[AMRadio] A bit of Viking Valiant help


Larry Szendrei ne1s at neandertech.com
Sun Aug 28 08:39:31 EDT 2005


More good advice from Gary and Jim - Check the final grid bias and clamp 
tube adjustments per the manual, the clamp tube screen dropping 
resistors (if the circuit uses them), and try a different 6AQ5 clamp 
tube itself.

73/GL,
-Larry/NE1S


Gary Schafer wrote:

> 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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> 
> 
> 
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