[Amps] Stock up on tubes?

[Roger D Johnson]

This is like swapping engines in a car. It sounds simple but is fraught with 
problems.

1. Input Z of 8877 is different requiring rebuilding of input circuits.

2. Power transformer will not handle increased requirement of more powerful tube.

3. Completely new pressurized cooling system needed.

4. Output circuit might not handle increased power.

5. ad infinitum

73, Roger

>> REPLY:
>>
>> If it were my SB 2xx amp I would replace the 3-500(s) with a single
>> 8877. It is a smaller tube so it should fit OK, and a single 8877 has
>> more power capability than a pair of 3-500's and needs only about half
>> the drive for the same output. The filament voltage is the same and
>> filament current is less, so no change is needed there.
>>
>> I have never actually done this so there might be a gotcha I'm not
>> aware of and you would need to proceed carefully, but if successful
>> you would have a much better amp. More headroom, better reliability,
>> less drive and in most cases an 8877 does not need a parasitic
>> suppressor. My homebrew 8877 amp does not have one and is perfectly
>> stable on all bands. I have seen numerous other 8877 designs that do
>> not have one either.
>>
>> Making a change like this is not for the inexperienced and if you are
>> unsure about it, you should probably find an Elmer to help you.
>>
>> 73, Bill W6WRT
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