Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: garyboy on November 07, 2012, 09:18:54 pm
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I need some help this is my first tube project build. I am building a twin tube distortion pedal w/ 12vdc heaters that i found a schem for on this site except i have V1's triodes wired parallel. the heaters are running off of a 18vac 1 amp wall wart tranny from Radio Shack. When i plug the tubes in and turn on the power i'm only getting 7.7vdc and 2.24 amps of current. I am testing with a pair of china 12ax7b so you can see the insides. You can barely see the heaters light up on the tubes and the plate voltage stays very high on my plates for a very long time then slowly drop over a lengthy period of time. I'm talking like 300+ volts on V1 pin 1 and 6 then will eventually drop down to 189V. I took some measurements with and without the tubes installed.
DC heaters
no tubes 24.07V
1 tube 13.44V
2 tubes 7.77V
Heater Rectifier
no tubes 26.72v
1 tube 25.51v
2 tubes 23.74v
I also attached a schem I hope this information helps any.
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Seems you have connected the LM7812 in a wrong way (may be now demaged)
look to the LM7812 datasheet for input - ground - output pins location
http://docs-europe.electrocomponents.com/webdocs/0daf/0900766b80daf4ad.pdf (http://docs-europe.electrocomponents.com/webdocs/0daf/0900766b80daf4ad.pdf)
K
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And here's a pic to go along with K's datasheet...
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Thanks for the help. Yeah that was it hooked up wrong and toasted the regulator. Installed a new 7812 w/ heat sink and it was defective so I have a 7818 w/heatsink for the moment till i get a few more 7812s. Simple rookie mistake. :BangHead: Oh well its works now for testing. Plate voltages drop after a few seconds. V1 138V, V2a 198V, and V2b 300V. Maybe i'll just install a resistor to drop the voltage to 12V instead of ordering a new regulator? I think 20R should be enough to drop it to 12V.