The AM Forum

So I found my Johnson 500 speech amp pooped out on my
yesterday...the 6B4 is a tired "old" Russian valve...
Looking at the current prices, they are pretty salty
bottles these days ($60-70)

SO...I'm considering a sub of a 6L6 or even a 6AS7
Anybody experience with such a mod?
73
8tow

It's sooooo easy to just pipe audio in externally to the grids of the mudulators. The driver transformer isn't the world's most beefy.

All changes can be reversible.

Oh, absolutely...I've migrated away from out-board audio
chains and like to stick to a basic D-104 input...Now, that
said, changing out the driver iron, adding PP drivers
(the first 2 stages are already modified to my preferences)
is a option...
One rout to take (since there is LOTS of room, is to put a
6AS7 (dual 2A3) in there with a new driver iron...
especially since 6AS7's are very reasonable... :) for now...

As you say, there are a number of different avenues that you could follow, depending on what tubes etc. that you have on hand there.

6080s or 6AS7 dual triodes would make good driver tubes. The EL34 would make a good substitute audio driver tube if one has access to one. I found some info online on using one to replace the 6B4. A triode connected EL34 and keep the original driver transformer. I have a 500 here and was always out on the look for spare tubes for it. Never had any trouble finding a spare 6B4s at hamfests in Ontario over the years.

http://www.diyaudio.com/forums/tubes-valves/14475-plugnplay-el34-kt66-6b4-se-amp.html

Al VE3AJM

...and it would give one time to look for an exact replacement by asking here on the forum - as Steve recently pointed out.

D104s?  simple FET mod in the base of the mic stand and you're good to go on most inputs to amplifiers that might be used to drive the mudulator grids

Al

Well, it just so happens I've got a half dozen Amperex EL34's ...hummm!
Thanks

Believe a 6AV5  connected in triode fashion is equivalent to a 6B4.  This thread states that a manufacturer indeed did that and sold them to the military as 6B4Gs

http://www.antiqueradios.com/forums/viewtopic.php?t=120373

Here is a lot more on the 6AV5:

http://www.tubelab.com/6AV5.htm

Jim
WD5JKO

I am interested in your information on the Johnson 500.  I am attempting to get mine back on the air. I replaced the electolytics and the HV transformer.  I plan to replace the rest of the caps when they arrive.  The pa grid meter reads about 1 ma when the transmitter is on. I once had a Johnson Challenger that drew plate current in stand-by which ruined the transformer because I had too high amp fuses.  Therefore, I am concerned.  Could the bad caps cause this?  In tune, the grid drive adjustment has no impact.  I am looking forward to getting back on AM. I was first licensed in 1959 when SSB was the new kid on the block.  I hope to join the AM crowd soon.  73, Don K5WSY

Don,

Definitely replace all of the suspect caps first because that is must  do basic maintenance.  Make sure the bias supply is producing proper output and that the bias level at the final is proper before first applying plate voltage.  The tune mode (reduces screen voltage thus limiting final current) will help reduce the possibility of meltdown during testing as will the plate overload relay.  A misadjusted keyer tube will result in VFO excitation even when not in transmit/spot position and a defective drive control will result in a lack of drive.  The Viking 500 VFO has the same undersized VR tube dropping resistor as the Ranger and Valiant so replace it with the same value (18K) but a larger wattage rating.

As to fusing, Jim Garland wrote an excellent article in the July 1997 issue of Electric Radio which provides a good tutorial on the use of input current limiters in the Viking 500 which also allows you to use fuses which are closer to the actual expected operating current instead of the original values which have higher current ratings to avoid nuisance blowing on initial start-up surge.

In another thread I detailed adding a capacitor with an isolating diode across the slow release relay in order to prevent arcing across the protective gaps due to a loss of slow release in the original relay.  PM me if you need more information.

And welcome to the group!  I grew up in the other end of the state in Gulfport where I was licensed as WN5NSC in 1975.

Steve,

Thanks for the information.  I'm afraid that my early radio interest was in talking rather than understanding all that was needed to keep things running.  Looking back, I should have been studying instead of ragchewing.  I was in high school when I got the 500.  It was a kit, but had been serviced by Johnson. My folks bought it from W & W in Memphis mainly to keep me at home and not out on the road.  I always got good signal reports.  I want to get the SX 100 up to speed after the 500. I can see that I have my work cut out for sometime.  Again, thanks for your help.  The Elmers that I used to talk with are gone now.  Best of luck.  73. Don K5WSY

Hi Don

From what you've described here, with the lack of grid drive to the final and no effect with increasing the drive control level, I would start by going through an alignment of the VFO and then the buffer and multiplier sections in the transmitter. All these sections in the 500 are gang tuned or track together, so if they are out of adjustment you wouldn't have the required grid drive to the final amp. The manual outlines how to do this.

All this alignment/adjustment can be done without the HV having to be on or activated, just the LV, by activating the spotting switch on the front of the transmitter, and as long as the LV power supply is working OK and the tubes in the low level Rf sections are good.

Replacing or changing out the electrolytics is a good thing to do down the line. Once you can obtain 12ma or more of grid drive for AM in the spot mode, you can go from there.

Al VE3AJM

Thanks Al,

I am going to a little more clean up and check LV values.  I was hoping it would go without too much adjustment.  I guess after 50 years it needs retuning like its owner.  Thanks again. I'll let you and Steve know when it is up and running.  73, Don K5WSY

You can replace the 6B4 with a triode connected 6V6 and it would probably work fine.

Tie the screen to the plate and the cathode to the heater terminals.

You can wire the socket so that you can plug in either a 6V6 or a 6B4 without changing anything.

I have a mod deck from an old BC transmitter where they did this.