LPB RC-6A Carrrier Current Vacuum Tube Transmitter Problems

Ever since you have been writing about your LPB tube transmitter, I have seen one or two RC-6A units on eBay.

It was tempting to buy one, since I am a big fan of tube transmitters, but the problem of the base-band crystal is scary and I let the moment slip away.

Is the power output variable, or is it a solid 6-Watts?

620 kHz was where I wanted to be.  I narrowly

missed a 620 crystal on E-Bay.  I have not seen one

again.

I had crystals for other channels long ago - most were bad

channels or didn't end on zero, such as 945.1 kHz.  I might

still have an 860 kHz rock around.  860 is a really bad channel

but OK for testing into a dummy load or just in the house.

I had probably mentioned that this RC-6A was made for just the low

end of the band.  It was difficult and tricky it pull it all the way

up to 1020.  Also, the tuning section is not in good shape.

But I still have a few easy ideas.

If it can't be fixed, I'll just turn it on once in a while and

watch the beautiful tube glow from the 6AL11s.

Carl - to answer your question - the power

output is whatever you get when the transmitter

is correctly tuned into the load.  It's bad for the final tube

to be out of tune, as you know.  So, I guess 5 or 6 watts

is about right.  It's not variable, but you could use an RF

attenuator -that is - probably build one for whatever

attenuation you wanted.

Bruce

Anyway, to make this short - one of the

6AL11s in the audio section had a round

burn mark on the side.  I pulled it out, and

the operating RC-6A "fixed itself." Great audio.  No

50 kHz spurs anymore.  (The audio section has 2

6AL11s in parallel, or "push-pull" or whatever - so that's

why it was possible to pull a tube and still have the

transmitter modulate.) 

2 questions

1)  Is it OK to run the rig on just

one 6AL11 in the audio section for

short lengths of time?  Does it even

make a difference if the other socket

is empty and it isn't operating?

Uh - that's two questions, so

3)  Did the 6AL11 fail by itself?  After all, it is

an old N.O.S.  Or did something in the

transmitter cause the tube to fail?   

We may never know the answer to that one. 

Neil?  I'm wondering what you think - but also

I haven't seen you on too much lately.  Hope

all is well.

Anyway,

Best Wishes,

Bruce

Bruce you've got me thinking back to the days of push-pull tube amps, of which I had four fabulous favorites.

The two I was afraid to change were the Heathkit 35-Watter and the Eico 50-Watter. The output tubes on those had to be balanced pairs and a pot was adjusted by meter to get the balance exactly right between the tubes or one of them could go nuclear and blow up the house.

Where I really went wild was with my two Grommes 10-Watt push-pull amps that used 6V6 outputs. I hooked up double size power supply with huge rheostats so I could play with both the filament voltages and the plate voltages.

I turned those suckers up until they were as bright as light bulbs and turned blue and orange before they made a "tink" sound and died.

I long for those great days of knowing even less about what I was doing.

I would have loved to be around when you

were conducting those experiments with

the audio amps.  It seems like so much

fun!

I am determined to keep this old RC-6A running.

A broadcast engineer friend of mine was mystified.

Push-pull audio amp with one tube removed

makes it work much better.  Why????  I don't know. 

I fear that something is wrong with the unit and it

is causing 6AL11s to fail.  But then you pull the

failed 6AL11 out and things get almost all better

even though one tube is doing all the work. 

(I know this is redundant.) 

Oh well. 

Anybody else out there?

Bruce

I had a one tube ham transmitter that

used a 6AQ5.  I hooked up the wrong

power supply and put 12 volts on the

filiment instead of 6.  I wondered why

the tube looked so bright.  (I liked the

brighter orange glow.)

Anyway I didn't make the mistake for

more than about 30 seconds.  The tube

seemed to be fine afterwards.

I would like to try to see what twice the

filiment voltage would do, but I can't

afford to replace whatever is damaged.

So I won't be trying it. 

Bruce

I am guessing, that like a light bulb, too much filament voltage would shorten the lifetime of the tube.

If it lengthens the lifetime of the tube then I'm wrong.

Someone once said that if you lower the voltage into a regular bulb, it will last almost forever.

So then, maybe running tube filaments low would keep them going.

and all is well here.

Comments:

You mentioned a "burn mark" on the side of the tube. If this is inside the envelope and is silvery then it is normal since this is the "getter" used to remove residual air from the tube.

It will make a difference whether the tubes are in parallel or push-pull. In parallel with one tube removed would mean less maximum drive power but the signal would still be linearly amplified up to a limit. About the only tube fault I can imagine in this scenario which would improve the performance with a tube removed is if the bad tube had a element to element short.

If operating push-pull then the audio would be very distorted if one tube is dead or removed. Without the schematic I don't know which (parallel or push-pull) is the case. Some push-pull circuits use complex phase splitter/driver circuitry and the bias can be upset if a tube is dead or gone. If you look at the schematic and see a center tapped output transformer with each leg connected to each tube plate then it is push-pull.

There was an episode back in my college carrier current career where I noticed the audio at the dorm drops sounded bad and I traced it to a push-pull amplifier in the studio which drives the phone lines. One of the tubes had a broken envelope and though the audio sounded "OK" to some ears it was very distorted and a new tube fixed this.

Neil

Neil: Wow.  Thanks for all of that info.

The tubes must be running in parallel. 

There is more to this story, but I'll save it

for now.  With only one tube going, I guess

I get 75 or 80 percent modulation.  Good enough

for these brief carrier current experiments. 

And yes, the "getter." Oh my, I had forgotten

about that.  I'll have to look at that tube again. 

So, and I keep forgetting this, the audio amp

has 2 6AL11s, so there are really 4 tubes, if

you go with the idea that each 6AL11 is a dual pentode.

Anyway Neil, I remember that you have a lot

going on there, so I'm glad to have heard from you.

And Carl, yeah I've heard stories about light bulbs

lasting for long periods of time.  Somewhere, in

Connecticut, in a barn where a rock group practiced -

there was a light bulb that was said to have been

operating for about 50 years or more.  It was not

a "frosted" bulb.  It was transparent.  So the filament

could be clearly seen.  Actually that was some time

back.  I don't know if it is still there.

Have you ever seen a light bulb oscillate just before

it fails?  That's fun to watch, although it hardly ever

happens anymore, I think.  It would pulsate at 3 or 4 HZ -

as if the filiment started to break, and then the heat/electric

energy would sort of weld it back together.  This would repeat

until the connection was lost.  I've also seen some bulbs

oscillate at audio frequencies (sometimes with multiple

harmonics) until they failed. 

Onw ham radio friend of mine had a scary experience

when he was a child.  He sat in his father's easy chair,

and turned the lamp on that was on a stand next to

the chair.  The incadescent bulb blew out with a big

bang - in other words, it exploded and blew glass all

over the place.  I still think that's really scary. 

Ugh.

Best Wishes to you both,

Bruce

I just want to thank you for your post

before I forget. 

I haven't read it all yet.  I'll probably have

to look at it for a while in order for it to sink in. 

Thanks again,

Bruce

http://www.w8ji.com/filament_voltage_life.htm

Is that neat or what?

By the way, a college prof friend

told me that there is a very unusual

charactistic or formula concerning the

filiment of an incandescent light bulb.

For the life of me, I can't remember

what it is. 

Bruce

Many thanks John WDCX for the educational Filament link. I spent a longtime reading it and thinking about it, and it reminded me of a whole other chapter in my mad psuedo-scientific days...

After fiddling with Knight Kit Part 15 transmitters and turbo-charged push-pull audio amps, I drilled holes in a cheap metal shelving unit and mounted tubes, transformers, huge rheo-stats and a row of meters and built an AM transmitter that helped me learn the "grid dip" method of tuning the final stage.

I knew so little, yet the damn thing put a signal at 1630kHz that I tracked on foot for over a mile using a first-of-a-kind German transistor radio and IT WAS JUST AS STRONG as when I started out. The audio was unreeling from a Wollensak tape recorder.

I paniced, dismantled it, and never grid dipped again.

Oh, but that's not even my point. What the filament link reminded me about was monitoring the powerline, and I did have a meter showing raw AC house-power (not to be confused with horse power).