[Amps] AL811, AL811H or AL572 mods to 6m

[Tony King - W4ZT]

Both of those amps are good candidates for retrofit with GI-7B tubes. 
Use a single GI-7B where there are two 811's or 572B's or two GI-7B's 
where there are 4 811's or 572B's.  You'll have lower capacitance to 
worry about, more stability and tubes that will easily take you to six 
meters. They're also inexpensive.

73, Tony W4ZT

Tom W8JI wrote:
>> The original capacitors are just fine if you have a tube 
>> load
>> impedance of 400-600 ohms. The added coil creates an 
>> L-network which
>> transforms the approx 4000 ohms of the 3-500Z down to that 
>> value.
>>
>> As I mentioned, this technique is covered in any recent 
>> ARRL handbook
>> in the HF amplifiers chapter. The example they give is for 
>> ten meters,
>> but the principle applies equally to six or any band for 
>> that matter.
> 
> The only problem doing that is stability.
> 
> It generally works OK to do that with a very stable tube 
> like a 3CX1500A7 or 3CX800A7, but tubes with long grid leads 
> can get a bit squirrelly.
> 
> 73 Tom