6AN8A or 7199 as input tube?

[AdmiralB]

With the resurgence in interest in pentodes as input tubes, why aren't people using these? Seems like you could get similar results as an EF86, and you have the bonus of the triode to use as a CF, to fix the high output impedance.

[JWK]

My thought is to use a mosfet follower after a pentode. The large signal swing with the cathode/heater voltage thing bothers me. With the mosfet follower I don't have to worry. With a low enough impedance and large current drive I'm hoping to get a 10K volume pot out of the deal. Might solve some of that low volume crappy sound. Just something I want to try, anyway.

[Amp Kat]

With the resurgence in interest in pentodes as input tubes, why aren't people using these? .

One of the main reasons is cost and as snapcase just pointed out in the theory and design 7199 thread the Sovtek 7199 is discontinued so as far as a production amp that's why but as far as a home brew I can't see why not if your willing to spend the bucks it's feasible. It could be a concern depending on how you set up the triode stage as a common cathode amplifier it would rock and as a Cathode follower you would have to make sure the plate voltage isn't to high to exceed the heater to cathode differential voltage. Nothing wrong with driving the stack with a higher impedance as it's been pretty popular itself and has worked very well for years. I think the driving the tone stack with a lower impedance is an overhyped expression and I actually prefer off the plate coupled version better in some situations. Another reason is a Pentode is noisier than a triode early in a stage it could be a problem to control but maybe with a topology where it's at the end would work better. It is a possibility but theres more than one way to skin a cat.

[DirtyGeorge]

I'm planning to use a 6AN8A for my next build. Pentode input stage into cathode follower into a Marshall tone stack.

I'm not sure what size plate load would be appropriate. The Sunn schematic, where this tube is used as driver/PI, shows 270k. Looking at the data sheet I would think that better linearity would be achieved using a much heavier plate load. I am considering 25k for this application. Does anyone have any thoughts on this?

Thanks,
DG

[Rob Mercure]

While the sovwrek tube may be discontinued there are scads of triode/pentodes that were designed for TV service that work extremely well for audio - and their available for just pennies from AES and other sources. The 6BR8 works very well and if you just want the pentode w/o the triode the 5879 was specificially designed for input and is a very quiet valve - I've built several amps with it (a bit more pricey than the TV tubes but still reasonable).

Rob

[Bruce / Mission Amps]

While the sovwrek tube may be discontinued there are scads of triode/pentodes that were designed for TV service that work extremely well for audio - and their available for just pennies from AES and other sources. The 6BR8 works very well and if you just want the pentode w/o the triode the 5879 was specificially designed for input and is a very quiet valve - I've built several amps with it (a bit more pricey than the TV tubes but still reasonable).

Rob

And the 5879 was used by Gibson in a some amps so there is existing schematic artwork to build from.

[Rob Mercure]

Like Bruce sez, lotsa designs to copy - RCA receiving tube manuals had them for years in the mid-late 1960s right up until they stopped publishing the manuals. Noise is energy in the system being dissipated in a manner that you probably don't want - and it can often be expressed as a percentage of signal level or energy used, etc. While sometimes the proportion varies with overall system energy usually it remains a somewhat stable fraction so if you can lower the overall system energy you can often lower the perception of noise.

Audio signal pentodes seem to run "quite quiet" at low Ep and Eg2 voltages - one of my favorite circuits using the 6BR8 runs the thang with about 45 VDC on each of these electrodes! While you lose gain pentodes have gain "to throw away" and you wind up with a really quiet amp. If I remember correctly Tektronics used the 5879 in one of the "3000" series 'scope's plug in vertical modules in the late 1950s/early 1960s and the schemo used to be available on the net. While this isn't an "audio" application it does provide some examples of very low voltage, very low noise, very high bandwidth use of pentodes. It also seems that I remember most input pentodes from that era being mounted in some sort of grommet or rubber block or other physical isolation shock assembly - could be as simple as isolating the tube socket mounting "ears" from the chassis with washers. Either that or you could cast them within a block of concrete but that may make replacement a tad difficult (yeah, I'm kidding but if you ran a 5879 at reduced heater dissipation and really low B+ you might have the encased tube outlast you!).

Rob