Re the EXO-45

2wo · 4807

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Offline 2wo

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on: December 24, 2011, 03:58:09 PM
So here is what I am thinking. Back when they were first released, I bought a pair of the BH5, nickle striped. These were intended for a 2A3 amp. The amp was about half built when Doc released the SR-45 parts. I couldn't  resist it. So I reconfigured the power supply and set it up with a pair BCP-15 plate chokes, BCP-16's on the grids and 6.8uf parafeed caps. The amp sounds outstanding, the only thing I can wish for is a little more power, sometimes. So the EXO-45 should be a good fit in this amp, freeing up the BK5 for the yet to be built 2A3 amp. So two things, with the EXO-45 do you think I need a change in value of the parafeed cap? and I am short a pair of plate chokes. If I remember correctly, you gaped the BCP-15's to handle the current of the 2A3 or 45 but I don't remember. I am inclined to leave them in the SR-45. So what to use with the BH5, BH6, EXO-003 or another pair of  BCP-15? Or do have another idea...John       

John S.


Offline Paul Joppa

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Reply #1 on: December 24, 2011, 05:40:45 PM
The appropriate parts depend on the operating points, at least as much as the tube.

For example, the SR45 operating point was chosen to match the 3K impedance of the BH-5. Switching to the 5K impedance EXO-45 will cut the power to 60% of what it was, unless the voltage and current are adjusted to suit the new impedance. The appropriate parafeed cap depends on the plate choke and the impedance of the output transformer; with the same choke the capacitor would be about 1/3 the value for 3K impedance.

So ... a decent answer depends on what operating points are chosen for the two amps.

Paul Joppa


Offline 2wo

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Reply #2 on: December 27, 2011, 10:54:24 AM
Thanks Paul,
losing 40% of the power I have now could be a deal breaker. With my speakers, its borderline already. I might have to rethink this...John

John S.


Offline Paul Joppa

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Reply #3 on: December 27, 2011, 12:23:05 PM
Yes, when you get fancy, voltage quickly becomes the problem. Direct coupling adds 150-200 volts. A C4S plate load adds another 200v. Shunt regulation adds ANOTHER 50-100v. You can get close to a 1000-volt power supply, even with just a 45 or 2A3!

The only way to get more power from less voltage is to reduce the load impedance and increase the plate current. Of course, shunt regulation eats up plate current, too...

Paul Joppa