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In Reply to: RE: changing a 45 amp to a 45a amp posted by sam@edelmannpgh.us on March 31, 2015 at 16:54:52
If you have built the amp exactly as shown above, you can get more voltage by changing the first cap in the power supply from 1.5uf to 10uf.(If it were me, I would just run them like a 45 and leave the amp alone. If I wanted 3 watts I would put in a 2A3.)
You can monkey around with modifying the amp and see if you like it. You will have to change the cathode bias resistor and stay within the Plate dissipation rating. I cannot find the Plate dissipation rating for the 45A, though it would likely be slightly less than the 15 watt plate rating for the 2A3. Lets say it is 13 watts and you want to run it at 10 watts.
For 10 watts, you would simply raise or lower the cathode resistor until it passes 30 mA.
ROUGHLY:
0.03 * 330 = 10 watts. 330v is the net of the 400 Plate voltage less the cathode voltage of 70.You will have to play around with the cathode resistor value and the first cap value to get the plate and current you want to get the dissipation. All that might get you a watt more. :-)
You can also, more easily, get more power without more voltage, just increase the current through the tube.
Best yet, don't monkey with it.
Edits: 03/31/15 03/31/15 03/31/15Follow Ups:
If you run at 325v and 30mA, you will need a higher impedance output transformer. I estimate a conservative 6800 ohms; you could probably get away with 6000-8000 ohms. With a lower impedance than that, you will have excess distortion - it may or may not sound good to you at 5000 ohms, but it won't sound like a proper 45.
Incidentally, to my knowledge only the Australian data sheet from 1941 gives a maximum plate dissipation for a regular 45; it's 10 watts. That same data sheet specifies 300v maximum voltage. Unless the 45A has a larger envelope, I would assume it has the same maximum plate dissipation.
At 300v/33mA a 5000 ohm transformer would be well matched.
Thank you all for your help. I was planning this as an exercise to compare the 45A to the 2a3, assuming it would require some small electrolytic changes. Clearly, this requires much more, and I think I will just enjoy the 45A's in the regular 45 setup. Just too much time and money for the amount of information to be gleaned.
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