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Author Topic: SB-220 madness  (Read 2066 times)
W1QJ
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Posts: 2947




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« Reply #15 on: June 19, 2019, 10:58:21 AM »

A shorted zener didoe would not cause this problem.  The only problem a shorted zener diode causes is high idle current and nothing else.  Those zeners usually short and rarely ever open up.  What makes you think the problem is not is not another parasitic choke issue?  What did you do about that before?  The relays rarely go bad and unless you can confirm it is bad I would not replace it as you are now shotgunning.  That is not the way to go about this issue.  Have you checked the .82 ohm grid shunt resistor which is also a glitch fuse?
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N6AF
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« Reply #16 on: June 19, 2019, 01:17:31 PM »

Nandu - Here's the latest update.  Once I cleaned up the relay contacts things settled down so you were spot on regarding the intermittent contacts depriving the amp of its bias, thereby allowing the plate current to go sky high.  In the process of fussing with the contacts I managed to ruin one 3-500Z tube.  It simply won't pull any current.  No internal shorts and the filament lights but it won't pull plate current.

So I substituted spare 3-500Z I had and the amp is running very well.  I am seeing 1250 Watts output average (CW) power on the Bird Watt meter on 28.200 MHz using 100 Watts drive from my K3.  This is into a dummy load with an excellent match so the results aren't high due to output mismatch.  It is the real deal!  This is with the loaded plate voltage at 2600 V and Ip = 800 mA.  I won't run it like that but it's nice to know it's developing healthy power even on the highest band.

So now I will order a new relay.  I just don't trust that the cleaned contacts will stay conductive and I'm tired of dealing with this.  Thanks for your advice regarding the relay.

Very 73  N6AF Chuck
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W3RSW
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Posts: 606




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« Reply #17 on: June 19, 2019, 01:54:10 PM »

Your relay replacement brings this to mind:
You’ve had those tubes in and out several times now in an old amplifier with old tube sockets, so probably ought to inspect and re-spring the tube socket contacts too.  Your supposedly defunct 3-500 may have just been in intermittent socket contact. Won’t hurt to re-spring or replace the contacts anyway.

In my old engineering experience with a down hole well logging outfit, Nevada (NTS, etc.) most of our problems were with intermittent or failed contacts. - Tube and relay sockets, all the Cannon connectors between rack components, scopes, E-Log panels, gamma and high intensity gamma tools. All the downhole calipers, neutron density tools, you name it.

The Cannon connectors even though based on military design were mated constantly and had surprising failure rates in the field. Panels were changed out in the trucks just about every job.
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Rick, W3RSW
W1QJ
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« Reply #18 on: June 20, 2019, 04:56:58 AM »

Chuck, you had an unusual event happen to you.  Carefully analyzing the data as you provided it I am thinking that this tube that you say bit the dust, was flaky and probably had an intermittent short of some kind.  Both tubes are subject to the same issue is something goes wrong with the amp and in your case only one tube had an issue.  I would be very curious to see the parasitic suppressors that you removed from the amp and examine both and try to determine if the suppressor that went bad was associated with the tube that went bad.  I have had SB-220 relays go  funky on me many times over the last almost 50 years of working on them  and I never had a problem like you have presented here.  I recall on several occasions that I did not have the 120vdc present and also that when I keyed the amp the bias arm of the relay did not come down and make contact with the B- pole of the relay.  Neither situation caused a run away out of control situation that you have seen with your relay situation.  On the other hand I have indeed seen run away current drawn with amps that had bad parasitic suppressors and shorted tubes or highly gassy tubes that would occasionally go plasma and draw lots of current. 

I am trying to rationalize how your relay went into transmit seemingly not being keyed  up.  I am thinking that you had a strange combination of events, firstly with a bad parasitic suppressor which you then changed and all seemed well and then suddenly another problem after a short period of all being well.   The second round seems like a shorted or gassy tube both being intermittent which is entirely possible.  I especially think this could very well be as now the tube is totally dead.  The short was probably cleared and at some point before it was cleared destroyed the grid or something. 

A new relay can't hurt and you may want to consider when you install the new relay to switch over to rewiring the relay for self bias.  This will save your filament transformer in the event of a G-F short.  Very simple to do especially when you are going to install a new one anyway.  Good Luck
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VU2NAN
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« Reply #19 on: June 21, 2019, 08:26:36 PM »

Hi OM Chuck,

Thanks for the updates. I'm sorry for the late reply.

My focus is on ruling out the probable causes of bad connections/contacts in the grid circuit.

Have you checked the contact points on the valve sockets as OM Rick suggested?

What about corrosion at the grid earthing points on the chassis? Bad solder connection on tube grid pins?

73,

Nandu.
« Last Edit: June 21, 2019, 08:28:59 PM by VU2NAN » Logged

WB4SPT
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Posts: 771




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« Reply #20 on: June 24, 2019, 01:10:48 PM »

Not sure what is happening, but UR making me glad I ditched the 120V bias, and just open the cathode to stop idle current.  and soldered the grids to chassis.  takes out a few failure points.
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AF6LJ
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Posts: 580




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« Reply #21 on: June 25, 2019, 06:24:40 AM »

A general comment regarding this problem.
I wouldn't rule out a bad Doorknob cap bypassing the plate choke to ground.
Before I put in another tube and put the amp back on the air I would check that Doorknob to make sure it is not open.
Those caps are getting awfully damned old.

1400W???
A little much for both the power supply and a few other things in the SB-220....
I run Oscar at just over a KW peak and all is good.
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Take Care
Sue,
AF6LJ

Don't Kalifornicate My Life
W1BR
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Posts: 4179




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« Reply #22 on: June 25, 2019, 06:50:43 AM »

The original Heath design for bias was poor.  There is a simple mod for the relay wiring that places a 100K resistor in the bias path to the tube.  I agree with Lou--loss of bias should only result in high idle current, not overly excessive destructive current.

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KB8VUL
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Posts: 278




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« Reply #23 on: July 04, 2019, 07:24:03 AM »

Well, a couple things here.
First is that cooking the parasitic resistors takes RF to do it as the wire around them is a DC short.  So plate voltage would have zero effect on the resistor.
That being said.
The amp was obviously oscillating hence the high plate current when it was keyed.
Bringing us to the relay.  Whats' the chance that the relay was somehow connecting the output directly to the input of the amp and creating a huge feedback loop allowing the amp to oscillate at whatever resonant frequency it could find?  Probably at the resonance of the tank circuit, but who knows.
You may or may not have been connected to the output keeping the watt meter from indicating RF being present, but then again, as things were getting wild, did you even notice?

I would dig through that amp pretty close and verify that the output and the input of the amp are fully isolated and that there is no direct path being created by the relays.
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W1BR
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« Reply #24 on: July 07, 2019, 10:05:58 AM »

I'd expect a properly designed GG amplifier using 3-500 tubes with grounded grids to be unconditionally stable.

Pete
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KM1H
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Posts: 5096




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« Reply #25 on: July 09, 2019, 04:28:15 PM »

I'd expect a properly designed GG amplifier using 3-500 tubes with grounded grids to be unconditionally stable.

Pete

There is noting on the market that fits that requirement, they are marginally stable and sometimes go bang.
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