there is an occasional crackle noise from the speaker. Does anyone know of a shop around the Knoxville TN. area that tests tubes? Sad thing is I had 3 nice tube testers(TV7D/U, HIckok 6000A and Eico 666) and sold them 6 months ago because in my head I would never need them.. lol.
Tube Testing
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kvflyer
Super Member
In addition to that, depending on the construction of the IF transformers there is a thing called silver migration disease or similar. The silver tarnishes in the mica capacitor inside of the IF can and causes noise that sounds like lightning crashes in the radio. I hope you find your problem and it's not SMD as they call it.
It's always the best starting point with any tube gear.
Was the first thing I did as well as cleaning the sockets.. Actually did it a few times.
Tube testers and their ability to cull out bad tubes can be iffy at times, but I truly believe if you're going to use tube gear and/or repair them, they are still an indispensable tool in your shop. They will help you find shorts and weak (testing) tubes. But peterh is right, they are not infallible. I recently had a 2-month old 12AX7A go wonky despite "testing strong" in my Hickok 800A. "In situ" really determines if the tube is up to the task in whatever piece of gear you use it in.
Buying a tube tester to work on an AA5 will cost more than a full set of tubes for an AA5. Hardly worth the expense.
In-circuit voltage checks will likely tell you anything you want to know about tube condition. Depending on the type of IF transformer though, the silver mica problem is very possible.
In-circuit voltage checks will likely tell you anything you want to know about tube condition. Depending on the type of IF transformer though, the silver mica problem is very possible.
Few testers, especially for output tubes, test them at the plate voltages and cathode currents that the tubes will see in the amp.
kvflyer
Super Member
In addition to that, those tubes get very hot and can often have grid emission after they get hot. Symptom is that the sound starts to get garbled after the set has been on for a few minutes. Some (Many?) tube testers cannot test accurately for grid emission. I have a Heathkit TT-1A and it has a "Grid Current" test. However... if you have sprayed contact cleaner on the switch, it will often show current on a very humid day. However, 50C5, 35C5, 50L6 etc. are available for reasonable money, NOS.
yeah I've run into that, the cathode voltage also drifts up as a clue that the tube is saturating.
the leakage tests in most testers I'm familiar with are simple DC resistance checks between elements. Sometimes on a meter, sometimes its just a neon that kicks on once leakage current exceeds however much. It can't really check for grid emission under running conditions very accurately. I've found it in-circuit, jumper the grid to ground resistor and suddenly the current drops back to where it belongs. It looks just like a leaky coupling cap so that usually needs to be unhooked to confirm.
the leakage tests in most testers I'm familiar with are simple DC resistance checks between elements. Sometimes on a meter, sometimes its just a neon that kicks on once leakage current exceeds however much. It can't really check for grid emission under running conditions very accurately. I've found it in-circuit, jumper the grid to ground resistor and suddenly the current drops back to where it belongs. It looks just like a leaky coupling cap so that usually needs to be unhooked to confirm.
I have a TV7 tester that I rarely use. Most of radios I work on are AA5's and I use a very simple little tester that just shows if the filament is good on radios I pick up. It's nice to have the tester for more extensive testing.
Mike
Mike