Hum in power amp

[Rattler66]

And don't forget to double-check the match of the power tubes, even if they are new. If one is pulling significantly harder than the other you won't get the benefits of phase-cancellation for the ripple in the first stage power supply node feeding the OT.

Good luck!

only about 2mA difference

[mykey]

my meter doesn't seem to work properly when checking the ripple, i'm getting over 900vac, can't be

how much DC is across the cap?

[Rattler66]

i'll measure in a little bit and post back

[Rattler66]

Here's a schem of my power amp section............If i pull out the pre and pi tubes I still get hum (not very loud but annoying enough)

Here are my thoughts:

1.can i ground the two grids to see if the hum is coming in on the grids or should i just disconnect the grid resistor?

2.i figure if the hum isn't in the grids then i can disconnect each B+ to see if the hum goes, that way i can see if it is high ripple. is this valid?

thanks

[Mark Black]

Don't ground the grids or lift the bias resistors - either way you're removing the bias voltage that keeps the power tubes in happy-land.

You could lift the leg of each 0.15uF cap on the 220K resistor side to eliminate the PI circuitry altogether.

[mykey]

[QUOTE=Rattler66;25666]Here's a schem of my power amp section............If i pull out the pre and pi tubes I still get hum (not very loud but annoying enough)

using your meter, you are close to solving this but you need to make some measurements.
lets see a schematic, including the power supply. if you are getting this hum without the preamp tubes, lets take some measurements in the power supply.
confirm that the meter is working correctly, what is the AC voltage measured from a 120V AC wall receptacle? please verify...
make sure the meter works right before going further.
you need to be willing to measure AC and DC voltages to solve this.

[Bruce / Mission Amps]

Here's a schem of my power amp section............If i pull out the pre and pi tubes I still get hum (not very loud but annoying enough)

Here are my thoughts:

1.can i ground the two grids to see if the hum is coming in on the grids or should i just disconnect the grid resistor?

2.i figure if the hum isn't in the grids then i can disconnect each B+ to see if the hum goes, that way i can see if it is high ripple. is this valid?

thanks

Use a .1uF-400v coupling cap with a grounded clip lead to send audio to ground through the grids... don't DC ground the power tube grids on this amp or they will turn on full blast and maybe blow up.

Is there any hum through the speaker with the power tubes out too?
If so you might have a OT to PT coupling problem.
That could mean relocating all the wires or more likely, one or both of the transformers.
If not, you could just have a simple short from a filament lug touching a grid... or a bad filament wire layout that is allowing the filament noise to go right to the power tube('s) grid(s)

[Rattler66]

i will try to get some measurements tommorow, i'm in the middle of installing hardwood floors, my house is a mess. thanks very much for the help

[Rattler66]

Use a .1uF-400v coupling cap with a grounded clip lead to send audio to ground through the grids... don't DC ground the power tube grids on this amp or they will turn on full blast and maybe blow up.

what am i supposed to expect with the above?


with the power tubes out there is no hum. i rewired the heaters, no wrapping around sockets and from what i see, no shorts

thanks bruce

[Enzo]

You wanted to ground the grids of the power tubes to see if the hum was coming in through the grids. That is a valid idea. But if you just ground the grids, you not only ground off any hum signal there, but also the bias supply, so the tubes would have no bias and would start to melt down.

By adding the 0.1uf cap in series with the ground clip, you block the DC path to ground, but the cap acts like a wire to the AC or signal at that point, so the signal is grounded without grounding away the bias. You can also "ground" signal at a preamp tube plate for example with the same method.

It is exactly the test you proposed without the DC problem that would result.

If the hum is coming in the grid, then AC grounding the grid should cause it to diminish. Of this has no effect, then the hum is from screen ripple, or mismatch in the power tubes.

If it DOES come in through the grid, then either it is noise from a prior stage, or the bias supply is hummy.

[Rattler66]

got it, thanks Enzo. BTW here is the scem

[Rattler66]

some more questions

I finally rebuilt the amp and still get the hum. I will AC ground the grids tonight to see if the hum persists. if it does is 10u,10u enough filtering for a bias supply? also i have about a 9-10ma difference between both tubes, I'm thinking that may be the culprit. if so i'll pop in other tubes, but what if they are still off, is there any other way to offset the imbalance (ie:balance control) thanks

[Rattler66]

ok i grounded the grids and it helped a little little bit. mostly on only one of the tubes. i'm really suspecting a mismatch i have a 10ma difference. i am going to invert the tubes and see what happens then i'll try the other set.

[mykey]

install the grid balance control as in
the fender schematics,
bias filtering should be 100uf, 10 uf
at least
you are right unbalanced grids will
cause a lot of hum
there are also several other sources
of hum starting with the AC heaters.

[Rattler66]

thanks i'll look up the fender schem. BTW-i started adjusting the bias and there was a ponit where it quieted down considerably, but one tube is run at about 60ma and the other about 54ma.(seems kind of high to me) It sounds great except for the small hum and the tubes are NOT redplating. so is this a balance issue?