Johnson Ranger modulation transformer kaput

[K3ZW]

My Ranger modulation transformer smoked today. One side is open circuit. Made my small home smell real nice. Memorial services to be anounced. :-)

I acquired a brand new Stancor A-3845 Mod transformer My Dad. My Dad is a ham. I think he said he bought just after the Civil War.  :-)   It is slightly larger than the origonal Ranger transformer. Stancor specs it at 25 watts. The spec sheet does not list the 1614 as a compatible tube.

I can make this fit, But is it worth trying ?

Anyone have the specs on the origonal Ranger mod xfmer ??

Is anyone out there rewinding or making replacements ?

Any comments or sugestions would be greatly apreciated.

[w4bfs]

I think from the Ranger tune-up threads that the A-3845 is a performance boost.

The burnout of the stock iron is disturbing and I would want a culprit under arrest before firing up with the new xfmr.

This seems like a good time to turbocharge the ranger

[K3ZW]

I have to add another victim. One of the 1614's is also kaput. The tube tester now shows it having low emission.
Several tube sites sugest that a 6L6GC is a suitable replacement for 1614's 6L6GC's are cheaper than 6550's
Also the 6550 draws 1.6 A filament current. 6l6GC & 1614 draw .9A filament current.

Whether I turbo or not I need to come up with some mod iron.

I am realy appreciating my DX-60 with it's controlled carrier mod set up.

[ke7trp]

Sorry to hear of your loss.  As you figured out, There are lots of different tubes that will work in the ranger, 6l6 Metal,  6l6GC, 1614, 8417, 6550, KT66, And more.  Mine has run for many years with some old RCA 6l6 metal tubes.

[wa3dsp]

The 1614 was intended to be a transmitting tube not an audio tube although I don't see any reason why it could not be used as one. There are no specs on the tube in audio service. I have it in a 1941 RCA transmitting tube data book. It is spec'ed for 35 watts input power delivering an output of 21 watts with .1 watt to the grid up to 80 MHz.  Reduced power to 120 MHz. It had a list price of $3.50 in 1941.

I don't know of any commercial equipment where this tube was used but I suspect it had some war use and afterwards there was probably a surplus which Johnson took advantage of.

It sounds like one of the tubes may have shorted. You should check the resistance of the bad transformer. The values from the manual are - Primary red to brown - 100 ohms,  red to blue - 100 ohms. Secondary Green to black - 28 ohms, black to yellow - 28 ohms   

My Ranger reads about 50 ohms each side of secondary rather than 28 ohms so it may be that some different transformers were used. What you are looking for is a big difference in one reading to the other. I suspect one of your tubes may have gone into conduction due to a short or lack of bias. Is your Ranger the old cathode bias arrangement or does it use fixed negative bias? The cathode bias used 200 ohms in the cathode - two 100 ohm resistors. If that is the case make sure there value is correct. If instead it uses fixed bias make sure there is about -26 volts at each 1614 grid. The fixed bias is fed at the driver transformer center tap. If one side of the transformer opened or one of the 100 ohm resistors feeding the 1614 grids opened that tube would draw excessive current.

Check the mod. current on the front panel meter it should be 60-70 ma with no audio.

The Johnson circuit is screwy and the best thing to do if you are capable is rebuild the audio section. I would use fixed screen voltage, I regulate it to 280-300 volts and adjustable fixed bias with a phase inverter rather than the driver transformer,

I would be interested in what you find.

73 Doug

[K3ZW]

Red to blue is open circuit. all the other leads are close to you you posted.
As far as checking modulator current. I will have to install replacement mod iron.
I see the Stancore A-3845 does not have a center tapped secondary. I guess using the modulator as a speach amp is out of the question. I did not intend to anyway.
Any I dea what the secondary inpedance of the Ranger mod transformer is ??

[kb3ouk]

The 1614 is just a premium version of the metal 6L6 that is rated for maximum output and longer life.

[K3ZW]

My stancore replacement has no feedback tap. Can I get around this ??

[wa3dsp]

Sounds like one side of the primary opened. It is hard to tell why. You did not mention if your Ranger is the original with cathode bias  or if it has fixed grid bias. There are many things that could have caused it including a shorted tube, lack of bias on the tube, or just a transformer that failed. In any event unless you have two good 1614's or you can get them you will have to select another tube. There are any number of examples of modulator updates for the Ranger on the Internet. 

As for the feedback winding you don't need to use it. Get it running first and then if you are not happy with the audio you can add negative feedback by injecting a sample of the high end of the secondary back to the cathode of an earlier stage. Again there are examples of how others have done this on the web.

I have run other rigs without it like the AF67 which is a 6146 modulated with 6L6's or 5881's and gotten very good reports. If you are a purist you will probably want to add it.

Also see - http://amfone.net/Amforum/index.php?topic=25744.0;wap2

[K3ZW]

This Just in ( Dididididididit)<  Walter Winchel

The ranger had fixed bias from the 6AL5. Biassss wich disappeared. I replaced the tube with a couple of 1N4007's and I now have -45v at the 1614 grids.
More good news. I still have 2 good 1614's. they both are still showing 80% on the ol'e Sencore test tuber. I guess it is now safe to intall my Stancore replacement mod iron.

[wa3dsp]

Wow, great find. I figured it had to be something like that. A shame it had to happen. There is really no protection for that other than watching the mod current on the meter. It would have been pegged over when that happened. When you are in there put back to back 1N4007's across the meter terminals. You wouldn't want to lose a meter!

Make sure you set the screen voltage on the new tubes for proper resting current as per the manual.

[K3ZW]

 I got the Stancor installed and I have it working with the origonal 1614's. The audio is OK but it sounds like there is some distortion. Screen voltage on the 1614's is about 200v when transmitting. I am still not sure wich tap on the Stancor A-3845 is best. Tried all of them and I dont notice much difference in audio quality or punch.

[wa3dsp]

What is the resting modulator current? I assume you set the screen tap to get the correct setting?

Didn't you say that one of the 1614's was weak? I would expect that both of them got pretty cooked with the bias loss. It may have taken a little while to burn up the transformer meanwhile the tubes are really cooking.

To really check the audio you need some measurement equipment. Perhaps someone else has used that transformer and already knows the correct taps.

[KM1H]

The 1614 is a metal 6L6 that was tested for RF at 8mc for the military and sold for more money. IOW it was on the high side of the bogey for emission.

Carl

[K3ZW]

Resting modulator current is zero.  I jumpered in a 50 k pot on the bias line and was able to adjust the resting mod current by adjusting the pot. Full bias is about -36v. wich yields a resting current of zero. the screens have about 200v on them. I havent found any instructions in my ranger manual on where to set the screen voltage. I havent looked in a tube manual yet.

[wa3dsp]

OK well you can go two ways here. The way Johnson did it was set the bias to a fixed value and change the screen voltage to arrive at the proper resting current. If you want to do it that way then you need to adjust the tap on the big power resistor. Set it for 55-70 ma (manual values) Of course you need to be careful and make sure things are off and discharged when adjusting the tap.

The other way to do it is to set the screen to around 280 volts and adjust the bias for the proper resting current. It should end up being around minus 22-26 volts. If you can't get the proper current within that bias range then you would have to adjust the screen up or down some. This is an easier way to do it as you don't have to mess with turning it off and on between adjustments, just set the pot.

I am assuming you have a fairly stock Ranger and the big variable wire wound resistor is still in there. I removed mine and will be running the modulator screens directly from the low B+ supply. When I get my project done in (hopefully) a few weeks I will document it and you can see what I did. You can't just remove the resistor as it is the HV bleeder and the cllamp tube also gets its voltage from the tap.

If you have the resting current at zero then there is no wonder the audio sounds distorted! It should be fine once you get it setup.

Doug

[wa3dsp]

Oh I forgot to mention. If you are using the screen voltage adjustment method you need to check the bias to be -26 or so. I think that is what the schematic shows - either -26 or -28.  It is not that critical, you will be setting the resting current with the screen tap. The bias is from a divider R52 47K and R53 4.7K off of the main bias line. Since you changed the 6AL5 to diodes (good idea) the bias might run a little higher.

[K3ZW]

I have set the screen tap to get 70 ma resting current. The audio has greatly improved as well. I have done the low voltage relay mods to the ranger. I am gathering the parts I need to install a zener regulated bias supply and eliminate the screen reistor.



I also had another success today. I on air testsed a home brew 4 X 572B linear on 75 AM.
I drove it to about 250W out put. Diving it with my DX-60B

[Steve - K4HX]

Congratulation. Nice amp!