[Amps] Pi-L Tank on TenTec Titan 425

Tom W8JI w8ji at w8ji.com
Thu Dec 21 22:51:34 EST 2006


> I have a nice Titan - works great for the most part even 
> with a
> 20 year old tube... however on 10 meters the "peak" on the 
> tune
> capacitor is right at the minimum capacitance point above 
> about
> 28.1 Mhz. Is there anything I can do, simply, to raise 
> this
> just
> enough to know I am tuned properly (i.e. requiring LESS
> capacitance)?

Spread the turns very slightly in the section of the tank 
coil for ten meters. You DO want the plate capacitor at 
nearly minimum C.

> I contacted TT service - they are a great bunch of people 
> for
> the most part... they said
> I spoke to the RF design engineer. If the coil is expanded 
> on
> 10
> meters in
> order to use more tune capacitance then the tuning will 
> change
> on 15 meters.
> If the tuning is changed on 15 meters then the plate choke
> might
> be damaged.

Nonsense. That's totally false. It can't happen.

> The suck out frequency for the plate choke is near the 15 
> meter
> band. As
> long as the tune cap will resonate there is no problem.

Series resonance in the plate has nothing to do with 
spreading or squeezing turns in the tank coil. They are 
separate issues. A choke goes series resonant because of 
distributed capacitance and inductance along the choke. At 
some frequency it acts like a group of back-to-back L 
networks with nearly equal ratios and this makes it have a 
very low series impedance. What you do with impedances 
outside the choke has nothing to do with how the choke 
behaves internally. The sole exception to this is if 
something external to the choke is directly coupled to the 
nmagnetic or electric fields. Any rig with a half way decent 
tank and choke layout would  not have an issue, and the 
choke should not be series resonant that close to 15 meters 
anyway.

> in manual is 5% of capacitance). Of course I could change 
> the
> series cap, 25p to maybe a 20 but finding one might be
> difficult

What 25pF series cap is this? Where is it at?

By the way, watch out for that circuit in the Handbook that 
shows an L between the tubes and the plate tuning capacitor. 
Unless you have a tube with a very high grid 
self-resonance...well up into upper UHF...and very good 
feedthrough isolation it will often destabilize the 
amplifier. It is often a bit tricky to suppress parasitics 
with a large L between the tube and the network input 
capacitor.

73 Tom 




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