The AM Forum

To support my new Tech license, a friend has loaned me a Drake TR-4CW transceiver.  He mentioned that it may need a couple tubes.  I checked it out and found that a couple of the tubes in the radio do not match the numbers in the manual or on the frame of the radio.  The tube in position V3 (Mixer/Cathode Fol.) should be a 6EA8 and currently has a 6GH8A.  The tube in the V14 (AM Screen Modulator) position should be a 13DE7 and contains a 12BH7A.  I was going to order the correct ones by number.  Have these been substituted for some other reason?  Thanks, Peter.

    Hi Pete! you sure joined up fast on here...like i told you that AM modulator tube is a 12BH7 because it was all i had at the time. note that the socket was rewired to use it. that's what i meant when i said i had to go back in and change it when you got the correct tube. if you plug  13DE7 in it now the radio will smoke, and not work... as to the other one (6EA8)? i did not notice, but it works just nice as is and would not change it. that radio was made for many years and it may have been changed by the factory, the manual might not reflect a later change, your radio being newer than the manual i gave you. let me know when you get the 13DE7 and 12JB6's- i will finish the AM mods and neutralize the new final tubes. above all don't sweat this stuff too much- this technology is far from being rocket science, amazingly reliable and robust, and easy to work on. fear not- i will get you on the air with a top notch signal and great audio. oh yes- drop by the shop later this week (i'll be out on a few big jobs most of the week) and i'll program some genuine ham frquencies in your HT1250 so you can get on the air...
best of luck
Chris J

Way to go Chris.  A new amateur getting some good ol' fashion help.  Welcome Peter!

   Howdy Dave! Pete is an old friend who recently decided to take the plunge and get his ticket. he's an all around Great American and will make a good addtion to our ranks. yesturday i had that TR4 on for a while checking it out. worked Ralph '3GL and Johnny Novice with it too.  no smoke came out so i deemed it airworthy. snowing again here but it's only 90 days or so till i get to Herkimer County. maybe i'll see you up there this summer?

According to the tube substitution lists, the 6EA8 is a replacement for the 6GH8.  It should work fine.

The tubes are on order!!  Thanks for the help.  Peter.

I plan on reporting you to the Drake On A Shelf Collectors Association.

 the radio in question here was found by Paul W2BP in a dumptster the day after a local hamfest at the 'fest site. it had been rained on overnight. the person who dumped it should be reported to the shrink wrap collector Mafia... while we are on Drake tech i have found on my TR3 and TR4 that after disabling the ALC if you short out the 27K resistor that feeds bias to the PA tubes it makes the thing much cleaner on SSB when you whack the tubes into grid current. next time i'm in one of them i'm also going to add a zener clamp on the bias. tests on stock rigs have shown me third order IMD as high as 20DB- yikes! has anybody on here ever put 6146's in place of the sweep tubes?

Chris,
this guy has done the 6146 surgery and documented it on his website...

http://www.k5dkz.com/t4x6146.html

Also, Drake had a tech bulletin about loading the transmitter to avoid impurities
At least they were aware of problem....

 http://www.wb4hfn.com/DRAKE/DrakeArticles/TransmittingTuningAlt.htm

The 6146 surgery looks more and more inevitable for us Drake fans.
6JB6's you get today probably have been re-labeled, etc.  Depending on constrcution of the tube results in low 10M output-lousy grid drive. 
Keep 'em cool
Jim KF2SY



 :'(

a copy of that tech bulletin came with my father's TR4CW when he bought it. the third order IMD on most of my Drake stuff is at best borderline ( by my standards). i have not had any trouble getting gennie 6JB6's lately though. only once about 15 years ago did i get burned. i bought some from Richardson Electronics/RF Gain Ltd. what i  got was tubes with a little PC board like "adaptor" soldered to the (cut off) pins. they not only did not fit the radio being too tall, but also would not bias correctly in addition to not being able to neutralize. the thought of using a single conduction cooled tube, like an 8072 (i have used then in other projects) occurs to me too. they have about 200 watts dissapation in a good heat sink and work very well at low plate voltage. EF Johnson used that setup in their early commercial SSB radios.

You are OK with the 6EA8, it is a replacement for the 6GH8, used them in the color TV's back in the 60's. Have FUN, and ask questions, we got alot of talent here. Great bunch of Guy's.

73,

Fred

KC1XF

I received the new tubes the other day.  I bought them from Vacuum Tubes Inc in Orlando Fla.  The tubes I received are NOS RCA.  They look great.  We'll see how they work.  Thanks, Peter.

Pete- maybe if you get time next Saturday you could drag the rig down to my Ma's house where i have my shop and we can get the tubes in, neutralized, and finish the AM mods i started on it. then you will be ready get that Drake on the air. i'll give you call during the week. by the way i took in an orphaned Underwood Carbine today too...like i don't have enough toys allready, HA!