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.



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!

[MWJB]

Why not make life easier for yourself, use a PT with a Centre tap & have the centre tap on a switch, one way the CT is grounded for full voltage, the other way introduces the 100v zener...or a PT with 2 different B+ secondary taps? Yes, voltages down the line will be affected but that's not necessarily a problem (if it is a problem IN PRACTICE add a preamp voltage drop switch).

If you want a tube rectifier, use a tube rectifier...or plug-in octal SS variants.

I'd be much more concerned as to how you are going to lay out all these tubes, sounds like a recipe for oscillations to me.

Switching from EL34 to 6V6 (subbing tubes) would be much more practical, you could probably work it so you have a cathode resistor that works with both. You wouldn't need such aggressive B+ dropping either (30-50v?). It would make layout more feasible too. Perhaps more like 5-6K on the OT primary would be a better compromise, or an odd speaker config/additional extension socket to correct impedances with the different tubes?

50W class AB from cathode biased EL34 seems rather optimistic, 450 on the plate, less 30-35v on the cathode puts your B+ more in the region of 410v? 30W-ish?

Forget "class", both scenarios will be cathode-biased, push pull, class AB.

The kind of signal voltages that your EL34 will want to see will turn your EL84 inside out.

[Bruce / Mission Amps]

Actually, you don't even had to change the PT... just install the zener and power resistor on the anodes of the FWB to ground and bypass them with a spst.

[ETR]

No worries about too high plate voltage on a tube with the cathode disconnected. Any cathode biased amp should be class A or close to it to avoid crossover distortion. I suggest a fixed bias octal section with a cathode biased 9 pin section. That should drop enough voltage across the cathode connections if you select the the right power transfomer for the job and run the cathodes separate.

[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?

[R.G.]

Actually, you don't even had to change the PT... just install the zener and power resistor on the anodes of the FWB to ground and bypass them with a spst.

Better yet, use a monster MOSFET as a switch on the - side, hook a resistor between drain and gate, and use a zener switched in/out of the gate circuit. When zener is out, drain voltage turns the MOSFET way on and the drop is low. Switch the zener in and you now have an active zener setup. All switching can be done with low voltage/low current switches.

[ETR]

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?

I don't think youre gonna find an OT that will work out with 2 primaries, but let me know if you do.

[MarshallPlexi]

No worries about too high plate voltage on a tube with the cathode disconnected.

So I can have high voltage on the EL84's without hurting them as long as the cathodes are disconnected?

What about the grids? Would leaving the grids of the disabled tube (disconnected cathode resistor) connected "load" the signal going to the grids of the working tube? I would imagine that the signal at this point in the amp would be of sufficient strength that loading would not be a problem, but still it comes up as a question in my mind.