Follow up questions to a Audio Asylum Thread For Paul

Hi Paul,

Here is a copy of the post that you made on Audio Asylum's SET forum. I have left none of it out and I hope that the context remains intact.

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(Topic) What KIND of 2A3, ...or 45 ....for that matter?

A special note for Jeff :^) ---

The amp Astroimage has is convertible by changing the cathode resistor and a resistor in the power supply, to get the current and voltage right for the respective tubes. (There is a tap change on the plate choke, too.) Both tubes are run at the maximum spec, 300 volts and 15 watts plate dissipation for the 2A3 and 275 volts/10 watts for the 45. However, it is possible to set the voltage and current for the 45 and simply plug in a 2A3. The 2A3 will then run at 10 watts plate dissipation, which I know Jeff prefers for longevity - especially important when using expensive tubes (!). The load impedance is then a little low by my preferred operating point calculations, but still well within the range that Jeff has advocated before. Power is intermediate, probably 2.5 to 3 watts.

Note for Astroimage ---

The high power available from the 2A3-S only comes when using it as if it were a 300B. That particular 2A3, like the JJ 2A3-40 and a few others, is able to withstand higher voltages and currents than the classic 2A3, and can be used in a few 300B amps - those with switchable filament voltage. Otherwise, to get the higher power you would have to design an amp specifically for that tube.
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If I am reading what you have stated correctly the Stereomour is able to run with 2a3 OR 45’s. By that I mean that I can power down, pull out the 2a3 and plug in the 45’s without any internal rework then turn it back on. To do this a few resistor changes are in order. Which resistors are changed and to what value do they change to? Specifics are necessary for me, I am a complete newbie. The Stereomour is my first kit and first time with a soldering iron.

Do I follow the instructions in the manual setting it up for a 45tube on pages 13, 50 and 66 substituting the new resistors?

Lastly, I do not have my amp yet, Eileen is working on it for me and she has been sweet enough to send me an advance copy of the very well done manual for me to drool on… round up tools… and look into any pertinent topic before I start building.

Yup, I read the manual… will keep my glasses on, put my left hand in my pocket, find a bleeder resister, and have a babysitter when I plug it in. The first manual that I have read in YEARS!

Brian

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