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previous: Carl Gigun Brutus,You sound like a kno... -- 10/3/2004 8:16 PM

Re: What tube amp to build next?

10/5/2004 2:01 AM
Brutus	Re: What tube amp to build next? Yes in general I'd say that you are right. In fact high voltage class A amps tend to eat tubes like crazy. The Matchless Chieftain is well known for doing this. So are some of the later Fender champs that had close to 400v on their plates. So yes, it definitely is a consideration. I like the point that Greg Simon makes about class A not really being class A because it has some relevance here. I think most class A PP amps are designed to only be class A for the first 70% of output power for the reason you mention -- too hard on the tubes when high voltage is present. So instead they run class AB1 for the last final 30% crank of the volume control. As I mentioned in my reply to Greg, class AB1 covers a lot of territory, and class AB1 biased very close to class A will sound more like class A then an AB1 amp biased colder to be just short of going into class B. It's not like you cross the line into class AB1 and then suddenly all AB1 sounds the same regardless of whether it is AB1 biased hot or cold. The tone of AB1 varies greatly depending on the bias setting within AB1. So in general, with high voltages, it is OK to cheat a little by nudging the amp into AB1 at high volumes so that you make your tubes last longer. Even doing this though will generate lots more heat than a more conventionally biased AB1 amp that is biased closer to class B. Brutus

10/5/2004 2:01 AM

Brutus

Re: What tube amp to build next?

Yes in general I'd say that you are right. In fact high voltage class A amps tend to eat tubes like crazy. The Matchless Chieftain is well known for doing this. So are some of the later Fender champs that had close to 400v on their plates. So yes, it definitely is a consideration.

I like the point that Greg Simon makes about class A not really being class A because it has some relevance here. I think most class A PP amps are designed to only be class A for the first 70% of output power for the reason you mention -- too hard on the tubes when high voltage is present. So instead they run class AB1 for the last final 30% crank of the volume control.

As I mentioned in my reply to Greg, class AB1 covers a lot of territory, and class AB1 biased very close to class A will sound more like class A then an AB1 amp biased colder to be just short of going into class B. It's not like you cross the line into class AB1 and then suddenly all AB1 sounds the same regardless of whether it is AB1 biased hot or cold. The tone of AB1 varies greatly depending on the bias setting within AB1.

So in general, with high voltages, it is OK to cheat a little by nudging the amp into AB1 at high volumes so that you make your tubes last longer. Even doing this though will generate lots more heat than a more conventionally biased AB1 amp that is biased closer to class B.

Brutus

Replies:

Greg Simon Hi Brutus. When I said Class A wasn... -- 10/6/2004 7:28 AM