> I am getting ready to rebuild a school club's SB-220
> amplifier and could use
> some advice. So far we plan to replace the power supply
> capacitors, diode
> stack and metering circuit, add a soft-start and keying
> interface (using
> essentially all of the Harbach SB-220-related products).
That's fine if you want a project, but in truth there is
almost nothing to be gained perfromance wise assuming some
components have not gone bad.
In other words, if you want a project to keep busy change
all that. If you want to improve performance or increase
reliability it's a waste of time. Either one is you choice,
but the only important thing is to replace any defective
parts.
There are only a few things in the SB-220 to do:
1.) It needs a properly rated HV glitch resitor, and that
doesn't mean sticking a small metal resistor in the line. To
be effective you ahve to increase the supply ESR, and the
resistor has to be able to withstand a glitch (3000V) from
end to end and not arc or explode.
2.) It needs the grids grounded to the chassis with the
shortest possible leads.
3.) It should have a negative rail clamping diode to protect
the m,eters and other components.
4.) You should probably get rid of the 120V relay voltage
inside the amp by adding an internal transistor buffer.
The rest is just emotional engineering that makes people
feel good about nothing.
> 1. Is there a procedure to out-gas, or otherwise prep
> these 'dormant'
> 3-500Zs for operation? Is it reasonable to hope that they
> will work after
> all of these years (they were good when put in storage).
One of the worse things to do is store large glass seal
tubes for years in any environment. It is a bit worse in a
humid or damp location because moisture deteriorates the
metallic coating used in seals.
The primary gettering in a 3-500Z is the gray powder on the
anode, and it is ONLY activated by heat. I recover old large
glass tubes by running the filaments and applying positive
grid bias. I run just enough HV to color the anodes a dull
red. I cook the tubes for several hours.
You can actually temporarily convert the SB220 supply to a
half-wave rectifier instead of a doubler. You can do this by
removing the transformer lead to the center of the capacitor
bank and adding a temporary similar diode bank to convert
the doubler to a bridge.
This will give you about 1000 volts DC for the anode on CW,
and 1500V on SSB. A variable negative voltage applied to
the filament center tap can be adjusted to make the tubes
dissipate a few hundred watts per tube. Watch the grid
current to be sure you don't over-do the grids.
I start out at about 1000 volts and 250mA Ip for a few hours
and then go up to 1500 volts and 300mA or so for a few
hours.
This will allow the tubes to safely getter at modest
voltage. It also bakes out the transformer at lower voltage
(the HV secondary has no DC in this situation, so it runs at
about half of normal peak voltages).
> 2. Is replacement of other stock parts recommended for
> preventative
> maintenance? In particular, I am wondering if the
> 5.1V/10W zener, the
> carbon comp. resistor in the parasitic suppressors, and
> some of the HV
> ceramic bypass caps are good candidates.
Carbon resistors do sometimes go bad from age. The mechanism
is the leads inside the resistor oxidize and lose contact
with the carbon substrate. I'd replace the parasitic
resistors with Ohmite OY series metallic composition
resistors. The HV multiplier resistors are also subject to
age related failures. I'd just test them to see if they are
good.
I would not worry about the zener or any ceramic caps.
> 3. Is it advisable to directly ground the 3-500Z grids,
> instead of the stock
> arrangement of grounding through a parallel RFC and
> capacitors?
Yes. Absolutely. Ground the grids.
> 4. Are there other modifications to the stock circuit,
> such as relay timing,
> parasitic suppression, etc. that anyone would recommend?
No, except buffer the relay with a transistor buffer so your
exciter is happier.
> 5. Finally, does anyone have a favorite HV DC probe
> (model, manufacturer)?
> I am going to buy one for this project and future amp work
> (under 10 kV OK).
I just bought a standard HV TV set probe from MCM
electronics. It works great, and so far I have not been
killed while using it.
73 Tom
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