Further questions on 1952 Gibson BR-9 amp

[Resonator Guy]

I have continued working on this amp while awaiting parts. I have it sounding pretty good with a 10" speaker hooked up to the original OT with the old non-functional 10" Jensen field coil still hooked up.

I was planning to replace the field coil with a 1000 ohm, 10 watt power resistor. I have done this already with a 1952 Gibsonette and a 1948 BR-9 and both work well, although the Gibsonette does hum a bit, as the field coil operated as a choke.

I saw in another thread that a choke suitable for a Deluxe would work instead of the power resistor and add some hum reduction. Antique Electronic Supply, where I get most of my parts, has such a choke rated at 50ma, 1500 ohms, 4 Henries. How does one determine if this is suitable as a replacement in one of these old Gibson amps since it has greater resistance than the power resistor? In each case the original speaker was absolutely shot and needed updating.

I did manage to get one that actually works, a 56 GA-9, so 25% I am guessing is a pretty average ratio. I am older than these amps, and I hope to keep working better than their speakers do.

[Enzo]

Did you bump the filter caps up to more modern values? That too would improve the hum level and be a lot simpler than a choke.


None of this is rocket surgery. 1000 ohms, 1500 ohms. COnsider the current flowing through the things. At 1000 ohms, each milliamp through it drops a volt across it. 5ma 5v, 10ma 10v, etc. At 1500 ohms, each milliamp through it drops 1.5v, so 5ma 7.5v, 10ma, 15v.

Now if your preamp tubes are colectively drawing 10ma from B+, and the B+ is 300v, does it really make much difference if the voltage at the other end of the resistor is 285 instead of 290? That is about the same difference as occurs when your mains voltage moves up or down 2 volts.

Put it together with the resistor, and upgrade the filter caps, then if it still hums, you can add a choke later.