[Johnson] Johnson Viking II repair

[Rodger Singley]

Bill,

Thanks for posting the information about the drive pot replacement and I believe the part you found is at Digikey, at least I couldn’t find it at Mouser.  That looks like a winner!  Several years ago I ordered some pots from Digikey when they weren’t stocking these 5 watt pots from CTS and what they carried then were 4 watt pots with some sort of resistance material plated on ceramic that had a ridiculous temperature rating that caused the wattage rating to drop quickly with increased ambient temperature.   At 110F the pots were de-rated to 1.5 watts but what you found are true wire wound types that will work very well.  I am going to order a few soon just in case they disappear from production.

I agree with your advice about not mixing tube types within the final although you can probably get away with it most of the time.  I bought a perfectly operating Valiant 2 that came with one 6146 and two 6146B in its parallel three tube final and it worked OK on all bands.

There is no need to perfectly match tubes in these parallel tube finals; the biggest concern is if one tube has greater gain it is going to tend to hog the current possibly causing it to exceed its ratings.  In push/pull service (like the modulator) mismatch can impact distortion however some people purposely put a “hotter” tube in the positive going side in an attempt to increase audio without driving the final to cutoff on the negative going peaks.  However even if you match a set of tubes now they aren’t going to age equally and tubes that are matched for equal idle current probably won’t be matched at peak current.  The best designed “sweep tube” linear I have seen is a Kenwood TL-911 I picked up at a hamfest a few years ago.  It uses five 6LQ6 tubes in parallel with a meter that can be switched to read total cathode current along with individual cathode current for each tube.  The screen voltage is adjustable individually for each tube to equalize idle current and a capacitor adjustment in each grid circuit allows setting each tube for equal peak current.  An electronic current sense circuit puts the amplifier in standby instantly if safe peak total cathode current is exceeded and also goes into standby if average current/duty cycle is too high even if the peaks are safe.  It shows what a company can do when amplifiers using sweep tubes were designed to be good instead of just cheap  😊

And the Viking 1 and 2 are probably the most rugged pieces of gear made by Johnson.  I have most of their stuff from the small Adventurer and Navigator through a Viking 500, Desk KW, Invader 2000, Thunderbolt, and 6N2 Thunderbolt but the component and construction quality of this early Johnson design is very nice.  They certainly weren’t designed with cheap as the priority.

Rodger WQ9E

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From: Bill Cromwell<mailto:wrcromwell at gmail.com>
Sent: Friday, April 21, 2017 11:47 AM
To: johnson at mailman.qth.net<mailto:johnson at mailman.qth.net>
Subject: Re: [Johnson] Johnson Viking II repair

Hi Rodger...
.........Mitch,

I found the "Drive" control pots at - I believe - Mouser and at
reasonable prices. In fact I looked at a 5 watt pot where the original
is rated at 4 watts. If a 4 watt device lasted 50 years then a 5 watt
pot shout last at least 50 more years! Maybe?

On those 6146 variants - it is probably good to have a pair of the same
flavor..maybe not a plain 6146 working with a 6146B. When I had a
problem I found a 'mismatch' and corrected it. I didn't do any of the
"matched pair" voodoo - I just changed it so the B model had another B
model for a mate. That matched pair stuff "might" apply to a push-pull
configuration and maybe not. Our finals are in parallel.

Good luck with the project Mitch.

73,

Bill  KU8H