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WESTERN ELECTRIC 91A CLONE DISCUSSION

Posted by maxhifi (A) on November 21, 2014 at 14:34:16

Been around the block a few times with audio, but never have owned a 300B amplifier. Have decided to start at the start, and build myself a Western Electric 91A clone. My logic is to start with this, and move to a more modern topology if I see a need for it after I experience what is possible with the original design.

Now, doing some research this really opens a can of worms, and there's a few points I would like to ask some advice on from more experienced single ended triode people. I am proposing to follow the original 91A schematic exactly, with the exception of a couple of points, and also have a couple of general questions regarding the original schematic.

1. I have read a lot of speculation as to the function of the 1MF capacitor between the voltage divider at the grid resistor of the 300B, and the self bias resistor of same, but cannot figure out its function and haven't read an adequate explanation of what it could be.

2. A lot of the "91a" circuits out there propose substituting a 6J7 or a 6C6 for the 310A, without any modification to component values. Are they really similar enough that this is a good idea?

3. I intend to not use the input transformer. I would however like to retain negative feedback from the plate of the 300B to the screen grid of the input tube, and need to figure out what sort of overall gain the amplifier has in this configuration. I am expecting it to be excessive, but am hesitant to change anything, since I don't want to mess with the negative feedback loop design.

4. Frequency shaping components in the screen grid circuit of the input tube don't have a real application if I will be using this as a power amplifier. am not sure what values to use, or maybe best to just eliminate them entirely. I read these components are related to the response of optical movie soundtracks, and thus not required.

5. Value of the choke to be substituted for "speaker field" ?

6. The coupling capacitors look very small, probably intentionally to reduce low frequency bandwidth. I will have to revise these to be larger, but don't want to ruin stability of the amplifier with negative feedback applied.

7. I understand that the brand of output transformer to choose is more or less like asking someone what is their favourite car, but feel free to state personal preference.

I realize I have more questions than answers, but any contributions are appreciated.

Best Regards

Max

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