audmod01
Super Member
I have a Fisher 440 Amp and a Fisher 490-T tuner/preamp chassis. The 440 Amp is similar to the amp which was normally supplied with the 490-T with the exception that the 490-T has only three 12AX7 tubes with their filaments in series as part of the cathode resistance of the 6BQ5/EL84/7189 output tubes. The 440 Amp was designed to have four 12AX7 tube filaments in series to properly load the cathode circuitry of its output tubes. Since a 12AX7 filament operates from 12.6V @ .15A that works out to be approximately 84 ohms with a 2W power rating. I believe I can add a resistor in series with the power from the amp chassis to provide the load the amp likes to see to the DC 12AX7 filament circuitry.
Alternatively, I could re-wire the 490-T chassis to put one of the 12AX7 tubes of the multiplex circuit in series with that filament feed from the 440 power amp circuitry. The needed 84 ohm resistance can reside either in the power amplifier chassis or in the tuner chassis.
I just submitted some orders for a set of EL84 output tubes and the 7487 driver tubes for the power amplifier chassis. I have enough 6AU6 and 6BA6 tubes on hand. I also had to order 12AX7s and a 6AQ8 for the tuner chassis. I have a 6AH6 that can take the place of the 6HS6 in the IF circuitry. The set came with the 6BE6 and the EM84-A eye tube so those are OK and check good on my tube tester. I am looking forward to getting this up and running to see how it sounds.
One thing that is nice about these cabinet chassis is that they are usually quite clean compared to the component series. I have seen far too many component models that have had drinks spilled down inside them with the corresponding corrosion of the chassis.
Joe
Alternatively, I could re-wire the 490-T chassis to put one of the 12AX7 tubes of the multiplex circuit in series with that filament feed from the 440 power amp circuitry. The needed 84 ohm resistance can reside either in the power amplifier chassis or in the tuner chassis.
I just submitted some orders for a set of EL84 output tubes and the 7487 driver tubes for the power amplifier chassis. I have enough 6AU6 and 6BA6 tubes on hand. I also had to order 12AX7s and a 6AQ8 for the tuner chassis. I have a 6AH6 that can take the place of the 6HS6 in the IF circuitry. The set came with the 6BE6 and the EM84-A eye tube so those are OK and check good on my tube tester. I am looking forward to getting this up and running to see how it sounds.
One thing that is nice about these cabinet chassis is that they are usually quite clean compared to the component series. I have seen far too many component models that have had drinks spilled down inside them with the corresponding corrosion of the chassis.
Joe