Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Diverted on August 17, 2024, 07:54:51 am
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Hi all, could use some advice.
I have a very old PP 47 amp here (Type I Rickenbacher) that has a bad OT. One half of primary reads about .5M.
From what I've read I'm assuming it has somewhere around 10K primary resistance. It also has a third tap on the secondary for the humbucking coil. 4 ohm voice coil.
I am trying to source a replacement and don't have much. But I do have a PP Magnavox OT from a 6V6 amp that I would guess has around 6-8K primary impedance.
Can I get away with this? I realize I will have to forego the humbucking coil without that extra tap on the secondary.
The 47 tube data sheets don't say much about 47 tubes in PP.
Thanks!
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I became addicted to schematics in '72 when TTL was born and bits were the cool kid on the street :icon_biggrin:
without an original wiring diagram showing Field 1, Field 2, and the relevance to the whole circuit, i'm gonna pass on the guess.
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I became addicted to schematics in '72 when TTL was born and bits were the cool kid on the street :icon_biggrin:
without an original wiring diagram showing Field 1, Field 2, and the relevance to the whole circuit, i'm gonna pass on the guess.
Field 1 and 2 is just the two leads off the FC.
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you're using a field coil or you've replaced with Resistors?
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I am running it with the field coil and original speaker. Replaced OT.
The voltages shown in the schematic are current, with FC speaker being used.
I previously had been running it with a different OT and resistors in place of field coil. There's no difference in volume now with the repaired original speaker than there was when I was using a 10K primary OT. Thanks.
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Why guess at the primary impedance of the replacement OT, rather than checking its ratio?
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Is it strange there's no grid leak on the preamp tube?
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:laugh:
it's all sorta strange.
but ya, i'd probably want the input cap coupled to removed any DC and I'd want the grid to be referenced to ground.
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Is it strange there's no grid leak on the preamp tube?
In most scenarios, the signal source that's plugged into the amp input provides a suitable grid leak resistance.
Bear in mind EVERYTHING was expensive back than; if a part was a 'nice to have' then to reduce cost of parts and assembly, it might get omitted.
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It is funny how things have changed, they were trying to save money on a couple resistors, and added another few lbs in transformers.
The humbucker coil is kind of a weird thing. So it's 4 Ohm, and your speaker is 8 Ohm?
There are quite a few push pull transformers in the 10-25k range, with multiple taps and fairly reasonable value. I wonder if it's possible to manipulate those to work with what you have. I would think 6-8k is kinda light for push pull, considering a single tube likes a 7k load. But it probably isn't being used at maximum efficiency, so maybe it's not a big deal.
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I can't get my head around a humbucking coil on the OT :dontknow:
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:laugh:
ya, me neither.
think it musta been of of those "special sauce" things from the 40's
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I wonder if it's more like a humbucking coil for the field coil?
I have seen videos with an extra coil on guitars with single coils or P-90's where they're just an extra coil inset in the back near the pickup. I'm not sure if they're wired through the guitar electronics, or they have a battery connected?
Your guess is as good as mine.
:w2: