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Old 20th April 2013, 05:40 AM   #11
Enzo is offline Enzo  United States
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Not sure what you are warning, though I agree the speakers need to be dealt with.

The voice coils are typical 8 ohm voice coils, and the amp has no idea what sort of magnets are on them.

But the power supply does expect to see the field coils - and the resistances are given. Without that 5k resistance from B+ to ground, the voltages will be way off, and the smaller one rides on the underside of the filter caps.

I used to rebuild old juke box amps, and enough of them had field coils that I still have a large 50w 5k resistor with clip leads for the purpose of substitution for the fielde coils while the amp was benched. That was the fix for replacing the FC speaker with a permanent magnet type, we added a power resistor in the chassis.
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Old 20th April 2013, 04:41 PM   #12
scrimpu is offline scrimpu  United States
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For now I am using the original speakers. They are almost 70 years old so I will be modifying it for permanent magnet speakers eventually. Does anybody here happen to know the power rating of the Jensen A-12 series speakers?
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Old 20th April 2013, 04:47 PM   #13
JMFahey is offline JMFahey  Argentina
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Very low, safe with 10W , would not surpass 15W under *any* circumstance and even less with a guitar.
Offer them in EBay and buy others.
As is they are worth some cash; once blown they are junk, even if reconed.
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Old 20th April 2013, 06:24 PM   #14
cancon is offline cancon  Canada
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The voice coil 10W, but the field coil...20W at least.

Quote:
Originally Posted by Enzo View Post
Not sure what you are warning, though I agree the speakers need to be dealt with.
Sorry, the words 'field coil' were lost somewhere in the ether...I meant to say that the power supply expects to see a field coil load, and often the field coil was doubled as an additional choke. When adding a resistor to load the field coil supply one should be careful about wattage choice and mounting style - it will get very hot.
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Old 20th April 2013, 06:42 PM   #15
scrimpu is offline scrimpu  United States
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Would I need to make any changes just connect it to the PI of an existing tube preamp?
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Old 30th April 2013, 05:52 PM   #16
scrimpu is offline scrimpu  United States
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The Hammond preamp is very strange, it looks to me like the 6SN7s are used as a PP output not a phase inverter. The signal is inverted in the previous stage. So now I think Ill keep the power amp circuit as is. Gut most of the preamp leaving V5 and the output/interstage transformer and use the left over tubes/socket as a preamp an PI for V5. Any obvious faults in this plan of action?
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Old 30th April 2013, 08:46 PM   #17
JMFahey is offline JMFahey  Argentina
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Fine.
In post #9 I suggested you consider that point where you injected your guitar the "original power amp input", although it physically lives in the "preamp" and feel free to do whatever you please whith what's "to the left of it", in the Preamp drawing, of course.

Would be the most practical combination: keep that killer Hammond power amp and add a "Guitar" type preamp.
One of the old Tweed Fender types, with 2 "channels" (4 inputs) "bright" and "normal" , 2 volume controls, 1 single tone, will probably be a *loud* mean killer Blues machine.
You want Metal?
Add some pedal or Pod in front.
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Old 30th April 2013, 09:28 PM   #18
scrimpu is offline scrimpu  United States
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Injecting into the original input gave me a very weak and distorted output, almost no gain at all. The input signal from the organ was way higher than a guitar. Would I need to make any changes to the power amp or can I leave it as is and use it with say a fender 5E3 pre and PI? Maybe a MV between them? And yes, a *loud* mean killer Blues machine is just what Im after.
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Old 1st May 2013, 01:24 AM   #19
JMFahey is offline JMFahey  Argentina
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Oh, that's (bad) news.
I understood you got low volume but good sound, sorry.

I wanted to keep it as simple as possible, and having an already working power amp meant saving a lot of workm but if it's not so, my personal decision (others may differ, of course) would be rather that endlessly chase our tails, strip it and straight build something tried and true.

A full 5E3 comes to mind, since you like it, with only 2 mods:

1) getting a "4x6V6" schematic is easy, just take a "2x6V6" one and double the number of tubes.
You already have the iron

2) to simplify things, I'd go for a cathode biased power amp.
Of course you'll need to halve the cathode resistor and double its power rating.

3) one doubt is about what actual power voltage you will have, since the original PSU is unconventional (and that's an understatement).

a) the transformer shows 2 x 430VAC windings ... yet it shows only 300 VDC +B
Well, it's a choke input PSU so DC voltage is close to *average* voltage, not peak .
Anyway that should give us around 400V +B .... how come it shows only 300?

b) fact is, it's wired as sort of a split supply, those tubes *are* cathode biased after all.
the cathode resistors are the 250 ohms fielld coils, which drop about 55V, so actual +B is around 360V.
See that the filter cap (C5/C6/C7) negatives are *not* grounded but "floating".
They are wasting 55V just to feed those electromagnet coils.
I would use "normal" (permanent magnet) speakers, wire a conventional (still choke input) PSU to get good 400V to ground, disconnect those cathode feedback taps at the output transformer and build a 5E3 type preamp and PI in those 2 octal sockets.
What do you think about that?

Just to feed my curiosity, please measure what voltage you have across the main filter caps, and across field coils.

And if you want to still use those field speakers, I'd build a small, extra 55V to 60V auxiliary PSU and stick it somewhere, just for the field coils.

That amplifier certainly was designed and built with the best of the best, spending whatever was necessary.
Kudos to Mr Hammond !!!!!
.
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Old 1st May 2013, 02:00 AM   #20
scrimpu is offline scrimpu  United States
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I was kinda hoping I could leave the power amp as it is and build a separate preamp as I have piles of old organ amps to work with already. I have not checked voltages, Im waiting until I replace the filter caps before powering it up again. I do plan on replacing the field coils as well which if done right should bring the B+ up a bit. Cathode bias is also a good idea since it sort of already is. Would cathode bias require well matched tubes since they would share a bias reistor? Half the tubes have been replaced over the years. This thing dates to 1946.
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