[Amps] 3-500z - intermittent filament

[Martin Sole]

Hi Terry,

I think you have experienced one or more not uncommon problems with a 
number of 3-500 amplifiers. The re-soldering of tubes base pins is a 
common sight and though highly inappropriate I think most  that I have 
seen have been done with regular soft solder rather than something with 
a much higher melting point. If you consider that something melted the 
original 'filler' then it indicates a whole lotta heat, either through 
inappropriate use or insufficient cooling for whatever reason.

There is another point to consider as well, I think. The tube bases get 
pretty hot, even in normal, reasonably cooled use. Over the years this 
heating and cooling of the base contacts may well lead to them becoming 
minutely distorted. I have found cases where I believe this has caused 
the very problem you mention, the contact becomes sufficiently high 
resistance to prevent the heater kicking in. On the Johnson 275 series 
sockets, I think, you can remove the spring clip that holds the two 
parts of the contact together and bend it to provide a firmer contact. I 
did that on my Henry 2K which was doing what you have indicated and it 
has been fine ever since.

I think it might also be useful to measure the voltage at the pins of 
the tube. This means putting your voltmeter on the tube pins NOT the 
base contacts. If you think about it the 3-500 series want 5 volts +/- 
0.25 at 14.6A, so just a little R in the base connections could see that 
fall below spec. My Henry promptly delivered several hundred watts more 
once the heaters pins were at operating volts rather than just the base 
contacts.

Martin, HS0ZED



terry at g4amt.com wrote:
> My TL922 is mis-behaving with one tube operating intermittently. Symptoms 
> were of open circuit filament but valve tested ok with ohm-meter and worked 
> when refitted. Next power up, tube failed to work. jump soldered link wire 
> to pins across bases to rule out base connections. Tube worked when pin and 
> base `encouraged` with screwdriver.
> Looks as though previous owner has attempted to resolder the filament pins 
> and I suspect a dry joint (?) within one or both filament pins.
> Is it safe to re-solder the pins ? Anything else suspicious I should be 
> looking for ?
>
> Many thanks
>
> Terry 
>
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