Q about strange tuning on a valve TX

[Brian]

One for the valve/tube techies.
I have built, maintained, restored, operated and fried many valve transmitters over the years.
As we all know, max output power always corresponds to the dip in HT current when tuned up correctly. Or as close to it to make no difference.
I have several transmitters and in each case this is true.........except for one.
It uses 3 x 807 tubes (or valves as we call them over here) in the output. When tuned up for the dip, it produces around 130 watts or so of RF. However, if I tune for max output, I can get 150 or more watts out but the HT is no longer dipped.
This doesn't make any sense to me.
It doesn't matter what config of load/tuning I do, the max output never corresponds with the dip.

It's not actually an issue as, I'm just curious as to why this is happening and if anyone else has seen this.

[Stretchyman]

Current dips when O/P circuit (tank) is resonant.

Max power may occur off resonance.

Str.

[Brian]

Current dips when O/P circuit (tank) is resonant.

Max power may occur off resonance.

Str.

Surely if the output network is tuned for resonance, the power would be max?
The current increases considerably when I tune for max power.

It's only this one transmitter that this occurs on.

Continues to scratch my balding head

[JimIO]

Sounds like tuning and loading interact more on that one than the others.

[NJQA]

I seem to remember an old ARRL handbook saying that when max power and plate current dip didn’t match, it might be an indicator of self oscillations/spurious emissions from the PA.

[Brian]

I think I might have figured out whats going on.
I didn't build this TX and so I presumed it was the plate current that was metered. It's not, it's the combined cathodes. 

[syfr]

Sounds like the final amp needs to be neutralized. Check arrl publications.

[syfr]

I seem to remember an old ARRL handbook saying that when max power and plate current dip didn’t match, it might be an indicator of self oscillations/spurious emissions from the PA.

Yup...ergo the neutralization process.