Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dsones on October 19, 2024, 07:25:18 pm
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Does anyone know if a hammond 290ax transformer can handle a 12bh7 tube. I build a princeton micro amp that uses a 12au7 that has a heater current of.3ma and 6.3V. The 12bh7 draws .6ma of heater curren and 6.3V. Some of the info I've read says you can substitue a 12bh7 for a 12au7 but you need to make sure the power transformer can handle the increased heater current. The spec for the hammond 290ax says 6.3V 2.5amps. Thanks
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Its the total current demand of all the 6.3V heaters in your amp that determines whether the PT can handle the load. There will be an amperage rating @ 6.3V published by Hammond. You can total up the demand of each one of your tubes with 6.3V heaters. Chances are good that there is enough headroom to sub that tube.
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The Hammond 290AX has a 6.3V winding good for 2.25A. 2X6V6 (.45A apiece) draw .9A for the pair. 3X12AX7 (.3A apiece) draw .9A, so there's 1.8A of filaments taken care of. Don't forget the pilot lamp with the jewel lens draws .15A if it's on that winding. That's way more than enough iron for that little circuit. Try 290WX, that's the tranny for a 6G15 Fender Reverb
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Here is some useful info.
with respect, Tubenit
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desones, I suggest you find a real 12BH7. The JJ 12BH7 are the same size as standard 12AU7, and don't have the long plate construction like the originals. The new JJ ECC99 might also be considered for your application.
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without the power tube plugged in the heater voltages are 6.3v. Once I plug in the power tube (12au7) the heater voltages jump up to around 10v. Any ideas what would cause this jump?
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How are you measuring? How many tubes? What tubes?
Don't measure from tube pin 1 probe, other probe ground. Put 1 meter probe to pins 4/5 and the the probe to pin 9.
Also try taking out all of the other tubes and put in only the 12AU7, now what's is read?
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I changed tubes to ones I know are good and I still have a problem. On the 6.3v heater circuit I'm getting 9vac and 17vdc. I get these readings at the pilot light and the tubes. I don't get it.
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start with a handful of batteries, 1.5vdc, 12vdc.....VERIFY your meter is "calibrated" for VDC. now switch to VAC and measure a handful of "wall-warts" that produce VAC... 14vac, 9vac...wall VAC. once you are confident in your equipment, then take out the AU7 and do as suggested above, measure at the "tube side of the socket" the heater pins voltage.
is this a DC lifted affair?
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Multimeter is accurate. With power tube in I get around 17vac. Without power tube I get what is expected 6.3v.
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I changed tubes to ones I know are good and I still have a problem. On the 6.3v heater circuit I'm getting 9vac and 17vdc. I get these readings at the pilot light and the tubes. I don't get it.
You didn't answer the questions I asked you.
Somethings not right. You can't get 6.3 with no tubes in and 17vac with the 12AU7 in.
Can you post some pictures of the amp?
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is this a DC lifted filament circuit????
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I'm trying to understand how an increase could happen :w2:
Double check that your meter is set to measure AC voltage for all filament measures.
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Try brand new batteries.
We've seen strange meter reader readings because of weak/low batteries several times.