Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Baguette on December 19, 2010, 04:42:30 pm
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So, I have this amp much like a Tweed Deluxe with 1 tube reverb. It was working perfectly until the reverb died silently. Did some checking and the culprit seems to be the reverb transformer (it's a regular Fender clone by Hammond, 22K / 8R). I measured the primary winding resistance and read 40 ohm. Did the same measurement on a brand new similar transformer and read a 1K resistance at the primary. I concluded the transformer heated and shorted.
The reverb circuit in the amp is much like the usual 1 tube reverb floating around. 320VDC B+ feeding the red transformer wire, blue wire to the plate of the AT7 side of a 12DW7, 1.5K cathode resistor. Nothing that seems harmful to my untrained eyes.
How comes the transformer died? Is it a common issue? Is there something wrong with this design?
I wouldn't want to put in a new transfomer and see it dying again.
Thanks for shedding some light!
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> the AT7 side of a 12DW7
There's no 12AT7-like side of a 12DW7. Which side is it?
> 1.5K cathode resistor
What's the voltage on that? (It should still bias-up even with the shorted RT; or a 1K resistor where the RT was.)
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He means the 12AU7-like side. Best way to test is with a new transformer jumpered into the amp. But first, can you post the tube voltages?
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> the AT7 side of a 12DW7
There's no 12AT7-like side of a 12DW7. Which side is it?
> 1.5K cathode resistor
What's the voltage on that? (It should still bias-up even with the shorted RT; or a 1K resistor where the RT was.)
Yes as TinyDaddy said I was meaning the AU7 side (low mu). The AX7 side is the recovery stage.
Here is the voltage I got with the dead transformer:
- 13V dropped on the 1.5k cathode resistor
- 320V on the plate
current flowing at idle = 13 / 1500 = 8.5mA
ipd = .0085 * (320 - 13) = 2.7W
The low mu side of the 12DW7 has a rated max ipd of 3.5W IIRC. The tube seems to operate within specs (and the tube still works fine BTW).
I just can't figure out why the little transformer died.
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Sometimes the transformer goes bad, usually because of no secondary load, same as the speaker transformer. The usual reason is, someone did not want Reverb so they unplugged the reverb tank.
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Yeah when you pull out the "To Pan" cable, you should also pull out the driver tube, or pull out the "From Pan" cable only.
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That's right. But the OT secondary was loaded all the time.
:icon_scratch:
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have you tested the reverb pan? it is possible the input transformer on the pan let go.
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Yes, good chance of that input coil being open. I have five reverb tanks in the shop all with open input coils. Its a shame someone dosent make replacements.
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The tank is ok.
The culprit IS the reverb transformer which has a shorted primary (reads 40ohm resistance on my DMM, whereas a new one reads 1k).
All I am wondering is wether the transformer is safe in this circuit I described above (which it seems to be) or not.
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> 8.5mA
Should be safe.
If you are spooked, raise cathode resistor to maybe 3.3K or 4.7K.
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Thanks PRR, will do that.
I'd rather have weaker reverb than an amp that blows transformer every month.