Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dsones on October 17, 2024, 05:56:02 pm

Title: Princeton reverb micro bias
Post by: dsones on October 17, 2024, 05:56:02 pm

I modified a Princeton reverb to use Rob Robinette's micro power amp.  I used the bias calculator and get a dissipation rate of 120%.  I'm using a 12au7 as the power tube.  Rob's instructions say if you are using a 12a*7 for the power amp and using both sides of the tube to list the number of tubes in the calculator as 1.  So it looks like I need to increase the 820-ohm 3-watt resistor to lower the dissipation rate.  Any idea what size a resistor I should use.  I added files with the calculator results.  Thanks

Title: Re: Princeton reverb micro bias
Post by: stratomaster on October 18, 2024, 02:01:21 am

You can draw load lines to make an estimate, but it's easy to just sub in a new resistor and measure. If the calculator is right then we're only talking about 18.4mA across the resistor, so a 1/2W is fine, especially for testing. You can sub in a 1W to help you sleep better at night if you feel like it. 3W is overkill for this application. 

As for values next standard up is 1k, so start there. Plus you very likely have a 1k 1/2W or 1W somewhere in your stash.  I'd be surprised if you need to go above 1.2k.