Tube Diving

[Diablo]

A buddy of mine let me know that he's been sitting on a large stash of tubes for a few years. He let me take home 8 cardboard beer cases full of tubes to sort through. The tubes came from the estate of a TV repairman / alchemist / mad scientist - apparently this fellow built his own x-ray machine in his garage to play around with. I sorted through a couple of hundred tubes last night and most of 'em don't do me any good. However, I did score on 16 12AX7 and 7025 tubes, a couple of 12DW7 and 6FQ7 for my Ampeg V4. I also found 10 EL84 tubes. Some NOS and some used/good - RCA, GE, Sylvania, Mullard, you name it. I'm still looking for some good power tubes (6L6, 7027, EL34), but I have a feeling that TVs didn't use 'em, so he didn't stock 'em. I've got 4 more cases to sort through of some really strange looking old tubes.

[Arthur Dent]

Very nice!

Don't automatically assume that tubes you've never heard of are useless. I'd be surprised if there weren't some very useful and beautiful-sounding sharp cutoff pentodes in there (EF86, 6AU6, 6BA6, etc.)

[Rob Mercure]

Diablo,

Get the RC-30 RCA Receiving Tube Manual and reexamine your tubes. A 6HK6 has a 42 watt plate dissipation vs only 35 for the 6550! I'd grab every large tube - "primitive: 2A3s are selling for fortunes for HiFi nuts- and any non-"signal" size tube with a number that begins below 6 (3DG4 for example) as well as any tube whose designation is totally number (5751 for example). I'd ignore most tubes with a last number that ended in 4 or less (some exceptions though such as 6X6, 6CA4, etc. - definitely those with low last numbers and plate caps). But don't discard plate caps for the larger tubes as these are going to be your highest power tubes.

Again, get the tube manual and this will steer you better than any other tome (although Sibley's "Tube Lore" is a great companion to the RC-30).

Consider yourself lucky - most folks are stuck in the "ghetto" of 12A_7s and 6_6s and EL_4s and never break out into designing with great, lesser known, tubes (try 6AV5 and 6AU5 for example) and there are scads of TV triode/pentodes that make great preamp tubes (6BR8 is a favorite) while some, such as the 20EZ7, are 12AX7s with different heaters (same guts as highly desireable tubes - Mullard made many of these) and tubes, such as 6AQ5, 6CM6, and 12AB5, are essentially 6V6s with miniature bottles!

And the great thing about having a good stock of interesting tubes is that you can set back 2-3 of whatever kinds you used in a design as your "living" stock and thus have a true "lifetime" supply of tubes for that piece of equipment.

Rob

[Steve Conner]

Yeah, there are lots of interesting tubes out there. In particular, TV sweep tubes make good audio power tubes. Traynor used to make a monster that put out (at least) 250 watts from four 6KG6 tubes.

Like most TV tubes, they came in different heater voltages, for instance the 6KG6 was also available as the 40KG6.

[Diablo]

I finally went through the rest of the boxes and hit the motherlode. I found 8 used 6L6G tubes (ST shape), RCA, Tungsol, Kenrad, Premier, Sylvania. I was nervous about using these old tubes with a high voltage amp, but I decided to give it a shot anyway. Plugged two RCA tubes into my Hot Rod Deluxe and set the bias to 66 mV, and they work just fine. I also came across a handful of 6V6G tubes (ST shape), and a dozen 5U4GB. I don't have an amp that uses a tube rectifier, but I will some day.

[michaelj]

sharp cutoff pentodes?

What would be the application for these in guitar amps? I have a small pile of 6AU6A in my stash as well as a bunch of 6U8A [medium mu triode / sharp cutoff pentode].

I have some 6DJ8s and 6SN7 etc but I think those are more for the HiFi crew.

Mike

[Rob Mercure]

Lessee, 6U8 is a triode pentode which could be used for pentode input and triode PI or perhaps cathode follower to tone stack. 6AU6 is perhaps the commonest used post WWII pentode in the US and losta line connected amps used the 12 V cognate (12AU6) for an input tube. While there's probably some guitar amp out there somewhere that used the 6DJ8 I'm not aware of it but the 6SN7 was used in Gibsons, Ampegs, and I believe at least one Fender (sorta like a 12AU7 or 6CG7 but not perzactly either). All you need is an output tube and you're rockin'

Rob