Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ernie_jr on November 10, 2011, 03:00:48 pm

Title: PI question
Post by: ernie_jr on November 10, 2011, 03:00:48 pm

For  the PI (12ax7 versions) of Ampegs, there is a 1K and a 220 ohm cathode resistors. Early designs use 1/2 watt but the new heritage editions used one watt resistors. Are 1/2 watt acceptable or were there problems and 1 watt is required.
Thanks,
Ernie

Title: Re: PI question
Post by: RicharD on November 10, 2011, 03:32:30 pm

Where's the circuit?  What's the voltage?  What's the current?  It's a 12AX7 not an AU so yeah, 1/2 watt is typically hunky dory but not enough info given to crunch the numbers.

Title: Re: PI question
Post by: ernie_jr on November 10, 2011, 04:11:41 pm

b15 heritage amp. voltage about 250
ernie

Title: Re: PI question
Post by: RicharD on November 10, 2011, 04:24:13 pm

I looked at all the B15's in doughs collection.  None use a 12AX7.

Title: Re: PI question
Post by: ernie_jr on November 10, 2011, 04:54:51 pm

look at the sb12,  (on Doug's schematic page) uses at 12axy, but same caps, resistors etc as what is used in the 6sl7 amps, sorry forgot to mention that. my main concern is do I need to use 1 watt cathode resistors as shown in the new b15 heritage version, or are 1/2 watt ok  as this was used in the 1960 build of the b15 and b 12.
ernie

Title: Re: PI question
Post by: RicharD on November 10, 2011, 05:03:32 pm

It shows 2V across a 1k resistor.  2/1000=.002A  2V * 2mA = 4mW.  1/2 watt way cool.  1/4 watt still cool.