Re: [Harp-L] 12AX7 vs 12AU7 power requirements

[Frank Evers <frank@xxxxxxxxx>]

Hi

Am Samstag, 28. Juni 2008 schrieb gary:
> I read on the list where someone said that a 12AU7 will draw more
> power than a 12AX7 and that the extra power draw could damage some
> cheaper equipment. What about a 12AT7 and a 12AY7 do they draw more
> power than a 12AX7?

It was probably me who brought this up. The 12AU7 actually draws more 
current. This current doesn't only run throught the tube itself, but 
also through a few resistors. These resistors heat up. They always 
do, but the more current runs through them, the more heat they have 
to take.
Usually an engineer will opt for a resitor, that can take at least (!) 
four times the heat thats expected to happen in normal operation mode 
just to provide an extra secutity bonus in case of shortages, etc.

With a 12AU7 when the circuit design is designed to hold a 12AX7 this 
bonus is probably not enough.
Scott is right, vintage point to point wired amps are usually built 
very failsafe, and you won't have to worry about breaking anyting 
while using a 12AU7. However today amps are build in a very 
competitive economical environment and manufacturers have to build 
their products at low costs. So the rule of thumb should be not to 
replace a 12AX7 with a 12AU7 in these amps.

Using an AT or AY should be ok though. Even if they draw more current 
than the AX things should still be within specs.

-- 
Gruß,Frank