My "New" 30K4

[K5UJ]

the impression I get from my reading here and there is the graphite anodes are okay in orange.   Everything else should be more reddish?

Get rid of those 866As... go solid state.  While pretty, mercury vapor rectumfriers have caused the death of many a transformer.

I hear that quite a bit but then there are other ops who claim there "ain't a thing wrong with m.v." and say they run them all the time for years no problems.  Maybe if you fire them up regularly and often they don't cause problems until they get old. 

I told the guys on 75 the other night that just getting to the point of getting RF would have taken me around 10 years.   the progress made with the rig so far in such little time is very impressive to me.

H.M. is great--I belive you get a lot for your money once you get something back from him and see all he has done.  Of course you don't get the same satisfaction and bragging rights that go with doing it all yourself, not to mention the education.   I sent him my 75A-3 and it is a joy to operate every day, but the next rx I get I want to fix up myself. 

[ke7trp]

4D21as are nice. I got a pile of eimacs boxed up in the closet. Might put a couple in the T368 today.

C

I too have noticed that Todd's 4-125 was glowing a bit more than Charleys. But that is just about the same amount of color that I routinely see on my 250TH when run at recommended voltage and plate current. That bright orange color must be near maximum dissipation.

Just thought I would pass this along to you lucky 30K owners

There is another tube out there in the 4-125 envelope that is not too well known.It is a 4D21A made by Eimac and Penta Labs. It has 175 watt plate dissipation where the 4-125 is 125 watts. The plate has fins but they are somewhat in a spiral rather than strait up and down. The filament current is the same. They are a bit hard to find but they are out there. It looks like a mini 4-400!

[w1vtp]

I have to agree about the 866(s).  I had two problems with my first 813 xmtr.  1) The fil transformer arced over inside to the case  2) The sockets developed a path from the fil pins to the chassis.

1) problem was fixed by taking the fil transformer apart and putting many coats of hi pot dope.  It was then put together and never had a problem

2) used the sunken garden approach to get the socket ≈ 1/2" below the chassis

That fixed the problem except I would have a flashover occasionally.  I ultimately did SS 866 replacements and NEVER had a problem after that.  If you really must have hollow state rectifiers, how about the 3B28 solution?  I have those in my Courier and NEVER had a flashover problem.

I love your approach - get it on the air but you might want to do some exploration on restoring the RF deck to its original condition over a few years.  I love these pics - sort of a vicarious trip where I would never be able to travel. I love the part where this transmitter was saved from the dump.

Al

[K3YA]

First of all, I would like to thank the numerous folks who have contacted me offering parts or other help with this project.  Your support helps fuel my enthusiasm.

Regarding the 866 tubes, mine have been replaced with 3B28 Xenon tubes.  Although the mercury vapor tubes do offer a far out blue glow that flickers with modulation in a most fascinating manner, they can be a PIA if used anywhere near there rated voltages.  In the past I had to sort through a number of old tubes to find a pair that would be reliable and not arc back on occasion.  Even with good tubes occasionally one would flash and blow out the fuses. In the case of the 30k or my BC610 that meant replacing expensive screw in fuses.  I found the same with 872's in 2-2.5 KV supplies. And, MV tubes don't like cold basements. The Xenon equivalent tubes have been quite reliable in my experience.  On the other hand I have used a pair of big old 575 MV rectifiers in my kilowatt and they have held up OK with regard to arc back as long as they don't get too cold.  So, I guess that I did ditch the 866's.

I don't think that I have ever seen a 4D21A in all my years.  Didn't even know they existed.  Thanks for the tip and I'll keep my eyes open for one.

Has anyone here ever used 4-125A's triode connected in a modulator.  That's what I put in this 30K4 in instead of rare 75TH's.   They are only idling at 30mA which seems kinda low.  2500V on the plates, g1 and g2 tied together at 0V referenced to the cathode.  Is this typical?

I'm stopping by the auxiliary radio storage facility in my parents basement tonight to see if I have a more suitable replacement transformer for the 3B28's.  Otherwise, I'll drill some new holes for an old Stancore that I found in my junkbox and try it the radio out again.

[K3YA]

The 30K4 seems to be operating pretty well now.  I replaced the 3B28 filament transformer and it didn't solve the problem.  Seams that I made a misdiagnosis and I later traced the problem to an arc over in the HV wiring.  Thanks again to all who offered me a replacement transformer.

So, here are a few photo's.  The Modulator Deck:

I hope to build up an internal audio driver on this chassis some day but for now I'm using an external hifi amp through an output transformer connected backwards to the triode connected 4-125's.  The low level part of the audio chain is shared by my other AM transmitters, and is also external to the rig.

Here are the plate and modulation transformers.  A real tight fit that resulted in the high voltage arc-over to the cabinet.  You can also see my collection of blown fuses in the photo.


The RF deck with quite a few original components missing.  Only one of the two channels is operational. The output network was returned to the original Pi network. I'm driving the grid of the final tube with an external exciter through a coil and link wound on a pill bottle, which is most certainly not how Art Collins would have made a coil.


So that's it.  I'm sure I'll do some more work on the rig in the future but for now I'll leave it alone and enjoy operating it.

[WA3VJB]

You've made great progress since the big road trip, Charlie !

I hope to hear you on this weekend - and maybe make a recording to send you of your efforts paying off.

[Todd, KA1KAQ]

Charlie, heard you on last night talking with K4ESE (I think?). Was in the house tinkering on a Mohawk receiver, you were both booming in. Thought about going out to fire up the big rig and giving you a holler, but you guys were in the process of signing out.

Glad to hear that it wasn't the rectifier transformer after all. Even if you had a replacement handy, it's always good if you don't need it.

Didn't get which knobs you were looking for or being sent, but give a yell if you need some as I have several spares available.

The 4-125s were sounding great here. As you said, you don't want to run out of audio and with a pair of those in the mod deck, you won't.