Hello, I have built an 2x 813 amplifier for HF. The bias is AB2.
When I give modulation in AM mode (with the driver) I receive some distortion in the final signal.
Possible a consequence of the AB2 stage?
Has someone a suggestion?
73'Paul PE1PAL
Is it GG or grid driven?
If grid driven, put a DC VOM on the grid, at the DC point of the RF choke and watch the grid bias level as you apply signal. It should be regulated and not fluctuate.
A better way for bias is to put back biased diodes or zeners in the fil center tap for a solid bias.
** Also, make sure the final is loaded heavily enough. Peak the plate tuning and loading for maximum output, then unmesh the loading cap until power drops off about 7%. Do this with full drive. If the final is loaded too lightly, this will cause good efficiency, but with distorsion. A heavily loaded amp has reduced efficiency - that's the trade-off.
Look at your RF waveform (100% modulated) at both the input and output. You will probably see where the sine wave starts to deteriorate.
Tom, K1JJ