Tube Amp

I don't know what's available in Oz, but some ARRL or RSGB handbooks from the '40s, '50s or early '60s will give you tons of sample circuits.

How low is low? Anything much under 100V gets tough to make any power.

You'll get the most power at a low voltage by using a tube intended for such use (kind of a DUH statement!). Look for a tube that has a higher heater current, since low-voltage tubes need more cathode emission to make decent power. That might be something like a 6CU5,

You'll probably only need one triode stage of AF amplification between the detector and the output tube. Something like a 6C4, 6J5, etc. If one stage isn't quite enough you could try a tube with higher amplification factor like 6AV6 or 6SR7.

BTW, you can talk tube radio homebrew to your hearts content on the glowbugs email list. Google should turn it up for you.

Mike

Hmm, I thought I hit SEND but who knows where it went. Anyway, I had another idea so I'll reply again.

First, what kind of low voltage are you talking about? Anything much under 100V is going to be very hard to make any power.

You'll want an output tube designed for low voltage operation (typically around 110V). Look for a tube with high heater current, typically 1.2A at 6.3V. That's because low voltage tubes need big cathodes to produce enough current at low voltages to make decent power. Examples would be 6CU5, 6W6, 6Y6, 6DB5, etc.

You'll probably only need one stage of AF amplification between detector and output tube. Look at triodes such as 6C4 or 6J5. If you need more gain look at a higher gain triode like 6AV6 or 6SR7.

I don't know what voltage source you have or are looking at, but I like the Triad N68X isolation transformer for low (tube) voltage use. You can get 150-160V out of a bridge rectifier and filter. You'll need a separate heater transformer, but the combination usually still ends up being cheaper and easier to obtain than a combined tube transformer with both HV and heater windings.

BTW, if you want to learn about tube radio homebrew you'll want to look into the glowbugs email list - best resource on the net that I know of. Google should turn it up for you.

Mike

Found one at

Going off topic a bit here, concerning the regen radio previously mentioned, that I've yet to build, there are some connections I'm not sure of, first is the A connection, this doesn't appear to be the antenna connection, second is the B+ 1 1/5 v connection. Do they mean A+ 1 1/5, even though it's not connected to the + filament pin?

Link to the schematic

Thanks, Dave Australia

"A" is the filament voltage, "B" is the plate voltage and "C" is the grid bias voltage (which is not present in the schematic you show). When you see things like B+1.5V and B+9V that's referring to taps on the B+ battery (many of them were made with such taps). Of course for B+1.5V you could also use a separate 1.5V battery. Here's a schematic that shows such a separate battery.

BTW, the amplifier you're looking at won't give anywhere near 250mW, much less 1W. The specs for the tubes show that they just get to

Mike

battery.http://www.schmarder.com/radios/tube/hikers1.htm

I'll use the 2 watt IC amp I have sitting around, easiest option.

If I have this right, and usually I don't, I connect the plus end of the filament battery to the terminal simply marked "A", and a separate "C" battery to the B+ 1 1/5v connection. Why is there only one connection for the bias battery?

Will I get good lifetime out of a 9 volt transistor battery? I intend on using a "D" cell for the filament, as I do with my other regen radio (which BTW, you will find on Dave Schmarder's web page, very first radio on his tube radio page).

Wire up the batteries exactly as shown on the schematic at the bottom of the page I linked

Mike

The "A" on the top end of the coil is the antenna. The "A" terminal strip is the 1.5V filament battery + connection.

The schematic is incorrect, so it's hard to tell what's up without guessing. Also, no tube type is shown.

The lower grid in the tube schematic is drawn out of place. It should be drawn above the grid that is connected to the grid-leak R/C parts. The grid with the grid-leak parts is called the control grid. The out of place grid is, I' sure, a screen grid. A screen grid requires a positive voltage, and in *most* circuits it is provided by a suitable resistor from B+. I don't really understand what the "B+ 1 1/2 V" note is intended to mean, *unless* it is the authors intention to denote it is 1.5V "B+" for the screen. In any case it indicates you should use a 1.5 V battery, connecting the + terminal to the screen grid, and necessarily connecting the - terminal to the E/B-/A buss.

ip

g.

grid

rid

and

't

Actually the circuit is employing a trick that was sometimes used in low voltage applications. The tube control grid is connected to a small positive voltage to help accelerate the electrons towards the plate to obtain reasonable plate current, and the tube screen grid is then used as the circuit control grid. Unless you've run across that trick you'd be justified in thinking "what the heck?!"

Mike

I'm planning on using a 1C5GT tube.

I had wondered about such a thing, but dismissed it. I need to try it and compare the result with the more conventional use of the grids.

Thanks

Also, I should have caught the use of that extra 1.5V cell to boost the B+.

.H"

ing.

Here's what I think. I don't claim it's the only way, or even the best way, it's just what I would be inclined to do. Take the money you would spend on the 1:3 interstage transformer and spend it instead on a power transformer and build yourself a battery eliminator that will give you 90-110V to run a power output stage. Tap it down to run your other stage(s). If you want, add a low voltage regulated DC output for filaments/heaters. Then just use RC coupling between the detector and the AF stage.

That's unless you were planning to use the radio away from AC power, in which case ignore everything I said.

Mike