# Tank circuit for two tubes

Discussion in 'Amateur Radio Amplifiers' started by WB5NEN, Sep 30, 2019.

1. ### WB5NENHam MemberQRZ Page

I'm learning. I can calculate values for a Pi network tank circuit using the method in Eimac's "Care and Feeding ..." That's for a single tube. Now, to show my ignorance (but that's how to learn). How do I calculate for a pair of tubes in parallel? That is, how do I modify my C and L values for a pair of tubes?

Wouldn't you just start by recalculating the impedance at the anode circuit of the pair of tubes which off the top I'd expect to be the same peak RF voltage over twice the single tube RF peak current with two of the same tube in parallel, biased the same and driven by the same signal. IOW, half the impedance of a single tube.

So, two matched tubes should draw twice the RF current of a single tube and it's RF voltage over RF current that determines the impedance at the anode(s). That's the input impedance to your PI network, I assume the output impedance is 50 ohms and you select the Q but something in the range of 10 to 20 seems pretty typical for loaded Q in an tube amplifier's PI network.

KK4RSV likes this.
3. ### WB5NENHam MemberQRZ Page

Is it really that simple? Peak RF voltage (unchanged) divided by twice the Peak RF current (for two tubes). I had wondered. So it looks like you confirmed. Thank you. And you have a nice station. I admire the TR4CW you have.

Thanks!

5. ### WB2WIKPlatinum SubscriberPlatinum SubscriberQRZ Page

+4+THE+

"Tanks for the mammaries..." Bob Hope's theme song.

"I need to know the tank Q."
"You're welcome."

WA7PRC likes this.
6. ### KD2ACOPlatinum SubscriberPlatinum SubscriberLife MemberQRZ Page

Goodness, Steve...
That’s two “tubes.”

K2XT likes this.
7. ### N8FVJHam MemberQRZ Page

Amateur Radio Handbooks have calculation tools. I would pick up a pre 1970 year.

9. ### W9WQAHam MemberQRZ Page

im far from an expert on this. yrs ago i struggled with handbook formulas and nomographs for pi net
inductance info. it seems that a 2nd tube cut plate resistance in half so i was uncomfortable testing with one 813 and then a 2nd.

i know that i had to mess with taps on the pi coil even though it would tune with thc cap at many tap points. its probably why my hb 2x813 amp is lazy above 20 meters but its an ongoing labor of love.
whomever inherets it may do better. i can whistle up appx 800 watts on 160 to 40, less on 20 and above

btw i do have 3 sockets wired in but 3 813's did not do well !!

10. ### KM3FHam MemberQRZ Page

Basicly there is double the plate current assuming 2 tubes vs 1, half the plate load resistance, double the Plate interelectrode capacity to account for.
813s were never very efficient above 20m.
Plate capacity to ground becomes a large part of the tuned output circuit you can't do anything about.

Last edited: Oct 14, 2019