The AM Forum

Well, finally saw the "smoke" today, after I had replaced the original, failed cap section.  What I am talking about is the series/parallel 150pf stack C38a-d going to the bandswitch.  The originals were those rectangular micas and were rated at 500v, so ordered some 1000v types thinking that is the problem...nope, think it is the current that is the problem.  Now, I need some info on just what, where, how I can procure the correct heavy duty caps.  I also replaced the 350pf stack C39a-d with 1000v mica, as it had a monster transmitting mica in that location that had been installed by one of the many owners of this rig, over the past years.  It didnt look good either, as looked abit swollen, so did that mod as stated previously.  If I could find the right value doorknob, I'd use it, but havent seen any of those values in any ads I have looked at.  So, anyone got any ideas that I can take a look at, hoping I can finally get this critter to keep on the air.  Any ideas appreciated..

John W6ZKH

Loading caps?
I do not think the value is critical.
I really dislike the switched in caps for loading, why not an air variable, they are not that large because the voltage is low...
You would think that would cost less then a switch and a bunch of fixed caps...

No they are in the plate tuning. If you do the math( PV+ RFV+ AUDV) PV=660vdc RFV= 8v X each watts at full output 150X8= 1200v AUDV = 4v per watt =600v total =2400v
This is a rough estimate I would use something around 2.5-3.0KV
Replace both caps the 80 and 40mtrs I think its 360pf and 150pf.
When Johnson made the Valiant they really went on the cheap side of things!!!!!
Terry N3GTE

Thanks Terry for the math, as I am no wizard, hi... Yes, I have replaced the 150pf with a true transmitting mica of 2.5kv and a 400pf replaced the 350pf SM's.  Kinda hard to stuff them in the tight space, but did it.  The 'multi-cap' stacked 600,900,1200 pf was also missing and had been replaced by various transmitting 2.5/5kv micas already. See photo before 150pf change, but he "red" micas are the ones that blew on 40M, as the 350pf (now 400pf)had been changed by previous owner. I did clean it up somewhat as had poor soldering job.  Need to get this critter back on the air again.

John W6ZKH

That is why I went all home brew, what a screwed up mess, I never heard of switched plate tuning caps in an AM rig.

  A Globe King 500 looks much the same as what was just pictured.

Jim
Wd5JKO

Many old manufacturers did that.

In desperation I defected and installed big honking Soviet doorknobs. No more failures.

Well, I have bit the dust and ordered the ruskie doorknobs after another failure of the 150pf/2.5kv Sprague mica cap today on the smoke test..  infact, even ordered some 330pf for the 80 meter side too... these are 10kv units...course wont be here until mid-late April, but what the heck anyway...  But one good thing, it does load up on all other bands, but funny thing...it wouldnt on 20M..but the manual says to operate on 10, 11 and 15 meters you set the fixed loading switch at "9" and go from there....well, that is how I have to do 20 meters also, and then get 130 watts out easily.  Audio sounds great without any mods using an amplified D-104.... at least to my ears using the SX-101a RX...  So will set this Valiant aside and hopefully at this Saturdays hamswap I will find the 4 tubes I need for the Halli HT-32a so I can finish up that project.  I am hoping to make the Hallicrafter pair of the HT-32a and SX-101a a vintage SSB setup.  To go with the Valiant for AM, am looking for another good receiver, like the NC-300 or Hammarlund HQ-170, etc. Maybe at the swapfest??

John W6ZKH

The guys at Woodstock who took the brown acid freaked out and were treated. Afterwards they were used to treat the new patients.

You lost me on that one !!

Perhaps a post made while chemically revisiting Woodstock

The meaning is found in the definition of "treated"!  ;D