[Amps] Need suggestions to address an interesting dilemma

w2cqm@juno.com w2cqm@juno.com
Sat, 30 Nov 2002 10:19:00 -0500


Perhaps someone can offer assistance  with an amplifier rebuild   problem
that has developed. I replaced a smoked Ameritron AL80 transformer with a
Heath SB1000 clone. (all voltages are tapped off the xmfr).   I found
that the filament voltage was high and that was addressed with short
lengths of nichrome wire on the secondary. Easy fix.  I never initially 
measured the high voltage  secondary  which  is the root of the dilemma. 
In any event, the doubler  rectifier board was completely rebuilt with
six (6)  filter caps rated at 450VDC (original units were 500VDC)  along
with bleeders. When I fired it up, I found that the HV was completely off
the amplifier's 3KV meter scale with only partial primary voltage.  Only
at that point did I  measur the HV secondary at 1500-1600VAC. Obviously
way too high both for the meter scale and the capacitors.   The
alternative is to reconfigure the diode alignment to a full wave bridge
however I'll be no where near the 2700VDC rating on the OEM Ameritron
amp. Power output will suffer with the single 3-500. This amp has been
retrofitted for dedicated 6m operation and I, based on a previous
project, achieved 600-650W output with a similar amp.    I could rebuild
the board and add a couple of more caps but the meter scale is calibrated
for 3KV so I'd be getting false readings with an additional 1meg
resistor.  I considered a choke input but decided I really didn't want to
add the additional "iron" to an otherwise nicely designed, compact unit.
There doesn't seem to be too many options open to me unless someone out
there can suggest  an approach that I'm overlooking. In a nutshell, I
need about 2600VDC developed from the parameter listed above.  I've
eliminated the use of a voltage divider resistor set up for obvious
reasons.  Any help would be appreciated. Best 73's Ron W2CQM/3


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