Plexi 6v6 hum issue

[Big_Mike]

So I built the Hoffman 6v6, and it is really an awesome amp and very quiet.  I made a few mods to the master volume, one was the addition of a 220K resistor in series before the master volume pot to preserve bass, the other is adding a small cap between lugs 1 and 2 of the master volume pot to preserve treble at low volumes.

I started with a 5p cap across lugs 1 and 2.  The amp was fine, very quiet, but I still felt that higher frequencies were more rolled off.  So I swapped out the 5p cap for a 7p cap, and I really like the resulting balance of frequencies at lower volumes.  The 7p cap seems to be the right fit.

Here's the problem.  After adding the 7p cap, I am getting hum from the amp.  I am not sure why swapping this cap would do this.  I suppose it could also be maybe a failure of one of the preamp tubes?  Anyway, I get a fair amount of hum with my Les Paul as I approach the amp, when it was dead quiet before.  This amp was dead quiet just a day ago, so I am trying to figure out what the issue is.  I literally have changed nothing other than the cap to the master.

I figured I would first remove the 7p cap and see if the hum goes away.  If not, I probably will try and swap some preamp tubes and see if that does anything.  Does this sound like a good start, or are there other things I should look at?

[shooter]

Does this sound like a good start,

yup, including the wires that got moved to get to the 7p, all the inadvertent things you did without knowing (un-intended consequences for actions )

your error, you tried preserving BASS and TREBLE 

[PRR]

5 or 7pFd across top of a audio volume control has "NO!" effect. it won't do anything until 23KHz! It is surrounded by 30pFd-120pFd of stray and grid capacitance. 50p-500p is more likely to "do something".

Unless your 7pFd is on leads many inches long, it's not your hum. More likely you caused a poor joint on a pot lug.

[Big_Mike]

I think I figured out the problem - I had recently moved my guitar rig to a new location, and it is on the wall opposite our refrigerator.  The refrigerator motor gives out a hum, which I think is being picked up by my guitar and the amp.

When I move the amp to a new location, the hum goes away.  Looks like everything is fine with the amp.

[Big_Mike]

Back to issues with this amp.  This is frustrating because this amp was dead quiet until I made an extremely minor addition of a cap to the master volume pot.

I thought it was due to a refrigerator being nearby, but I moved the head and cabinet to another room and had the exact same hum issue. 

The hum increases in volume as I change the gain knobs, which leads me to believe it is something possibly in the first tube.  I am going to try and swap that out.  One other thing that I noticed on idle is that the 6v6 power tubes make a slight buzz/hum sound that they did not make before.  So it could be a 6v6 issue as well. 

I think the first step is to do some tube swapping and see if it is a tube issue.  Process of elimination, right? 

[shooter]

Process of elimination, right? 

I call it half splitting, you did that by noting the volume pot interaction, so go left and fix, don't go ALL OVER.  Once the left is behaving, 1/2 split the rest.  In math there are an infinite number of "1/2 splits" in a finite range, but amps should only need a couple 

[Big_Mike]

Process of elimination, right? 

I call it half splitting, you did that by noting the volume pot interaction, so go left and fix, don't go ALL OVER.  Once the left is behaving, 1/2 split the rest.  In math there are an infinite number of "1/2 splits" in a finite range, but amps should only need a couple 

Sounds great.  I am going to check out swapping the first tube, see if that works.  The other possibility is one of the 6v6s may be acting up.  I don't think it is a heater issue, the amp worked perfectly until two days ago and I have not touched the heaters at all.

[Big_Mike]

One other thing to note... previously, when I had no hum, I would turn on the amp, it would have a very low hum, then it would go away.  Now, the hum stays.  I am not sure the power tubes are matched (I bought them at Guitar Center about a year ago, so probably not).  Does this sound like a power tube issue possibly not cancelling out the hum from the power transformer?

[dude]

I would take the caps and whatever else you did to the master "out", if hum's gone you're done. If not, replace one 12Ax7 at time with know good tube (you only need one good 12Ax7), still hum? Then replace both power tubes with know good, if you don't have extra power tubes, tap on one, then the other, any change in hum when you do that? If a change "might be" one or the other 6V6 is bad.
You should put in 1 ohm resistors, 2% or 5%, between the pins 8 and ground of 6V6s, lift the ground from pin 8 add a 1 ohm R and attach the other end to ground. Turn on amp, take a mA current reading across the 1 ohm R's. Should be about 5 mA different or less, you'd have to have a 10 mA difference or more to get maybe a hum.
Possible that your 6V6s bias drifted too, but I'm sure the Hoffman's layout has this bias set-up on his layout and schematic along with a pot to adjust the bias, should be around 20 to 27 mA's  according to your Plate voltage. Use this chart,     http://www.tedweber.com/webervst/tubes1/calcbias.htm 

[shooter]

since you moved right of the  above noted problem;

slight buzz/hum sound

are they red-plating?
what's the 1ohm R's read?
does the math put them anywhere near Max plate? - no? fix 1 thing, then the next, otherwise you get caught in an endless loop 

[Big_Mike]

The first test I did tonight was I removed each of the preamp tubes and fired it up with each one removed.  The humming sound was constant even with a preamp tube removed.  It was not as loud obviously without all of the preamp tubes inserted, but it was there.

The power tubes are not red plating, but I suspect this may be a power tube issue. 

Tomorrow I am going to try to insert a fresh 12ax7 preamp tube into each slot and see if that does anything.

[labb]

If it hummed with the pre amp tubes removed, replacing them with new ones will not solve the problem...Have you tried chop sticking the amp to see if you can make the hum change?

[dude]

From my experience, since the amp played with no hum before you made changes, go back to were there was no hum. It's usually the last thing you did...?  Maybe a solder joint was weak, maybe a loose 6V6 socket (use an ice pick to the sockets to close the pin sockets, loose pin contact can cause this). Also, try a different cord if hum only with guitar plugged in.


It does sound like a power tube issue, could be one 6V6 is weak or bad, recheck all wiring around the tubes. Do they bias equally within 5 mA's

[Big_Mike]

Thanks for all the help, I figured out the problem - moved a couple of wires near the heaters, cleared it up well.