Triode Pentode combo tubes

[TheGimp]

I have a few (ok, near 100) old combo tubes like the 6AT8, 6BL8, 6GH8, etc that are triode/pentode combo tubes that I got from an acquaintance who used to run a TV repair shop.

These seem to be designed for about 40MHz operation, in mixer circuits.

How useful are they in audio circuits? Particularly I'm interested in trying them with the Pentode strapped as the source for the triode as the gain stage ala Mu Follower.

I've also got some pentodes like the 6JC6, 6GM6, 6GY6 that look like they might work but would require an additional triode since they are pentodes only.

Any guidelines would be appreciated.

Or should I just e-bay them and go for more traditional circuits?

[Shoog]

The ECF80 is a dual triode/pentode tube. It is unusual in that it runs both sections at about the same current. It makes a great Mu-Follower with the pentode strapped as a triode on top.

Shoog

[kavermei]

If you keep the tubes you can make something quite unique on-the-cheap, and it can sound excellent but might require a lot of tinkering & learning. There is no such thing as a triode or pentode that works only at high frequencies.

The more traditional circuits or tubes will lead you to outstanding results faster, but may not be as cheap, or intellectually satisfying.

[Tubes4e4]

Hi Shoog,

Originally Posted by Shoog

The ECF80 is a dual triode/pentode tube. It is unusual in that it runs both sections at about the same current. It makes a great Mu-Follower with the pentode strapped as a triode on top.

It makes an even greater µ-follower by using the pentode as what it is - a pentode.

Regards,

Tom Schlangen

[Radioman62]

6U8A is another Pentode/Triode combo I have gathered about two handfuls of. They are Sylvania Goldbrands and have goldpins I have only done some building as a preampstage for a guitar so far but this seems to be a good tube.

Here is some curves on that one and equivalents 6U8A/ECF82/PCF82/9U8A pentode section strapped as triode

[sphinx]

Originally Posted by TheGimp

How useful are they in audio circuits?

Very useful! The Dynaco ST-70 uses them ( a 7199 ) to drive it's push pull output stage.

[Shoog]

Originally Posted by Tubes4e4

Hi Shoog,



It makes an even greater µ-follower by using the pentode as what it is - a pentode.

Regards,

Tom Schlangen

Indeed you are right, but its slightly less straightforward.
I have used mine in a version of the FVP5 with one pentode as a CCS on the bottom, and the other pentode triode strapped as a bootstrap voltage follower on the top. The triodes are in the two critical positons in the front end and as CF in the middle*of the output stage.
Sounds great, and quite simple really.

Shoog

[Jeb-D.]

Originally Posted by TheGimp

I have a few (ok, near 100) old combo tubes like the 6AT8, 6BL8, 6GH8, etc that are triode/pentode combo tubes that I got from an acquaintance who used to run a TV repair shop.

These seem to be designed for about 40MHz operation, in mixer circuits.

How useful are they in audio circuits? Particularly I'm interested in trying them with the Pentode strapped as the source for the triode as the gain stage ala Mu Follower.

I've also got some pentodes like the 6JC6, 6GM6, 6GY6 that look like they might work but would require an additional triode since they are pentodes only.

Any guidelines would be appreciated.

Or should I just e-bay them and go for more traditional circuits?

I've used the 6bl8 for audio and think it can be a very good sounding tube in some applications.

[Yvesm]

You may build a PP amp, a bit more complicated and less powerfull than a Dynaco:

6L6 Push Pull



Yves.

[m6tt]

In some cases the pentode is more like a power tube than a preamp. Could make a nice little one tube per channel headphone amp with two SE transformers. These sorts work well for reverb drivers in guitar stuff as well.