Humdinger question

[passfan]

So if I soldered pins 1 and 3 of a 100 ohm pot to pins 3 and 4 of the socket and put pin 2 of the pot to ground this would be the humdinger?

I built a new shielded wire and unsoldered the old, resoldered in the new. Same humming. This was the shielded wire between the pot and the circuitboard. Should I try the other shielded wire between V2 and circuitboard? Could this noize be caused by a bad capacitor?

You can check your shielded wires quickly by just lifting the ground. If there's a problem it's going to get better, if not it won't change a bit. A humdinger works to eliminate a slight hum due to winding imbalance. It will not wipe out a serious hum issue due to a grounding problem or worse. Best to find the problem and not be so quick to mask it. I've built this circuit and played it for a few months, other than hiss it should be noise free without a humdinger.

[surfsup]

Also, I notice crimp terminals used on the main buss ground to chassis down by the input jack. It's always good to cut off the blue plastic and solder the wires in the crimp.

I did solder the wire into the crimped terminal ring. I plan to get some nylon lock nuts too but they are all tight.

I will lift the other shielded ground and check after spending a little time with the family. Been ignoring them all day! thanks for explaining that better. I was procrastinating replacing that one!

[martin manning]

I have 3-4 jumped on the EL84 small bottle, not the EL34. The EL34 is in series with the EL84 socket and only one tube can be in the amp not both. Here is a jpg of the hookup schematic and you can see the actual wiring in the photos at my previous link:

http://chicagocadcam.com/ChrisHahn/octal.jpg

Ok, so you can run EL84 or EL34 with the octal socket wired in parallel with the noval. Interesting that the diagram (linked above) takes the octal's suppressor grid to ground, putting it below the cathode's voltage. Is that required to make it work with the same Rk as the EL84? Also, it looks like the heater ground reference is not elevated when the octal socket is in use. You'd have to tie pins 7 (or 2) and 8 to get that to work. Can you run a 6L6 in there?

[passfan]

I have 3-4 jumped on the EL84 small bottle, not the EL34. The EL34 is in series with the EL84 socket and only one tube can be in the amp not both. Here is a jpg of the hookup schematic and you can see the actual wiring in the photos at my previous link:

http://chicagocadcam.com/ChrisHahn/octal.jpg

Ok, so you can run EL84 or EL34 with the octal socket wired in parallel with the noval. Interesting that the diagram (linked above) takes the octal's suppressor grid to ground, putting it below the cathode's voltage. Is that required to make it work with the same Rk as the EL84? Also, it looks like the heater ground reference is not elevated when the octal socket is in use. You'd have to tie pins 7 (or 2) and 8 to get that to work. Can you run a 6L6 in there?

They have a trimmer circuit mod that allows you to adjust the bias to interchange tubes out. I set my bias resistor to use either EL34's or 6L6's by just swapping them out on the one I had.

[martin manning]

Thanks, passfan. I'm not familiar with this amp (obviously!). Do you know what's up with putting the suppressor grid below the cathode voltage?

[surfsup]

I have the EL34 in there now and I think you can put in any octal tube since the bias is adjustable (via a pot)

I'm not sure about your other question as I'm a noob. I am also wondering if you guys took a look at the voltage readings, even coming off the tranny, the readings are 10-20% lower than what they are supposed to be. Not sure what's up with that...

[martin manning]

Yes, the voltages are low across the board. What's your line voltage? Your PT has a 115V and a 125V primary voltage option right?

BTW, I see I was wrong about the elevated heater ground reference using the octal, it'll still be tied to the top of the cathode resistor with the EL84 socket empty.

[surfsup]

Line voltage is 125.1v last time I tested the outlet...(a few days ago)

Today its 125.2 (i used the black and white on the Hammond 269EX for 125V)

[martin manning]

On the schematic it looks like they're putting 120V on the 115V primary, and you have 125 on the 125. Hammond literature says they added the extra primary option in August 2009... The schematic is dated October, 2004, but shows the dual primary (?).

[passfan]

Thanks, passfan. I'm not familiar with this amp (obviously!). Do you know what's up with putting the suppressor grid below the cathode voltage?

Not a clue. Someone smarter than me will have to answer that. The designers hang out at the ax84 site and have published a paper on their designs which can be downloaded there in the amp sections.

Try this link http://www.ax84.com/hioctane.html

[martin manning]

Following that link, there is a document describing the paralleled output tube socket mod, but no answer to my question. I'm guessing that it is done that way so that a metal envelope 6V6 can be used in that socket; wherein pin 1 is connected to the envelope. Grounding pin 1 works okay for the EL34, and has no effect on glass 6V6 or 6L6 since that pin is NC.

[surfsup]

Well I lifted both grounds and the amp still hums. I also broke the jumper between pins 3 adn 4 and connected pin 4 to ground. Still a hum.

Anyone have any other ideas? Any comments about the low voltages across the board? I would really appreciate it.

EDIT - I also replaced the shielded w/ground to the tube pin 7 - still humming. Actually I think its a bit worse now but less static. More hum, less static.

[Ian444]

It may be my monitor colors, but it does look like you have 1M resistors instead of 100K (R18/R23) in the anodes of the first valve stage, and also 1M's instead of 220K for R20/R21? I could be wrong but its worth checking.

[surfsup]

It may be my monitor colors, but it does look like you have 1M resistors instead of 100K (R18/R23) in the anodes of the first valve stage, and also 1M's instead of 220K for R20/R21? I could be wrong but its worth checking.

Thanks for looking. They are all correct. The yellow printed line on a blue resistor makes it look green...

[marcoloco961]

Pin 3 on a EL84 is the Cathode and should not be tied to pins 4+5, it should go through a Big wattage resistor and a cap in parallel and then to ground. Remove it and try again. It is hard to believe that the noise was not present in both channels if this is the problem.

:

Pin 3 to 4 is a cathode reference for the heater circuit and raises the heater reference above ground to reduce hum. A standard trick and implemented here in an unusual way by the folks at the AX84 project.

Ooops. My bad. I have never seen that before. Every time I think I know something, I find out just how much more there is to learn.

Sorry Surfsup.

How does AC heater voltage on the Cathode not cause a hum. I would think it would make it buzz like a son of a gun. Time to read some more I guess.