[AMPS] Problem with SB-220 Bias Diodes

[measures]

>
>> 1. Replaced the capacitors with new capacitors and bleeder resistors. 2.
>> Replaced the existing rectifier board with a new board.  This board
>> includes the replacement of the zener with 8 - 1N4005's. 3. Installed the
>> Parasitic Suppressor Kit.
>
>I've never seen his kit. I can't believe all the people making money 
>selling things that aren't needed!
> 
>> Here is my problem. About a few months ago I was operating normally and
>> the H.V. power supply failed. There was no "flash" or "Bang" ... just
>> quit.  I have had the "flash" and "bang" when I first fired up the amp
>> when I bought it and operated it without any of the mods being installed 
>> That's one on the main reasons why I initially installed the Parasitic
>> suppressor kit.
>
>The flash and bang with virtual certainty was not caused by a 
>parasitic. You've been duped by the sales pitches to sell parts. The 
>most common cause of a flash and "big bang" is gas or barnacles 
>(whiskers) inside the tube.
>
>The tube getters itself during an arc, and then when you replace 
>the parts taken out from the arc and power the amp back up, in 
>many cases it doesn't bang again for a while. 
> 
>> I opened up the amp and found that one of the 8-1N4005 bias diodes had
>> failed.  It was quite obvious ... charred and black circuit board at the
>> diode location.  The H.V. fuse was still intact.  I called Harbach and he
>> couldn't think of anything obvious that would make the diode fail and he
>> concluded that possibly the diode may have been faulty.
>
>A number 30 wire 1/2 inch long is a silly thing for a fuse. The 
>response time of any metallic fuse is very long compared to failure 
>time of any semiconductor.
>
>The best thing would be to add a current limiting resistance, like a 
>ten ohm HV pulse rated resistor (RCD 175P series is good). You 
>can continue to use the fuse, but at least add the resistor. 
>  
>> I replaced the diodes and operated normally for a few days and the same
>> thing happened.  A bias diode had failed again.
>
>You have a bad tube(s). Maybe a seal is leaking. You had one in 
>the beginning, you have one now. You will continue to have one.
> 
>> It should be noted that I always operate the amp in CW/Tune (1kW)
>> position. If I operate the unit at SSB (2kW) the H.V. Fuse will open with
>> a "flash and a bang" ... yes ... even after the mods.
>
>Yep. That's because the voltage breakdown of a tube or tubes is compromised.
>
>Look at it this way. Suppose you did have a "parasitic". Why would 
>the parasitic make the tube arc? 

It doesn't.  The big-bang arc is from the B+ circuitry to gnd.  The arc 
leaves no arc marks inside a shorted tube.  

> The emission is limited by the 
>filament area and power of the tubes to maybe ten amperes or so 
>per tube maximum. The HV breakdown of a good tube should be 
>over 11 kV, since in normal operation peak anode voltage 
>approaches twice the DC supply voltage and when underloaded it 
>can be even higher.
>
>When you get a big bang like that, it is because the HV is faulted 
>to "ground" through a low resistance non-current limited path. If 
>that low resistance path is through a normal tube, current can't ever 
>reach more than 20 amperes.  

The tube is not in series with the B+ to gnd arc path.

>Diodes will take twenty amperes for 
>a reasonable period of time.
>
>The big bang occurs because you are dumping hundreds of 
>amperes of current from the supply through a bunch of 
>components. Unless you have a dielectric failure that creates a 
>path through those diodes, you have a gassy tube or tubes.
>
Why don't grid/filament shorted tubes show gas after they are removed for 
high-potting? 

>The SB-220 sets itself up for problems by not directly grounding 
>the grids. If the grids were directly grounded, any fault path through 
>the tube would be less likely to involve the filament because the 
>grid would be a "shield". 

A directly grounded 3-500Z grid is resonant c. 80MHz.  Above that freq., 
the grid does Not act as a shield.  SB-220s oscillate at approx. 110MHz.  
The 3-500Z is rated at 110MHz max. for Oscillator and Amplifier service.  

>You can still have excessive current out 
>the cathode if the tube is gassy, but at least you have some 
>reasonable chance of the current being limited.

€  Say what?
>
>You've made all the mods, now all you need to do is put in a good 
>set of tubes. The mods will make you "feel good" about the 
>amplifier, and the tubes will fix the problem.
>
€  not a sound bet.  My guess is that lower VHF Q parasitic suppressors 
are needed to reduce the gain of the tubes at 110MHz.  

>Be sure the new tubes high-pot to at least 11 kV, and be sure you 
>occasionally get the anodes red during operation to keep the tubes 
>gettered. 

€  Has anyone seen leakage current decrease after the anodes are cooked?

>And by all means, add some current limiting in the HV 
>rail.
>
€  Peak current limiting is good engineering practice. 
>     
>
>
> 
>> I haven't HI-POTTED the tubes yet but they look clear (no blackening).
>> 
>> I'm sorry I'm so long winded here but this is my first posting to this
>> group after monitoring it for a while.
>> 
>> Does anybody have any idea why the bias diodes would fail?
>> 
>> Thanks,
>> 
>> John (VE7JDB)


-  Rich..., 805.386.3734, www.vcnet.com/measures.  
end


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