[Boatanchors] Re: Valiant Question/Help

[Mahlon Haunschild]

Forgot to mention:  the PA bias pot should be a 5K 4 watt unit, while 
the mod bias pot should be a 5K, 2 watt unit, by the book.

Mine were seized up as well.  Strongly suspect that, in the typical 
Johnson vs. Collins (read: Chevrolet vs. Cadillac) design philosophy of 
the day that they were undersized for the service demands placed on 
them.  Replacement with higher-valued units would be good; wire-wound 
units would be ideal.  This assumes that you could fit the new controls 
into the available space, of course.

My Valiant manual shows the newer bias control components in the 
pictures, parts lists, and schematic.

regards,

Mahlon - K4OQ

Mahlon Haunschild wrote:

> Tom,
> 
> It's not a modification, it's a production change.  Somewhere around Oct 
> 1958 of Valiant production the tapped resistor arrangement for deriving 
> modulator and final bias was changed to the dual pots that you see in 
> your Valiant.  This eliminated interactions between final grid current 
> and static modulator plate current.  Description of this appeared in 
> "Viking Valiant Information Bulletin #1" which came out at that time.
> 
> There's a copy of the bulletin on BAMA.
> 
> regards,
> 
> Mahlon - K4OQ
> 
>> --__--__--
>>
>> Message: 4
>> Date: Sat, 17 Apr 2004 15:03:08 -0500
>> To: boatanchors@mailman.qth.net
>> From: Tom Norris <r390a@bellsouth.net>
>> Subject: [Boatanchors] Valiant Question/Help
>>
>> I am going over a Johnson Valiant that I came across
>> a while back and recapping and replacing what needs
>> to be replaced, sent the bezel to Dee Almquist for
>> refinishing, etc. But have a question about the bias
>> circuit.
>>
>> The schematic shows a tapped resistor for bias adjustment.
>> This TX has a pair of 5k pots, one each for mod and RF
>> bias. Is this a "normal" mod? Looks like a 5 watt pot for
>> audio and a 10 watt for RF. For some odd reason both pots
>> have seized and need to be replaced - just wondered if I
>> *really* need to look for an nice, fun to adjust tapped
>> resistor as stock, or just replace the pots, which are much
>> safer to adjust. Just looking for an opinion.
>>
>> Power seems low right now, all tubes that did not check
>> good were replaced, as were any out of spec resistors.
>> Other than the currently broken bias pot which might
>> be contributing to this, are there any other "gotchas"
>> I need to look for? Everything else *looks* normal
>> other than bias.
>>
>> Thanks for the help - just not familiar with Johnson gear,
>> can repair most Collins gear with my eyes closed.
>>
>> Tom
>> KA4RKT
> 
> 
>