New Production 6AS7/6080 Tubes?

[karl]

First, an introduction. I just joined recently and ordered a Crack kit. I have had a "love/hate" relationship with headphones for years that has turned into definite "love" recently. I want to see how an OTL amp compares to my Headroom Little.

Second, Is anyone making new production 6AS7s or 6080s? I would think there would be sufficient interest due to demand by the OTL power amp owners and the units being made by Woo Audio.

Regards,

Karl

[Paul Birkeland]

The old stock supply of 6AS7's and 6080's is absolutely huge.  Everyone with an OTL amp that uses this tube could go out and order 50, and there would still be tons left. 

There are Sovtek 6AS7's ($45), Chinese 6AS7's ($15), and old stock tubes ($5-$40).  I can't quite figure out who is spending nearly $50 for the new production, although perhaps the sections are more closely matched.

[Jim R.]

Anybody know the history of this tube, where it was used and what the  variants were specifically for?  Always curious about tube lore and history.

-- Jim

[Paul Birkeland]

The 6080 was designed as a low-ish voltage high current series regulator (essentially a high current triode with very low plate resistance) designed in the late 50's/early 60's.  These are really tough little bottles that can burn of a ton of heat, and  since you can boil a lot of current through them, they work pretty well run as cathode followers.

Since their inception, these have been used pretty frequently as output tubes in OTL amps, as their plate resistance is nice and low, and the tube cost is pretty low.  Unfortunately, the matching between halves is fairly marginal (it wasn't important for the original design), so push-pull OTL can be kind of tough.  On the other end of the spectrum, some people have made simple push-pull amps out of these, as a 2.5-5k output transformer is easy to get and perfectly reasonable as a load. 

There are other similar tubes like the 6336, or the paralled double triode 6C33.

[Jim R.]

Thanks, Paul.

Looking around I see that there really is no shortage and prices are quite cheap as a result.

-- Jim

[Paul Joppa]

I think there was always a market for multiples, to get more current from a regulator. The data sheets often have extensive information on using cathode resistors to force some balance between sections. I strongly suspect that the existing old stocks have been picked over and what's left as "new" old stock are those that were rejected for bad balance!  :^)

Here are the variants I am aware of:

6080 - industrial version in cylindrical bottle, very common
6082 - same except 26.2v 0.6A heater
6520 - 6AS7 with balanced sections - caveat emptor! *
7105 - 6080 with 12.6v/1.25A heater

* I've seen tubes labeled "6520/6AS7"; the one I bought a decade ago was nowhere near balanced. I tested a half dozen examples at that time, and found only one which had reasonable balanced sections. The cathode follower is a good application if the sections are not well balanced.

In looking up these tubes, I spotted the interesting 7802, like a 5998/WE421 but with even  higher mu and transconductance (hence lower output impedance - 50 ohms?). High transconductance means it really would rather oscillate than amplify, but careful layout and grid stoppers should make it worth a try if you can find one.

Also of course the 6BX7 has the same pinout and can easily run at 30mA, might be an interesting substitute.

[tdogzthmn]

I think someone should make a list of compatible tubes that work well with the Crack.  I have been looking around on ebay at various 6AS7G/6080 tubes but I dont know which ones are reported to sound better than others.  I am also interested in Capacitor upgrades and want to know which are recommenced.  My goal is to build a Crack then begin upgrading the various parts until max it out or I go broke trying.    

[Grainger49]

I think someone should make a list of compatible tubes that work well with the Crack.  I have been looking around on ebay at various 6AS7G/6080 tubes but I dont know which ones are reported to sound better than others.  I am also interested in Capacitor upgrades and want to know which are recommenced.  My goal is to build a Crack then begin upgrading the various parts until max it out or I go broke trying.    

Take a look here:

12AU7 Equivalents

I have started a list of input/driver tubes that can be edited if I am wrong about some.