Building a JMI Vox AC30/6

[pdf64]

Are the EL84 control grids lug 2 definitely very close to 0V DC?

Individual cathode bias resistors with each require their own bypass cap. eg 220R // 47uF.

Another option to enable monitoring of each EL84, whilst retaining the shared bias response, is to insert 1R cathode current sensing resistors between each lug 3 and the 50R // 220uF cathode bias.

Unfortunately the rough handling of shipping can bugger up valves, they may have matched fine at the vendor's facility.

[pdf64]

...
your old numbers for example;
31.3VDC / 50 ohms = .626 divided by 4 (tubes) = 160mA per tube!!
you had about 400VDC plate so 400 * .16 = 64WATTS from a 14W tube, boom, send in the new quads!!!

Just to note that when the cathode voltage measured 31V, the 50R cathode resistor was actually open circuit, so current was 0mA.

[shooter]

so I can maybe alligator clip a 100 ohm resistor,

That's what I do, "roll resistors" till I find the closest. and YES, all the voltages change, NORMAL.  GET the bias to 10-14W a tube
Don't worry about G2's current, it's about 5% of the math you're doing, since your 100+% off it's just wasting time n cooking tubes

[Willabe]

Individual cathode bias resistors with each require their own bypass cap. eg 220R // 47uF.

Another option to enable monitoring of each EL84, whilst retaining the shared bias response, is to insert 1R cathode current sensing resistors between each lug 3 and the 50R // 220uF cathode bias.

That's what I told him to do, use the 1 ohm on each tube before the 50R/220uF.

You really want to do that so you know for sure what each tube is drawing by itself.

[shooter]

agree, but he simply needs to get it to the point where tubes ain't frying, the amp works as designed....Then maybe mod it properly.

[four_corners]

Woohoo! We have a non-burning up AC30!

After swapping tube placement and V6 still smoking, i started looking right there, and there was a cold solder on the jumper between pin 1 and 2 (*facepalm*).

PDF64 mentioned I should check the mains transformer HT winding resistance, which was 52ohms and he said it should be at least 120ohms. I had two 10W 15ohm resistors, so I put them in series between the rectifier output and the reservoir HT cap (C39) and it cooled things down a bit.

So new EL84 numbers:

Cathode - 10.05 VDC
Plate - 331 VDC
Screen - 320 VDC

______________


Only thing left is to figure out what the heck is going on with V7 (Vibrato preamp and gain).

heaters are fine, but getting wacky voltages for everything else:

pin 1:  22 mV DC (changes +/- 30mV with the Vib Speed knob, I put a push/pull pot instead of rotary)
pin 3:  31 mV DC
pin 6:  about the same as pin 1
pin 7:  -670 mV DC
pin 8:  1.3 mV DC


These are the expected voltages:

pin 1: 140 VDC
pin 3: 1.2 VDC
pin 6: 200 VDC
pin 8: 18 VDC

I tried swapping tubes, and it wasn't the tube.

[four_corners]

One thing to add, I realized I had put the 30ohms of resistance between the rectifier output and the reservoir cap before finding the error I had made on V6, so I decided to quickly desolder it, and just put the direct wire back to see the voltage without the added resistors, and this is what I got...

WITHOUT 30ohm:
Cathode - 10.3 VDC
Plate - 339 VDC
Screen - 327 VDC

WITH 30ohm:
Cathode - 10.05 VDC
Plate - 331 VDC
Screen - 320 VDC

Maybe this is splitting hairs, but which seems more "ideal" for a classic AC30 with the 47ohm cathode resistor?

[shooter]

"ideal"

as a Tech, anything less than MAX dissipation
as a Guitarist, anything that get's the girl


 

[four_corners]

"ideal"

as a Tech, anything less than MAX dissipation
as a Guitarist, anything that get's the girl

 

I trust your expertise as both a tech AND rockstar

Now to just track down this Vib/Trem pre/gain stage voltage issue...

Tube is glowing and heater AC voltage is correct, and it is not the tube as I've swapped tubes in V7. I went through and reflowed all (I think) the component around that area.

My plate and cathode voltages are basically non-existant. Plate voltages (1 and 6) will modulate +/- but in the low mV's, and the cathode pins are both way too low.

I'll just start working backwards from V7 pins 1,3,6,8  I guess? Cathode resistor and cap, plate resistors, etc. Would be cool to know more than just the correct voltages directly at the valve, but I guess all I really need to know is where the voltage is "coming from" and try to find a big voltage drop/s.

Hoping to have a fully functional Vox AC30/6 by the end of the day! My first ever amp build. Thanks for all the help everyone, and humoring my dumb questions!

[SEL49]

You have no plate voltage on V7. That explains why none of the V7 voltages are good. So, you gotta find out why the plate voltage is missing. I suspect a missing wire.

First, check the voltage on V9 pins 1 and 6. What have you? This will provide a good clue about the missing voltages on V7. Once you provide the voltages for V9 it should become easy to find the cause for the missing voltage on V7.

[four_corners]

You have no plate voltage on V7. That explains why none of the V7 voltages are good. So, you gotta find out why the plate voltage is missing. I suspect a missing wire.

First, check the voltage on V9 pins 1 and 6. What have you? This will provide a good clue about the missing voltages on V7. Once you provide the voltages for V9 it should become easy to find the cause for the missing voltage on V7.

Okay cool, I'll go from V7's plate. All the other tubes seem to have the correct voltages, including V9.

[SEL49]

Check the wire I've indicated in the pic. You can also check for zero resistance from the bottom of R39 to the bottom of R60. I suspect a missing or broken wire.

[SEL49]

BTW, hi rez pics of your build would be verrrry helpful.

[four_corners]

Check the wire I've indicated in the pic. You can also check for zero resistance from the bottom of R39 to the bottom of R60. I suspect a missing or broken wire.

Well... you were spot on! I did not have that wire from R39 to R60, as I remember someone somewhere saying it wasn't needed (maybe it was for a different build, or a mod of some sort), and then I found another layout that also disregarded that wire. Wouldn't be surprised if the first person said it wasn't needed because they referenced that same layout!

I can officially say all tubes are performing as they should be! Now time to fully test this bad boy!

[four_corners]

A quick video for everyone who helped! And a couple photos of the head.

https://streamable.com/n2abc7

[pdf64]

It sounds and looks amazing, you should be proud of yourself, well done!