Does gain of tube matter with a cath. foll.

[traynor]

I built an amp using a pentode as the first gain stage. I had to add a 6AV6 to the amp for the cathode follower. This time around I was thinking of using a 6U8 or 7199 that has a triode. The only problem is the triode that comes with the pentode is a Med Mu. Is there a difference when using different gain tubes as cathode followers. I don't have an amp here that i could test this out.
Thanks
Brent

[mooreamps]

I built an amp using a pentode as the first gain stage. I had to add a 6AV6 to the amp for the cathode follower. This time around I was thinking of using a 6U8 or 7199 that has a triode. The only problem is the triode that comes with the pentode is a Med Mu. Is there a difference when using different gain tubes as cathode followers. I don't have an amp here that i could test this out.
Thanks
Brent

I believe the only thing that will matter, is in which the circuit is going to set up for correct bias for the stage. In this , I would be looking more at the plate resistance of the stage in the selection of the tail resistor.

For example, typicially I'll use a 56K tail resistor for the 12ax7 configurated as a C/F, mostly because the plate resistance for that tube is approx. 66 K ohms. This, in my mind, sets up the stage as close to a 50/50 split between the plate resistance and the tail resistor. So, if i were to use this as a "rule of thumb" for other types of tubes, I'd be looking at the plate resistance of that tube.

So, if you wanted to do this with a pentode, I'm just thinking for openers, you would want to have that set-up in a "triode" configuration, {ie for a lower plate resistance} and set up accordingly. One would want the lower plate resistance for any C/F, as that would allow you to source more current, hense the lower driving impedance, which is what we use C/F stages for, in the first place.


So, according to my data sheet for a 6V6, in triode mode, it shows the plate resistance to be 1960 ohms. Now, I've not ever set-up a pentode as a cathode follower, so these values are un-tested. As a side note, my first choice would be to use the 12au7. I show it's plate resistance at 7700 ohms. But, even that is still much lower than the 12ax7 which is listed at 62 K ohm in my RCA data sheet... Also I'm looking at the cost/benefit aspect. You get "two" C/F stages inside one 12au7. You get only "one" C/F inside a 6V6. Also, would there really be any real "better" audible difference between the two ?? But, if you want to do for the educational benefit, then yes, absolutly go for it.....




-g

[black_labb]

he was looking to use a pentode/triode tube instead of the pentode in the preamp, not the 6v6 if i understand correctly. the 6v6 was just the output tubes from the amp.

very good tip though. very helpful to me too as im using some 6bl8's in an amp im going to be making, and am considering wiring the triode in one half as a cathode follower.

[jrfrond]

Mu doesn't really matter, because the voltage gain of the stage is 1, no matter what. However, the impedance, hence the CURRENT GAIN (which is what a cathode follower, aka "BUFFER" provides) will be different for each tube type. The lower the plate impedance, the higher the current drive capability. For tone controls, as is commonplace, I don't think it will make much of a difference.

[6267]

http://www.freewebs.com/valvewizard/accf.html
http://www.tubecad.com/2007/04/blog0104.htm

some stuff I found, the valve wizard site is quite good. I hate doing math by hand (yes, I'm a youngin'), so I write the equations in as calculator programs. Tubecad has some good equations for basic triode voltage amps in one of his articles too.

[black_labb]

having done some reading (thanks for the links 6267). the only issue i have is reguarding elevating the heater voltage. would you usually use caps to block dc, and use the bias or similar to raise the voltage? or would you elevate all the heaters (which should help minimise hum).
using 6bl8's it is pretty crucial as they are rated at only 100v cathode to filament voltage.

[defaced]

There's alot of talk on the SLOClone forum about elevated heaters. Here are two schematics I've pulled from there (untested by me, but used by others).

In high gain amps it's common toe rectify and filter the first two stages, then run AC on everything else what is what the second schematic is showing.

[Steve Conner]

When I did that Crown hi-fi amp I just elevated the whole heater string, power tubes and all, to +40V.

The effect of a low-gain (more precisely, low-gm) tube in a CF is to increase the output impedance and hence lower the gain below 1 when it's loaded. It might go down from 0.9 with a high-gm tube to 0.8 with a low one, but it's still hardly noticeable. A 12AT7 is a good compromise.

[mooreamps]

The effect of a low-gain (more precisely, low-gm) tube in a CF is to increase the output impedance and hence lower the gain below 1 when it's loaded. still

I don't suppose you would post your mathematic calcuations ; that would illustrate this point ?

-g

[black_labb]

thanks for the help. think i may just elevate the whole heater supply 40-50v

[Steve Conner]

The output resistance of a cathode follower is roughly 1/gm, and the loss of gain follows from the existence of this resistance, and Ohm's law. For a more detailed discussion RTFRDH4

[tubeswell]

I must download those manuals this w/e - I desperately need to do some more reading

[traynor]

Thanks for the replies. I was out of town, otherwise I would have thanked you all sooner.
Looks like using the "med-mu" triode section of a tri/Pent tube 6U8 or 7199 as the C/F won't make much of a diff. This gives me the ability to use one tube. Instead of using a "5879 Pent" with a "Hi-mu" 6AV6 as the C/F.This will now cut down on a 7 pin socket.


Brent