Humdinger question

[surfsup]

I have just built the p1xHO kit, and am experiencing some noise that sounds like hum.

With no guitar, gain stage 2 for the 2nd 12ax7 seems to cause all the noise. With gain 1 pot max and vol max, gain2 at zero, it is quiet. As g2 pot is turned up, the noise is introduced.

Then i removed the second tube and again, very quiet.

Then i removed the first tube and put tube 2 back in and again there is hum/static but not as loud as before (i assume because the first gain stage is bypassed with no tube)

I am thinking a humdinger might help?

I read this as a 100ohm pot wired at the lamp, with the sweep to ground or the cathode of a power tube. Which is better, ground or cathode? If cathode should i connect to the cathode of tube 2?

Does this prob sound like a humdinger is the solution? Thanks!

[Phil_S]

OK, good diagnostics, but more are needed. I'm assuming you are discusing a Hi-Octane amp? This is 2x 12AX7 and one EL34.

First, the hum to which you refer is 60Hz (50Hz in GB), usually a bleed over from the filaments. 60Hz ia about A# below the low E on a guitar. You think you have this? I'm thinking you don't. Static isn't part of 60Hz hum, the kind you might be able to cure with a humdinger.

Have you swapped V1 and V2 to see if the problem follows the tube? If so, get another tube. If not, your problem is between V1 and V3. Please answer this question before we continue to guess.

Consider posting over at AX84.com where they have expertise on this specific amp.

[passfan]

Sounds more like you have a ground issue in the preamp section. Double check all grounds with the layout drawing from AX84. Just for giggles swap out v1 and v2 to check that it's not a bad tube. The HO is a high gain design so expect some hiss. Heater hum is a deep hum. A grounding issue hum will be higher pitched.

[passfan]

I have just built the p1xHO kit, and am experiencing some noise that sounds like hum.

With no guitar, gain stage 2 for the 2nd 12ax7 seems to cause all the noise. With gain 1 pot max and vol max, gain2 at zero, it is quiet. As g2 pot is turned up, the noise is introduced.

What happens with a guitar ?

[surfsup]

Thanks for your replies. I did post at ax84 but the forum is down quite a bit (currently down for me this AM). Plus it's nice getting other opinions. In answer to your questions:

With a guitar plugged in it is the same result.

I swapped tubes 1 and 2 (the 12ax7s) and still the same thing with Gain2 control. Gain1 doesn't contribute any noise whatsoever with either tube.

It seems to be localized to tube socket 2 with whatever tube is in there, a static is introduced.

With G1 and G2 at max and VOL=0 as I turn up volume, it starts getting unbearable at setting 2. basically instant noise from 0 on, and never turned it to 10 because its really pretty loud by 7 or 8 and it was after my kid went to bed last night.

[Phil_S]

Can you give us a voltage chart for all tube pins? Omit the filaments. I am thinking the problem is confined to one section of the dual triode and this might give us a hint. Also, while you are probing, what happens to the noise as you probe, better or worse?

I am also thinking, this is a cold solder or missed/wrong ground. Go over the amp with a fine tooth comb for construction errors. Take the schematic and a highlighter. Mark the schematic as you verify. Do it twice -- once from input to output and a second time in the reverse order.

[surfsup]

okay I'll probe with it running here in a moment. I also reflowed all the turret solder grounds along the copper ground line. Same problem. I did take some pics of the guts, to save space here, all the pics are viewable in one webpage:

http://chicagocadcam.com/ChrisHahn/amp.html

[surfsup]

Here are the values, I am posting two values separated by a colon here they are:

My value : schematic value
B+ 228.2 : 264
B1 220.5 : 254
B2 212.3 : 250
B3 198.5 : 234

TUBE V1 12ax7
01: 118.3 : 139
02: Loud lots of buzz can't get reading
03: 0.954 : 1.13
04: 6.7
05: 6.7
06: 143.4 : 165 hum increases slightly while reading, static at first touch only
07: 0
08: 1.45 : 1.82
09: 6.7

TUBE V2 12ax7
01: 212 : 250
02: 113.3 : 129
03: 114.1 : 130
04: 6.7
05: 6.7
06: 113.1 : 129 static at first touch only
07: loud buzzing can't fix on a reading
08: 0.81 : 0.98
09: 6.7

TUBE V3 EL84
01: NA
02: 6.4
03: 6.67 : 6.62 (I have a jumper soldered btwn pins 3 and 4 per layout (but this is not on the schematic))
04: 6.67
05: 6.67 slight buzz increase
06: NA
07: 212 : 257
08: NA
09: 207.3 : 246

OCTAL
1: 0
2: 6.7
3: 211.7 (pin7 on V3)
4: 207.4 (pin 9 on V3)
5: 6.2 (pin 2 on V3)
6: NA
7: 6.7
8: 6.7 (pin 3 on V3)

[martin manning]

My bad... EL84 not 34!

[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.

The Layouts are sometimes a little hard to read compared to the schematics, you should always compare the two. I usually lean towards the schematic when there is a question.

Just a good read on the EL84 data sheet should have set you straight to start with. Check the Garage talk section and check out the reading material on steroids post. Look for Duncan amp site (I think) then look for the TDSL Personal edition 1.1 program and the load line calculator. TDSL is a tube data program that is a must have if you are going to build amps. The load line calculator from Martin is the bomb also. Do a search for it and download. Then order Merlin's book so you can understand how to use the Load Line Calculator. LOL (Just got mine for Christmas)

[surfsup]

Are you counting the EL34 tube pins from the correct reference point? Pins 1 and 8 are jumpered to put the suppressor grid at the same potential as the cathode. If you have 3 and 4 jumpered you have..

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

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.

I specifically asked this question prior to soldering it in and received two responses that said to do it. Are you sure?

Author: MaxPower (registered user: 136 posts )
Date: Thu, Dec 23rd, 2010 @ 13:39 (6 days ago)

--------------------------------------------------------------------------------
Normally pins three and four are connected. That's what the two boxes with the letter A in them mean. If you're using the 269EX and follwoing the plans, then you should connect them.

I thought the same and at least knew to ask why I didn't see it on the schematic but saw it on the layout. There are two [A]'s at pin 4 and 3 on the schematic. Schematic is here:

http://ax84.com/hioctane/AX84_Hi-Octane_101004.pdf

I did order Merlin's books and read most of the preamp book. Soon the power supply book...my brain is ready to explode but I'm getting there.

[passfan]

Just for giggles lift the shield ground on your shielded wires between the gain 2 pot and v2 and check that nothings shorted under your heat shrink. You should be able to pull the grounds one at a time off the main buss and verify. 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. A loose wire at this point could cause a lot of noise and buzz. Even better here would be a solder lug and star washers on the bolt. This needs to be a really solid ground.

[surfsup]

passfan, I was kinda wondering if there was a short under the heatshrink too. I'll try that. I just finished taking and editing more pics of the tube sockets: not for the squeamish! The solder looks crazy in super up close.

http://chicagocadcam.com/ChrisHahn/tubeshots.html

[passfan]

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.

[surfsup]

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?