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Re: Field coil replacement in Gibson BR-6

8/10/2004 9:56 AM
Wild Bill	Re: Field coil replacement in Gibson BR-6 There were two kinds of field coil speakers, as far as I know. One was used as a power supply choke and had a DC resistance of a few hundred ohms or less. The other had a resistance of several thousand ohms and was hung across the B+ to ground. Sounds like yours is the power supply kind, which is a lot more common. You don't have to worry about exactly matching the old inductance value - you just need a value that will work! Is this a choke input filter or a pi-net with a cap on both sides? You need to know this to choose a suitable choke value and rating. If it's a choke input then the choke will have to handle the entire current of the amp. If it's a pi-net with the plate tap on the input then the choke is after that point and the total current is the screens and preamp loads. What are the output tubes? Tube charts will give you an idea of how much current is involved, in either case. If it's a choke input then you need a minimum amount of inductance for it to work. The formula for critical inductance is: L= B+ divided by the current in ma. So for example if you have 400 volts and 80 ma of idle current then you need a choke of at least 5 Henries. Any thing smaller and the choke won't work. The B+ will rise like a cap input filter. Worse yet, when the amp starts to crank the current will rise high enough for the choke to snap into play and the voltage will nosedive from 1.41 x RMS to .9 x RMS, which will make the amp sound really weird! So figure out the critical inductance and then choose a choke of at least that amount or the next standard value that can handle the average current. I'd look at the tube tables to figure the max peak current and subtract the idle currents and then split the difference. The odds are you'll have to choose a choke rated higher than this figure anyway, 'cuz that's what's offered in the mfgr's catalogue. When in doubt go bigger but be reasonable. A choke rated for max peak current and inductance will be a big expensive monster that will eat up an unnecessarily large amount of chassis real estate. If the choke is after the 1st cap then choose anything bigger than 6-8 Henries good for the screen current. I use the Hammond 156G as a standard up to 50 watt circuits. It's rated at 9 H @ 40 ma of continuous current. The screen current bounces up and down but the choke never even gets warm. If I was choosing a choke for a production run it might be worth doing a bit more measuring to choose the cheapest choke available but for homebrew projects it's not gonna save you a beer or two so who cares? If you're running a pair of 6V6's and the choke is after the 1st cap/plate load a resistor is ok. I think a choke sounds better but maybe not enough to matter to most folks. If it's a choke input situation then you must use a choke or your B+ will be sky high and will perhaps blow the filters. Hope this is useful... ---Wild Bill

8/10/2004 9:56 AM

Wild Bill

Re: Field coil replacement in Gibson BR-6

There were two kinds of field coil speakers, as far as I know.

One was used as a power supply choke and had a DC resistance of a few hundred ohms or less. The other had a resistance of several thousand ohms and was hung across the B+ to ground.

Sounds like yours is the power supply kind, which is a lot more common.

You don't have to worry about exactly matching the old inductance value - you just need a value that will work!

Is this a choke input filter or a pi-net with a cap on both sides? You need to know this to choose a suitable choke value and rating.

If it's a choke input then the choke will have to handle the entire current of the amp. If it's a pi-net with the plate tap on the input then the choke is after that point and the total current is the screens and preamp loads.

What are the output tubes? Tube charts will give you an idea of how much current is involved, in either case.

If it's a choke input then you need a minimum amount of inductance for it to work. The formula for critical inductance is:

L= B+ divided by the current in ma.

So for example if you have 400 volts and 80 ma of idle current then you need a choke of at least 5 Henries. Any thing smaller and the choke won't work. The B+ will rise like a cap input filter. Worse yet, when the amp starts to crank the current will rise high enough for the choke to snap into play and the voltage will nosedive from 1.41 x RMS to .9 x RMS, which will make the amp sound really weird!

So figure out the critical inductance and then choose a choke of at least that amount or the next standard value that can handle the average current. I'd look at the tube tables to figure the max peak current and subtract the idle currents and then split the difference. The odds are you'll have to choose a choke rated higher than this figure anyway, 'cuz that's what's offered in the mfgr's catalogue.

When in doubt go bigger but be reasonable. A choke rated for max peak current and inductance will be a big expensive monster that will eat up an unnecessarily large amount of chassis real estate.

If the choke is after the 1st cap then choose anything bigger than 6-8 Henries good for the screen current. I use the Hammond 156G as a standard up to 50 watt circuits. It's rated at 9 H @ 40 ma of continuous current. The screen current bounces up and down but the choke never even gets warm. If I was choosing a choke for a production run it might be worth doing a bit more measuring to choose the cheapest choke available but for homebrew projects it's not gonna save you a beer or two so who cares?

If you're running a pair of 6V6's and the choke is after the 1st cap/plate load a resistor is ok. I think a choke sounds better but maybe not enough to matter to most folks. If it's a choke input situation then you must use a choke or your B+ will be sky high and will perhaps blow the filters.

Hope this is useful...

---Wild Bill

Replies:

magamps Bill,I believe that the for... -- 8/10/2004 1:47 PM