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
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