6K7 rf remote cutoff pentode

[sean k]

I just dig the idea of using one of these metal can tubes with the grid on the top cap so the signal can go straight from the input, through a resistor to the tube.

So whats going to happen to a normal guitar type signal when it hits the grid of an RF signal pentode? Is it still going to amplify but possibly have a huge bass roll off...what else can I think of? is the spacing of the grid too close for audio frequency?

I mean' if thats a complete disaster I think I've also got an old metal can 6SJ7 sharp cutoff pentode and that might be a better choice but if theres a chance the 6K7'll work...

I've also, from the same radio, got a 6F5, which is a single medium mu triode which is also a metal can with the grid off the top.

Then to make matters worse I've got a 72-0-72 which I was hoping to full wave double and use a 6H6 for the first two diodes of the bridge.

All this is about a mad old ham radio sitting on the top of a toilet cistern in a cafe I go to and it's such mad professor I've just gotta use this weird old normally trash can stuff to try and make something.

[Steve Conner]

Should work fine. The late Fred Nachbaur used the 6K7 in the front end of his "Dogzilla".

DOGZILLA - Input Stage

[sean k]

Good Golly, that is such the right circuit to show me, Rock on in Heaven Fred!

Not only will the 6H6 be a close enough substitute for the 6AL5 but the 6F5 will likely be able to take the place of the 1/2 6SN7... and my 72-0-72 doubled up should give me 200VDC which is what hes got on his b+. So I've basically got a tube compressor... Actually I do have a couple of the sovtek 6SN7's so maybe I could use the 6F5 after the pentode, as a cathode follower, to drive a tone stack then go into the 6SN7 as the output and CV driver.

Hmm. Thankyou Steve!

Heres another question. I'd have 4 valves at 6.3V filaments and three at 300mA and one at 600mA so if I run the filaments in series at 24V the current draw would be 1.5A/4? (.3 + .3 + .3 +.6 /4) ... 375mA?

[booj]

Heres another question. I'd have 4 valves at 6.3V filaments and three at 300mA and one at 600mA so if I run the filaments in series at 24V the current draw would be 1.5A/4? (.3 + .3 + .3 +.6 /4) ... 375mA?

No. Maybe best to look at the filiments as resistors. R=E/I, so 6.3/.3 would be a 21 ohm resistor. So now you have (3) 21 ohm reisistors in series with one 6.3/.6 or 10.5 ohm resistor. Total current draw with a 24 volt source will be I=E/R, or 24/73.5, or 327 ma. Your voltage drop across the 3 tubes would be E=IR, or .327*21, or 6.87 volts and the voltage drop across the big honker would be .327*10.5 or 3.43 volts.
This isn't exact of course because if you don't run the filaments at exact voltage you won't get exactly the same current draw.

[J M Fahey]

Hi seank
As booj told you, don't complicate matters; in other way: run them in parallel and 6.3 volts.
That radio is a treasure trove: for me tubes are tubes and all can be used, reclaim them , some odd sockets if necessary and maybe some good capacitors, if any.
I'm afraid electrolytics and very old paper in oil ones are trashbin stuff.
Ah! RF tubes means they are good at RF (duh!), what for example, 12AX7's can't do, but *all* are flat down to DC.
Tubes by themselves are neither fat nor thin ; circuits that surround them are.

[sean k]

It's just that the power transormer I'm going to use hasn't got filament windings so it's a matter of finding the relevant filament transformer to suit whatever I decide to do and 6.3V transformer of a couple of amps are somewhat scarce as off the shelf items... in a country on the bottom of the world with 230VAC mains. Actually I can get a multi tapped 0, 6, 12, 15 at 30VA so maybe that'll do if I can keep the total filament draw at under 2A and hope that the 6V regulation is a little wide. Basically keeps me down to a preamp unless I use two filament trnsformer because now I'm thinking, the power tranformer has a 90VA rating, I could easily squeeze in a couple of 6F6's for reverb driving and an SE output.

Nah, I won't stress and over complicate. I'm at a stage now where I just go ahead and do things because that which I need always seems to arrive in good time. Intention drives definition whereas over definition tends to muddle intention.