My amp tech tells me that Baldwin did this to their Sylvania 6BQ5's. Apparently, they were selected tubes (aren't they all) and very closely matched at the factory. They've been tested on a Maxi Matcher, 539C, and a KS15750L1. They're close on all three, but I'm not getting sweaty over it. Any thoughts?

Often OEM tubes have a paint swab, dunno if it really means anything other than to ID it as orig, although date coding system is generally different from replacements as well...

I had two assembly line flats of 100 ea 11FY7 from the old GE plant in Portsmouth... Every one of those tubes have a green paint mark, I doubt any of them are special...

Tom

"Painted tops" (usually exhaust tips on miniature tubes) did indeed represent "selected tubes."

Tomer relates the entire story in his book, Getting the Most Out of Vacuum Tubes (Gernsback, 1960). The practice was very common among older B&W TV sets. You'll often see tubes with their exhaust tips daubed in what looks like red or white paint. Many circuits "skated the rim" as far as workability went, and without the cream of the tubes plugged into them, they'd give trouble. When the cream of the tubes began to age out, they gave trouble anyhow.

With audio output tubes, I'd agree that they were tested for precise matching if used in the output of Baldwin electronic organ amplifiers. If used for other purposes (tone generator oscillators maybe?) in other parts of the circuit, they may have been tested for other qualities.

Larry

Yeah, green paint on top of the getter. Pretty interesting because I've never seen 6BQ5's like this before. They'll have a use in one of the amps I have around here. Appreciate the inputs. Don't know if they were in an oscillator, though. You never know with tubes coming out of an organ amp.

I dug out some RCA 12AX7 tubes from a stash of loose ones to test. They all had the nibs on top painted red. I was thinking, Oh, here we go, extra special tubes. All had one section bad. Maybe the paint indicated they were half tubes?

Those sections were probably worn out by marginal circuits that worked the hell out of them.

Larry

Could also be QC folks testing parts before they were installed in equimpent. especially high reliability equipment. Decades ago, I peered into the ancient EKG machine at my doctor's office and saw there was a 6SN7 glowing in there. Guess I wanted it to be a good one.

That begs the audiophool question-do some doctors prefer tube EKGs over transistor EKGs?

If that's the kind of question audiophools are asking these days, it may actually come under the heading of improvements...



Larry

Does that mean my box of NIB Baldwin (made by GE) 6SN7's are specially bred in a lab from new DNA?

Carl

Not unless the exhaust tips are painted red. Trouble is, you'll have to remove the base or break off the keyway pin to see...



Larry

The Holland 6BQ5's in my Hammond tone cabinet damn near as old as me are naked.