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first post... please be gentle ;)
I have a mid-30s era guitar type amp that i'm trying to restore. i *think* the original speaker had a coil @ 1500 ohms. i'm not having an easy time finding an exact match. i have located a unit that specs @ 1K. I presume this will "work", but what is the down side of this mismatch?
any feedback greatly appreciated.
Follow Ups:
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The voltages to the driver tubes will be a little bit higher. Not too much given that the 500 is small compared to the other resistors that go after the FC. Even if the 56 and 57 draw 20mA together, which I doubt, that would only be 10V.
Like was suggested you can just add that much in series with the FC if you are concerned about it but you probably get more variation with different rectifier tubes and changes in line voltage.
You also need to consider how much current the FC needs to operate properly. The 50K to ground after the FC could be adjusted to set that with the caveat that the power supply might be strained if the new FC needs a lot more current than the old one. If that is the case you might consider a separate supply for the FC and put a 1500 ohm R in it's place.
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You might add 500 ohms to the field to get a match.
If the field goes across the plate supply with a dropping resistor, you just might add a little resistanct to make your driver match.
Do you have a schematic, or know where the field coil goes in the amplifier?
let's see if this will work. i believe this one is most like mine.
I have similar amps in my main system - autoformer phase splitter, 6B4G's (6.3v version of 2A3). I had them at church (nice big hall) for a demo on some JBL's, and some git-tar picker decided to run his Martin through one.
The tone! You should have a winner there.
no idea how it's going to sound with the pie tin over the speaker.
I may have to consider building up something using 6A5 types as i'm not very familiar w/ pre-50s era stuff & continue to read good things... and there must be a reason 2A3s are so expensive !
so... it appears that installing a field coil speaker is doable as per the above, but now there is another wrinkle. one example of an original speaker is pictured above. at first I thought the transformer was an OT. turns out to be a choke. how would I determine the value for this piece of the puzzle ?
as always thanks in advance.
jerry
(should read 6B4 v 6A5)
I have no real experience w/ FC's, and will defer to the judgement of others. Probably the easiest thing is to put a resistor in place of the FC and use a modern permnanent magnet speaker.
Sovtek 2A3's are a bargain, actually cheaper than their 6B4G's by a bit. Not sure about NOS 6B4G's, but they're not cheap. Your amp is built for 2A3's, would take modification to use 6B4G's. Just use less expensive Russian or Chinese 2A3's.
Wow. Cool amp.
If there is no dropping resistor you should add one so that the resistance is about the same. With a field coil speaker the field coil is part of the power supply as well as part of the speaker, as you can see from the schematic. The voltage is probably going to run high anyway due to the higher mains voltage we see in our homes nowadays (125 VAC). Without the proper resistance in the field coil the B+ would be even higher.
If you ever find the exact right field coil speaker with the right resistance on the field coil you can remove the extra resistor.
if you think THAT's cool, get a load of the cab & speaker cover!
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