What is the modulation transformer you're using?

If it's a mains transformer it could be going into saturation. Proper
modulation transformers are designed to accomodate a d.c. bias through the
windings which normally requires an airgap in the core. In an ungapped
core the dc bias will drive the core somewhere up the BH curve and the
effects will be exactly as you describe - clipping on one of the peaks

I don't have exact specs on it. It's a Stancor, an actual modulation
transformer with isolated secondary so that the RF final could be run off a
different B+ supply if desired. I bought the transformer from a ham who had
pulled it from an AM rig that used sweep tube outputs, and he had assured me
that the match to a 6L6 would be good on both primary and secondary. Perhaps
I can get a mH reading on it with my LCR meter, and someone can do the math
for me.. I haven't used that math in many many years, and it's left me, I'm
afraid.

Brenda Ann wrote:


It's a Stancor,

Have a number? some of us have old references that might list the specs...

best regards...
--
randy guttery

A Tender Tale - a page dedicated to those Ships and Crews
so vital to the United States Silent Service:
http://tendertale.com

"Brenda Ann" wrote in message
...
What is the modulation transformer you're using?

If it's a mains transformer it could be going into saturation. Proper
modulation transformers are designed to accomodate a d.c. bias through
the
windings which normally requires an airgap in the core. In an ungapped
core the dc bias will drive the core somewhere up the BH curve and the
effects will be exactly as you describe - clipping on one of the peaks

I don't have exact specs on it. It's a Stancor, an actual modulation
transformer with isolated secondary so that the RF final could be run off
a
different B+ supply if desired. I bought the transformer from a ham who
had
pulled it from an AM rig that used sweep tube outputs, and he had assured
me
that the match to a 6L6 would be good on both primary and secondary.
Perhaps
I can get a mH reading on it with my LCR meter, and someone can do the
math
for me.. I haven't used that math in many many years, and it's left me,
I'm
afraid.


Not withstanding the other suggestions, you may also want to try swapping
the connections on the modulation transformer - see attached sketch.

Figure 1 gives the worst case condition when both Modulator and PA dc bias
currents add to increase core flux density. In Figure 2 the dc bias currents
are opposing and help and reduce the flux levels in the core.

Also I would very much recommend putting resistors and decoupling capacitors
in the cathodes of all stages - particularly if you do use a bridge
rectifier to increase the B+ supply voltage as suggested. See maxhifi's
sketch newamp.jpg for details.

Just a final thought - is the modulation transformer intended for single
ended or push pull operation?