Re: [Amps] AL-811H on 6m

[R.Measures <r@somis.org>]

On Aug 10, 2004, at 7:43 PM, Crawfish wrote:

> One of the folks on VHF Contesting list is wanting to put his AL-811H 
> on 6m. I told him that 2 each 811A's would work, but IMHO, the output 
> capacitance would be too high to work on 6 with all 4 tubes. I would 
> appreciate opinions, here or off-list. Thanks in advance.

-  If a tube has an RL of 2000-ohms, for a tank Q of 10 (Eimac method), 
 XC1 (reactance of the Tune-C) would be 2000-ohms/10 = 200-ohms.  At 
50MHz, 200-ohms of XC is presented by a 16pF capacitor.  Thus, if the 
anode-C of the tube is 5pF, the Tune-C would need to cover 11pF.
-   If 4 of the same tube-type were connected in parallel, RL would be 
2000-ohms / 4 = 500-ohms -- so, for a Q of 10, XC1 would need to be 
50-ohms, which is 64pF at 50MHz.  Since 4 of these tubes would have an 
anode-C of 4 x 5 = 20pF, the Tune-C would need to cover 44pF.
-  If 20 of the same tube-type were connected in parallel, RL would be 
2000-ohms / 20 = 100-ohms, XC1 would be 10-ohms, whixh is 320pF at 
50MHz.  Since the anode-C is 20 x 5pF = 100pF, the Tune-C would need to 
cover 220pF.

Thus, the only limit on paralleling tubes is that a suitable Tune-C 
must be provided.  In the above case, a 6 - 25pF variable would be 
suitable for one tube, and a 25 - 250pF would work with the 20-tube 
version.
cheerz, Joe
>                                                                        
>                                                    Joe Crawford W4AAB
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Richard L. Measures, AG6K, 805.386.3734.  www.somis.org

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