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Triode Pentode combo tubes

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TheGimp

TheGimp

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Joined 2009
#1
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?
 
#3
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.
 
#7
Tubes4e4 said:
Hi Shoog,



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

Regards,

Tom Schlangen
Click to expand...

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
 
#8
TheGimp said:
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?
Click to expand...

I've used the 6bl8 for audio and think it can be a very good sounding tube in some applications.
 
#9
You may build a PP amp, a bit more complicated and less powerfull than a Dynaco:

6L6 Push Pull

Sch-V21.gif


Yves.
 
#10
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.
 
#11
World Audio Design used the ECF80 as a phase splitter in several of their kit amps. The first time I saw this asymmetrical design I was surprised, but actually these simple amps gave quite good results at a comparatively low cost.
The attachment is just a snapshot of the circuit as I'm not sure of the current copyright status of these.
 

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#13
World Audio Design used the ECF80 as a phase splitter in several of their kit amps. The first time I saw this asymmetrical design I was surprised, but actually these simple amps gave quite good results
Click to expand...

I have an old Scott "laborotory reference amplifier" that uses the same circuit. It does work very well and I have built a few duplicates of it that all worked very good too. The Scott used a 6BL8 but I used a 6U8 in my clones because I have a bunch. The 6GH8 works there too.

I have the schematic (and the entire Scott manual) but I am currently 1200 miles away from home (and my lab).
 
#18
Just one thing (sorry to rain; well drizzle only ...) on the parade -

I have used a number of pentode-triodes in various circuits. My favourite is a pentode directly coupled to the triode as a cathodyne phase splitter (in preference to the circuit shown earlier). Only, I found some of these tubes to be hum-sensitive to heater induction, even in power amplifiers, to the extent that I had to use dc on heaters. Being primarily intended for rf use, I guess not as much attention has been given to heater insulation as in audio tubes. (By that I am not saying that the characteristics are in any way unsuitable for audio; on the contrary.) Depending on tube construction, heater leads can occasionally run within a fraction of a mm of a grid lead.
 
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