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Author Topic: SB-221 Grid-Filament Short After Replacing Filament Transformer  (Read 6785 times)
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KN4SK
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« on: June 25, 2016, 10:41:57 AM »

I'm trying to keep my dad's linear running, ham radio is his one remaining passion in life. Thanks to the forum for the help in the past, unfortunately I'm back for more.

Jan 2015 I replaced the tubes with a matched pair from RF Parts. Everything ran well until late last year when my dad told me it stopped working. Looking into it I saw the filament transformer had clearly gone to the heavens, brown goo was all around it and the filament secondaries had no voltage output.

I got a new one wound from Hammond/Peter Dahl and installed it the other day along with a new C4 rectifier filter cap. After a hiccup or two it was wired in and the amp looked OK DC-wise on the filament network (5V) and the relay/cathode bias network (115V DC or so and T/R relay working).

Last night I drive it up to my dad's, we hook it up, we start to tune it and the T/R relay stops working. Some time later I diagnosed it down to a grid-filament short on one of the tubes.

At this point I'm speculating that maybe there was an intermittent grid-filament short that overstressed the filament transformer and that short is now permanent. I can't think of any other explanation.

I'd like to make sure there aren't some lurking gremlins in there before I replace the tubes. Is there anything else I should be thinking of?

FYI, I replaced the HV cap and resistor bank and installed the Harbach rectifier board in 2014.

Thanks to all.

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N8ETQ
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Mort


« Reply #1 on: June 25, 2016, 02:20:25 PM »




 Yo'

    Sounds like you did everything you can.
That Harbach Board is the "Bomb". The only
other thing you should verify is that the
"Plane" of the Anodes should be vertical.

     Not horizontal. Depending on the sockets
it may have been assembled incorrectly.
I Don't think the 572 likes being horizontal,
amps that run them vertical seem fine.

     Also make sure the Fil choke isn't
shorting to chassis somewhere. If the tube
is truly shorted you should be able to see
it with your ohm meter.

    you can test him with the one remaining
jug and measure bias and fil just to make sure.
I think in another post you mentioned that
u did recap the bias supply.

    Prolly just got a poorly assembled 572B,
Not uncommon. Aggrevated by horizontal operation.

  see pix.


  New Jugs
  Harbach board
  Bias caps

  Should be good to go, May want to strap
a couple diodes across the meters. Harbach
Soft Start is nice too..



GL

/Dan



* 572 002.jpg (581.27 KB, 3648x2736 - viewed 453 times.)
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Tom WA3KLR
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« Reply #2 on: June 25, 2016, 02:26:05 PM »

Perhaps the new filament transformer is beefier than the original and causes higher inrush current to the cold filaments than before.   The inrush current is stressful to the filaments; they will flex.  Look into having a step-start circuit added to the primaries of both the HV and filament transformers if not already incorporated.  
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73 de Tom WA3KLR  AMI # 77   Amplitude Modulation - a force Now and for the Future!
KN4SK
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« Reply #3 on: June 25, 2016, 03:29:23 PM »

I didn't mention that I grounded the grids but that's the only other mod Ieft out.

The sockets are vertical and the tubes are 3-500z's, not 572's but I don't think that matters in this context.

A soft start sounds nice but I figured since the original tubes from the 1960's lasted almost 50 years, a new pair ought to last 20 but maybe I'm too optimistic about current tube quality?

Thanks for the other ideas, I'll take a close look at those areas as well.
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W3GMS
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« Reply #4 on: June 25, 2016, 04:42:04 PM »

As you said, who knows the tube quality these days.  You could have just got a bad tube and such things do happen.  On my AL-82 which also uses a pair of 3-500Z's I did develop a grid to filament short early on, but I believe that occurred due to the tubes being over stressed.  Ameritron did not have the power output meter calibrated and I was running a lot more power than the tubes were designed for!  I calibrated the front panel watt meter against my Bird 43 and life with that amplifier has been extremely reliable for the last 15 or 20 years that I have owned it.  I actually had two 3-500Z's develop a grid to filament short due to the electrical over stress.  I was not happy with the original slow start circuit for inrush limiting so I re-designed that circuit early on.  The new circuit takes care of the HV and filament supply as well.  Not saying that inrush is your problem, but like Tom said, due to the beefier filament transformer the filament could be flexing more and contributing to the short. My circuit uses a relay to short out a resistor one the inrush time period has passed.  You could also use a thermistor for a quick solution.  The amplifier should work at reduced power with one tube. 

Good luck and great job on keeping your Dad on the air!

Joe-W3GMS           
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N8ETQ
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Mort


« Reply #5 on: June 25, 2016, 05:38:15 PM »




    Sorry Man,


    wrong amp on my part, still gud info
on the 572B and any other Chinese tube.
Been distracted of late, Too much stuff
going on.

    Your running that off 220V right?
Currently thats really about 240V +.

    Lost my dad in "77...

GL

/Dan



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KN4SK
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« Reply #6 on: June 25, 2016, 06:00:10 PM »

The soft start circuit is pretty simple, especially given the kit available from Harbach. Maybe I'll toss that in for good measure.

We are running the amp off of 220V.
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KD6VXI
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« Reply #7 on: June 25, 2016, 07:15:19 PM »

 The transformer for filament on the heathkit sb220/1 is barely adequate to start with.   This is a good thing.   It doesn't require soft start.  The fil xformer is current limited,  and the mag field won't support huge inrush.   Rich Measures has 'measured'  it,  and it's a non issue.

Also,  always starting in the CW position will help the plate supply. 

Remove that - 110 volts bias on the third leg of the relay.   Wire the relay to short out a c.  10k ohm 10 Watt or greater resistor in series with the cathode bias.   Now you don't lose a filament xformer in a flash bang or other event.

You can also wire a fuse in series with the cathode bias.   Exceed cathode current,  fuse pops,  no more drive until fixed.   Maybe not a great idea for a sight limited ham.

You stated you already grounded the grids directly.

Lastly,  before the anode.choke, wire a series 10 ohm 20 Watt wire wound.   Bypass with a. 01.  This will help on the low bands,,  and if you have an "event",  the R will keep current down until a fuse / breaker can blow.   The extra L of a wire wound helps the barely adequate plate choke deal with rf on 80.

--Shane
KD6VXI
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W3GMS
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« Reply #8 on: June 25, 2016, 09:09:00 PM »

The transformer for filament on the heathkit sb220/1 is barely adequate to start with.   This is a good thing.   It doesn't require soft start.  The fil xformer is current limited,  and the mag field won't support huge inrush.   Rich Measures has 'measured'  it,  and it's a non issue.
--Shane
KD6VXI

He is not using the original filament transformer, so the inrush current on the sub may be much higher based on its construction. 
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KD6VXI
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« Reply #9 on: June 25, 2016, 09:48:49 PM »

Very good point Joe,  I didn't take that into account!

--Shane
KD6VXI
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W9GT
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« Reply #10 on: June 27, 2016, 04:10:24 PM »

I recently had very similar problems with two amplifiers.  I had a couple of what I thought were very good Eimac 3-500Zs that I obtained from a web swap site and plugged them in only to experience very extensive and expensive failures.  My rebuilt AL-80A experienced a flash burn-out of a 1.5 ohm w.w. resistor, total zapping of the grid current meter, and shorting of the bias zener diode all resulting from what I believe was a grid to cathode short.  I also plugged in an equally  nice-looking 3-500Z into my home brew pr of 3-500Z amplifier and a similar fireworks display wiped out the fil/bias transformer (SB-220 part), the bias zener diode, and numerous other components.  (If you have tried recently to find those 10W zeners, you know they are hard to find and expensive).

This disaster all happened right after I recently had a total replacement of my right hip, adding to my frustration while not wanting to do a lot of repair and fussing to keep my station on the air and just to enjoy it while recovering.  Needless to say, it seemed as though Murphy prevailed in every way possible.

I am happy to report that I got a couple of Chinese tubes from RF Parts at Dayton, as well as a replacement fil/bias transformer and got the home brew amp back in business.  I also obtained a replacement meter and other parts from Ameritron and repaired the AL-80A.  Lesson learned...BE CAREFUL USING "PRESUMED GOOD" USED TUBES.  It can get very expensive and time-consuming to repair the resulting damage.  I would also suggest using a power transistor as the regulating element controlled by a small and inexpensive 1 w. zener for the cathode bias circuit.  A little more rugged and less expensive.

I am also happy to report that I am recovering nicely from the hip surgery and doing well.  Now, I hope that the radio equipment hangs together for awhile :-).

73,  Jack, W9GT
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Tubes and Black Wrinkle Rule!!
73, Jack, W9GT
Jim, W5JO
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« Reply #11 on: June 27, 2016, 08:54:11 PM »

If you buy tubes from sources that are of unknown quality it might benefit you to have the tested before sparks fly.

New tubes should not be a problem but who knows with some of the manufacturers out there.

These people might save a transformer for you.  I used them recently and they had quick turn around.


http://www.ebay.com/itm/Power-Tube-Testing-Service-for-Medium-Power-Eimac-Tube-4-1000A-3CX3000A7-YC156A-/251805119299?rmvSB=true
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Patrick J. / KD5OEI
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« Reply #12 on: June 27, 2016, 09:37:14 PM »

I don't always use a relay and resistor, especially for small loads. That's a small load there 150W.
There are these little things - inrush limiters, a kind of NTC.
http://www.ametherm.com/
I have been putting them in whenever something is opened up for repair -receivers, audio gear, etc. It does away with the cool UUHNNNT power supply noise when firing up but it's worth the loss of coolness. They are 100% made in Carson City, NV. USA.
There are hundreds for all applications and full load amps from <1 to >35. The trick is to pick one that has the proper cold and hot resistance so the end voltage is correct and not really affected by the device. It's important to select the right one. The calculator here is what I use. - but it's not working now? nice. oh well
http://www.ametherm.com/inrush-current/

I also been getting my 3-500ZG from ESRC. They give a filament time warranty, none of that pompous "we tested it before we shipped it so tough luck" malarchy.
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w1vtp
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« Reply #13 on: June 29, 2016, 12:23:34 PM »

Do consider a soft start that includes the filament xfmr. 
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