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John Boy
- Thread starter Mudfoot
- Start date
Shadetree Mechanic
Delaware Base Station 808
I don't think that I would be reaching across it with the covers off......
These amplifiers were built with a fatal flaw that causes them to be unstable. The tubes are high gain tetrodes and the output load tuner has been placed in the bottom of the amplifier. You can bet it's not in a shielded compartment and there is coupling between the input and output circuits. You'll also find no attempt has been made to neutralize this circuit or provide the screen with reasonably regulated voltage. That makes them prone to oscillations at lower drive levels or on SSB and the unstable screen voltage reduces linearity at higher drive levels.
The very nature of Tetrodes cause them to oscillate. There is a Dynatron region characteristic common to all tetrodes.
https://en.wikipedia.org/wiki/Tetrode
With a little skill, the right layout and grid resistor, these tetrodes can be made to run fairly stable even without neutralization. That all goes to hell when you place the output load capacitor in the same RF compartment as the input circuit. Plus you get a lot of RF heating on the feed-through bushing connecting to the cap, due to high current.
Yep, many of them small Tets ive seen around over the years would oscillate and latch like a SOB. We always called them 250s popcorn tubes anyways. Most dudes don't get high gain= very low drive ..
Yeah, that's his name brand.
Most puzzling bad habit we saw in them had to do with the output coax that leads from the front-panel Load control to the rear-panel antenna relay. The braid on this coax would not be connected at either end. Not connected to anything. This makes a capacitor out of the center wire and shield of this piece of coax, and couples the amplifier's output RF to the braid. Makes the outer surface of the braid into a "feedback antenna", radiating RF into the input circuits. The fix, no surprise, is to ground both ends of this coax, to the Load control at one end and to the antenna socket at the other end. Both. Reduces the "jumpiness" of the tuning knobs when you turn them in a big way.
For a long time, John Boy's tetrode boxes were built with a particular odd quirk. He shuts off the positive screen-grid voltage to place the tubes in standby. A perfectly-good method, but the way he went about it caused the filter cap for that voltage to remain hot until it bleeds off.
After you unkey. Made the tube oscillate for a fraction of a second ever time you unkey.
The fix was to move the filter cap's negative lead from ground to the other relay wire, so the screen voltage goes to zero as soon as the relay releases.
Required one mod. A single insulated tie point to support the new connection.
Some of his tetrode amplifiers had other quirks that reduced stability, but that one was pretty unique.
73
Most puzzling bad habit we saw in them had to do with the output coax that leads from the front-panel Load control to the rear-panel antenna relay. The braid on this coax would not be connected at either end. Not connected to anything. This makes a capacitor out of the center wire and shield of this piece of coax, and couples the amplifier's output RF to the braid. Makes the outer surface of the braid into a "feedback antenna", radiating RF into the input circuits. The fix, no surprise, is to ground both ends of this coax, to the Load control at one end and to the antenna socket at the other end. Both. Reduces the "jumpiness" of the tuning knobs when you turn them in a big way.
For a long time, John Boy's tetrode boxes were built with a particular odd quirk. He shuts off the positive screen-grid voltage to place the tubes in standby. A perfectly-good method, but the way he went about it caused the filter cap for that voltage to remain hot until it bleeds off.
After you unkey. Made the tube oscillate for a fraction of a second ever time you unkey.
The fix was to move the filter cap's negative lead from ground to the other relay wire, so the screen voltage goes to zero as soon as the relay releases.
Required one mod. A single insulated tie point to support the new connection.
Some of his tetrode amplifiers had other quirks that reduced stability, but that one was pretty unique.
73
with all the Chinese talk I best stick with a pill box. 
Especially after Shade said not to be reaching across it.
Especially after Shade said not to be reaching across it.
Shadetree Mechanic
Delaware Base Station 808
Those tube amps can make over 1000 volts sometimes. We heard a story a while back about a guy who was working on his amp and was electrocuted. Better safe than sorry. I am going to see if I can find a video about safe electrical practices.with all the Chinese talk I best stick with a pill box.
Especially after Shade said not to be reaching across it.
I've heard BBI tell horror stories about a guy with a rope chain.
I'll stick with crap Chinese pills
I'll stick with crap Chinese pills
The power supply for the amp I am sloooooowly picking away at making runs at 4500 volts. Then again I used to work on transmitters that ran at 2700 volts and 5000 volts so I am well aware of the precautions. HV does not scare me but I have a healthy respect for it and I want to remain healthy around it.
This goes beyond one hand in the pocket!
No kidding. Definitely no loose jewellery or even long shirt sleeves to get snagged. I don't own a long sleeved shirt anyway. ALWAYS check and recheck meter settings before connecting leads and preferably connect leads and THEN apply power. Remove power THEN disconnect leads. ALWAYS check the quality of your meter leads too for dried or cracked insulation and for God's sake do not use regular meter leads. I make my own from 10 Kv HV wire as a minimum. I have one set that is 15 Kv rated. I always stop and think what I am doing and don't want anybody around as a distraction when working on something like that. Many times I have sat with a yeah yeah yeah look on my face waiting for my wife to finish telling me something before I check high voltage as I simply do NOT want any distractions. She might be a registered nurse but I don't need to see how good her CPR skills are. LOL