Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Deric on October 09, 2018, 08:37:13 pm

Title: 12ax7 filament as Power Tube Bias Resistor
Post by: Deric on October 09, 2018, 08:37:13 pm

Working on a Montgomery Ward 35 JDR 8421.  Have searched but can't find a schematic.  It is very similar to a Danelectro Commando


http://www.blueguitar.org/new/schem/misc_amp/dano_commando.gif


In the Commando, the 4 6v6 output tubes are cathode biased using the filaments of 2 12ax7 tubes in series as a cathode resistor.  In the Montgomery Ward there is a 39R resistor in series with the filament of 1 12ax7. 


When calculating plate dissipation, can I just use the voltage drop across the 39R or should I be using the voltage drop across the 39R and the 12ax7?  If I use the 39R and 12ax7, what is the resistance of the 12ax7?  I'm guessing 84R ish.


Second question, what's max safe dissipation for a cathode bias 6v6 with 290vac on the plates? Right now it's ~34mA per tube.


Third question, the 12ax7 is getting ~12vdc.  If I want to bias cooler by increasing the 39R the voltage to the 12ax7 will drop.  How low can I go before I have issues with the 12ax7?  The Commando schematic shows 19.5vdc for 2 12ax7's so I'm thinking if I stay around 10ish for 1 I should be OK.


Thanks!!!

Title: Re: 12ax7 filament as Power Tube Bias Resistor
Post by: PRR on October 09, 2018, 09:26:58 pm

> with 290vac on the plates?

AC??

Why would you want cooler bias?

Max dissipation is in the Data Sheet. But....

I've seen these four-6V6 w/12AX7 bias amps run for decades without problem. Don't fuss. Leave it be.

Title: Re: 12ax7 filament as Power Tube Bias Resistor
Post by: Deric on October 09, 2018, 10:00:59 pm

Doh!  Not AC but DC. 

Quote from: PRR on October 09, 2018, 09:26:58 pm

I've seen these four-6V6 w/12AX7 bias amps run for decades without problem. Don't fuss. Leave it be.

Good enough!!


Thanks!