[AMRadio] A bit of Viking Valiant help


Gary Schafer garyschafer at comcast.net
Sat Aug 27 16:48:47 EDT 2005


I agree with Jim. I would check to see that the clamp tube adjustment is 
set correct first. Sounds like you don't have enough drive and the clamp 
tube is not cutting off the screens.

73
Gary  K4FMX


Jim candela wrote:
> 
> 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: amradio-bounces at mailman.qth.net
> [mailto:amradio-bounces at mailman.qth.net]On 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" <jpl15 at panix.com>
> To: <amradio at mailman.qth.net>
> 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
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>>
> 
> 
> 
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