Regarding the SB-220 questions and answers that have been posted here...
There was a method to measure the Z of a transformer by shorting the
secondary leads and measuring the current in the primary while bringing
up the primary voltage.
I would like to know if this is a "normal/conventional" method or
something that someone has just experimented with. I'm just looking for
conformation from one soures or a possible text reference that I could
look to for more details.
Re self biasing tubes...
>If I was going to do a mod, I'd get rid of the whole 130 volt supply,
>and let the tube self-bias off. Than a more conventional 12 volt
>relay could be used.
About ten years ago I built a clone of an SB-220 using the same basic
design less the -130 volt supply. There was a considerable problem in the
amplifier trying to cutoff the tubes in the standby mode.
The first design just opened the "cathode" (center tap of the fill xmfr)
across a 50K 10wt resistor in standby. The "diode hash" type noise in the
receiver was horible and made the amp un-usable. Increasing the value to
200K reduced the noise to a relative S-4 on the receiver from the
starting S-9 +20.
To completely eliminate the noise, I rectified a small negative voltage
from the 24 volt relay circuit (surplus relays) and sucessfully cut the
tubes off using the conventional method.
My question is: Is the static/idle noise a common occurance/problem
for tubes not completely cut off. Your thoughts on possible other sources
of this noise? BTW, the amplifier had considerable parasitic protection
built in from the start and never shows any stability problems. Although
not in current use, I still have the operational amplifier closeby after
faithfull years of service in the 40 meter war zone at full output.
Last question: Does/Is there a source of exact size replacement parts for
the SB-220? I have one here with a questionable fan that I would like to
replace without physical mods. I've read here about a company(?) called
Harbach that might provide these parts. Also, if the meter ever goes away
because of a problem, I'd like to have a spare... any help?
Thanks Much
73's
Skip wv6f
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