My '73 Bassman 50 went up in smoke, and upon inspection, the leads to the pilot light and the 100 ohm resistors were charred.
I bought a Hammond replacement transformer PT290EX but it has an extra wire, a green/yellow wire. It seems like I was sent a 290DEX with a 290EX sticker on the transformer.
Please correct my assumptions. This green/yellow wire is the center-tap for the heater/pilot light circuit. All I have to do is wire it to ground.
Does this mean I do not have to replace the 2x 100 ohm resistors coming off the pilot light? I can go without the 2x 100 ohm resistors?
Can I wire this GRN/YEL to ground with the RED/YEL wire?
Also, how short do I cut the excess length of the wires coming off this new transformer? I twisted the two reds together and the two greens together, but they're still long.
And I guess the most important question: Why would my amp go up in smoke in the first place?
The following are the mods done on the amp:
The Bass channel was modded to a 2204-like cascading preamp circuit.
The Normal channel was modded to a simpler AA164 preamp that only uses two triodes.
bonano wrote: ↑Mon Jul 15, 2024 4:36 pm
...
Please correct my assumptions. This green/yellow wire is the center-tap for the heater/pilot light circuit ...
It's a moment's work to check the veracity of that assumption.
Note that the 290DEX is intended for a 240V mains.
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When your Pt smoked did it blow the mains fuse, or did the two resistors just burn up?
Either way did you pull all the tubes out and then see if you still had filament voltage on pin 2 and 7 of either output tubes?
When those two 100 ohm resistors burn up 99% of the time you have a shorted output tube that took them out.
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If the wires between transformer and pilot lamp assembly have overheated, but the wiring from the lamp to the valve socket looks ok, then the pilot lamp assembly seems the most likely suspect.
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If you end up needing to use the hammond and want to verify if the green/yellow wire is heater center tap, meter both green wires for ohms. Then meter one green wire and the green/yellow wire. If it's half the ohms of both green, then it's the center tap. Ideally you'd cap it off with heat shrink and use two 100 ohm resistors at the lamp ,as an artificial center tap , for a bit more saftey. Or check out the rob robinette of elevated heaters.
Like was mentioned above. You.should verify the original transformer is OK.
Vince- wrote: ↑Thu Jul 18, 2024 2:05 pm
... If it's half the ohms of both green, then it's the center tap. ...
The winding resistance is 60 milliohms.
Surely multimeters would need an order or 2 of magnitude better resolution for low ohm readings before they've got a chance of determining that?
If there's a very low resistance reading between the greens and the green/yellow, then it's almost certainly a CT.
To verify that voltage would need to be applied.
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