6026 Sub-mini tube - useful output tube??

[moonbird]

Hello all --

I have been looking at the 6026 triode sub-mini tube. Would this be a possible output tube for a small sub-watt pedal based amp?? I am thinking maybe a ss pentode front end (5899 or 5840). However, I can't (as yet) do all the nuts and bolt inerds. Might be fun? thx.

[Cliff Schecht]

There are a few problems with that tube. The first problem is the really low plate voltage it can handle, 150V max. Your next issue is fighting the really low interelectrode capacitances. This is great for RF work but not so desirable for audio work. You either have to limit the bandwidth yourself (kills the sound) or use some kind of snubber circuit (i.e. a grid stopper) to reduce the bandwidth. Having too much gain at high frequencies tends to cause oscillations and you will always be fighting against this.

Then again, they do give specs for a class A1 amplifier. I wouldn't expect more than a lonely Watt out of this tube but who knows, it could sound pretty cool. Just be prepared to fight a little bit to get what you want (especially if you go for high gain).

[moonbird]

I am not much of a high-gain nut at all - its all about novelty and tone for me -- thats what big SS PAs are for right?? I love tube sound but sand is just better/cheap/lighter for big transparent amplification in the final analysis ...

Any helpful hints!!!

Paging Frequency Central !!!!!!!

[frequencycentral]

What Cliff said, anything you design around it will be dicatated by the properties of the tube itself. Yeah, expect a watt of slightly more. Designed for use in 'radiosonde' - a balloon-borne instrument platform with radio transmitting capabilities. If you have some anyway, worth a try to see what you come up with.

[PRR]

Why this specific tube?

There's dozens of small tubes.

A particular point: Read the fine print on the RCA sheet. "...applications... where a tube life of only a few hours is required." A radiosonde is a balloon. You send it up to see which way the wind blows aloft; and in this case to send back temperature and other info.

The 3 Watt rating seems excessive to my eye. I believe it is a fine 1.5 Watt tube. If the tube were readily available (on a Saturday night!) then 2W might give reasonable stage-life. 3W is really a one-night-stand (even one-set) rating.

It's actually hard to hit any 2W-3W dissipation in any useful SE audio amp without grid current. I'm plotting numbers beow 0.3 Watts output, with distressing (even for gitar) THD numbers. (The tube is SO optimized for flogging that it is hard to swing it the other way.) (For triode audio POWER we want a LOW-Mu triode, Mu nearer 5 than 20.)

Anyway.... triodes are mellow, rubber-hammer, HI-FI.  Rock-n-Roll demands nasty naked pentode hardened-iron-hammer outputs!

The overall size of a tube amp is usually NOT about the tubes, but about the other parts. Especially power transformer. And HEAT. If you have a USAF budget and can get exact-right super-small power supply, maybe the tube is a limit..... usually not.

Assemble parts for 150V 10mA, PT and a half-Watt OT. Include jacks, knob, etc. Now trial-add three different tubes: 6026, 35C5, and 6AU6. It's hard to believe this would all fit in less than 2"x4"x4", or that the tube makes a difference. The 35C5 and kin are known-GOOD small audio power devices proven in a million radios. Worked at less than a Watt the tube life should be tens of thousands (not "a few") hours. The 6AU6 won't go much over a third Watt but is extra easy to drive and I've never killed one.

[PRR]

 > fighting the really low interelectrode capacitances

It's not a fight.

The cap is not that low. Half a 12AX7. A hair higher than a 6V6, and since moon is talking "power" a 6V6 may be a good benchmark.

Wiring capacitance in audio chassis is typically larger than tube capacitance. With too-tight layout, it may be higher for the sub-mini than for a typical mini.

If bandwidth were really too large, you'd add resistance or capacitance to taste. Just enough and no more. Classic Fender stages have 15KHz-25KHz bandwidth. Some older Gibsons were narrower, sloping by 6KHz. You can add grid-plate capacitance but nobody does. You can add series grid resistance, or you can shunt a cap plate to ground (or cathode); these are common in over-thought guitar amps. You typically have more than one or two stages, and typically only one or two top-audio roll-offs. A lot of drivers and outputs are flat far beyond 50KHz. The preamp and OT provide plenty of top-droop.

There's other reasons why I think there's other tubes which will can be made to make music quicker and with less fuss. And I think the 6026 can be used as a low-rated half-12AU7 and play today. But the size is not a major bonus, and the capacitance is no demerit at all.

[moonbird]

Hmm - never looked at it quite that way -- however I think I will pass on this tube. Thanks for the advice all!!

[zombiwoof]

Those mini-tubes are being used in pedals today, I think Siegmund was the first to use them that way, and others have followed (like Seymour Duncan, who is using them in at least one pedal).  In fact, I think that very tube is one of them.

Al