Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: acheld on February 10, 2023, 12:54:50 pm
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Greetings everyone. This has probably been answered before, but I can't seem to find a generic answer, so . . . And all of my assumptions are suspect . . .
I need to build a standalone preamplifier with 5 12AX7 and one 12AU7 tubes.
The filament spec is simple, 6.3 VAC@2.1A.
The 12AX7s should be run at about 250 VDC, and the 12AU7 lower at 200VDC.
It's not so clear to me the anode current -- somewhere in the range of 3-5mA for the 12AX7s and about 20-25mA for the 12AU7 (?). So the VA spec for the HT side should be about 10VA (250VDCx.04A)?
Using the above, I should be looking for a transformer with 10VA on the HT secondary, but at what AC voltage?
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what type rectifier?
choke or CAP/resistor after rectifier?
these each add their own math to the equation
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You want buzz? Then you want filtering. Large resistors are far cheaper than large capacitors. So it is economic to allow large voltage drop in your filtering. RDH suggests 20%. But you are actually going to use the transformer you can buy, which is likely even higher. So find a transformer and then figure the filter drop to make your "magic voltages".
250VDC plus 20% drop is 300VDC, which with solid-state rectifier needs at least 212V AC. (Not a common part, so expect to be blowing much more than 20% in filter.)
> about 10VA
The AC VA will be higher than DC Watts because cap-input rectification is abusive. But this will probably vanish in the overall budget and nearest-available-part process. (If you then want DC heaters the AC-to-DC correction may matter.)
AND-- you can buy 110V-240VAC to 12V DC regulated supplies VERY inexpensively. Also 12VDC to 300V DC step-up units.
> choke...
It would be unlikely to use a choke for this small load. It would need a LOT of Henries to do any good, and today fat Aluminum (caps) is cheaper than fat iron (chokes).
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I need to build a standalone preamplifier with 5 12AX7 and one 12AU7 tubes.
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It's not so clear to me the anode current -- somewhere in the range of 3-5mA for the 12AX7s and about 20-25mA for the 12AU7 (?). ...
One 12AX7 stage will pass around 1mA:
- Think about your probable supply volts, your desired plate voltage, and your planned plate load resistor
- 250v supply, 150v on the plate? With a 100kΩ resistor? 250v - 150v = 100v, 100v / 100kΩ = 1mA
5x 12AX7s at 1mA per triode is 10mA.
Your 12AU7 will probably pass about 1mA as well.
- Yeah, the data sheet (https://frank.pocnet.net/sheets/093/1/12AU7A.pdf) shows 10-20mA is possible.
- But you have a 100kΩ resistor between the plate & that 250v supply: 250v / 100kΩ = 2.5mA
- The supply voltage & plate load resistor dictate that you can't reach 3mA, much less something higher.
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This is very typical of g-amp design. This is 410V supply and you aim lower.
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Great replies.
This pre-amp will be SS rectified for sure. (KISS !). And no choke for this . . .
The truth is that my main design philosophy has been to lift bits and pieces of amp designs from here and there, and there is always a PT to fit the scenario.
For this project, I went out and bought Merlin's "HiFi" preamp book -- but while there is a lot of great information there, it is not as practical as his guitar amp or power supply books.
LOL, I'm sure there is a tranny out there that meets my need -- I just don't know which one it is.
Thank you! Each one of your answers was helpful. I'll post a schematic once I've actually done it, though not sure how interested folks are in this forum about hifi gear (which I'm building for daughter and SIL).
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Any powertransformer from a radio with single ended audio power stage would very probably work and give leeway to your filtering, too. They are good and cheap.
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Only be caute and verify your donor radio didn't used an autotransformer instead of a safe transformer with separated primary and secondary windings
Franco
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This Weber PT (https://www.tedweber.com/wrvbpt/) is perfect for your project. Cheap too! I used it in my Revibe (http://sluckeyamps.com/revibe/revibe.pdf). Simply juggle the dropping resistors until you are happy.