Why no single triode OTL power stages?

[leadbelly]

Feel free to go away and start as many "Why OTL's stink" threads as you want.

 

[andrei]

I guess you don't like hearing what I have to say. Sigh...

OK, if you are set to build SE cathode follower with a triode, check out 6528 tube. With two sections parallel, you get Gm = 47mA/V, Rp = 120ohm with mu = 9. Significantly better than 6C33 (Gm = 40mA/V, Rp = 130ohm, mu ~ 2.2) for cathode follower duty.

 

[audio3chung]

6c45

Hans,
Thank you for the reply.
Howabout using 4 x 6c45pi as CF output stage.
Ra=1.3K, Mu=52 Gm=45mA/V,R out=Ra/(1+Mu)/4
Is it correct the output impedance=6 Ohms
without NFB

Regards CHOW

 

[tubetvr]

Is it correct the output impedance=6 Ohms

Yes it should be something like that but it will not give much power, max current is specified to 52mA for each tube or 208mA for 4 tubes in parallell, this only gives a output power of ~0.17W.

Much better to use feedback or even better to use a SEPP stage ala inverted Futterman, preferably with 6C33C.

There are better tubes "available" like the 6528 mentioned here but it seems to be made of unobtanium therefore discussing these rare beasts is more of an academic excercise.

Regards Hans

 

[audio3chung]

6c19

Dear Hans

I know 6c33c is out most suitable for OTL,
but 6c19 only cost for a few HK dollars from China,
And I got 40 of them, that's why I want to scratch the
SE1.5WOTL.

Regards CHOW

 

[tubetvr]

If you want to use 6C19 you either need to use a lot of them or use feedback, I would use feedback and that is also obviously what Rozenblit is using in his SE OTL.

Regards Hans

 

[EC8010]

I don't want to water anyone's firework, but my understanding is that OTLs become more practical the bigger they are. You end up having lots of valves in parallel to handle the power and by doing so, you get a reduced output resistance even before feedback. However, I keep on wondering about a low power OTL using 6C45 in Class A2 with perhaps only 60V across them and driven by something beefy, another 6C45, or (dare I say it?) a source follower.

 

[tubetvr]

However, I keep on wondering about a low power OTL using 6C45 in Class A2 with perhaps only 60V across them and driven by something beefy, another 6C45, or (dare I say it?) a source follower.

It would work but it doesn't give higher output power as the allowed max anode current is only 52mA per tube. You can probably exceed this by up to a factor 2 if you can live with the shorter life but it would still only give very low output power as a CF.

The problem is not to get the tube to draw high current as such but to have reasonable reliability.

Regards Hans

 

[EC8010]

Oh dear, I just did a few sums and you're right. The 52mA limitation rather puts the kibosh on the whole idea.