The AM Forum

Gentlemen,
 Am a newbie as to tube rigs. I have a Valiant II that basically has been rebuilt.  When running full output tuned as it should l have low audio, when I crank it up, it flat tops. Now the few QSO's I have had said it sounded fine, but the scope shows flat topping, and the REA shows more neg peaks then pos, even if I flip the phase. If I back down on the drive, and get it down to where she starts producing power around 20 - 40 watts, audio is beautiful on the scope, nice positive peaks over 100%, and the negs in line. This is a stock radio, no audio mods, except the clipper circuit is disabled. Any words of wisdom would greatly be appreciated.


73,

Bill
KA3MFN

Scope pix?

Are the tubes known to be good, tested??

How did you bypass the clipper?

How do you "flip the phase"?

Do you mean RANGER II?
A Valiant II is capable of up to 120watts of carrier...


                     _-_-

The Rob Lowenstein audio mods... which bypass the clipper should be used and followed.
published in ER in the 90's   avoid the many bad audio mods that have been published

check the polarity of cathode by pass caps, and all values of coupling caps.  .01 to ,o15 is plenty big enough.
check your tubes on a good tube checker like a hickock..  use the Life Test for filament emission
or reduce the fil voltage to the next lowest voltage..and watch the Gm.  new tube rock solid, good used tube
slight drop ( 10 %)  soft tube big drop.  a reasonable match in Gm for the mod tubes is required, as full filament emission is a must !

measure your mod transformer windings
against the manual values !
check for shorts to ground and shorts between prinary and secondary

check your mod bias  voltage and resting current  ( is your meter shunt reasonably accurate ?)
and Your PA bias.   If your Final Amplifier bias is wrong.. then the class of operation is wrong too !

You should Never attempt to modulate your transmitter
when the resonant plate current is not in the 300-330 Ma range.
The mod transformer needs to work into its rated impedance
with is determined by the plate voltage and plate current.
So load it correctly.  you should see 140 watts or more of carrier
if you do not   then something is wrong ( meter shunts are notorius)

You must have enough grid drive, in order to modulate correctly.
The rule is to saturate the grids... so observe the grid drive while transmitting
and make sure it right. to low no modulation.. too high.. short tube life.

unlike the old viking,  the valiant audio driver xfmr is well spec'd and unless it
failed, should not need replacement. but anything old can fail.
including me