Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: AlNewman on September 08, 2024, 11:42:56 am
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I have a power transformer from an old Hammond organ I am trying to figure what it would work best for.
I have a schematic, where it has loads on both the B+ line and CT. Are these loads calculated in series?
So from my calculations I see around 133mA current on the B+ side, and approximately 120mA on the CT side.
Would that mean total current could be calculated as 253 mA?
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I have a power transformer from an old Hammond organ I am trying to figure what it would work best for.
The PT will work best for the number & type of tubes it supported in the original piece of equipment.
Except here, there was also a speaker field-coil in the mix, and that was used to generate fixed-bias voltages in a back-bias type circuit. You may be hard-pressed to get a similar output stage going without adding yet another transformer to develop a small fixed-bias supply.
The Golden Rule of Salvaging is keep the entire power-supply/phase-inverter/power-section intact (here, that also requires retaining the original field-coil speaker), and modify only the preamp as-needed to get a good guitar-oriented sound.
That's because as you strip away bits from the original circuit, you find the "cool old PT" delivers the wrong voltages, or is missing a bias-supply, or has much higher DC voltage output that you expected, etc. "Buying the right part" is usually cheaper in time & money than trying to make a salvage-part work right.
... it has loads on both the B+ line and CT. ... I see around 133mA current on the B+ side, and approximately 120mA on the CT side. ...
Look at the 5U4 rectifier: the current out the 5U4 plates is 100% equal to the current pulled through the PT center-tap. So there is "130-something mA" being delivered by the PT.
What you're not seeing is each one of the Voltage-nodes marked (+290 and -19 and +250, etc) have circuitry attached that is typically drawing some amount of current from that node. Further, probably none of the voltage-nodes are precisely the value marked on the schematic. Therefore we have to be cautious inferring much from voltage-drops we calculate from the schematic-figures.
It would help to know what-model Hammond organ this powered. But I would guess 325vac tube-rectified without a speaker field-coil will net about 400vdc to a power supply. 120mA seems like it would be right for something like a Deluxe Reverb; however, you need to add a transformer to get a fixed-bias supply to build that amp.
EDIT: See Merlin's post about adding a bias supply. --HBP
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I was looking at a diagram as attached, where it seems like the center tap and b+ are in series to make a circuit. The original power section was a push pull 6v6, with an LTPI, and about 20 pre amp tubes, and a field coil.
So with no load, the transformer with the 5U4 creates about 464V DC. It's just a regular transformer, what I'm trying to figure out is if the load through the center tap is in series with the load through the B+.
It weighs about 15lbs, so I'd like to get the most out of it. If a deluxe reverb is all I can get, so be it. I think I can find a way to get a bias supply if I need to.
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Actually, I answered my own question. The center tap could float, and it wouldn't make a difference as long as there's a reference to B+.
:icon_biggrin:
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for something like a Deluxe Reverb; however, you need to add a transformer to get a fixed-bias supply to build that amp.
A regular bias supply would also work (e.g. Deluxe 6G3), no need for another transformer.
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I tested, (quickly), with a 1500R load from B+ to B- and got 317V drop, or .211A.
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I tested, (quickly), with a 1500R load from B+ to B- and got 317V drop, or .211A.
A short circuit would produce even more current... I'm not sure what you were trying to establish?
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fwiw the 5U4B datasheet I looked at has 275mA as typical operation characteristics
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Edit: I'm an idiot
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:laugh:
i resemble that remark quite often
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I should have said I had a 317V drop across the resistors, so the B+ voltage dropped from 464-317V.
I would have left it longer, but I was running 60W through 30W of resistors.
It's ok to be an idiot, that's how we learn.
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that's how we learn.
:laugh:
I already lern't it, it's just the recall process after 40 years of learning other things that gets in the way
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... The original power section was a push pull 6v6 ...
That's 90% of the load on the PT, nearly always.
20 preamp tubes might still only be 10-30mA, and less than one of the two idling 6V6s.
... for something like a Deluxe Reverb; however, you need to add a transformer to get a fixed-bias supply to build that amp.
A regular bias supply would also work (e.g. Deluxe 6G3), no need for another transformer.
Thanks for the correction! I should have been thinking of the 6G3, Princeton amps, or a Marshall.
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that's how we learn.
:laugh:
I already lern't it, it's just the recall process after 40 years of learning other things that gets in the way
I learn something new at least once a day. And forget at least 2 other things.
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... The original power section was a push pull 6v6 ...
That's 90% of the load on the PT, nearly always.
20 preamp tubes might still only be 10-30mA, and less than one of the two idling 6V6s.
True. But they weren't all 12ax7's either. I've actually got the board here with most of the labeled tube sockets on it, maybe all. Quite a few high current tubes. Only 2 12ax7, 1 6c4, 1 6 bj7, 2 6ba6, 10 12au7, 3 12BH7. I'm not the one who salvaged the organ, but it could be likely there was also a leslie speaker piggybacked on the power supply. If those tubes were designed properly, and I wouldn't be surprised at all if Hammond designed their circuit properly, that compliment of tubes would eat up a lot of current.
The field coil would also draw a lot of current. I've used transformers from radios with field coils, and amps that ran at 250-280V on paper were well into 300V without the field coil.
It was stable with a 211mA draw at 317V. No heaters on the 6V tap though.
I'll have to go through the schematic and do some math, or stock up on resistors and do some more tests. It is a monster transformer, close to double the size of my super reverb. I want to put it to good use.
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Pic