Lo Plate Voltage: Opinions needed

[paully]

Working on an Ampeg V4 guitar amp's reverb circuit, and at power up I get the right supply voltage, but after the plate resistors, the (recovery) 12AX7 plate reads way low, less than half the voltage at plate 1 of the 6CG7. It should be 3 volts more! The cathodes all have proper readings, it's that one plate. I used the voltages from the original Ampeg schemo. Wondering if it could be a misprint there. Otherwise, I'm guessing a bad 12AX7(?). But that correct cathode voltage has me scratching my noggin. Opinions much appreciated. Thanks.

EDIT: problems solved, image removed.

Best, Paul

[boxymoron]

Calculate your cathode current. It should be almost identical to the plate(I think). 270Kohms is a pretty large plate resistor calue, so youre def. gonna see a big voltage drop there. Ohms law V = IR is your best friend.

That plate voltage on the schematic looks suspect. (120-115 volts)/270Kohms = 1.85*10^-5 or .0000185 amps which seems like a very low current for a 12ax7 (which are typically biased close to 1 ma or .001 amps). Its hard to tell the values on the schematic

[boxymoron]

The original schematic calls for a 12DW7 type tube, which has one section similar to a 12AX7 and a second section (pins 1,2,3) which is VERY different than a 12AX7. Also, the original schematic calls for a 300Volt supply, not 120 like your schematic.

[paully]

Thanks Boxy, but you might be looking at the driver tube, which is indeed a 12DW7 in the V4 and this amp. It's not actually a V4, but a hybrid using the reverb circuitry from the V4. Here are the shemos from 2 different sources. They both concur with the 12AX7 as the reverb recovery tube for the V4.

http://www.schematicheaven.com/ampegamps/v4preamp.pdf Piazza draft
http://www.drtube.com/schematics/ampeg/vt22-v4-74.gif Ampeg (this schemo shows the voltages)

AAR, same problem but different tube(6CG7). I was reading the wrong pins. 12AX7 reads to spec. The low voltage is after the 120K plate resistor at pin 1 of the 6CG7. I'm also having a problem with the 10K/5W plate resistor. Slightly low voltage and very hot, even at 1/2 to 3/4 line power.

That 6CG7 is a real old tube with some mileage. Might just be time for an upgrade . Thanks again!

Best, Paul

[boxymoron]

Dunno, this one: Schematics :: Ampeg V4.com
clearly shows a 12DW7 as the "recovery" tube (V5), but yeah that 10K resistor is gonna get hot regardless, here the power equation is your friend P = I^2R (or V^2/R).

So, assuming your 6CG7 were a straight wire you have max dissipation of P = 400^2/10K = 16 watts.

In reality the tube will be biased close to this: Iq = V/( Rk(mu+1)+rp) ), so using the 6CG7 datasheet and the values in your schematic: 400/(( 220(20+1)+7000) ) = 400/11620 = 34mA, or 11.8 watts!! (in reality the plate resitor will limit this current) which is why your resistor and poor ol' tube are cooking.

Your tube should be biased to no more than 20 mA following the specsheet guidelines, so taking a reasonable bias of 10mA your cathode resistor value should be more like: Rk = ((Vb/Iq)-rp)/(mu+1) = ((400/.01)-7000)/(21) = 1570 ohms :)

[paully]

Thanks, Boxy

Originally Posted by boxymoron

Dunno, this one: Schematics :: Ampeg V4.com

>>>>>>>>>>>>>>>>>>
Ya, I don't know where Dr. Tom dug up that paper , but it's pretty different from the standard(117 VAC only) V4 schemos. The schematic links I posted are my templates for the reverb section, and are is WAY different from that one.

Your tube should be biased to no more than 20 mA following the specsheet guidelines, so taking a reasonable bias of 10mA your cathode resistor value should be more like: Rk = ((Vb/Iq)-rp)/(mu+1) = ((400/.01)-7000)/(21) = 1570 ohms :)

>>>>>>>>>>>>>>>>>>
I replaced the 10K resistor with an 8.5K 10 watter, and the voltage is exactly to spec(270) , but with the increased quiessant current flow, it also gets pretty hot. Gonna take your suggestion and mess with the cathode side. I still have a problem with the other section of the 6CG7. BTW, got a new 6CG7 on the way. I think that's the problem there. Gotta check the cathode voltage against the plate voltage again. I really appreciate the suggestions. Thanks again.

Best, Paul