John,
I am not a Valiant owner, but you sure seem to have a annoying and
expensive problem.
I keep thinking that your DC operating point is wrong somehow. Since the
finals are class C, there should be enough bias to completely cut off the
finals in the absence of drive. Maybe try this on CW mode and see what
the
key up cathode current is. I think it should be zero milliamps.
Today's line voltage is typically higher than back in the 50-60's, so
spec bias may not be enough today. Got a Variac? What is your filament
voltage? Seven volts is too high. Also the screen voltage might be a
little
high too. I'd concentrate looking at this in CW mode, and Linear SSB
mode. I
seem to recall that a Valiant has some kind of switch to allow a SSB
adapter
to drive it and the 6146's run in linear mode. If you have no cathode
current in CW key up, and linear mode is idling fine without a rising
cathode current over time, then the tubes, and DC parameters are not the
problem. I suspect however that you will have trouble with these
tests, and
the problem is seen static, in the absence of RF drive. I might be
wrong, so
read on.
If the problem is seen with drive, and on the lower frequencies as
you
mention, then think that the RF plate load impedance is wrong, and tube
efficiency is so poor that they get hotter and hotter by the second,
and go
into a thermal runaway. I see this problem when trying to make a poor
6DQ5
work on 6 meters in my Gonset G-76. No can do, 6 meters on a G-76 is a
joke.
I am not sure what a Valiant can do, but my guess is that 200 watts DC
input
on AM should give you about 150 watts RF output (150/200 * 100 = 75%). A
6146 normally should not show any color in the plate. Are yours
blushing? If
the RF plate impedance is wrong, and efficiency is poor, then it's
time to
scrutinize that pi-network for problems. Some of the loading
capacitors may
have gone south, and are only used on the lower frequencies. If you
suspect
a VHF parasitic oscillation, look for sudden jumps in grid current , or
cathode current while adjusting the controls for plate and grid
tuning. Also
a neon bulb near the plate of a 6146 (can mount to a pop sickle stick)
should glow orange. The color turns more purple at VHF.
Keep in mind that this is general advice, and that others with more
first
hand Valiant experience may nail this with one stab instead of me with my
buck shot approach. Hope this helps.
Why would this be:
"It is my understanding that sometimes subbing in the "'B" for
the early 6146 just doesn't cut the mustard."
Could it be that we need to adjust the DC parameters for the 'B' like
lowering the screen voltage a little, or a little more fixed grid
bias? Or
is this tube unsuitable for use in a Valiant for some other reason?
The 'B'
was used a lot in AB1 liner use with plate voltages up to 1000 volts.
Maybe
the 'B' is not tame in a Valiant, and wants to go into a destructive VHF
parasitic oscillation often enough to make us curse them?
Regards,
Jim Candela
WD5JKO
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] Behalf Of GBrown
Sent: Saturday, August 27, 2005 1:01 PM
To: Discussion of AM Radio
Subject: Re: [AMRadio] A bit of Viking Valiant help
Just a wild a-- guess, but I thought that the Valiant used only
6146's, not
the "B" 6146. It is my understanding that sometimes subbing in the
"'B" for
the early 6146 just doesn't cut the mustard.
Regards,
Gary...WZ1M
----- Original Message -----
From: "John Lawson" <[EMAIL PROTECTED]>
To: <[email protected]>
Sent: Saturday, August 27, 2005 1:56 PM
Subject: [AMRadio] A bit of Viking Valiant help
I'm in the middle of a long, slow project to get my Valiant back in
service. I've changed out the bigger power tubes, solid-state
replacements
for the 866s, and partially re-capped the rig, specifically the RF power
section.
After finding (expensively) out that 'W' 6146s just don't work, I
replaced the finals with new 'B's and power is now being transferred to
the antenna / dummy load.
Main bug:
On the bands below 20M, however, and getting 'worse' as the frequency
gets lower, the rig "runs away" after 15 - 30 seconds - the plate
current
rises fatser and faster and then needs to be shut down, lest another set
of 6146s gets wasted. ;}
I have neutralized the transmitter per the manual, seems ok. Bias also
set per sepc. LV rectifier tubes have been chacked (on a Weston 686) and
seem within spec, though their performance under load is measurable only
in situ.
I left the doorknob cap alone because it's fine electrically and not
cracked or discolored.
Do the sypmtoms sound familiar to anyone? Before I spend a bunch more
hours wearing out the transmit switch, I thought I'd ask those who
have a
lot more experience than I do with the Valiant.
I can add more info/observations as needed, specific caps replaced,
etc.
Thanks all!
Cheers
John KB6SCO
______________________________________________________________
AMRadio mailing list
Home: http://mailman.qth.net/mailman/listinfo/amradio
Help: http://mailman.qth.net/mmfaq.html
Post: mailto:[email protected]
______________________________________________________________
AMRadio mailing list
Home: http://mailman.qth.net/mailman/listinfo/amradio
Help: http://mailman.qth.net/mmfaq.html
Post: mailto:[email protected]
--
No virus found in this incoming message.
Checked by AVG Anti-Virus.
Version: 7.0.344 / Virus Database: 267.10.16/83 - Release Date: 8/26/2005
--
No virus found in this outgoing message.
Checked by AVG Anti-Virus.
Version: 7.0.344 / Virus Database: 267.10.16/83 - Release Date: 8/26/2005
______________________________________________________________
AMRadio mailing list
Home: http://mailman.qth.net/mailman/listinfo/amradio
Help: http://mailman.qth.net/mmfaq.html
Post: mailto:[email protected]
