Power tube as a R-C stage

[Andy Le Blanc]

I was looking through a HP audio oscillator schematic and I noticed they used a 6f6 power tube as a R-C stage.
It had a 10k 20 w plate resistor and the screen was handled thru a 10k 10w/ 25k 10w divider, the 25k being on the ground side.
Both plate and screen were supplied from the same 390v source, and it was cathode biased with a 820r 2w.

looked like it would be fun and simple to play with.

Where in guitar amp would it be most usable?

I was thinking maybe to replace a cathode follower with it for giggles...
Maybe try driving a verb tank...

[gingertube]

Andy,
Power Tubes suffer noise from random low level grid current variations so I would use it toward the end of the pre-amp section somewhere.

Ideally I would have it as the 3rd stage. That would allow you to take advantage of it's lowish output impedance to drive the tone stack or use a Baxandall (or other) tone stack in lieu of the FVM stack .

If it were the 3rd stage then it is likely that there would be an interstage attenuator feeding it. That would allow a low Rg1 value (bottom resistor of the attenuator) to keep that grid noise low.

Having said all that it is going to have pentode level gain so you might not want 3 gain stages. Using it as the 2nd stage in a 2 stage preamp might suit OR keep it as the 3rd stage but triode strap it.

ASIDE:
My favourite little amp at the moment runs 6J7 pentode => 6J7 triode strapped => 6V6G with an 80 rectifier.

So there are my "random ravings".

Interested to see the suggestions of others.

Cheers,
Ian

[Reeltarded]

This is why I love this place.

You guys are crazy.

[Cameron]

I was looking through a HP audio oscillator schematic and I noticed they used a 6f6 power tube as a R-C stage.
It had a 10k 20 w plate resistor and the screen was handled thru a 10k 10w/ 25k 10w divider, the 25k being on the ground side.
Both plate and screen were supplied from the same 390v source, and it was cathode biased with a 820r 2w.

looked like it would be fun and simple to play with.

Where in guitar amp would it be most usable?

I was thinking maybe to replace a cathode follower with it for giggles...
Maybe try driving a verb tank...

Also check out the Sound City SMF amp. It has 2 el34s as a phase inverter

[Andy Le Blanc]

Glad to have been referred to that soundcity amp, every now and again something jumps up as "unique".

No worries around random ravings, I been digging a bit.

One application using a power pentode R-C stage is to meet the requirement of an enormous drive voltage for directly heated power triodes, if you were
to try it you could implement this as a buffer stage after the phase inverter to deliver the needed drive voltage.

Another rational I've found is that using a power tube in this manner allows you the benefit of a not only a low output impedance, that doesn't suffer the same detraction's as a follower, but also a very low distortion figure.

So its usable as a low impedance, low distortion, voltage driver.
It makes sense I saw it in a audio freq. generator.

I got the remains of some old twin kicking around the bone pile, thats sockets and transformers, might be worth a go to explore some more.
Always nice to find something new to try.

[Cameron]

Glad to have been referred to that soundcity amp, every now and again something jumps up as "unique".

No worries around random ravings, I been digging a bit.

One application using a power pentode R-C stage is to meet the requirement of an enormous drive voltage for directly heated power triodes, if you were
to try it you could implement this as a buffer stage after the phase inverter to deliver the needed drive voltage.

Another rational I've found is that using a power tube in this manner allows you the benefit of a not only a low output impedance, that doesn't suffer the same detraction's as a follower, but also a very low distortion figure.

So its usable as a low impedance, low distortion, voltage driver.
It makes sense I saw it in a audio freq. generator.

I got the remains of some old twin kicking around the bone pile, thats sockets and transformers, might be worth a go to explore some more.
Always nice to find something new to try.

The only problem is ...I think the shitty tubes we have today. No matter what tube I try, it picks up every thump or vibration..kind of like a bad power tube...but its the PI.

[Andy Le Blanc]

sounds like your running it?

Its a chore to go thru a pile of tubes tapping for noise and all that.
matching might not be as much of an issue, you can get after ac/dc signal balance in the circuit but to find a couple quiet tubes could be hard.

be interested to see just how practical it really is, because of the micro-phonic issue, once I get it up and running and start wingin bench tubes and different tube types thru it see what happens

[gingertube]

Andy,
I noted your mention of using a power tube as a driver for a filamentary triode.
I have just such a project which is a just a pile of parts at the moment, I'm doing a "Super Champ" with an 845 tube into a Hammond 125ESE Output Tranny for 20 Watts Single Ended. The 845 will be driven by a 6V6. The 845 is running at around +850 Volts so not a good project for a beginner.
Cheers,
Ian

[JazzGuitarGimp]

Hey Guys,

Pardon my ignorance. When I think of RC, I think of a resistor / capacitor network, or an RC time constant. Clearly, this is not what you're referring to. Care to enlighten me?

Many thanks,
Lou

[Reeltarded]

It is what they are talking about. They are crazy! (and Ian is doing an 850v 20w one.. THAT IS CRAZIER!)

[JazzGuitarGimp]

Come on. Miles..... methinks you're getting into the holiday cheer justa wee bit early!

[martin manning]

Maybe not... I'm guessing it's resistive-capacitative coupled. Not that he isn't getting into the holiday cheer...

[gingertube]

It is what they are talking about. They are crazy! (and Ian is doing an 850v 20w one.. THAT IS CRAZIER!)

Hey - I resemble that remark.

Actually the local guitar god spotted an 845 sitting on my shelf last time he was around and immediately said "I want and amp with one of those", to which I replied "OK, that could be fun". So is it me or him that is crazy? (Both maybe).

Yes RC coupling into and out off the power tube stage. On that "Super Champ" project 6V6 RC coupled to 845 filamentary triode. We can do that coz I'm not attempting to drive the 845 into grid current and so blocking won't be a problem, 20 watts will do me, no point in trying to go A2 for 25 or 30 W.

Some of the HiFi guys use power tubes such as the 6V6 for their line-stage, Using a pair of EL84 for the phase splitter in a parallel push pull power amp output stage (like the EL34 example posted above) is reasonably common.

Cheers,
Ian

[JazzGuitarGimp]

Okay, I get it now - thanks for the clarification. And Miles? I take back all those good things I said about you and the holiday cheer!

[Reeltarded]

I am not a drinker, not even a little bit! I came by my cheer honestly..

I hit a whiskey truck. Contact buzz.