Dual Bias supplies on a ST70 ?

[TomMcNally]

Hey while you gurus are on the bias supply subject ... I have a 25 watt tube type AM transmitter that uses EL-34's in the modulator, and the bias supply winding of the power transformer is open. I was thinking of squeezing a reverse connected 12 VAC filament transformer in there and using the 115 side for the bias ? Good idea ? Anyone have a better suggestion ? I just wanna fix it and eBay it out. The rest of the tubes are Compactrons ... 7984 final - I happen to have spares.

... tom

[WA4SWJ]

Tom,

You could do what Shannon does I think in the eiclone - just connect a .1 ufd cap to one of the HV leads of the power transformer to a diode then to a filter net and a voltage divider to develop grid bias voltage. One thought though - is it the final you're trying to bias? When running CW it might be running Class C. In that case the transformer might be better.

You must be having fun with all those projects!

[mesherm]

I did that on my "plinker" amp where the power tranny didn't have a bias winding. I used a 12 volt tranny in reverse hooked up to the 6 volt filament line and used a small full bridge diode package on the primary lines. Worked like a charm and was dirt cheap from MP Jones.

[WA4SWJ]

Tom,

It's the fixed bias eiclone version. See page 18 of the eiclone docs at the upper right. It shows the bias rectifier and filter arrangement. I re-read your post and it looks like you're biasing the modulator. Should work fine and would take up less space than a transformer.

Regards,

[EWBrown]

I would SWAG that the bias voltage would be for the EL34 modulator section, the RF stage is probably class C and low or no grid bias voltage.
(BZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZTTTTTTT!!!!)
Sounds like some of the stuff I used to screw around with about 40 years ago... In those days it was more a combination of dumb luck and guesswork than actual engineering knowledge. And I learned that RF burns can hurt like hell Yellow_Light_Colorz_PDT_29
AN/ART-13s and teenage kids don't necessarily play well together...

:o Whoops, technical foul-up there... :o

Class C amps are biased even more negatively than class A, AB or B...
I was thinking of some old grounded grid scheme using quad 811s in an HF "linear" (and thusly NOT class C) amp. Now back to your regular program...



ARC-5s were a lot safer, but not as much fun... There were seemingly enough of those WWII vintage aircraft transmitters made to supply every American man, woman, child and dog with one...

Someow I survived and didn't blow too many things up in the process Yellow_Light_Colorz_PDT_06

/ed B in NH

[TomMcNally]

Oh yeah - sorry, I didn't mention it - the bias is for the EL-34's ... when I got this thing they lit up cherry red with a nice hot tube smell for a few seconds before I shut it down. I haven't messed with it in awhile, the thread here reminded me of it. Thanks for the ideas, maybe I'll mess with it tonight.

[TomMcNally]

OK - I dug out the transmitter - I had already installed a reverse wired 12 Volt transformer, which I had forgotten. I got the schematic after I did this, they are using -75 V on the final, and -30 V to the 47K resistors that go to the grids of the EL-34's ... my voltage is way too low ... -40 and -10 ... time to troubleshoot and probably get a bigger transformer. I have plenty of room under the chassis, but don't want to load the filament winding too much by putting a 6V filament transformer on it. Thanks for the replies.

... tom

[EWBrown]

If you can find a small 240:120 VAC step down / isolation trannie, only a few VA are needed, 120VAC to the 240VAC side, and thusly 60VAC out.
Biasing only needs a few mA, more is lost to resistive voltage dividers than the tubes' grids will require.

I've seen little compact PC mount trannies of this description, they're about half the size of a deck of cards, and some double sided sticky tape on the flat side is more than good enough to hold 'em in place... Just mount it in a location that doesn't get too warm...

BTW what make of transmitter is this one? Sounds like a fun boatanchor project for the AMers on 3885 and 7290

/ed B in NH KB1MZ FN42cs

[TomMcNally]

It's an LPB (Low Power Broadcasting) It was intended for "Carrier Current" AM broadcasting for college campuses and the like, they couple the RF into the power lines. These type transmitters are still made, solid state of course, and are used for "TIS" (Travelers Information Service) and theme parks and the few remaining drive in movies use something like this instead of speakers. I doubt the audio is very hi-fi, the input transformer is a little thing. Otherwise, it's fairly well made.

Thanks for the transformer tip ... I probably have something I can use ... I have some small dual primary transformers I could use back to back, and one of my friends offered me a small isolation transformer. Just gotta get into it.

I have more unfinished projects around the house ... everytime I go searching for parts I find something else.

[EWBrown]

In the bowels of the deeper recesses of my cellar, I have a couple of small synthesized "talking house" AM transmitters, probably about 10 mW out on a good day... The synthesizer is weird, it has the requesite thumbwheel switches to set the frequency, but the oscillator uses a manually adjustable slug-tuned coil, with an LED idiot light to tell you when the loop is locked. I f they weren't free, I wouldn't have taken them... I considered cobbing up a 10W linear power amp for these things, but as most of my screwier project ideas, it never got off the ground.

The idea of these xmitters was to connect it to a loop cassete recorder and let the house deliver its own real-estate sales pitch.

I look at it as just a new spin on one of those old "phonograph oscillators" of decades gone by.

/ed B in NH

[erichayes]

Hi All,

A high school classmate of mine, a PG&E engineer, gave a talk to our repeater club years ago on Carrier Current Communications the power companies use, instead of or in addition to regular modes. I can't remember the details, but I recall being fascinated with how they coupled the carrier to a 750KV transmission line (the step-up and step-down transformers act as chokes to the RF, so they can't transmit or receive on the low voltage sides).

Kinda like kissing a duck's butt: ya blow and ya kiss, but ya gotta be quick.