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 Post subject: SB-401 Tune Up Procedure
PostPosted: Jun Wed 19, 2019 4:27 pm 
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Joined: Jun Fri 23, 2017 5:36 pm
Posts: 45
I am posting this as a possible help to anyone working on a SB-401. I have finally got mine working really well (thanks for the help to the forum). I have come to realize that some of the problems I had were because I was not watching the grid current closely enough during tune up. My other tube final radio is a Kenwood 530 which does not even offer a grid current meter indication so I am not used to having to monitor grid current. Based on my experience I suggest leaving the meter on "grid" until you have a fairly good tune of the driver and plate. Keep the current below .5ma! This radio can develop huge amounts of grid current and its easy to over drive the finals while tuning. Start with a low setting of the "CW Level" control (I suggest no more than 9 o'clock position). If you can't get any output you can then advance it somewhat. Once you have good settings (peaked output and grid below .5ma) you can switch the meter to plate and dip it. This requires an external power meter so you can see output while you get your initial settings while watching the grid current. If you hear any snapping sounds this is a very bad thing! I believe the instructions suggest using the meter to monitor relative power. I think this is a very bad idea.

BTW I have seen posts asking if the SB-300/401 are worth restoring. My view is yes, for sure! These are fine functioning radios though they do lack some "modern" features but that's what makes them fun.

Just a suggestion, and I hope it helps someone. This is a really great forum!

Bill N5VR


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 Post subject: Re: SB-401 Tune Up Procedure
PostPosted: Jun Thu 20, 2019 10:46 pm 
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Joined: Jun Sun 19, 2011 2:31 pm
Posts: 6513
Bill,

I fully agree about these early SB series rigs being worth restoring and they still make very usable radios for both CW and SSB.

One note is that you probably aren't running as much grid current as you think because the grid current reading on the SB-401 is 1 mil full scale, the scale that reads to 10 is used but you have to mentally multiply by .1 to get the actual grid current reading. The SB-1xx transceivers are the same where a full scale reading of 10 in the grid position is actually 1 mil. In CW mode you want a reading of around half scale (.5 mil) on the grid current meter which is just starting to push the final from class AB1 towards AB2.

The grid current meter should at most just barely flicker on SSB and much greater drive to the final will result in the ALC taking control.

Rodger WQ9E


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 Post subject: Re: SB-401 Tune Up Procedure
PostPosted: Jun Thu 20, 2019 11:37 pm 
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Joined: Jun Fri 23, 2017 5:36 pm
Posts: 45
Hi Roger,

Yes, I was aware of the way the meter reads. It is ironic that I make this post about being careful to not run too much grid current because early on I could never get any grid current and I posted on this topic here. As I worked my way through the restoration the grid current finally showed up. I noted from time to time a snapping sound would seem to come from the final cage but I could never catch it in the act and its occurrence seemed somewhat random. The last time it happened it blew the mains fuse (4A). Eventually I figured out (at least I think) that this was because I was way over driving the finals. My theory (no proof) is that the arc is actually happening inside one of the 6146s. That 6CL6 will develop way more drive than the 6146s want and this happened when I was tuning up but not watching the grid current.

One other lesson is that even my 10:1 tek probe and HP scope loads the final grid circuit. I got the final alignment with nothing attached and using the "grid current" method described in the Heath alignment procedure.


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 Post subject: Re: SB-401 Tune Up Procedure
PostPosted: Jun Fri 21, 2019 12:18 am 
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I wonder if that snap you got is from the final trying to take off from excessive drive? The 6146 tubes are designed to run at around 3 mils of grid current each as a safe maximum in class C service and will withstand more briefly without damage so the small availabe extra current available from the SB-401 shouldn't hurt a good 6146. You might have a final tube with an internal issue. If the final tubes were used and came from another rig then they may have been damaged before you bought your SB-401. My first Johnson Valiant manual came with the letter from RCA stating that 6146 tubes subjected to excessive grid drive weren't eligible for warranty replacement and apparently that was a common problem in the early days when many hams tried to drive their 6146 tubes to the levels of grid current they were used to with the older 807 tubes.

Although the plate capacitor has plenty of spacing in the SB-401, increased RF output will create higher peak voltage in the tank circuit and you may have a bit of debris or something on the capacitor or built up damage from previous debris/arc events so take a close look at the plate tune capacitor plates.

I have a couple of different sets of the SB series twins and they are some of the better products from Heathkit. The original SB-300/400 was a little awkward with its control/separate setup but that was fixed with the later versions.

Rodger WQ9E


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