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60 Cycle Beast from Hell ...Please help!

6/27/2000 10:53 PM
Stan Bailey	60 Cycle Beast from Hell ...Please help! Hello, I built an amp that has a gain stage + tone stack + volume + gain stage +phase inverter + mosfet source follower + push- pull output. I have two more gain stages that are switched in between the last gain stage and the PI of the arrangement above for distortion. I already played this amp at a couple of gigs using the clean channel with only a slightly noticeable hum. The problem is when I switch in the other two stages, the hum is amplified to the point where the dirty channel is not usable. Too bad, because the distortion does sound really good. Anyway, here are some of the measures I took to try to remedy the problem: 1) All grounds, including input and output jacks, have their own 18 awg wire to star ground. 2) Negative side of FWB has wire going to - terminal of first filter cap (110 uF @ 600V) then a wire from cap to star ground. 3) DC filament supply with twisted wires and referenced to ground. I did the test where I disconnect the filament supply to see if hum stopped, but it faded out slowly. This would eliminate the heaters as the culprit, correct? 4) Tried lifting circuit ground from chassis ground but amp became unstable and hummed like a madman! 5) Paralleled all filter caps with known working caps. 6) All grid wires shielded and grounded at quiet end of circuit. 7) When probing with a cap to ground, the second gain stage's grid had the biggest effect in eliminating the hum. Also, when I turn off volume just prior to second stage, the hum is a barely audible. 8) Swapped all tubes. After all that, I didn't hear any notible decrease in hum.The only other thing I can think of is a leaky transformer. Could that cause this much hum and still be working otherwise? Man, I'll probably end up in the mental ward on this one. :^O I'm ready to do a complete rebuild if I can't figure this out. If anyone has anymore suggestions I would be really grateful. Thanks for your time, Stan.

6/27/2000 10:53 PM

Stan Bailey

60 Cycle Beast from Hell ...Please help!

Hello,
I built an amp that has a gain stage + tone stack + volume + gain stage +phase inverter + mosfet source follower + push- pull output. I have two more gain stages that are switched in between the last gain stage and the PI of the arrangement above for distortion. I already played this amp at a couple of gigs using the clean channel with only a slightly noticeable hum. The problem is when I switch in the other two stages, the hum is amplified to the point where the dirty channel is not usable. Too bad, because the distortion does sound really good. Anyway, here are some of the measures I took to try to remedy the problem:

1) All grounds, including input and output jacks, have their own 18 awg wire to star ground.
2) Negative side of FWB has wire going to - terminal of first filter cap (110 uF @ 600V) then a wire from cap to star ground.
3) DC filament supply with twisted wires and referenced to ground. I did the test where I disconnect the filament supply to see if hum stopped, but it faded out slowly. This would eliminate the heaters as the culprit, correct?
4) Tried lifting circuit ground from chassis ground but amp became unstable and hummed like a madman!
5) Paralleled all filter caps with known working caps.
6) All grid wires shielded and grounded at quiet end of circuit.
7) When probing with a cap to ground, the second gain stage's grid had the biggest effect in eliminating the hum. Also, when I turn off volume just prior to second stage, the hum is a barely audible.
8) Swapped all tubes.

After all that, I didn't hear any notible decrease in hum.The only other thing I can think of is a leaky transformer. Could that cause this much hum and still be working otherwise?
Man, I'll probably end up in the mental ward on this one. :^O I'm ready to do a complete rebuild if I can't figure this out. If anyone has anymore suggestions I would be really grateful.
Thanks for your time, Stan.

6/28/2000 12:25 AM
Carl Z	Stan; Check your input jacks to start with...especially if you're using those plastic Cliffs jacks. Those suckers can give you all kinds of weird problems when the contacts get dirty. Another possibility is that you might have a problem with your guitar. Have you tried injecting a signal from another source into the amp? If all else fails you may need to check it out with a scope to find the problem. Carl Z

6/28/2000 12:25 AM

Carl Z

Stan;

Check your input jacks to start with...especially if you're using those plastic Cliffs jacks. Those suckers can give you all kinds of weird problems when the contacts get dirty. Another possibility is that you might have a problem with your guitar. Have you tried injecting a signal from another source into the amp? If all else fails you may need to check it out with a scope to find the problem.

Carl Z

6/28/2000 4:04 AM

Stan Bailey

Hello Carl,

quote:
" Check your input jacks to start with"

Did that already.

quote:
" Have you tried injecting a signal from another source into the amp
If all else fails you may need to check it out with a scope to find the problem."

Yes, I have a generator and a scope. I get a bunch of 60HZ crap at the 2nd and 3rd stages, but my troubleshooting skills aren't good enough at this time to find the cause. The amp is a previously hacked up princeton that was modded by Dan Torres ( nice guy, but…) I worked with some of the components he had in there from his "princetone" mod. I should of just ripped everything out, but I thought I could save time by using the awkward positioned terminal strips . Anyway, I tried to get the lead dress right but it got pretty tight in there. There is no hum at all when I plug my d-clone pre-amp into the PI of this amp, so I'm just going to redo the amps pre-amp section very clean like.
Thank you!
Stan

6/28/2000 3:00 AM
Brad Myers	I just fought one like this. I converted a Pro Reverb & cascaded the channels for a 4 stage, two tonestack amp w/ reverb & line in/out. I fought hum for 6 months (about 3 attempts opening/closing the chassis up). I had lowered the B+ to around 260V (used half the PT secondary) and hum was a bear. I increased the supply filter cap values & it helped minimally...not much. The hum was just enough to show it's presence in clean mode and a bear in higain mode with all 4 stages in. The culprit? The B+ supply uses a 2.7K wirewound to start the supply drops (instead of a choke). And I, in an attempt to maximize the B+ to the preamp tubes (my first tube amp!) connected all these stages together in FRONT of that resistor...big mistake! I moved one wire over (that fed those stages) and it was amazing how the hum virtually disappeared in clean mode and was BARELY audible in higain mode! So...I lowered the value to about a 1.5K, 10W wirewound and everythings happy now. How I got the tip to do this was by going back & cracking the old college circuit analysis text book & read the section on power supplies, rectification & LC/RC networks and discovered how important that first series impedance (choke OR resistor) is in dropping off the ripple...absolutely critical...and more so...in high voltage supplies. Best to you... BradM

6/28/2000 3:00 AM

Brad Myers

I just fought one like this. I converted a Pro Reverb & cascaded the channels for a 4 stage, two tonestack amp w/ reverb & line in/out.

I fought hum for 6 months (about 3 attempts opening/closing the chassis up). I had lowered the B+ to around 260V (used half the PT secondary) and hum was a bear.

I increased the supply filter cap values & it helped minimally...not much. The hum was just enough to show it's presence in clean mode and a bear in higain mode with all 4 stages in.

The culprit? The B+ supply uses a 2.7K wirewound to start the supply drops (instead of a choke). And I, in an attempt to maximize the B+ to the preamp tubes (my first tube amp!) connected all these stages together in FRONT of that resistor...big mistake!

I moved one wire over (that fed those stages) and it was amazing how the hum virtually disappeared in clean mode and was BARELY audible in higain mode! So...I lowered the value to about a 1.5K, 10W wirewound and everythings happy now.

How I got the tip to do this was by going back & cracking the old college circuit analysis text book & read the section on power supplies, rectification & LC/RC networks and discovered how important that first series impedance (choke OR resistor) is in dropping off the ripple...absolutely critical...and more so...in high voltage supplies.

Best to you...

BradM

6/28/2000 4:24 AM

Stan Bailey

Hi Brad,
Thank you for responding.

quote:
"The culprit? The B+ supply uses a 2.7K wirewound to start the supply drops (instead of a choke). And I, in an attempt to maximize the B+ to the preamp tubes (my first tube amp!) connected all these stages together in FRONT of that resistor...big mistake!"

I used a 100 ohm resistor in place of a choke, but I subbed in a 10 henery 120 ohm choke with no reduction in hum.I didn't see too much ripple after the 2nd, 3rd, and 4th drop.
As I mentioned before, I am just going to rebuild the pre-amp section since it is not as clean as I would like it to be.I think rats would like it though 8o)
Thank you, Stan

6/28/2000 6:47 AM
JohnC	Try grounding all power supply filter caps to a star point near the power tranny/ rectifier, then ground all signal related stuff (cathodes, vol pots and jack) to another spot far away at the other end of the chassis. This always works for me. I think star grounding power supply and signal circuits together is a sure way to induce hum in sensitive gain stages John

6/28/2000 6:09 PM
Carl Z	Grounding is definetly a tricky beast! It seems that no two amps will respond the same to the same grounding scheme. Basically, you've got to screw with it a lot to find out what works best. The method I've got that seems to work well for me is to use a two ground point system. I run a bus ground for all my pots and input jacks and run this to the input stage local ground. From there the entire preamp is run off a bus to it's own ground point which I'll call point "A". all the filter caps for the preamp run to this point with their own wires as does the phase inverter. All the power supply caps that are coupled to the power tubes, including the screen grids go to star point "B". The heaters are just grounded to some convienient location. I know this isn't exactly a textbook grounding method but I've found that any improvements are marginal at best and a clean layout is easier to deal with than having a rats nest of wires running all over the chassis. Carl Z

6/28/2000 6:09 PM

Carl Z

Grounding is definetly a tricky beast! It seems that no two amps will respond the same to the same grounding scheme. Basically, you've got to screw with it a lot to find out what works best.

The method I've got that seems to work well for me is to use a two ground point system. I run a bus ground for all my pots and input jacks and run this to the input stage local ground. From there the entire preamp is run off a bus to it's own ground point which I'll call point "A". all the filter caps for the preamp run to this point with their own wires as does the phase inverter. All the power supply caps that are coupled to the power tubes, including the screen grids go to star point "B". The heaters are just grounded to some convienient location.

I know this isn't exactly a textbook grounding method but I've found that any improvements are marginal at best and a clean layout is easier to deal with than having a rats nest of wires running all over the chassis.

Carl Z

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