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July 04, 2020, 11:27:04 PM

Author Topic: Stereomour II upgrade to use 300B  (Read 1414 times)

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Offline JGK

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Stereomour II upgrade to use 300B
« on: August 23, 2015, 12:12:54 PM »
Currently I have an 8 watt 300B amp that I use with B&W 90dB sensitive speakers. I then purchased and assembled a Stereomour II 3.5 watt with JJ2A3-40 tubes. The difference in power can be heard in the low frequencies. I would like to know if there would be an upgrade to use 300B tubes in the Stereomour II?

Offline Paul Joppa

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Re: Stereomour II upgrade to use 300B
« Reply #1 on: August 23, 2015, 04:13:00 PM »
The Stereomour does not have enough power supply voltage or current capacity to produce greater power than 3.5 watts.

Here's an analogy: race car tires will not make a Corolla go faster unless you also replace the engine and transmission with higher power/higher speed components.

Tubes in SET amps are typically about 25% efficient. So a tube that produces 3.5 watts output will dissipate about 15 watts in heat. A tube that produces 8 watts must dissipate around 32 watts as heat - which it can only do if it has greater voltage and draws greater current - in other words, a more powerful power supply. (You would need plate chokes and output transformers capable of the greater power as well.) The transformers will weigh about twice as much. When you use a 2A3-40, which is capable of dissipating up to 40 watts, into a 2A3 amplifier, it has the 2A3 power supply and dissipates the same 15 watts as a 2A3. It will probably last longer since it is not being cooked at its maximum rating, but it can't create any more power.

Hope that clarifies things - this is a common source of confusion.
Paul Joppa