813 homebrew amp-- go for three or four tubes?

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K1ZJH:
I'm starting a homebrew 813 amp.  It was going to be a two holer, but since the power transformer can handle it, I can expand to three or four tubes.  I'm not sure if going to four tubes will buy a lot.  On the other hand, I have the tubes, sockets and parts...

Has anyone had any luck using four of these tubes on 10 meters, or is that a fool's errand?  I am familiar with tapping the tank coil down for the plate tuning to improve matching to tubes with high plate capacitance.

How important is grounding the metal shells on the base; the sockets will be recessed about 1-1/2 inches.

Pete

KA2DZT:
I think that metal base is connected to one of the pins which would get grounded.

The more tubes you put in parallel the lower the plate load.  Lower plate loads require more capacitance for both tuning and loading.  The tank coil requires less inductance to maintain the right Q of about 12.

Plate load is the plate voltage divided by twice the plate current.

Fred

K1ZJH:
Hi Fred

Plate load for two is around 3500 ohms, so I'll be around 1750 or so... which is doable using a Pi L on 80 and 160 to keep the load capacitor size reasonable, and then going over to a Pi Net for the upper bands.  I was more concerned with the combined plate capacitance, which is going to be close to 60 pF  ???  That is value is over the design value for a Pi Net with a reasonable Q at that load impedance.  I am still very curious how others have made out using two or three tubes in parallel on 17 meters and above.

73

Pete

KD6VXI:
I have seen an abortion of 813s on 'ten' meters.  It used ten tubes!  So yes, 4 will work.

If you want whisper quiet, I'd go for the full four tube amp.  It might not buy you much as far as power out, but it would sure help with plate dissipation and lower the demands for any blower!

Plus, four looks cooler!

--Shane
KD6VXI

KD6VXI:
One way to get low plates impedance to work, the old L pi L trick.

You can simulate that with the g3sek spreadsheet.

Looks like you'll need about .3 uh of 'step' inductance.

This is a run for 29.3 mhz at 2.5kv and 1.6 kw output.


With a .3 uh step inductor, your C1 is now 32.9 uH.

This is failing to put any parasitic suppressor in the anode leads. I can simulate those as well.

Pictures / screens hots included below.  If your operating parameters are grossly different, let me know and I'll run your numbers instead.  Also, only includes 10 pf stray c.  1=12, at the design frequency.

--Shane
KD6VXI

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