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I have an LPB RC-6A carrier current
transmitter that I just took out of
the box from our move a year ago.
I just started running it into a dummy
load about 5 minutes ago. I have crystals
for 860 and 1020 kHz. These are not great
channels, although 1020 is very quiet during
the day. I really want to be down in the
bottom of the AM BCB somewhere.
Anyway, I’m getting a good carrier. I have not
tried audio yet. The transmitter has some odd
problems – but it should operate. It is a vacuum
tube model from 1971. It uses 3 6AL11 compactrons
and is built like a tank.
I also made a very simple line coupler. It isn’t the
greatest thing in the world, but it is safe and does
work after a fashion. Oh – and the transmitter runs
about 6 watts out. Many of you will remember that
I did experiments with this unit in the past.
So, we’ll see what happens.
Best wishes to all.
Brooce, Part 15, Hartford, CT
Total posts : 45366Only about 30 to 50 percent modulation.
I’ve got ideas.
More later.
Brooce, Part 15, Hartford, CT
Total posts : 45366told me he saw this thread, so I guess
I’ll just add – that – I’m going to look
for some more 6AL11s. Perhaps the
6AL11s in the unit aren’t good.
That’s by and far the easiest thing to do.
I think some time will go by before I
report on this again.
Brooce, Part 15, Hartford, CT
Total posts : 45366
Total posts : 45366It is easy to suspect that tubes are bad but in my experience tube failure is usually not the cause of problems.
If you lack access to a tube tester there are some other things which you can do to check tubes. Let’s look at how they fail:
The filament is open. Easy to check since if it lights then the filament is OK.
Air got into the tube. This will cause a white deposit to appear usually at the top of the tube bulb. This is rare unless the tube was smacked.
Shorts. These will produce crackling and popping in the signal from the tube. In some cases sparking can be seen.
Tube has high hours of operation or high power operation. These will have a brownish deposit on the inside of the glass bulb located next to the plate. Often a shadow of the plate can be seen. If the brown deposits are easily visible then the tube is probably at the end of its useful life.
Brooce, I know that visual inspection is difficult for you but maybe someone can help you. Also, give a shout to your local hams on a repeater and ask if any have a tube tester. I, for example, do and so do many others.
It is probably simplest for you to just replace the tubes but I don’t think this will solve your problem. A very common failure in old tube equipment is caused by bad or leaky capacitors. When caps become electrically leaky they disrupt the biasing of the tubes and can cause poor performance or even damage. The best way to deal with this is to replace all caps with modern ones of equal value and equal or greater voltage rating. This is not very expensive, just time consuming. Here’s a link to a discussion of capacitor types and failure modes. You might also enjoy looking at his restoration vids, good information.
Neil
Total posts : 45366That’s right, most tubes in a receiver or audio amp can last forever and the ones that go bad are often used in transmitting on ICAS – intermittant ratings, for example tube CB and ham linears. In that equipment, holes can be burned in the plates and glass rippled or melted from the heat
That’s not even normal, but when something goes wrong, like loss of grid or cathode protective bias it can happen.
Now LPB equipment is probably rated conservatively, having to run continuously, so the tubes probably have enough headroom at 5 watts output to be safe under all conditions.
I’ve had radios with 60-70 year old tubes that work fine, and often it’s capacitor problems, electrolytics and then those paper and wax coupling caps that dribble when they get hot. I’ve rarely seen any tube problems in an All American Five set, even on the audio output tube and rectifier that run the hottest. I don’t think I’ve ever seen a 12AV6 go bad from normal listening.
Total posts : 45366Like Nate states, I have radios here that have tubes in them that are 50-60 years old that work fine, many of the tubes are OEM. Capacitor failure causes many of the tube failures due to shorting and the often increased current draw on the tubes in the circuit.
Lack of use can also cause problems with the tube becoming gassy… sometimes just letting them “burn back in” will return them to a useful state.
The all American Five radios are a great, fairly simple receiver design that have proven themselves over the decades.
In transmitter equipment, mismatched outputs, over drive etc are hard on the final tubes and cause the plates to heat beyond safe limits. But even then, proper care and use can allow these tubes to last a lifetime with normal use.
All that said, it is nice to have solid state smd devices that are useful for part 15 that can run off of a battery or solar power easily. I am not sure of their repairability in 60 years though.
Total posts : 45366WDCX, for the link, and Neil and Nate and Craigf for
the comments.
Actually, Neil and Nateand Craigf, I completely agree
with all three of you.
I beleive the reality of the situation is that
the RF section is working fine and the
audio sections are running way out of tolerance
for some reason. I think there might be some
damage due to aging of the unit and also
because it has been shipped to friends and
back to me a couple of times.
But there is nobody here to do the work of
recapping and measuring voltages, etc.
So the only thing I can do is to try some
other tubes. I even have reason to beleive
that the transmitter may be so out of wack
that it might be hurting the tubes in the
audio section. I took out a tube that did
have internal damage, i.e., brown metal
coatings on the inside – for lack of a better
description.
So, if I can’t get it checked out, I will try
some more 6AL11s, just for fun.
(Because, there is a teeny chance that I
might have gotten a bad batch of N.O.S. stock.)
It probably
won’t be soon – I go slow – – I’ve owned this
great little piece of gear for 35 years. Maybe
I’ll work on the coupler. I don’t really have
any kind of a station – this is just for fun.
I’ve got a Gates Studioette board sitting down
in the cellar not too far from the transmitter.
Maybe I’ll mess around with that for a while.
Oh yeah – I love All American Fives, by the
way. I have quite a few stories about playing
with those over the years.
Thanks again guys! Oh – and please
excuse any spelling errors, etc. I’m
typing this kind of fast and have to go.
Brooce, Part 15, Hartford, CT
Total posts : 45366Thank you for the video on capacitors.
I learned a lot and it was fun!
Well, you never know. Maybe i might be
able to recap something some day.
Best Wishes,
Brooce
Total posts : 45366brooce, i live in CT and am pretty confident i could recap that, for a nominal price of course because it is time consuming. i also have a crystal for 1000khz and 1640 that i’ll sell to ya on the cheap, but in central CT, watch out for the evangelicano station. they cover a chunk of the river valley
Total posts : 45366I will keep you in mind! I can’t do anything right
now, but it would be fun to talk to you via E-mail
or landline.
In Hartford, there is a pirate on 1640 (just for kicks
I thought I’d mention that.) Oh yeah, also 1620,
maybe 1590 for a while, 101.7 FM, 103.3 FM, and a few others
that come and go.
I have been AM BCB tuning almost every day as of late, and
have gotten all kinds of things.
BACK to the subject – this particular RC-6A is only a
“low and of the AM BCB rig.” – So 1640 wouldn’t
fly. You know what I MIGHT DO at some point is to
just wire an LC (appropriately designed and slightly
tunable) in where the crystal is supposed to go. You know,
like a ham radio VFO. I know of hams have done
that with old vacuum tube transmitters. It’s not like
this thing is going to run 24 hours a day, 7 days a week.
I would love to talk to you and depending how
far apart we are – I wonder if you and I have heard
a lot of the same AM and FM stations.
So because of vision limitations at the moment – typing
this is a little peculiar. So I’m gonna run.
I hope we can contact each other directly.
Brooce, Part 15, Hartford
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