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 Post subject: SB-200: Odd failure
PostPosted: Oct Wed 07, 2009 4:24 pm 
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Location: FORT WORTH
I was tuning up on 20M when a firecracker went off in my SB-200. It was the 33Ω grid resistor on one of the 572-Bs. I replaced it and its mate on the other tube which was out of spec.

I always wonder when a part fails, what could have caused it. I decided to take the tubes out and test them. Something caught my eye in one of the tubes. Two springs were bouncing around in the tube! They were the ones that hold the filament tight. The filament had slumped down onto the grid. It was an almost new Taylor.

Anyone have a similar experience?

drbill


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 Post subject:
PostPosted: Oct Wed 07, 2009 4:41 pm 
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Location: Somers, CT
Strange it did it while tuning. Almost sounds like the RF voltage at the plate got high enough to allow a flashover, destroying the tube. I've also heard that parasitics can cause the same problem.


Pete k1zjh


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 Post subject:
PostPosted: Oct Wed 07, 2009 5:02 pm 
Silent Key

Joined: Jan Thu 01, 1970 1:00 am
Posts: 34328
Location: Sandpoint, IDAHO 83864
That is a very common thing to happen when the amplifier breaks into a VHF parasitic oscillation.
Curt

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 Post subject:
PostPosted: Oct Wed 07, 2009 6:16 pm 
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Location: Monterey California USA
Didn't Rich Measures have a kit to correct this issue? The famous kit which caused such a squabble at ARRL?

This is a classic example of the result of a VHF parasitic, in my experience.

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 Post subject:
PostPosted: Oct Wed 07, 2009 6:36 pm 
Silent Key

Joined: Jan Thu 01, 1970 1:00 am
Posts: 34328
Location: Sandpoint, IDAHO 83864
Yes he did. I never did understand the controversy that went on over his article that was published in QST some years ago. But I ordered a kit of parts from him for an SB-220 that had been going into VHF parasitic oscillations on a regular basis for the previous owner. He got tired of replacing 3-500Z tubes in it and moved up to a Henry amplifier. After I got the parts installed, it never did that again.
Curt

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(Connoisseur of the cold 807) CW forever!


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 Post subject:
PostPosted: Oct Wed 07, 2009 9:08 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2653
Location: Powder Springs,Ga. USA
My SB-200 did the same thing. I installed a 50 ohm 25 watt wire wound 'glitch resistor' and back to back 1N4007s across the meter. The tubes checked no shorts in my tester but bad in the amp installed horizontally. New Chinese 572Bs direct from China and all is OK.

I had previously installed new filter caps and bleeder resistors along with CL40 inrush resistors in each 120V primary. I added my own 'soft key' circuit as well.

Image

http://home.comcast.net/~suptjud/Heathkitsb_200wdenhanced.jpg

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Last edited by Terry Judkins on Oct Wed 07, 2009 11:47 pm, edited 1 time in total.

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 Post subject:
PostPosted: Oct Wed 07, 2009 9:41 pm 
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Location: Somers, CT
Well, I don't don't intend to take sides in the Tom Rauch vs. Rick Measures dispute, but here is Tom's take on catastrophic 3-500 failures being caused by other than VHF parasitics. FWIW. :wink:

http://www.w8ji.com/SB221/sb-221.htm

Pete k1zjh


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 Post subject:
PostPosted: Oct Wed 07, 2009 10:58 pm 
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Joined: Oct Sat 20, 2007 3:36 am
Posts: 14393
Location: Southern NH, 03076
Its simply the junk tubes you used. I wouldnt buy anything from RF Parts. Send them back for a refund.

The ones on Ebay direct from China are the ones I regularly buy and havent had a failure yet.

Stay far away from Measures voodoo science.

Unwind one end of each parasitic suppressor coil and measure the resistors. Replace if out of spec.

Carl


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 Post subject:
PostPosted: Oct Thu 08, 2009 12:41 am 
Silent Key

Joined: Jan Thu 01, 1970 1:00 am
Posts: 34328
Location: Sandpoint, IDAHO 83864
Well, I am not about to reopen this old controversy, but the SB-220 that was given to me had suffered from VHF parasitics several times in the past. It would take out the tubes and at one time it took out the plate RF choke and the power supply diodes and the arc burnt away a quarter of one of the plates on the tuning capacitor. The previous owner insisted on using either Amperex or Eimac tubes in it.

When I got it, I replaced the diodes and installed Rich Measures kit and now it is as cold as a cucumber and I can not even force it into taking off on its own.

So I don't care who was right or wrong in the controversy. I did what I thought at the time was the proper thing to do, and it fixed the problem, and that is all I was asking for. So I let it be with that.
Curt

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(Connoisseur of the cold 807) CW forever!


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 Post subject:
PostPosted: Oct Thu 08, 2009 1:44 am 
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Joined: Oct Sat 20, 2007 3:36 am
Posts: 14393
Location: Southern NH, 03076
Ive had hundreds of both the SB-200 and 220 series come thru here for repair or 6M conversion. The first thing I do is rip out all of the voodo science crap on the very few that have it.

For HF all it requires are new suppressor resistors.

The resistor sets the suppressor Q, not a stupid piece of undersized nichrome wire which wastes over 100W of power on 10M.

The parasitics war went on from the mid 90's to 2006 when Measures got kicked off the forum which included many amp company, power tube and university engineers trying to rebut his fantasies.

Carl


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 Post subject:
PostPosted: Oct Thu 08, 2009 5:52 pm 
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Location: FORT WORTH
Wow. This has been very enlightening. I'm sure glad I brought it up. Many thanks.

drbill


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