homebrew help

[496dart]

Im having some weird thing going on with my amp project. The power and output section is done up to the master volume. It is exactly like a Jcm800. i have normal voltages at the tubes , 467 at the plates and -42v . The bias is reading WAY off. It reads -220ma . Ive checked my wiring a couple times and it looks good. the bias pot works and measures 25k .tubes are new JJ el34.
any ideas ?

[Amp Kat]

What are you reading the bias with and how ? -42 at the grids seems right. 222 ma's could be the total of 4 tubes if your measuring across the whole transformer. One tube at 222 ma's should blow the HT fuse if one is present and the tube would be big time cherry red. Then again what's not finished on the amp ?

[496dart]

What are you reading the bias with and how ?



-42 at the grids seems right. 222 ma's could be the total of 4 tubes if your measuring across the whole transformer. One tube at 222 ma's should blow the HT fuse if one is present and the tube would be big time cherry red. Then again what's not finished on the amp ?

a multi-meter--transformer shunt method.

this has 2 el34's. the fuses are good and the tubes are not super hot or glowing red. They look normal.

If you look at a jcm 800 scem, my amp is the same , its built from the Master volume pot on. In other words the preamp /p.i. section is not in it yet.

[Enzo]

Just for grins, reverse your meter leads and see if the readings change.

[496dart]

tried it. same thing

[Enzo]

Did you swap places with the tubes? Just because they look good doesn't mean they are. Also, just because they might be new doesn't mean they are good. If the odd reading travels with the tube, you know the tube is the problem. if the odd reading remains no matter what tube is installed, then something is funny in the amp.

OK, voltages:
You got 467 at BOTH plates? You got 450-467 or some such volts at BOTH screens? Got -42 at BOTH grids? Verify with an ohm meter zero ohms to ground from BOTH cathodes. Or 1 ohm if you added one of those. In fact, a 1 ohm resistor in the cathode leg is a good idea even if you plan to use other methods. It will come in handy in situations like... this one. It provides a backup reading.

ANd to be a nit picker, your bias is not way off, your idle current reading is way off. It might be caused by faulty bias, but the ma reading is tube current, not bias. Yes I understood what you meant.

WHICH JCM800 50 watt? 1987? 2204? 2205?

Looking at the 2204 which sounds closest to what you describe, the MV is before the PI, but you said the PI wasn't in yet. SO exactly what is there at this point? If there are coupling caps from the PI or where the PI would be goung to the power tube grids, ground the PI end of them. UNLESS they are wired to plate voltage for the PI. In that case, disconnect the PI end of them so you can ground them. That will stop any picked up noise or oscillation from unterminated inputs to the power amp.

[496dart]

this the 2204 model.

i found the problem--my DAMN METER all of a sudden wont read milliamps ! It reads everything else !! So i went and bought a $3.99 meter to make sure....and it works fine.
28ma / 34 ma on the tubes .

this is a $60 Radio shack meter--why would that section quit ? the fuse is good.

thanks anyway Enzo

[Enzo]

My meter has more than one fuse inside. If I blow the current fuse, ither things still work.