beta followers

[baggystevo82]

Hello,
I'm still quite new to tubes, and circuits in general to be fair. I've built a KT88 Williamson kit (Ella) and am very happy with it, but after reading through Morgan Jones's book a few times and reading lots on forums like this, I'm hungry to make improvements and start from scratch! I've seen that SRPP stages are very very commonplace in PP KT88 circuits (what I want to stick to for now) and wondered if a beta-follower would be an improvement in this situation. In Morgan Jones book he suggests that beta-followers carry the best traits from mu-followers and srpp stages in general, but then never mentions it again. Is this because it's just more complicated and he wanted to keep his designs for the book simpler? Or is it just because it's no good in this situation. Also there's very little information on beta-followers on the internet, so I'm guessing it's not widely used. Any suggestions from all you helpful experts out there?
Thanks for your help,
Steve

[Eli Duttman]

Aussie DIYers have documented SUCCESS choke loading a SRPP. That idea follows the same general trend as a beta follower, but is simpler to implement.

A plate loading choke whose inductive reactance at 20 Hz. is at least 3X Rp is used in place of the upper triode's cathode resistor. A variable resistor is used in series with the choke and adjusted to make the total resistance of the LC combination = to the lower triode's cathode resistor.

More gain and improved linearity are observed when a SRPP is choke loaded.

[fdegrove]

Hi,

An implementation of the beta-follower idea can be found here:

MANFRED HUBER

Cheers,

[baggystevo82]

Hi,
Thanks, sounds like a nice simple idea. I take it you mean the var resistor is used to match the dc resistance of LR combination to the bottom triodes cathode resistor? Or am I getting my letters mixed up or just misunderstanding...
Cheers,
Steve

[baggystevo82]

Cheers Frank,
As it happened that was the only implementation I found earlier too, hehe. Apart from a few theoretical articals on tubecad. Do you think it's a worthwhile thing to try in your opinion then?
Thanks again,
Steve

[fdegrove]

Hi,

Do you think it's a worthwhile thing to try in your opinion then?

Well....IMO it mostly depends on what you need it for.
Like yourself, I haven't tried it either but it seems attractive enough to give it a go.

Cheers,

[Eli Duttman]

I take it you mean the var resistor is used to match the dc resistance of LR combination to the bottom triodes cathode resistor?

Correct. Sorry about the typo; my bad.

BTW, this is 1 time that the inductance of an "ordinary" wire wound pot. is beneficial.

[baggystevo82]

I was thinking a pair of 6j5 or a 6sn7 in beta-follower driving a KT88...not sure what before that though, if anything (I need enough gain not to need an active pre with a cdp as source) all run off a phase splitting transformer and doubled up to make a PP amp. That's my thoughts at the moment
Thanks,
Steve

[baggystevo82]

Correct. Sorry about the typo; my bad.

Ah cheers. Wouldn't the inductance change depending on where the wipers were in the pot? or would it be too negligable to worry about. Ahh just scanned down the page and saw it says at least 3*Rp so that makes sense now.
Thanks again

[Eli Duttman]

Steve,

I have not tried to work out how much gain the "SRPP" will have to supply. However, the ECC99, 6n1p, 12AV7, and 12AT7 with mus in the 20s, 30s, 40s, and 60s respectively should provide you with a good choice.

BTW, are you going to triode wire the KT88s or run them UL? If you are triode wiring, I suggest you acquire a set of EI KT90s manufactured before the current quality control problems set in.