[AMPS] More SB-220 amusement(!)

[Rich Measures]

>At 09:58 AM 9/18/97 -0800, you wrote:

NOTE -- Although the e-mail input from Mr. Smith was not posted to the 
[AMPS] group, I'm guessing that other amplifier owners might like to be 
included in the ongoing analysis and repair process.  Thus, I am posting 
this reply to the group.

>>>On Thu, 18 Sep 1997 10:39:52 Pete Smith <n4zr@contesting.com> writes:
>>>>
....... .....
>> Check the tube with a high-Pot.  If you don't see over 6 kV breakdown 
>>between the grid and the cold filament, the tube may have been damaged by 
>>a VHF parasitic.  
>>
>Easy to say, but how many people have a hi-pot tester.  

I don't have a clue how many do.  However, high-pots  are easy to build.  
An article on building one is available on my my Web site.  Repairing or 
building an amplifier without a high-pot. is difficult at best.  C and H 
surplus on Colorado Blvd. in Pasadena, CA reportedly has some 4500Vrms 
low current 50/60 Hz transformers in stock.  

>Not me, for one.
>In the history of this amp with me, there has not been a single, sudden
>transitory event that plausibly resulted from a parasitic.  

What seems plausible is worthless compared to what one measures.  
If the 0.82 ohm grid current meter shunt resistor opened, there are three 
possibilities:
.  1.  Anode short:  -  During the 90s, Eimac produced some 3-500Zs with 
weak spotwelds on the anode cooler assembly.  If some of the weak 
spotwelds let go, the anode cooler tilts and touches the grounded grid.  
Since the only dc path between ground and the negative side of the anode 
supply is through the 0.82 ohm resistor, it can burn out.  
.  2.  Gas :. -   With anode V applied, the inside of the tube will glow 
with a blue color.  It may be a good idea to connect an electric heater 
in series with the electric mains for this test since, with a large 
amount of gas present, something is needed to limit the mains-I.  If you 
see whitish deposits inside the envelope, a large amount of oxygen gas 
was present when the filament was operated.  
.  3.  VHF Parasites. -   Measure the R of the VHF parasitic suppressor 
resistors.  If the R is substantively higher than the marked value, a 
parasite is indicated.  Using a high-pot -- provided the vacuum checks 
good -- if the grid/filament breakdown potential is lower than normal, a 
bent filament is indicated..  An alternate test is to use a C meter to 
measure the grid to filament C.  If the C is greater than 10 pF, a bent 
filament is indicated.   
[NOTE - Bent filaments can be caused by other than a VHF oscillation.  If 
the 3-500Z has been operated horizontally for more than c. 4k hours of 
filament run time, the filament might be able touch the grid.] 
... ... ..
>>I would change the 220 from V-cutoff bias to R-cutoff bias.  In the stock 
>>config. a shorted tube will destroy the unfused, filament transformer. 
>
>Sorry, I don't understand what this implies, ...

On standby, +110V is applied to the fil. CT to cut off the tubes.  If a 
grid/fil. short occurs, it places a short on the +110V supply, which 
crispy-critters/smokes the unfused transformer that supplies the ac 
voltage to the supply.

>...or how you would go about it.

In it's stock configuration, +110V is wired to the relay coil and to one 
of bias section contacts on the relay.  Remove the wire that brings  
+110V to the bias contact.   (the bias contacts are the middle section of 
dpdt contacts).  Check to see if one side of the coil is still wired to 
the 
+110V supply.  

>The popping was very muted, and clearly from the metering board.  There was
>no color or other transient stuff happening with the tubes.  The anodes
>still look entirely normal.
>
Which, if any, resistor(s) on the metering circuit board have changed R?  
The 1.0 and 0.82 ohm units are of special interest.  

>... and things operated just fine thereafter through an entire CW
>sprint.  Doesn't sound very plausible that it was killed by a parasitic and
>then recovered for that duration.
>
Intermittent parasites don't necessarily prevent an amplifier from 
subsequently functioning normally.  
>>
>>>...and maybe the meter also unless there were protection diodes
>>>across it.
>
>There are.
>
good

>>>Other things probably blown are one or more of the bias diodes on the
>>>Harbach board.
>
>What's the cause?  

All of the grid current and all of the anode current passes through the 
bias diodes. Bias diodes can short during an instanteous current surge in 
grid or anode current. 

>If I replace the metering resistors and the resting
>current looksw OK, does that rule out the diodes?

Yes.  If the bias diodes short, the ZSAC/'resting current' would increase 
to several hundered mA because there is zero volts of operate bias.  //  
I do not normally replace a resistor unless the measured resistance is 
out of tolerance.  

>I've read and re-read those articles -- and much of the debate here and
>formerly on rec.radio.  It just doesn't sound like what you describe, Rich.

Full blown, big-bang ones get most of the attention, Pete, but there are 
the also-rans.  

> I'm agnostic on VHF parasitics, but they are clearly not the only thing
>that goes wrong in HF amps.  

Clearly.  However, the naysayers have tried to cloud the matter with 
furious, spurious arguments -- for a reason that isn't terribly difficult 
to decipher.  One of the naysayers even accused me of having said that  
'parasitics bend grids'.  ROFL, L.  The whole biz reminds me of the 
goings on during Richard Millhouse Nixon's final months in the 
Whitehouse.  
- Amplifiers can fail from a number of causes.  Parasitics are just one.  
To the best of my knowledge, Kenwood (Trio) is the only manufacturer who 
will currently say anything about parasitics.  In the old days, Kenwood 
talked about "normal arcing" on page 14 of the 922 owner's manual.  Now 
Kenwood says that the TL-922 does not have any more parasitic-arcing than 
other brands.  However, the 922 is out of production --  and all 922s in 
the field are safely out of warranty.  .  

>This just doesn't sound like it.  There were
>no tuning anomalies (except the gradual one I described), ... ...

The gradual element is a bit of a puzzler.

>...no sudden
>phenomena, and no arcing. 
>
No one can know the likely cause until you conduct the measurements.  
>>>
>>>Probably time to think about replacing the tubes!
>>>
>>Not unless the grid/filament breakdown voltage is abnormal.  
>
>Any way to test this without a hi-pot tester?
>
yes.  With a C-meter.  See above.  
>>>
>>>>Any advice greatly appreciated.
>>>
>>>Here is some free advice. When something STARTS to happen, shut down
>>>immediately to investigate. The SB-220 is a very forgiving amp and often
>>>gives warning signs....just heed them.
>
>But how do you characterize the situation if you don't test and observe?
>
- If you want to put the problem to rest, you gotta test. 
Rich---

R. L. Measures, 805-386-3734, AG6K   


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