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Old 12th May 2005, 01:01 PM   #1
krzanik is offline krzanik  United States
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Default What's a Field Coil Amplifier?

I recently purchased a "HAMMOND H-AO-40-1FIELD COIL AMPLIFIER" from ebay. The tubes are: eight 6V6's, two 6SN7's an two- 5U4G's! So it appears (I have not recieved the amp yet) that there are two completely seperate (dedicated rectifier and power supplies) channels on the amp. The seller states that the amp has been tested and works "perfectly". It appears to be circa 1940-1960 era. I was wondering if anyone has had any experience with, or information about, field coil amplifiers? Is this a type of push-pull topology? Any info is appreciated as I could not find anything on the forum or on the web. Cheers. Dylan.
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Old 12th May 2005, 03:16 PM   #2
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A Field Coil Amplifier is one which is designed to power loudspeakers which use electromagnets rather than permanent magnets.
Many times, the loudspeaker's electromagnet (field coil) serves double duty as a power supply choke.

These amplifiers are useable with regular PM loudspeakers if you install a choke in place of the loudspeaker field coil.
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Old 12th May 2005, 04:37 PM   #3
krzanik is offline krzanik  United States
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Default Field Coil

Frank, Thanks for replying, I was starting to think I asked a really strange question. I was wondering if you had any advice on determining the size (Henries and Ma) choke to use in place of a electromagnetically driven speaker. I assume the choke would be placed in the power supply section, like in a Pi filter? I don't want to screw up and damage my speakers. Also, you said "Many times, the loudspeaker's electromagnet (field coil) serves double duty as a power supply choke". Does this imply that the amp may already have power supply chokes (I ask this because it looks like, from the picture, this amp already has a couple of chokes on it). So if I determined that this amp already has chokes in the power supply do you know how I would determine if I can use my (fixed magnet) speakers directly with the amp. I have a couple of old drivers I pulled out of a junk amp for just these types of experiements, so I could hook these up to it and if they get fried it would be no big loss. Lots of questions... sorry about that. Thanks again. Dylan.
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Old 13th May 2005, 09:23 AM   #4
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Model number would suggest that this unit comes from an early organ console or speaker cabinet although I can't find the A-40 listed anywhere. These units normally powered a number of speakers of different sizes with separate channels for reverb etc.
It's likely that your unit also supplied power to the rest of the organ which would explain the pair of 5u4s. If this is the case, you need to watch that ht rails don't go excessively high due to reduced loading.
I suspect that this amp is more likely to be configured as 4 channels (pairs of 6v6 in pp) than a monster ppp stereo affair. The number and size of the output transformers will tell all. Unfortunately, you might find that only one channel is full-range though. Good luck and let us know what happens when it arrives
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Old 13th May 2005, 09:28 AM   #5
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It's possible that the amp uses one of the 5U4 tubes to power the field coil. If that's the case, you can simply remove the tube and connect your speaker to the secondary winding of the output transformer. If the field coil is part of the power supply, you can wire a choke (10 henries or so) to take the place of the field coil.
You should trace the circuit from the amplifier's output connector(s) to find the pins that connect the output transfromer to the voice coil.
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Old 13th May 2005, 02:55 PM   #6
krzanik is offline krzanik  United States
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Default Field Coil

Thanks to Martin and Frank for the replies. You've given me a lot to think about. I definately have some research ahead of me, but I like that. Once I have the unit I am going to do my best to draw a circuit diagram of it (if I can't find one on-line). Once I know what transformers etc., have, I can determine whether

(a) I can reasonably addapt the amplifier to fixed magnet speakers, or

(b) wether I should use the parts to built something like a s5U4 rectified single ended 6V6 amp. I found a schematic for a SE 15 watts per channel amplifier that uses 4 6V6's per channel. It looks interesting.

I'm slowly finding some information on-line about converting field coil ampls to fixed magnet. I wonder about the sonic performance of a converted amplifier though. Thanks again and if anyone has additional information please post it. I appreciate the help. Thanks.
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Old 13th May 2005, 04:31 PM   #7
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I think your amp might have come from a Hammond H40 or HR40 speaker cabinet with the later single chassis type amplifier.
model info here
If that's the case then you are the lucky owner of a nice amp with serious audio potential. These amps did use 4 6v6 per channel in a ppp configuration with one 5u4 each for power and would have decent transformers for full range audio.
Possible schematic here but this is for the two chassis version
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Old 13th May 2005, 10:12 PM   #8
krzanik is offline krzanik  United States
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Default Filed coil

Martin, thanks for the schematic link. I believe Figure 19B is the amp. The "top view of the amplifier" provided looks to be a match for the layout of the one I purchased. The diagram indicates the same number and location of tubes. If this is confirmed (when I get the amp) you have saved me a lot of time and effort. Your mention of "serious audio potential" is what I am hoping for. If this is the amp I bought, it appears that there are two (one 6 and one 10 Henry) chokes in the power supply section which, according to an earlier reply, would make adapting to fixed magnet speakers easier (and less expensive).

What is the deal with the two seperate out puts, one for bass and one for treble though?
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Old 14th May 2005, 03:49 AM   #9
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Sorry, I missed that one - it would appear that there is some sort of active crossover thing going on and that the treble channel has a smaller output transformer and different operating points for the output tubes.
I guess you might be looking at replacing the output transformers with full range items after removing the filter network between v2 and v3. Worth a go as is though - the transformers might even be the same type.
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Old 14th May 2005, 04:22 PM   #10
krzanik is offline krzanik  United States
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Default Schematic thoughts

Quote:
after removing the filter network between v2 and v3
Yes, the reverberation curcuit will definitely go!

Any thoughts on this schematic?

6V6 SE

Double parallel (4-6V6s per channel) single ended amplifier. 15 Watts per channel. It seems like the only thing I would have to buy are the output transformers and new caps. I could adapt the power supply to tube rectification via the 5U4s and use the chokes in a pi filter configuration. I wonder how it would sound? If anyone has any thoughts on the curcuit please let me know. Thanks
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