Dentron Linear Amplifiers, my current collection, thoughts, walkthru and a little Dentron history.

[@AB1Vampire]

Tram Dr, this was best video yet. The history of the MFJ/Amp Supply/Ameritron plus the Dentron line-up was much appreciated.

[@tramdr]

Thanks

[@keyboarderror1]

Thanks for the informative video. I've had an MLA-2500 for over 30 years. Still works good. I swapped out the plate capacitors a couple years ago. It holds the high voltage a lot longer when you power off now. I came into a second set of tubes from an amp which went through a fire. I haven't tested them as I'm not sure if the fire temperature and exposure to the fire department's hoses damaged them. I got to them within a day or two and cleaned them up best I could. The seals seem intact. No idea if they're safe to power. I guess I'll find out when my originals burn out. I've read somewhere you're not supposed to run them with more than 50ma of grid current, which is hard to read since the meter is way over scaled. It'll put out over a kilowatt, but not for long. Mine seems to like to run around 600 watts before the ALC picks up if I'm using it. The startup inrush eventually ate the power switch, so I've got that bypassed and just plug it in for now. On the hunt for a new switch, or maybe I'll build something. Also maybe build a soft start circuit. I've seen where people build big amps and they'll use a ceramic cone heater as the soft start resistance. Something to think about anyway.

[@tramdr]

Agree with everything you said. Dentron used a minimum cap value in the MLA-2500 HV bank. With both of mine, I am upgrading that value. Here is one for you though. The schematics show that the HV caps have a 100K bleeder resistor across each cap. Thats pretty heavy bleed. However on one of mine it has the 100k's which would bleed them quick, but the 2nd one used 470k bleeders which is almost 1/5th of the bleed of the 100's. If your caps are now bleeding slow, check for the 100's and make sure they are all good. Also agree on the grid current, especially now that those tubes are unobtainium.

[@keyboarderror1]

@@tramdr Good to know. It's not real slow discharging, but a bit slower than the originals which I chalked up to leakage. It didn't blow fuses. I also added the suggested .01uf 2KV caps across each of them for an RF bypass. Maybe that helps with the parasitics. The one thing I can't figure out is where to put the suggested 10 ohm 10 watt resistor (r22) in series to limit the anode current for a tube arc. I'm afraid of getting an arc to the case from wherever i put it. It might need some high voltage heat shrink for insulation but not sure. It seems like some of those connections are awful close for two thousand plus volts. Perhaps I'm just paranoid.

[@MM0IMC]

I believe that some of the Yaesu amps that used the 572B tubes, also had problems with the filament sagging on the higher bands and higher output power. I built a 20m amp with a pair of 813 tubes, but vertically mounted. I had thought about building it with the 813's horizontal, but decided against it...

[@tramdr]

I heard that too.

[@va3dxv]

Yeah man the 572B issue was the filament and grid structure. It wasn't upside down though, it was just 90 degrees. You'd want the grid and cathode elements in the tube running in the vertical direction. If they're horizontal they can sag and short out when hot.

[@tramdr]

Yep, now that you mention it, 90 degrees off, you are right.

[@kevinroberts781]

I still have my elkin 6 tube and elkin 8 tube amps from the 80s. I used them on my base station.

[@tramdr]

elkins are top of the line.

[@1112223333111]

they had a 8873 8874 and 8875. the 8873 was 200w plate in the sb230 and the 8875 was 300w and the 8874 was basically 3cx400a7 at 400w and a normal cooling design

[@tramdr]

Yep

[@1112223333111]

@@tramdr have a close look at pin 5 on the sockets. they like to crack loose from the ground

[@danieljohnson8437]

THX.DAN

[@tramdr]

welcome

[@MM0IMC]

10:23 Is that a soft start timer tube, next to the relay?🤔

[@tramdr]

No soft start in any Dentron that I know of. It's a Tube warmup delay though. Keeps a mud duck from transmitting before the tube is warmed up. That would kill those expensive tubes.

[@MM0IMC]

@@tramdr there was someone looking for one of these recently.

[@keithweiss7899]

Great history! Thank you Doc!

[@tramdr]

My pleasure!

[@MM0IMC]

1:34 Unfortunately, it seems MFJ has downgraded the Ameritron brand name, with poor quality control.😥

[@tramdr]

MFJ is another company that is known to cut corners to make a dollar and you are right. Hams call MFJ Mighty Fine Junk.

[@MM0IMC]

@@tramdr I call it More Foxtrot Junk, the F is a b-omb though...

[@marksandquist4275]

nice information and history. i think some models had a cw setting that reduced voltage

[@tramdr]

Some did, some didn't. The MLA-2500 non B first version did not, then the MLA2500 non B 2nd version did, and then the MLA-2500B did and they did some updates with the Bias too.

[@markviers998]

Interesting history, thanks.

[@tramdr]

Our pleasure!

[@maninthevan53]

So could you reduce the filament voltage

[@tramdr]

Sure, there are a few ways to do that, but most people want Mo Power and also, if the tube is already used to running 'Hot' with the high filament voltage, the output is going to drop a lot when you now reduce the filament voltage to the proper level.

[@bblod4896]

I see the same issue with solid state amplifiers. Running the power supply voltage at 15 to 16 vdc to get another hundred or so watts. Thanks Professor.

[@tramdr]

and not to mention those that go 18V and I have seen some go up to 22. Same principle. Mo watts, short life.

[@spaceflight1019]

"The light that burns twice as bright lasts half as long." Blade Runner

[@spaceflight1019]

Appropriate that you're featuring the Dentron amps because there's a current DXpedition to Clipperton Island going on now, and they're on 28.485 as I write.

[@tramdr]

Good timing.

[@spaceflight1019]

@@tramdr I just got home from two weeks in Fort Myers and the propagation is just as bad as when I left. I took a Cherokee AH-100, Azden AZ-10, and a Radio Shack DX-390 along. Most mornings I could hear stations in New England and Ontario on 38 lower working European stations but I couldn't hear them. The day before I came home was the only time I heard anything on 10 FM. Some evenings on 40 meters were pretty good for European stations but most of the time it wasn't worth it. This solar cycle is strange, the flux is relatively high but I can't hear anything beyond about 6-8000 miles. And I can't hear the DX. Bouvet, Crozet, and now Clipperton Island seem to be out of range.

[@bucketofguts]

I have a lot valves. Never seen a "This Side Up" Who new? Keep on Trucking! 73 Doc!

[@tramdr]

I was the same, but I did know that some tubes don't like to go Horizontal, the 3-500Z's for one. On many glass tubes, the grids are built like my main fake Christmas tree is. Basically hangs down from the top. If you mount it sideways.....it doesn't hang straight, and you got real problems. I think it was right here on this channel that someone mentioned about the Dentron having the 572B's upside down. My first thoughts on that were are you stupid? 572B's are one of the few tubes that can be mounted horizontal. Heathkit has a mobile amp with them mounted horiz. However, after looking it up, there is an Up and Down correct way to mount the tubes Horiz. and Dentron did it wrong.

[@andyr8625]

Do you repair tube amps? 🙄

[@tramdr]

Sorry, no.

[@andyr8625]

@@tramdr ok thanks

[@kevinraynor9116]

Had one but got stolen. Very nice

[@tramdr]

Dang, sorry to hear that. Curses to all the thief's and scammers out there. That includes you too Mr. Treles H.