Power Supply Question

[MarshallPlexi]

Hey guys,

I'm working on an amp design that will be 50W Class AB Cathode Bias with a pair of EL34's and 18W Class A Cathode Bias with a quad of EL84's. There will be a switch (S1) that switches the power supply voltage and filtering. The B+ for the Class AB operation will be 450 and the B+ for Class A is 310.

I am reducing the B+ for Class A operation by use of a 100V zener diode in series after the rectifier and using a 100R resistor to simulate the feel of a tube rectifier.

Since the power supply voltage for the plates and the screens will change depending on the operating class selected I opted to run the preamp power supply off the rectifier so it's voltage will be consistent regardless.

[img:944:333]http://bradjackson.net/Images/power_supply.gif[/img]

I'm still trying to work out the S1a switch so that the center tap of the OT can tie to it and feed the plates. Obviously as it is written, the S1a switch will not work right since the zener diodes would be after the plate connection. I still have to work that out.

One of my concerns is regarding the plates and the screens on the tube that is NOT selected. Since I'm using one OT that has a 6.6k primary I will have the plates and screens of both tubes connected at all times. Switching of the cathodes of the EL34's and the EL84's will be what turns each tube set on and off. My question is this:

When in Class AB operation, the cathodes of the EL84's will be disconnected BUT the plates will be be seeing the full 450V of the amp. Obviously this situation is reversed when in Class A mode. The cathodes of the EL34's will be disconnected BUT the plates will be be seeing the full 310V of the amp. My only concerne is that the max plate and screen voltage of the EL84's is 300V. Should I be concerned about this since the cathodes will be disconnected and therefore no current will be flowing or does that even matter?

Thanks for your help guys!

[xk49w]

Regarding the switching this might work: Remove s1b and tie all those connections together. Configure s1b so that it alternately grounds the upper and lower filter caps. The non-active section will be attached at the output but will be floating, disconnected from both ground and the input.

Regarding the other, you will have the heaters still energized on the otherwise inactive tubes? I wonder if you could find a 3PDT, switch the B+ as well to each section?

[austinb]

I have tried disconnecting the cathodes before and that didn't work. The tubes still conduct somewhat through the screens and grids to the plates.

[MarshallPlexi]

Okay so here's my thinking. Since I want the filtering to be different for each section:

Class AB EL34 - 100uF Plate Cap - 10H Choke - 50uF Screen Cap
Class A EL84 - 30uF Plate Cap - 1k Resistor - 30uF Screen Cap

The preamp power supply will start from the 50uF cap on the Class AB power supply and look something like this:

Resistor - 33uF - Resistor - 33uF - Resistor - 33uF - Resistor - 33uF

I think I'm going to make two separate power supplies. I can just wind the PT with dual High Voltage Secondaries that way I can really tweak the power supply for each section and play to each sections strengths.

So assuming that there are two separate power supplies that are both active once the Standby switch is thrown, would simply lifting the cathode resistor of the unused output tubes be sufficient to turn each section off? I would have full plate voltage and screen voltage going to each set of tubes continually. Also would I need to switch the grids of the output tubes as well as the cathodes, or would running them in parallel (leaving the grids of the EL34's and EL84's connected) work fine without causing any undue loading of the signal.

Note: As far as I can tell, the OT will have to have a dual primary in order to keep the power supplies separate. This is cool because I can tweak each primary for each operating class.

Thoughts? Concerns?