Where have all the pentodes gone?

[dhuebert]

I recently got a Stromberg-Carlson AU 33 PA amplifier. 6SJ7s on the input and 6L6s on the output. Unfortunately this one has seen a number of repairs and is no longer very original. Anyways:

My question, after reviewing a number of older schematics is: Why don't people use pentodes on the input anymore? Everything, including the Dynaco clones, which used to have pentode inputs have gone to triodes. Anyone shed any light?

The reason I'm asking is: I'm thinking of a preamp with a little compression built in using a 6SK7 and just wondering...

Don

[Shannon Parks]

I recently got a Stromberg-Carlson AU 33 PA amplifier. 6SJ7s on the input and 6L6s on the output. Unfortunately this one has seen a number of repairs and is no longer very original. Anyways:

My question, after reviewing a number of older schematics is: Why don't people use pentodes on the input anymore? Everything, including the Dynaco clones, which used to have pentode inputs have gone to triodes. Anyone shed any light?

The reason I'm asking is: I'm thinking of a preamp with a little compression built in using a 6SK7 and just wondering...

Don

Hi Don,

There's always a multitude of reasons, but I would guess a couple factors. However unfair, one reason would be that small signal pentodes aren't seen as 'sexy' and aren't viewed as sonic nirvana. For example, 6AU6's sure seem to be a great voltage gain tube and there's plenty of them out there (I have a small box myself). Not sure about noise and microphonics on these as I'm a 2nd generation tube DIYer with no experience with them. Being a 7-pin tube and needing to hit the 'sweet spot' for operation is another factor. Whereas the small signal dual triodes are pretty flexible.

The second reason is the lack of good modern production. EF86's were a great tube, but where can I get a good new one for cheap? 7199's?

I say experiment away. Plus you can always triode strap them for a totally different tube!

Shannon

[EWBrown]

There are eastern european and russian made EL86 tubes available, I've used them in one of Uncle Ned's ST70 replacement boards. THe pentodes offer a lot more signal voltage gain than do triodes, and in the case of the very simple ST70 circuitm, this gain is needed in order to obtain full input, without using a real "hot" preamp.

Similarly, the SCA35 uses 7199s, in order to haev the extra gain that gets otherwise lost through the passive tone control and balance circuitry.

If one uses an LTP phase inverter instead of "cathodyne" (which offers no voltage gain), then an all-triode circuit is practical, as in Shannon's ST70 driver and the one that Roy Mottram has available (all 12AT7s).

The Heathkit AA150 used 6AU6s as gain stages, with decent results, and 6EU7s for the phono preamp statges - these are basically 12AX7s with a 6V filament and a different pinout in order to reduce hum and noise.

/ed B in NH