chokes and power supplies

[Steve-O-Rama]

I am new to the tube amp world. Currently, I am having a pair of tube amps built for me right now by a couple of great gentleman in Washington state. I am NOT technically advanced so what I did was propose ideas I read on various sites and periodicals to see if they were technically feasible. I do not know what the values of the components being used are (i.e. tranformer impedances, etc) but I will give you a brief rundown of the circuit design - it is an approimately 80 watt class A amplifier using 8 EL34 tubes in a push-pull parallel design running in triode mode per monoblock. The input stage uses 2 EC99 tubes per monoblock. The amplifiers are a 2-stage design. There are also interstage and phase splitter transformers along with separate power and filament transformers for each stage along with, of course, the output transformers. The power supplies are solid-state rectified with choke filtering. I have three questions that I hope you guys can answer for me.
First, I have read some stuff on plate-load chokes and grid chokes but I am not sure where they apply in an amplifier design - i.e. input, driver and/or output tubes. My guys are using pi-filters right now. I would like to know how if these chokes could apply to my design, and if they could, whnere in the design.

Second, I was told by a couple of tube amp builders that for the purest tonal quality the best way to go is to use a self-bias circuit rather than a variable bias design. I would like to know if there is a way to design a variable bias circuit such that there is no loss in tonal quality, if any of you guys feel there is in fact a loss of tonal quality between the two designs.

Third, I have read some differing opinions on power supplies - i.e tube, solid-state, hybrid. I would like to know if any one could offer me tips and/or resources as a starting point for building exceptionally robust tube rectified, choke filtered and hybrid tube/solid-state, choke filtered power supplies for my design that could be switchable for fine tuning the sound to a particular musical genre and/or recording style - i.e. switching on the fly between a tube rectified, choke filtered power supply, a hybrid tube/solid-state rectified, choke filtered power supply and the solid-state power supply that the amps are already using.

I know these ideas are off the wall and my description of the circuit is quite vague so I do apologize for not being more technically advanced. However, I am really trying to see how far I can take this design and I live by the philosophy that if I don't ask the question I will never get a possible solution. Any information you guys could give me would be wonderful.

 

[audiousername]

First, I have read some stuff on plate-load chokes and grid chokes but I am not sure where they apply in an amplifier design - i.e. input, driver and/or output tubes. My guys are using pi-filters right now. I would like to know how if these chokes could apply to my design, and if they could, whnere in the design.

The plate-load choke replaces the plate-load resistor of the input/driver stage. The idea being to increase linearity and maximum output swing. Grid chokes are usually only used replace the grid resistor of the ouput stage to provide a low dc-resistance path from the grid to ground. Grid chokes are not very common. You need neither since it appears to be an interstage-coupled design. The IT coupling provides inductive loading for the driver valves and a low-DCR path to ground for the output valve grids.

Second, I was told by a couple of tube amp builders that for the purest tonal quality the best way to go is to use a self-bias circuit rather than a variable bias design. I would like to know if there is a way to design a variable bias circuit such that there is no loss in tonal quality, if any of you guys feel there is in fact a loss of tonal quality between the two designs.

Self-bias is less efficient than (adjustable) fixed bias, since the cathode current of the output stage passes through the self-bias resistor and dissipates power. Also, you will require a higher B+ voltage for fixed bias, unless the operating point of the output valves is changed. On tonal qualities between the two... I offer no comment

Third, I have read some differing opinions on power supplies - i.e tube, solid-state, hybrid. I would like to know if any one could offer me tips and/or resources as a starting point for building exceptionally robust tube rectified, choke filtered and hybrid tube/solid-state, choke filtered power supplies for my design that could be switchable for fine tuning the sound to a particular musical genre and/or recording style - i.e. switching on the fly between a tube rectified, choke filtered power supply, a hybrid tube/solid-state rectified, choke filtered power supply and the solid-state power supply that the amps are already using.

Thermionic rectifiers have a greater voltage drop than SS rectifiers (several tens of volts vs. ~0.6V), and will result in a lower B+ to your amp if you use the same transformer and filter. Also they will result in pooerer regulation (the power supply will sag more when current is drawn). Furthermore, the peak current ability of a most thermionic rectifiers is considerably less than SS rectifiers, so more PS modifications will probably be necessary than only swapping in a different rectifier. In terms of switching 'on-the-fly', I don't think it is a particularly good idea to go meddling with 400-odd volt power supplies while they are on.


If there's something I've said which isn't clear, ask. Everything I've said is rather general... A schematic would help greatly!

In any case, if the amps are still being built, it would probably be a good idea to listen to them for a while, to see how they're like and what you do/do not like... then attempt any modifications

 

[Steve-O-Rama]

Thanks a lot for the advice! I will discuss these issues with my design and production team A.S.A.P.