[Amps] 3-500z - intermittent filament

[Robert B. Bonner]

Martin, Terry, Group,

Good morning guys. Like I have mentioned before when I was back at the RADIO
SHOP (Drake Collins Kenwood, Yaesu etc factory service) while in college in
the late 70's I had to perform surgery on a lot of SB-220's.

History Lession:

The Johnson sockets had been designed for the 4-65, 4-125 and not even the
4-250 sized tubes let alone the 4-400's.  Since the 3-400 and 3-500 plugged
in directly many ham manufacturers cut corners and put the $5 sockets in
instead of the much more expensive aluminum Eimac airsystem sockets for the
4-400 or the fancy less expensive SK-410's.

The original Drake L4's had Sk-410's the L4B the Johnson sockets, cheapo.

Well the amplifiers where where air blew sideways across the tubes and pins
instead of through the sockets or didn’t have enough airflow due to NOISE
CONCERNS developed a syndrome.

Somewhere along there we also got the 1500W output increase.  These two amps
were 2KW PEP input amps.  When operated at balls out power the TUBES didn’t
cool adequately and got hot enough to either melt the solder out of the tube
pins, or take the temper out of the socket retainers which then caused a
high-z situation that melted the pin solder on the filaments.

THE PHENOLIC Pin holder base Eimac put on the 3-500Z was there to stabilize
the pins and ad some cooling flow if this situation arose.  (Cheap fix)

However in the normal air system socket base it blocked the airflow
slightly.

Syndrome Fix:

If you are resoldering pins in 3-500's bases, you have to be very careful to
not melt the tube base seal.

I did this by heating the removed larger pins with a torch in an old tube
socket and setting a tube with clean and fluxed pins into solder bath filled
pins and allowing to cool. If you have a lot of repairs a steel fixture
could be machined to effect the repairs.

It has been noted here that the spring clips can be individually replaced on
the sockets, I preferred to replace the whole now $20 socket.

You guys are right on it.

BOB DD 

-----Original Message-----
From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
Behalf Of Martin Sole
Sent: Tuesday, February 06, 2007 5:13 AM
To: terry at g4amt.com
Cc: amps at contesting.com
Subject: Re: [Amps] 3-500z - intermittent filament

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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