Need power amp suggestions

[Ripthorn]

Hi guys, I want my new amp to have a hi/low output power switch and I have two different thoughts of how to do it. The first is to have two separate power amps. These would both be submini tubes (5672 in pp and 5902 in pp) sections. The problem with this is that the 5672's (low power) would need a load resistance of approximately 35-40k and my OT (hammond 125c) can only get up to about 27k. Is there a way that I can increase the load resistance other than just getting a different OT?

The other thought I had is to make the 5902, which is originally in class A/B, into purely class A. From what I understand, this will lower the efficiency, and as such, with a constant B+, it will lower the output power. My question is exactly how to do this. I am thinking that I would just lift the connection of the cathode electrolytics to ground, so that there is just a cathode resistor to ground. Does this sound like it might work?

Really, I am looking for suggestions. If you have ideas of a different way, I would be interested in those too. I just got done getting the preamp just the way I want, so now I am moving on to the power amp. Ideas? Comments?

[andrew_k]

The problem with this is that the 5672's (low power) would need a load resistance of approximately 35-40k and my OT (hammond 125c) can only get up to about 27k. Is there a way that I can increase the load resistance other than just getting a different OT?

The specs show a primary imp. of 22k when using taps 5 and 6 and a 4ohm speaker. Use that same setup with an 8ohm speaker and the power tubes will see a reflected impedance of 44k.

[Wayne]

You could just use the 5902's and use a zener diode to drop the supply voltage for low power mode. At that current, a 5W zener could handle the job. As for how much voltage to drop - zeners are available in a wide range of voltages - tune to your taste!

W

[Ripthorn]

Those are both great suggestions. Don't know why I didn't think of your suggestion, Andrew.

Wayne, that is another great idea. I am planning to run these things around 120V, but I have some 39V zeners, so a couple would give me less than half the voltage. I will have to think about which one will work best, but thanks to both of you.