[Johnson] Valiant Chirp - update

Al Parker anchor at ec.rr.com
Sat Jan 3 13:43:24 EST 2009


Hi Chris,
    Things are a lot different on 80m, the VFO is on 160 then, vs on 40 when 
xmtg on 40.  But, we've recognized that already.  I don't think your test 
proved my diagnosis, but didn't disprove it either.
    I'd suggest that you try grounding the variable cap directly by using 
stl (or SS) washers iin place of the fiber ones.  That'd probably be easier 
than my method.  Tom, W8JI, did that to cure the problem.  I'm not sure if 
his post was on the bottom of the notes file I sent you, so here is the 
latest.
    Anyhow, glad you're getting into it and enjoying it.  Keep us posted.
73,
Al, W8UT

----- Original Message ----- 
From: "Chris Kepus" <ckepus at comcast.net>
To: <johnson at mailman.qth.net>
Sent: Saturday, January 03, 2009 1:16 PM
Subject: [Johnson] Valiant Chirp - update


> Again, thanks to everyone who contributed ideas re my chirp problem.  A
> special thanks to Al, W8UT, whose Valiant problems and fixes message 
> archive
> immediately became one of the hottest request items on the lists! :-)
>
> The info in that archive and Al's message pointed out that the chirp is
> likely being caused by RF feedback from the amp into the VFO, especially 
> on
> 40 M.   I managed to get an hour or so last night to play so I loaded up 
> on
> 80M CW.  Wow!  It was like a different rig.  Had some real nice QSO's and 
> NO
> chirp reports!   My monitor receiver also confirmed a nice T-9 signal.  It
> appears that RF feedback is the villain.
>
> My next task will be to fashion a grounding method for the tuning shaft by
> copying Al's method.  I'll report back then.  I receive my Johnson list
> messages in digest form so there may be some delay in my responses to
> comments.  Please reply direct to me for a faster response.
>
> 73,
>
> Chris
>
> W7JPG
>
>


--------------------------------------------------------------------------------


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> Home: http://mailman.qth.net/mailman/listinfo/johnson
> Help: http://mailman.qth.net/mmfaq.html
> Post: mailto:Johnson at mailman.qth.net
> 
-------------- next part --------------

From: "Garey Barrell" <k4oah at mindspring.com>
To: "John King" <k5pgw at yahoo.com>; <Johnson at mailman.qth.net>
Subject: [Johnson] Re: [Boatanchors] Viking Valiant shaft coupling
Date: Friday, January 04, 2008 6:32 PM

John -

Those are just about impossible to find.  They may even have been custom 
made for Johnson. 

They are unfortunately a very common restoration problem, and result 
from someone unfamiliar with Johnson Rangers and Valiants.  If you 
remove the front panel with VFO dial and fail to support the heavy 
vernier, the weight will break that coupler almost every time.  There is 
a hole in the chassis of both the Ranger and Valiant to allow you to 
reach in and loosen the couple setscrew so it can come off the variable 
cap shaft when the vernier is removed with the dial, but most don't know 
why that hole is there!!  :-)

I made one once by purchasing a 3/16" - 3/16" coupler (the small side) 
and a 1/4" - 1/4" coupler, drilling out the rivets and reassembling them 
with 2-56 hardware.  I put one in a Ranger I was restoring and a friend 
used the other in a Valiant!

73, Garey - K4OAH
Glen Allen, VA


============
Here is the guy who sells the long screws.

Cal J. Eustaquio
cal.j.eustaquio at lmco.com

=============
Note to Walt--
>     Bought a Viking Valiant, at Shelby, Sept. 2000, hoped to get it on the air quickly, but
> haven't.  The plate mtr shunt had bn replaced by a short pc of solid wire,
> mtr read zero, of course, but did hv output.  I put in a proper .2 ohm,
> still no indic. on the mtr.  Turned out the wire from the meter function
> switch to a gnd lug had never been soldered, & didn't make contact for the
2
> positions it was needed.  It was said to hv modulator problems in the early
> stages, not xfmr, I found 2 inproper tubes there, but haven't wired up a mic
> yet, so still needs attn.

===============
From: W4UOC at aol.com
Date: Wed, 8 Jul 1998 
Subject: Viking Valiant Osc. problem.

Any suggestions on this one...

I have a Viking Valiant transmitter that works fine except for one small
problem.
When I use the xtal and key the transmitter the xtal osc. does not shut off!
I can her it in the receiver and the the meter in the osc. position shows it
is running.  It keeps running until I switch the xtal switch to the empty xtal
position.  Howeve, it only shows grid drive when the key is closed.  Is this
normal.... I doubt it....!!!!

I do not have this problem when using the vfo position.  It is not the keyer
adjustment (R39).  

All voltages and resistance measurements look normal on the osc and keyer
sockets.

I suspect that the -6 vdc at pin 1 (control grid) of the osc. may be changing
values and causing the problem.
=============================
Date: Wed, 8 Jul 1998 
To: Old Tube Radios <boatanchors at theporch.com>
From: Rodger Singley <rbsingl at rs6000.cmp.ilstu.edu>
Subject: RE: Viking Valiant Osc. problem.

In my case, replacing the keyer tube took care of that particular problem.
This doesn't necessarily mean the keyer tube was bad but replacing it with
another with slightly different characteristics took care of the problem.
My Valiant had the additional annoying habit of producing a roughly 250 khz.
(NOT Mc.) oscillation in the final, produced about 3 watts out according to
the scope/dummy load measure.  Made it really difficult to use an antenna
tuner/swr meter at low power settings (and it of course did not show up on
the spectrum analyzer since it was well below the fundamental and the lower
frequency display of my analyzer unit but was clearly identifiable on a
receiver which tuned this range). Now if someone could get rid of the last
of the chirp on 40 meters (due to fundamental getting into the VFO) I would
really be happy.  Mine is much better with additonal shielding but does not
sound nearly as nice as my Viking 1 or 2 units.

Rodger WQ9E
====================
From: "Freeberg, Scott (STP)" <scott.freeberg at guidant.com>
To: Old Tube Radios <boatanchors at theporch.com>
Subject: RE: Looking for Johnson Viking Valiant info
Date: Mon, 15 Mar 1999 13:23:35 -0600
X-Mailer: Internet Mail Service (5.5.2232.9)
Reply-To: scott.freeberg at guidant.com
Sender: owner-boatanchors at theporch.com

Hi Ken,

Congratulations on getting your new Valiant.  It is a great transmitter and
alot of fun to operate!   I bought one 2 years ago and continue to use it
weekly on cw and AM.  Here is a couple of things I found with mine and would
recommend you look into also.

1.  Replace the 866 high voltage rectifiers with a pair of 3B28's .  Don't
wait for one to fail, do it now.  While the 866's are more nostalgic and
prettier to look at,  they have a tendency to fail shorted and could
possible take out your hv transformer.  One of my 866's shorted but did not
damage the transformer.

2.  Since you are poking around, open up the VFO cabinet and replace that
18K resistor that is in series with the OA2 voltage regulator.  I don't have
the schematic at hand, but its the 18K series resistor feeding the VFO
voltage regulator.  Replace it with a 5 or 6 watt part.  The resistor in
there is underrated and is slowly burning up.  Mine finally burned up and
damaged several tubes.   I put the replacement 6 watt part on a new terminal
strip mounted under the chassis.  There is a convenient rubber grommet to
run the wires through into the vfo compartment.  Helps reduce heat in the
vfo compartment.  This is a known problem to Rangers and Valiants.

3.  You mention recapping.  I would only recap if something is wrong.  If
the caps are working fine,  there may be no immediate need to replace them
unless you just want to replace them all.  

4.  Buy a manual.  You can get a nice manual from W7FG manuals, plus others.
There is other good information in there besides the schematic.

Good luck with your Valiant.

73,  Scott WA9WFA in Saint Paul Minn
==============================================
VFO
Found this in my files.
Hope it's useful.
Ed - VA3ES

-----Original Message-----
Sent: Wednesday, April 19, 2000 12:55 PM
Subject: Re: Ranger VFO problem


The VFO housing can be removed by the following steps (these are from
memory, my complete description is on another computer and can be found no
doubt in the archives.)

0)  Do NOT remove all the screws visible on the top of the VFO housing.  Do
NOT remove the screws at the sides of the housing.  Do NOT remove the VFO
tuning  shaft coupling.  Do NOT remove the circle clips from the phenolic
adjustment devices.

1)  Mark the orientations of the adjustment devices on the top of the
housing.

2) Unfasten the connections from the top front of the VFO housing to the
top rear of the front panel: there are at least one terminal strip with
pilot lamp connections and a very important ground connection.

3)  Visually locate the four  studs at the lower edges of the VFO
housing.  You will remove a nut from each of them from the underside.

4) Invert the transmitter and remove the cover that protects the
bandswitch.  The phenolic adjustment rods each have a metal spring in the
end.  These springs engage the adjustments in the VFO housing.  These
springs must not be lost and must be in place upon re-assembly.  A conical
shaped hole in the end of the rod ensures that the rod with its spring
engages the slotted end of the adjustment.

5)  Locate and remove the four nuts and lockwashers that fasten the housing
to the chassis.

6) Upright the chassis and remove the housing by wiggling and pulling
straight up.

Here are some steps you may wish to carry out while you have the thing
apart:

0)  Do NOT move any of the adjustments.  OR, set them all to one
orientation for re-assembly and be prepared to do a complete
re-alignment.  If  the rods have become mis-aligned, do not be alarmed. It
is possible to re-align all adjustment rods before fastening the housing
with its nuts.  Let the thing sit on the mis-aligned adjustments and turn
each one to get it to snap into place.  This is best done with the chassis
in normal horizontal position.

1) Move the 18K VR tube dropping resistor outside the VFO housing.

2) Move the VR tube outside the VFO housing, or replace the VR tube with a
suitably cooled or heat sunk solly state device.

3) Clean and apply DeOxit and/or ProGold to the variable capacitor ground
wiper.

4) Apply a SMALL drop of oil to the VFO capacitor shaft at the front of the
bushing.

5) Apply DeOxit and/or ProGold to the tube socket pins.

6) Test and replace the VFO tube with a new one.

7) Clean, lubricate, and treat with DeOxit and/or ProGold the Bandswitch.

Upon reassembly:

1) Clean the bottom edge of the housing for good contact with the chassis..
Similarly, clean the chassis at the are where the housing will contact
it.  A pencil with rubber eraser end is good for this.  Vacuum out any
abrasive residue.

2) Ensure that the adjustment rods are lined up with the adjustment devices
in the VFO.

3) Reverse the removal procedure.  (Hey, I copied that from a military
manual that tells how to remove and replace a helicopter turbine engine.)

4) Ensure that the pilot lamp connections are restored and that the
grounding strap is connected from the VFO housing top front to the rear top
of the front panel.

5 If you replace the VFO tube with a new one, allow some hours of burn in
before completing final frequency alignments (alinements sic.)

Roy Morgan
Keep em glowing!  K1LKY since 1959
7130 Panorama Drive, Derwood MD 20855
301-330-8828


==============================
From: "Rodger" <WQ9E at DTNSPEED.NET>
To: <BOATANCHORS at LISTSERV.TEMPE.GOV>
Subject: Re: [BOATANCHORS-TEMPE] E F Johnson Question - Need parts
Date: Saturday, January 27, 2007 12:13 PM

Bruce,

Sorry I cannot help you with a spare but when you find a new transformer
carefully check the wiring from T2 to the 866 sockets.  On earlier Valiants
(both kit and factory) the wiring from the transformer to the 866 cathodes
is standard hookup wire with very thin insulation which is not sufficient
for the high voltage that appears.  This wire will arc to ground where it
runs near the chassis and blow fuses which is probably what was happening
when the former owner decided to add the 20 amp fuse which then turned T2
itself into a fuse (or smokebomb).  If you have the standard hookup wire
replace it with wire designed to carry higher voltage.  If this is what
happened, there is a small chance that T2 is okay except for the 5 volt
winding being open.  If T2 is otherwise okay, you can disconnect the 5 volt
leads and use solid state replacements for the 866 rectifiers.
Unfortunately, it is usually the transformer primary that gives up.

Also, you should loosen the insulating coupler in the VFO drive before
working on the Valiant.  When you turn the Valiant upside down the front
panel will try to flex and the coupler will try to resist this flexing-and
lose.  I found this out the hard way the first time I worked on a Valiant.

The Valiant was my novice transmitter and I have a couple of them today.
The 18K resistor in the VFO and the 866 cathode wiring are common problem
areas.  Also be aware that the meter shunts (particularly in kit built
units) may be far from correct so you should check these for accuracy while
going through the transmitter.  Mouser and others have precision low value
resistors you can use for shunt replacement.  If the VFO tuning is stiff
and/or does not feel smooth the ball reduction drive needs to be cleaned and
lubricated.  I have had to do this on one of my Valiants, both Rangers, and
my Viking Pacemaker and I imagine my soon to arrive 500 will need this also.
It is easy to clean, just keep track of the sequence of parts as you do
disassembly.  Some people have had problems with the fixed output loading
capacitors but so far this has not affected any of my Johnson transmitters.

Good luck with your repair!

73, Rodger WQ9E

> -----Original Message-----
> From: Boat Anchor Owners and Collectors List
> [mailto:BOATANCHORS at LISTSERV.TEMPE.GOV] On Behalf Of hudlerb1 at netzero.com
> Sent: Saturday, January 27, 2007 8:40 AM
> To: BOATANCHORS at LISTSERV.TEMPE.GOV
> Subject: E F Johnson Question - Need parts
> 
> Hello,
> Now that I am officially a true boat anchor owner, and a member of this
> fabulous list, of
> few questions please.
> I've got a pretty nice Valiant I, but it needs repairing. Some idiot put a
> 20 amp NO BLOW
> fuse in the primary of T2 and of course it is SMOKED. I think I'll have a
> rough time
> replacing this item, what do you folks suggest?
> Is some one able to rewind the thing or just keep poking around for one
> that is being
> parted out. I think this is a common problem, so the remaining parts are
> going to be
> scarce and expensive.
> Any other alternatives, such as two or more separate transformers?
> Please help! I'd LOVE to get this thing on the air again.
> Bruce WA3MKC
> 
=====================================
From: "Rodger" <wq9e at dtnspeed.net>
To: "John King" <k5pgw at yahoo.com>
Cc: <boatanchors at mailman.qth.net>
Subject: Re: [Boatanchors] Valiant line cord
Date: Friday, August 10, 2007 11:16 PM

Hi John,

You never quite know what you will find after years of "service".  I use 
inline fuses installed under chassis to avoid drilling holes.  You 
should only fuse the black lead in your new 3 wire installation and this 
is the lead that should go to the main power switch, the neutral lead 
should never be fused or switched.  While you are in your Valiant, take 
a close look at the wiring from the low voltage transformer to the 866A 
cathodes.  In early Valiants (both kit and factory wired) they used 
regular hook up wire which will often arc to the chassis given that it 
has the plate voltage also present on it and if this occurs it will take 
out your low voltage transformer.  If it is regular wire either replace 
it with better grade wire or sleeve it with external tubing.  Of course 
if you replace the 866A's with solid state replacements you can unhook 
these wires.

Also, before you do any major work on the Valiant, loosen the screws on 
the insulating coupling in the VFO drive.  Once the Valiant is out of 
the cabinet, the coupling will try to hold the front panel from flexing 
when it is turned over and the coupling loses every time!  I had to 
rebuild the coupling on one of my Valiants after the previous owner 
learned this lesson the hard way.

You might also want to replace the dropping resistor in the VFO assembly 
with a higher wattage unit if this has not been done yet.

Enjoy the Valiant, I used one as a novice and it is still a favorite of 
mine.

73, Rodger WQ9E

John King wrote:
> Here is a Valiant story:
>
> Tonight I pulled a Valiant, that I bought two years
> ago, out of the cabinet and was really suprised. I had
> never plugged it in or attempted to use it. A previous
> owner had removed the original line cord and the fused
> plug on the end and replaced it. There was no strain
> relief grommet and the solder connections were all
> that were holding it.
>
> The line cord installed is a zip cord with molded two
> prong AC plug on the end. It did not have a three wire
> line cord or a three prong plug. To top it off THERE
> WAS NOT A SIGN OF A FUSE BETWEEN THE TRANSFORMERS AND
> THE AC WALL PLUG. I could not believe my eyes!!!!
>
> Now I will put a strain relief grommet and three wire
> AC Cord on the Valiant. I will ground the green wire
> to the chassis and fuse the other two wires by
> installing fuse holders in the rear apron of the
> Valiant.
>
> Hopefully the transformers and most parts are OK. I
> will replace the electrolytics and a the coupling caps
> as well as a few others.
>
> I am still shaken up by how someone rigged this thing
> up and hopefully I can make it safe and reliable. 73,
> John, K5PGW   
>
>
From: "Ed Tanton" <n4xy at EARTHLINK.NET>
To: <BOATANCHORS at LISTSERV.TEMPE.GOV>
Subject: Re: Help EFJ Ranger II VFO
Date: Saturday, December 18, 2004 9:09 PM

Keep in mind folks that there is a (relatively) simple way to replace the
18k resistor permanently. Open the compartment and SHORT the resistor with
a soldered, insulated, jumper wire. This is much easier than trying to get
another resistor into its place.

You then find THE wire that goes into the VFO compartment (e.g. to that
now-zero-ohm resistor) and interrupt it. Add a NEW  ~10W (or more, total)
18k (total) 'resistor' (2 x 5k, 1 x 8k/etc. 3 to 5W each) in series with
that wire and its original source. There was one spare terminal on a
terminal strip somewhere nearby, and I then ran the resistor-combo from the
source to that terminal strip, and attached the wire to the VFO compartment
to the other end. Works like a champ.

When I get some time, I am going to remove the 0A2 altogether from the VFO
compartment, and add a combined 20-25W (5 x 20V 5W ea 1 10V 5W-or just
leave it at 100V combined with the 5 20s) of zeners to that same
resistor/VFO-wire junction; to ground.

I have to go in anyway because the darn cam has slipped, and has to be
reset/ remounted/ repositioned/ whatever; and since there's nothing like
seeing the switch actually in the right place; and it wouldn't hurt to get
in there and respray the contacts and relube the switch detent path, I
might as well do it all.

And... if I'm going to do that, I might as well remove that VR tube in the
process (and add those zeners.) If I'm feeling REALLY ambitious, I'm going
to 'select' the most stable 6AU6 I have out of the 5 or 6 new ones around
here before I button the thing up again.

And finally... as long as I'm going to do all that, I might as well drill
and pin the darn cam to the shaft once it's 'right'. Unless someone has a
better idea for permanently keeping that *%^#$*^$ cam in place?




73  Ed Tanton N4XY <n4xy at earthlink.net>

Ed Tanton N4XY
189 Pioneer Trail
Marietta, GA 30068-3466

website: http://www.n4xy.com
===============================

Al your memory is pretty good.  I believe the resistor is an 
18 K resistor and can be reached by removing the side of the 
VFO box.  It is the left side when you are facing the radio 
panel that will expose the resistor..

You can just clip the resistor out leaving a bit of the lead 
wire in place then solder new insulated wire to the 
remaining pieces on the terminal board and tube socket.  You 
can then run new wires out of the hole in the chassis.  If 
you move that resistor out of the box so it can cool with 
more air moving about, you can use a 2 watt.  In my 
experience, you don't really need a 10 watt, but you can buy 
some carbon film 3 watt and it should be sufficient.  The 
real key is to get that resistor out of the closed box where 
heat does build up and destroy it.

If everything is all right with the VFO, it should not 
chirp.  But I have heard many of them in past years that 
did.  I have never explored why they do, but I have heard 
them back when they were new radios.  Completely removing 
the VFO is a bit of a challenge and to do it, you will need 
to take the panel off.  Sometimes the caps in the divider 
can be bad, they should be C 12, 13 and C 14 and 15 but that 
may not be the source of chirp.  That could be something 
near the keying tube.  If the VFO is oscillating, I would 
not go to the trouble to remove it just to explore.  You 
might create more trouble than you have.

There should be about 4 resistors associated with the keying 
tube, check their values against the book.  As I recall the 
one in series with the wiper of the pot is about 1 meg.  If 
it has gone high, then that could be part of the problem. 
The cathode of one section of the 12AU7 is attached to about 
260 volts negative through a resistor of about 18K, be sure 
it is good.  Even a dirty contact on the mode switch or 
keying pot could be the culprit.

Good luck Dave.

Jim
W5JO


> Hi Dave et al,
>    I don't see the Johnson list, so don't know what responses have been there.
>    The Valiant/Ranger, etc., VFO's have a dropping 
> resistor in the line to the VR tube which is inside the VFO box IIRC.  The 
> resisitor is abt 12k or 15k, 2 watts, and goes high in value as it gets very hot. 
> Then the VFO chirps.  I can't remember if the resistor is inside or 
> outside of the box, but it should be mounted outside, and be a 10watter if 
> posible, at least 5w. Some guys have moved the VR tube out of the box, but I 
> think just getting the dropping resisstor out helps the drift.
>    I suspect the 122 VFO is much the same.  I've been into  both the Ranger
> & Valiant to make the resistor change, without removing  the VFO, but it's
> tight, and tough to get into it.
    It should not chirp.  That dropping resistor usually fixes the chirp.
> 73,
> Al, W8UT,
> New Bern, NC
>
>> All: I have several questions about the Valiant II VFO, 
>> inspired by the
>> exchange on the 122 VFO a bit earlier.  I've never gotten 
>> the pull out of
>> this VFO; that won't help much if I want to run the SSB 
>> adapter with it.:
>>
==========================================
12/5/07
Hi agn folks,
    Well, I think I've made the cure, thanks to all.  I did get 3 responses 
from the BA list, and 2 from the Johnson list.  4, 2 from each list, said it 
was 40m RF getting back into the VFO, which was operating on the output 
freq.
    3 suggested fiddling the mechanical switch that changes the VFO freq. 
from 1.8mc to 7mc on 40m & higher freqs.  That would keep the VFO from 
operating on the output freq. exc. on 160m, where it doesn't seem to be a 
problem for anyone.
    Tom, W8JI, correctly identified the actual cause of the problem by 
sniffing with a probe.  The final tuning capacitor, C8, is isolated from 
ground for some reason.  Usually a capacitor rotor and shaft is grounded. 
In this case the shaft, which is about 8" long, and goes thru the front 
panel to it's knob, picks up RF from the final tank coil which is 
conveniently close coupled to the capacitor rotor and shaft.  I quote Tom's 
analysis here:
-----------
"I got out my H field probe and started sniffing around
inside the cabinet and found all of the problematic current
was coming from the shaft that runs back to the plate tuning
capacitor in the PA tank. I measured almost 1/2 amp of RF
current on that shaft on 40 meters in mine!! It was making
the VFO shaft hot with RF."
------------
    I made myself a crude but sensitive probe and also found a large amount 
of RF on the shaft.  I did as he did, with a long insulated screwdriver 
grounded the shaft - the frequency shifted slightly, and the chirp was gone 
(maybe).  I say maybe because I my case once I had slid the chassis about 
10" forward out of the cabinet, there was no chirp to be found.  Ain't these 
things fun?  Anyway, to lengthen the story, I really didn't want to horse 
the thing all the way out of the cabinet and remount the capacitor to ground 
it, as Tom had done.  If I had a strip of phosphor bronze I would have made 
a grounding strip for the shaft to rub on, and mount it using existing 
hardware that would't require getting underneath.  I almost started making a 
spring strip from a hacksaw blade, to mount a graphite motor brush on to rub 
on the shaft & ground it.  But the motor brush pair nad a nice 3" long 
stranded copper wire, probably a bronze alloy.  So I cut it off, twisted it 
arouond the 1/4" shaft 4 times, crimped teh ends in a spade terminal and put 
teh spade under a VFO shield screw, with some "preload" to keep the coils 
tight on the shaft.  A cupla drops of DeOxit ProGold, and it checked out 
fine business with the RF probe, and even after going back in the cabinet. 
It may not be a long term permanent fix, but I think it'll do the job for 
many months.  I could have remounted the capacitor in less time, but just 
don't feel strong today.  I've horsed it around enough in the last month.
    Thanks to all who helped.
73,
Al, W8UT
New Bern, NC
www.boatanchors.org
www.hammarlund.info

Hi agn,
    After all that, I forgot to say I've posted a pic of the fix:
http://www.boatanchors.org/valiant40mchirp.htm
click on the thumbnail to get a full sized one.
73,
al, w8ut
---------------------
12/05/07
Hi Tom,
    Thanks for clearing my thinking about this.  You can be sure that the 
next time I have this beast out of the cabinet I'll be properly grounding 
that capacitor, with SS washers.
    Thanks again for your help.  Your solution, and the other guys who 
recognised the problem, sure saved me some time and head scratching.
73,
Al, W8UT
New Bern, NC
www.boatanchors.org
www.hammarlund.info

----- Original Message ----- 
From: "Tom Rauch" <w8ji at contesting.com>
To: "Old Tube Radios" <boatanchors at theporch.com>
Sent: Wednesday, December 05, 2007 8:18 PM
Subject: Re: Valiant chirp - fixed (long) rev.


>> I remain preoccupied with the fact that the capacitor was insulated from
>> the chassis yet grounded to the chassis.  Were they trying to avoid the
>> very circulating currents that they created?
>
> Whatever the root cause, the designer intentionally isolated the frame 
> (rotor) and brought it down to the area of the loading cap with a small 
> buss wire. It wasn't uncommon to do things like that just from habit.
>
> In all the PA's I've ever designed or looked at, probably  a few dozen low 
> power HF tube applications in the lot, harmonic suppression and stability 
> were always improved with short direct to chassis grounding of the tuning 
> cap. This is because the excitation path in single-ended designs is 
> through the time-varying cathode to anode resistance of the tube.
>
> http://www.w8ji.com/Vacuum_tube_amps.htm
>
> There are also other paths creating even larger rotor to chassis currents. 
> The loading cap itself is grounded to the chassis, and the return path for 
> circulating currents is through that cap. So you have tank Q and 
> circulating currents flowing through that cap.
>
> When they put a small skinny long buss wire from the rotor (frame) of that 
> tuning cap to the chassis, the frame of that capacitor lifts appreciably 
> in RF voltage. That voltage is what drives or diverts tank and PA currents 
> down the tuning shaft. It isn't the tank coil via induction, it is the 
> direct wired connection to the poorly grounded capacitor frame (rotor) 
> that causes the problem. While it might have been habit from other 
> designs, like push pull amps or breadboard amps, in a metal chassis 
> single-ended amp, floating the tuning cap frame cannot ever be the best 
> way to arrange things.
>
> Perhaps when the rigs were new, the shaft bushings further up that shaft 
> kept it grounded. The VFO shaft bushings were also probably clean and 
> shiny. Maybe that hid the problem.
>
> Whatever the case, the problem absolutely is not instability in any 
> external stages. The floating also didn't make the PA better. The PA stage 
> got better, not worse, when I replaced the upper insulated washers with 
> stainless steel washers (stainless because they will not corrode). 
> Harmonic suppression improved also.
>
> It's a very simple basic problem. There are a few amperes of RF current 
> flowing through the plate tuning cap. That cap is only grounded through a 
> long skinny buss wire. This means the rotor, since the thing was first 
> fired up, always had a voltage difference between the cap frame and the 
> chassis. That voltage would increase with frequency because the impedance 
> to ground would increase with frequency. On 40 meters the front panel has 
> enough current flowing to lift the shaft of the VFO away from the ground 
> potential of the VFO box. That directly couples the PA output back into 
> the VFO grid, and it pulls the oscillator.
>
> Since there is no reason to have RF currents flowing out that shaft, I 
> grounded the frame (rotor) of the tuning cap with metal contact directly 
> to the chassis with zero lead length by removing fiber washers and 
> replacing them with stainless. This is the way any single ended tank 
> should be constructed, unless it is built on a wood chassis.
>
> 73 Tom 



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