Input Tubes?

[d1983]

I've noticed quite a few designs using a pentode as an input tube.
I was just wondering why this is. I thought that pentodes were high in 3rd order harmonics and therefore would sound bad.
That being said there must be something there that people like about them.
My second question is: Since pentodes must have something to offer, can anyone also suggest some good input tubes, or your favourites (pentode or not)?

[Tom Bavis]

Originally posted by d1983
I've noticed quite a few designs using a pentode as an input tube.
I was just wondering why this is. I thought that pentodes were high in 3rd order harmonics and therefore would sound bad.

Only when driven hard - in an input tube, the voltage swing will probably stay in the linear region. Some high transconductance pentodes can swing well over 100VRMS at 5% distortion (that's nearly 300V P-P!) - they would make good drivers for power triodes. See http://www.pmillett.com/pentodes.htm

For low level preamps, triodes have an edge in noise, though...

[engels]

Although I usually read that pentodes at the input are noisy I've had good results with different types. The recommended types are - of course - EF86 and 6SJ7. I was using metal 6SJ7's and they were dead quiet on input. The total gain is less than from two triodes in series but usually sufficient to drive power tubes without distortion problems.

I think it's better to have one tube instead of two - shorter signal path, less dirt.

The only problem is you cannot throw in tone controls like with double triodes (impedance and gain problems) but for minimalistic designs pentodes are very cool - like SE with EF86 -> volume -> KT88. Who needs tone controls anyway??

I was ripping many German (Semens, Grundig) reel to reels for parts and they always use EF86 as the first tube. They use shielding though - the tubes are wrapped in a sort of steel sheet (almost glued to the glass).

Older pentodes like 6L7 or 6K7 are IMHO not that good. I also tried several loktal pentodes: 7B7 and 7C7 are very similar to 6SJ7 (and usually cheaper).

[Wavebourn]

Originally posted by engels

Older pentodes like 6L7 or 6K7 are IMHO not that good. I also tried several loktal pentodes: 7B7 and 7C7 are very similar to 6SJ7 (and usually cheaper).

6L7 is not a penthode, it is a multigrid converter. 6K7 is a tube with uneven first grid for long smooth nonlinearity, it was used in AGC stages (stages that change gain with bias voltage variations).

[Miles Prower]

Originally posted by d1983
I've noticed quite a few designs using a pentode as an input tube.
I was just wondering why this is. I thought that pentodes were high in 3rd order harmonics and therefore would sound bad.

It's possible that they would. It is also possible to find a loadline that doesn't cause all that third order distortion. Nor is it automatic that triodes, especially the high gain triodes, won't likewise generate lots of third order distortion. With pentodes, stiffening up the screen voltage helps greatly, meaning use a voltage divider rather than a sieries dropping resistor. Also, those small neon panel lamps make good voltage regulators for small signal pentodes.

That being said there must be something there that people like about them.
My second question is: Since pentodes must have something to offer, can anyone also suggest some good input tubes, or your favourites (pentode or not)?

They can get you good gain margin for gNFB in one stage, as opposed to using two or three triodes. As for small signal pentodes, 6SJ7's and 6AU6's are pretty good.

[ray_moth]

Actually, the important thing is to have the sreen regulated w.r.t. the cathode. With small signal pentodes, it's usually enough to use a series resistor brom B+ to the screen and a decoupling cap. from screen to cathode. In fact, it's better to do it that way if any part of the cathode bias resistor is unbypassed.

[richwalters]

Don't forget using the standard signal pentode configured as a triode. I do this on all from EF86 through to ECF series. The improved b/w with a much lower internal R is a distinct advantage and sorts out the thd issue. On some video pentodes strapped as triodes, 25dB gain is obtainable at excellent performance.

The harmonics issue with pentodes can be often tuned out by carefully adjusting the screen voltage. Eg. The 7199 app note (triode elect) illustrates somewhat haphazardly that it isn't really repeatable for consistent performance. Those who don't have a thd analyser, I find making the screen volts lower than the anode voltage by 1/3 is a good start but this doesn't always work for video tubes.
The mistake many make is to run pentodes designs into symmetrical clipping on a scope by adjusting screen grid volts but this doesn't always optimise thd.
The screen grid is a pain....the voltage ill defined v.s performance and it's always been like this. Decoupling to cathode or ground; some tube app circuits prefer one or the other.

One of the reasons pentodes claim to sound "eh" is simply the subsequent circuitry dampens the anode impedance and capacitance so much that b/w suffers. The layout plays an important part and must be kept short.

richj

[planet10]

Originally posted by engels
I was ripping many German (Semens, Grundig) reel to reels for parts and they always use EF86 as the first tube

My source for EF86 too.

The RH84 variant i'm using right now has EF86 (trioded) on the front end.

dave