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Field coil replacement in Gibson BR-6

8/9/2004 4:25 PM
Keith	Field coil replacement in Gibson BR-6 Howdy, I'd like to replace the old field coil speaker in my 1950-ish Gibson BR-6. The speaker is a Rola 10" and is in pretty rough shape (several tears, really wimpy sounding). My question is dooes anybody know offhand what the inductance of the coil is so I can install a choke of similar value? I did this replacement on another BR-6 for a friend several years ago and just used a resistor of the same value as the coil, with no choke. That amp just screams, and is still working fine. No increased hum was apparent. Was this OK, or should I really use a choke? BTW, I don't own an inductance meter, or I'd just measure the thing. Thanks in advance, Keith

8/9/2004 4:25 PM

Keith

Field coil replacement in Gibson BR-6

Howdy,

I'd like to replace the old field coil speaker in my 1950-ish Gibson BR-6. The speaker is a Rola 10"
and is in pretty rough shape (several tears, really wimpy sounding). My question is dooes anybody know offhand what the inductance of the coil is so I can install a choke of similar value? I did this replacement on another BR-6 for a friend several years ago and just used a resistor of the same value as the coil, with no choke. That amp just screams, and is still working fine. No increased hum was apparent. Was this OK, or should I really use a choke?

BTW, I don't own an inductance meter, or I'd just measure the thing.

Thanks in advance,

Keith

8/10/2004 9:56 AM
Wild Bill	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

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 1:47 PM
magamps	Bill, I believe that the formula for critical inductance in the way that you've set the formula would be: L= B+/I*1130 Henrys with current in Amps. For calculating critical inductance in a 60Hz full wave rectifier circuit, the formula is: Lmin = Rtotal/1130, Where Rtotal is the sum of all series resistances in the load side of the circuit. Typically, we'll use the "load resistance" of the transformer secondary...straight Ohm's Law... for that figure unless there is a lot of other R series floating out there. Mike

8/10/2004 1:47 PM

magamps

Bill,
I believe that the formula for critical inductance in the way that you've set the formula would be:
L= B+/I*1130 Henrys with current in Amps.

For calculating critical inductance in a 60Hz full wave rectifier circuit, the formula is:

Lmin = Rtotal/1130,

Where Rtotal is the sum of all series resistances in the load side of the circuit. Typically, we'll use the "load resistance" of the transformer secondary...straight Ohm's Law... for that figure unless there is a lot of other R series floating out there.

Mike

8/10/2004 8:00 PM
Wild Bill	Hi Mike! Never seen your formula before but it looks to be a bit more precise than mine! That factor of 1130 with Henries will give a slightly smaller end value than 1000, which is the factor in my formula to use milliamps. Using ma is easier to work out in your head and since your answer will be slightly larger it will always work. A technician's rather than an engineer's formula, if you like! You're looking to choose a stock choke from a catalogue rather than having one made to your design. My formula was found in the 1967 edition of the American Radio Relay League Ham Operators Handbook. I've kept it all these years for the great tube info it contains... ---Wild (and a bit sloppy) Bill

8/10/2004 8:00 PM

Wild Bill

Hi Mike!

Never seen your formula before but it looks to be a bit more precise than mine!

That factor of 1130 with Henries will give a slightly smaller end value than 1000, which is the factor in my formula to use milliamps. Using ma is easier to work out in your head and since your answer will be slightly larger it will always work. A technician's rather than an engineer's formula, if you like!

You're looking to choose a stock choke from a catalogue rather than having one made to your design.

My formula was found in the 1967 edition of the American Radio Relay League Ham Operators Handbook. I've kept it all these years for the great tube info it contains...

---Wild (and a bit sloppy) Bill

8/11/2004 1:52 PM
magamps	Hey Bill, Mine came from Practical Transformer Design Handbook. I didn't use the one that I normally use because it is even more convoluted....but sure enough...it matches yours for 60 Hz IF it includes "a 25% safety factor" according to Reference Data for Radio Engineers (4th Edition) ...although you're really trying to filter 120 Hz rather than the 60 Hz fundamental. Lmin = K/f*Rload Henry K = 0.060 for full wave single phase 0.0057 for full wave 2 phase 0.0017 for full wave 3 phase Rload = the sum of ALL resistances on the secondary side to include rectifiers, ESR of caps, load and winding DCR. Mike ( a bit anal engineering type)

8/11/2004 1:52 PM

magamps

Hey Bill,
Mine came from Practical Transformer Design Handbook.

I didn't use the one that I normally use because it is even more convoluted....but sure enough...it matches yours for 60 Hz IF it includes "a 25% safety factor" according to Reference Data for Radio Engineers (4th Edition)

...although you're really trying to filter 120 Hz rather than the 60 Hz fundamental.

Lmin = K/f*Rload Henry

K =
0.060 for full wave single phase
0.0057 for full wave 2 phase
0.0017 for full wave 3 phase

Rload = the sum of ALL resistances on the secondary side to include rectifiers, ESR of caps, load and winding DCR.

Mike ( a bit anal engineering type)

8/11/2004 7:49 PM
anonymous	Now Mike, a real "plumber" would build a HUGE amp that needed a 3 phase supply, just to use that particular constant in the formula -'cuz it's neat! Years ago at McMaster U. all we artsies scorned our own parties in favour of the engineers. Much more impressive and way more fun! ---Wild Bill

8/11/2004 8:00 PM
Magamps	LOL....actually even for smaller amp...3 phase would be pretty cool. A lot less filtering required because there is a LOT less ripple to deal with. HMMMMMM..... OK...so we start with 60 Hz single phase and "fake" 3 phase with a Scott T connection...then into a 3 phase power transformer.... I'm gonna do some research.... BWAHAHAHAHAHAHAHA Mike

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