:From: Earl Morse <n5tu@wt.net>
:I acquired a SB220 that hasn't been powered up in a
:long unknown length of time.
:Should I continue slowly bringing it up on the variac?
Hi Earl,
You can bring the unit up very, very slow, but keep in mind
you should do a complete update to the amplifier. That
would be the addition of many items described on rich's
web page.
You know, parasitic suppression, power supply rework,
general updates which include protection when faults occur.
Dealing with the old power supply becomes an issue when
the original bleeder resistors are on board. They should be
replaced first. I use an older Heath variable HV power
supply to slowly ramp each capacitor back up to full voltage
while watching it's current draw. Then each cap is bled down
and tested. If everything goes well, you can reinstall the caps
with the better bleeder bank and move on to the other items.
:Will sustained low filament voltage bother the tubes? Or
:should I remove the tubes for this process?
Nope. but you can remove the tubes. Be sure to leave
the mode switch in the CW position for most of your
initial testing. It's a set of lower voltage transformer taps.
Try to cold start an SB-220 in the CW Mode switch
position if you can.
I wouldn't fire the unit up without doing most of the mods
first. I've done a lot of SB-220's, two in the last year came in
with tube grid/cathode shorts. The updates saved me a
lot of grief and probably a filament transformer. All for
the cost of a few resistors. It's a deal you shouldn't pass
up.
:BTW, I'll switch it over to 220VAC after I get it checked out. Don't
:think my 120 line in the shack will be able to tolerate this amp at full
:power.
:Earl N5TU n5tu@wt.net
Most of my SB-220 initial work is done on the 117 vac taps, then
changed to the 220 volt taps later. An updated SB-220 will draw
less than 3.5 amps @ 117 vac with the t/r relay keyed, but no signal
applied. Makes it handy for small variac work, but be sure to use
an isolation transformer if you can. Please be safe...
cheers
skipp
________________________________________________________________
YOU'RE PAYING TOO MUCH FOR THE INTERNET!
Juno now offers FREE Internet Access!
Try it today - there's no risk! For your FREE software, visit:
http://dl.www.juno.com/get/tagj.
--
FAQ on WWW: http://www.contesting.com/ampsfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|