Triode, pentode

[Number 9]

In glancing over the tube discussion boards for the past year, it seems that many people prefer "triode" mode to pentode sound-wise. This may not be true, just an observation. As for single-ended and push-pull, will not go there - this is not the forum for that debate.

:headscrat
q: Has anyone ever tried rebiasing their Mac tube amps from pentode to triode mode? If so, what's the scoop?

I fully understand this would amount to sin and heresy, paricularly with a Mac collectible amp which we all know is primo engineered, but it seems that some people are doing this with Fishers and Scotts with good results.

For example, there is the Fisher 400-X that Paul Grzybek of Bizzy Bee in Chicago offers. He puts in EL-34s and biases them to triode mode using a Fisher 400 receiver. Also found an article where someone converted the Scott 299C to triode operation with great results.

 

[Dan ʕ•ᴥ•ʔ]

Never owned a Mac amp that I did that to, but I did do it to a Marantz 8B I once owned. It was simply a job of moving a few resistors around on the output tubes and then re-checking and adjusting bias, which was easy on the 8B because it had individual bias pots and its own built in meter . Other than the output power dropping from 35 wrms p/ch to 17-18 wrms p/ch, the difference in sound was obvious. In Triode the presentation was more natural and a bit relaxed. However, bass became a bit tubby and loose on some material when pushed. In Pentode, it reminded me of a good solid-state power amp. In Triode it came *closer* to the magic of real Triodes, but I never could seem to get a good mix of both worlds. I wanted the dynamics of a A/B Push-Pull amp and the natural midrange and top end of a Class-A Single-Ended. The closest I've come is a Pass Labs Aleph-30, which although is solid-state, is Class-A and Single-Ended.

 

[Number 9]

Celt, that's the other thing you made me think of - some modders also change the A/B biasing so that amps run more in A class. This too, seems to work well in some cases.

I have a feeling, after changing to Triode, and re-biasing for Class A, the amp probably would not sound like a Mac amp anymore.

 

[ron-c]

Mc tube amps use the Unity Coupled output design which uses the cathode and the plate equally to produce power. This is a fundamental difference from other designs. The only similar design is Circlotron.
Of course you can remove say half the output tubes and let one tube complete both sides of the waveform.

Ron-C

 

[Number 9]

So Ron-C, are you saying that you just can't do it because of the unity-coupled design?

 

[ron-c]

Yes, it is not that simple.
Ron-C

 

[Dan ʕ•ᴥ•ʔ]

We learn something every day! I did not know that!

 

[CarlV]

Well that will save me a lot of grief. I would have a lot of no-fun trying to figure out why it didn't work if I tried it! I have a single ended EL84 and Celt is right about S.E. pentode's being tubby with full gain. 2/3rds is real sweet though and plenty loud on the speakers I use.

Carl

 

[Number 9]

Carl. SETs I've heard tend to lack guts in the bass. Beautiful midrange though.

I think the output tubes come into play too. Some people have their favourites, be it: EL34s, KT88s, 6L6GCs in PP, 45s, 300B, 211, 2A3 and 845 for SE.

PP designs in triode mode seem to be gaining in popularity on the discussion boards as best of both worlds, though with a twist - with differential circuits and transformer-coupling. Pentode does not seem to have much of a following.

 

[CarlV]

The EL84 pentode S.E. is just a part of my collection, a cute little Italian guy and it is real sweet sounding. A keeper to me.
In this room I'm in I play these I built:
http://www.audiokarma.org/gallery/showphoto.php?photo=780&password=&sort=1&cat=500&page=1
Through these:
http://www.audiokarma.org/gallery/showphoto.php?photo=778&password=&sort=1&cat=500&page=1
If you heard this combination you would find no lacking in bass whatsoever, or volume either. Just it's silly little 175 mid horn.
This will be corrected with one of my pairs of 375's and a pair of
500hz Edgarhorns as soon as I can afford them.
I went to the H.E. show earlier this year and heard both PP and
series SET's. Going by my ears both are a waste especially PP IMO. You want SET with power go for an 845 SET amp(s). Or even one of Jack Elliano's newest designs. But this is just me and my ears talking.

Carl

 

[Number 9]

The new MC2275 McIntosh is intoducing at CES will likely be Unity-coupled. This amp will be a keeper no doubt, but I wonder if the SE vs. PP, Triode vs. Pentode thing will come into play with some buyers. The die-hard-in the wool tube types seem real fickle.

I know when I glanced at some Mac sales literature from the '50s and '60s, Mac seemed to put some effot into explaining Unity-coupling and why its good for you. Perhaps its time to toot its horn once more.