Looking for transmitting pentodes. / Sale and Trade / Forums

Forums

4hv.org :: Forums :: Sale and Trade

« Previous topic | Next topic »

Looking for transmitting pentodes.

previous 1 2

Move Thread

LAN_403

Proud Mary

Sun Nov 01 2009, 12:11AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Dear ol' Arcs,

here is my view of a serious valve-based Tesla arrangement:

1. A variable frequency oscillator (VFO) tunable by a variable capacitor (VC) across 200 - 600kHz. Here is a good case for a small RF pentode such as EF91, which should be in an aluminium screening can. Ideally, the entire circuitry for the VFO should be located in a diecast aluminium box, and should be as 'bomb proof' as possible. A useless, crappy VFO will begin to change frequency as your hand approaches it. (so-called 'hand capacity'). A competent professional design will drift only 20 or 30 cycles in 24 hours, and will be unaffected by hammer blows/shrapnel impacting on the case. You'll be doing well as a beginner if your VFO comes out between these extremes. Allow 10mA of your HT+ for the VFO. Your output signal on to the next stage may be 10V. Choose a type with a low heater current to minimize thermal drift.

2. The buffer amplifier. has several important functions: to act as a cut-out and reduce the chance of unstable feedback from the (next) Power Amplifier stage back unto the VFO, and to supply sufficient grid drive for the Power Amplifier e.g. plus/minus 100V Here I would suggest a cheap beam tetrode such as 6V6, but as we are not in the audiophile world here, almost any medium power - 4 or 5W - will do.

3. The power amplifier. The simple VFO/Buffer scheme I've suggested will power anything from the famous tetrode 807, (say 60W) up to the legendary 813, and beyond.

Most of the women and men on this forum would find good quality point-to-point valve construction simple and rewarding, so what is against it?

Money.

It is not expensive to power the VFO and the Buffer (say 240-0-240V/50mA and 6.3V 2A for both) but once you start buying your HT+ transformer you will be in for a nasty shock (

) A 3000-0-3000V 500mA xformer has to carried by two men.

Of course, you can try mucking about with MOTs - knock the shunts out of them, and so on - but this is not an approach for serious men.

So what is to be done? (The question framed by TV designers around 1950)

The answer is, of course, the 'sweep tube' - a valve which is not much more than a switch, which will keep the HT requirements to less than 700V - within reach of mains voltage multiplication - but which will conduct heavily when we tell it to do so.

Free of the need for a colossal and expensive HT transformer, you can mount your sweep tubes in pairs to get up into the kilowatt range if that's what turns you on.

Anyway, Arcs, that's enough chatter for now. I never send good money after bad, so the day you send me your best design attempt at the Variable Frequency Oscillator will be the day I send you the high quality parts to make your vision into reality.

Don't worry if you get a few bits wrong, son. But I'd be a mutt to send you 50 bucks worth of bits without
some clear demonstration that you knew what to do with them.

No need to tell anyone else that I've helped you out,

your Pal,

Christopher ("Harry")

Arcstarter

Sun Nov 01 2009, 02:48AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Chris, thank you very much, again

.

I have in fact never used that method of feedback. That is, i have never used a buffer. I have only messed around with the single tube kind. Basically an Armstrong oscillator.

I do not want to take anything for free, and i do not necessarily expect to make a VFO. This is why i am trading for big tubes, and letting you know that they will be run hard

.

The circuit i am using, at least for what i plan to attempt first with a pentode, is very very basic. You have a power supply on the plate (that uses an inductor, and a resistor to block RF), and the cathode is grounded. The resonator is connected to the plate, and the feedback is capacitively coupled, via some sort of antenna, or wire around the output of the resonator, or even a tiny 'capacitor' formed with two little plates, with air dielectric.

It is simple, but it yields good results. The main reason for doing this, it simply to try different things to get best efficiency, and... for fun

. that is about it.

Though, i do have some tiny, maybe 4 watt output tubes, for preamps, that i can tinker with.

Edit: sorry about that, i forgot to mention the reason i wanted a pentode in the first place, and why i mentioned the small tubes.

At some point, i would like to attempt modulating the screen of the pentode, for audio modulation with some of these small tubes i have. In my opinion, that would be a kind of neat project to embark on

. Not so much unique, but a learning experience nonetheless.

Also, if i get some money, about 50 bucks, at some point, i will probably buy a GU-81M. Though, it is really kind of that point that saving up for a car is a good idea, i would probably still do it. What is more important than learning, is my excuse :D.

another edit: Just in case you where wondering, i would like to pretty much replicate this:

This time, though, i want a pentode. They have higher gain, so there is no feedback coil, just capacitive. So, that being the case, that was essentially an Armstrong oscillator.

Arcstarter

Fri Nov 20 2009, 02:06AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Ok, so i have still been unable to get any large pentodes.

If you have a fairly large pentodes (around 2-3kv 250ma plate current, GU-81M kind of tubes) I will buy them. I would send a cashier's check or money order, from Western Union. I do not have much money to spend. If you have smaller tubes, such as the GK-71 or in that range, i *may* be interested, depending mostly on the price.

Thanks for looking,
Matthew.

Proud Mary

Sat Nov 21 2009, 01:21AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Matthew, trust me, if you haven't got a lot of experience with thermionic valves, you will be throwing good money after bad by buying a giant valve which you will have no way of powering, and no way of designing all the supportive circuitry that such valves need.

Why not try something more modest, learn about how changing the different parameters of the grids affects operation, and so on.
Then, with a little understanding of how valves work, you will be in a much better position to make the giant thermionic extravaganza of your dreams.

As you may know, I always put my money where my mouth is, so if you's like to attempt a serious design for a small VTTC, I'll be happy to send you a new RCA 807 (Va 600V, Ia 100mA) and a ceramic socket as a kind of advance Christmas present.

Why go for second bes,t Mathew? Better to have a properly designed and carefully crafted VTTC driven by 60W RF from an 807, than to dream of utterly fantastic sparks at RF frequencies which will only end in loss and disappointment.

You mention 813, which is a valve I have known very well for many years.* To keep it within its safe operating area involves a whole raft of sub-circuits which are not shown in simple student circuit diagrams/schematics.

*813 was generally used in Uk military ''Class D" transmitters (HF truck-towed mobile) until the 1980s because of its ability to conduct massively in the event of EMP, without much short-term damage.

Anyway, the offer is there, Arcs.

best wishes,

Stella

Appendix afterthought: If you take the case of screen grid modulation of an 813, you will need a chunky valve to do it - say an 807 - together with costly chokes, and its own PSU, say, 400-0-400 at 75ma, plus LT, plus neon voltage regulators, and so on.

Anyway, Arcs, I'll leave it to others who perhaps know better about what you are after here. S X

Arcstarter

Sun Nov 22 2009, 12:21AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Well, i don't have a ton of experience, but i would say i *at least* have enough not to kill a tube right off the bat.

I have messed with the grids of tubes plenty. Varying current, and voltage... So on and so forth. Also made that two channel amp with some old tubes i scavenged. It worked well with cool plates, and the worst part was the hum. The 'lytic capacitor was too small and crappy

.

Also, i am not going for spark length. I would expect no more than a couple of inches, but the current density is magnificent. Not to mention the incredible plasma it makes in vacuums. One of the main reasons i am making on is because of that, that the very high frequency will excite bulbs and other things brilliantly, and easier than other things such as Tesla coils. I plan to try to vacuum the air out of a jar or something of some kind and experiment with different pressures and maybe i will try to charge it with argon... That brings up the question, anyone here have a compressor from an air conditioning system or a refrigerator? If so, i am *very* interested

.

I made a successful high freq base fed LC resonator with a small 600v some 100ma sweep valve that has visible holes in the cathode, and the control grid is half dead. Came from a dead television.

Unfortunately, i am looking for something with higher plate voltage and current, now. I have tubes that small, but these things are fairly inefficient and need more power. that is a generous offer, and i do thank you

.

Also, to drive the tube to oscillate at the resonant frequency is not hard, at all. Simply an antenna/capacitive coupling with biasing on the grid is enough. The scope approved :P. And modulating the screen grid is not as hard as you make it seem. That part would basically be a preamp with a dual triode and a pentode (two seperate amps) just like a normal run of the mill amp for a guitar or whatever. You can also use transistors.

Proud Mary

Sun Nov 22 2009, 03:11AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Arcstarter wrote ...

And modulating the screen grid is not as hard as you make it seem. That part would basically be a preamp with a dual triode and a pentode (two seperate amps) just like a normal run of the mill amp for a guitar or whatever. You can also use transistors.

Whatever you say, Arcs! Tell us when it's up and running!

previous 1 2

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob