My first class or thoughts on repairing a tube linear amplifier for dummies. First off,
WARNING WARNING WARNING WARNING WARNING WARNING
Tune Linear amplifiers contain LETHAL voltages, and one wrong move will take you out, permanently. A small JB-12 almost took me out as a kid, 50 years ago.
With that said, today just going thru the simplest of amps in engineering and design. A single stage 2 tube grounded grid amplifier, a Browning Golden Eagle Mark 100 which is an earlier version of the Browning 180. It is as simple as they come. No thrills, no spills, just an amp that takes four watts in and does about 50 watts out. No preamp, no turbocharging, no 'Supermod', no bias, no screen, not even a Tuned Input Circuit.
The Browning 180 just has a power supply that provides about 800v's for plate voltage, 6vac to light the tube filaments, and a high voltage center tap at 400v to run the tube keying circuit. That's it for the power supply. When in operate mode, the HV is on the tubes at all time. However, the Tube cathode which is basically the ground, is only grounded when the keying relay is keyed. When the relay is keyed, the choke L1 which is connected between the radio input and ground is now in circuit as the radio Input and choke both are connected to the Tube Cathode when the relay is keyed.
So enough about this amplifier, here is my very simplified thoughts on repairing an amplifier.
First and foremost, one should determine what exactly the amplifier is either not doing right, or is doing wrong. Common things are:
It blows fuse
No power at all
Tubes not lighting
No Output
Low Output
With Ham amps that usually have meters that read Voltages, Plate Current, and Grid Current, it is a lot easier to see what is going on with the amp, However, since hardly any CB amps have meters for that, we have to use a little common sense and get creative.
If an amp is blowing a fuse, you have a short. Shorts are usually easy to find. Just start disconnected stuff until the short is no more. With Tube amps, the most common cause of a short is a shorted Tube. Hence take the Tubes out, fire up the amp and see if the short is still there. If it is, start working your way backwards. Next disconnect the HV power supply cap bank. Still short, check the power supply diodes for a short, and even if you don't find a shorted one, disconnect the diode bank and try it. If the amp is still shorting out with the Diodes disconnected, sorry to tell you, cancel Christmas, you probably have a shorted Transformer. Replacement transformers usually aren't easy to find, nor cheap.
No Output? I've said many times in many video's the first thing I would check is your Input SWR. Just to pick a point, under 3:1 you ae probably good. A little higher, it may be the problem and a lot higher, it is a problem. If your radio has a SWR meter built in, you can use it. If it doesn't or you are using a driver amp to drive the amp you are working on, put an SWR meter inbetween the radio or driver and see what it reads. There are other clues too. If your amp had an amp meter or grid meter, those should be reading correctly if the amp has HV and is getting drive to the tubes. If the amp had a HV meter, it should drop a little if the amp as getting drive. If you had an external line amp meter, which I wanted to show in the vid, but I ran out of time, it should show that you are pulling good amps when the amp is getting drive. If all the voltages are there and the amp is getting drive, the tubes should be getting hot, especially if there is no output. I look at all these things and check my Input SWR after I made sure my voltages are OK.
Filament Voltages. With glass tubes, if the Tube has the orange glow inside, the filaments are ok. If not, the most likely suspect if a bad connection between the Tube and the Tube socket. Most likely culprit is a dirty tube pin. I use a Dremel with a soft brass wire brush head and a very light touch. I want to Brush away the dirt and grime, but keep in mind, most tube pins are coated with an extremely thin coat of conductive material. You brush too hard, which is not hard at all, you remove the conductive material. Old school Hams did not like Brushing tube pins at all for that reason. Also look for broken wires and cold solder joints if not lighting.
No output is probably the hardest to track. If the tubes have voltages and drive, but no output, they are eating that output and should be getting hot. could be a short or open in the Tank or output circuit, and even more likely, a bad connection at the relay contacts. Clean those relays. For bad ones, I use a Dremel with the soft brush again. Check the output circuit, Tune and Load cap, and coil for shorts, bad connections etc. I have also seen bad coax and wiring cause the output to fail.
I had more, but I am out of space, so more to come. Good Luck.
8 янв 2024
