Allen Old Flame 60 cycle Hum issue PICS ADDED!

[benabloom]

Hi Gurus,

I am trying to diagnose a Hum problem I am having with my Old Flame.

The issue is , as I turn the master volume up (this amp uses a PPI MV ), I get a strong 60 cycle hum. At 12 oclock the Hum is present and at full CC position the HUM is loud and proud. This hum does not change in intensity if the preamp volume is on or not. If the reverb knobs are on or not there is no change.

I first changed the Rectifier tube = No change
Next I swapped the Power tubes with new tested ones = No Change
I swapped the PI tube = No change
I swapped the rest of the pre-amp tubes one by one = No change

NOT TUBE RELATED me thinks

Because this Hum occurs ONLY as I turn up the MV, it seems that it is completely independant of the pre-amp. My initial thought is that it is a grounding issue in the poweramp section of the amp. but I don't know how to trouble shoot, or what else it could be? This a problem that is not intermittent, but has developed in the last 6mos. I have changed Power tubes a few times in the last few months to try new flavors, and wonder if in sliding tubes in and out, something could have been disconnected/dislodged on the power tube socket?

Whatcha think?

Thanks for the help.

[Structo]

Howdy.
Here is a good hum troubleshooting guide.

http://www.geofex.com/ampdbug/hum.htm

[mark norwine]

Over the years I've had dozens of customers bring me their homebrew amps because of "60 cycle hum".......and in almost every case, the issue was 120 cycle hum.

60 Hz is ground related, filament related, bias related or induction related.

120 Hz is power supply

So......my first question: are you certain you have a 60Hz problem? Have you measured it with a meter or a scope?

[benabloom]

Over the years I've had dozens of customers bring me their homebrew amps because of "60 cycle hum".......and in almost every case, the issue was 120 cycle hum.

60 Hz is ground related, filament related, bias related or induction related.

120 Hz is power supply

So......my first question: are you certain you have a 60Hz problem? Have you measured it with a meter or a scope?

Hi Mark. I am not certain that it us 60 v 120, however it is a low (bassy) Hum. How can I meter this? I don't have a scope. This is not a homebrew it was built by David Allen. Thanks for your help.

Ben

[mark norwine]

Many DMM's can measure frequency.....can yours? Put it right across the speaker jack & see what you get

[benabloom]

Thanks I'l try that tonight. and report back.

[TweeDLX]

Ben,

I have an older Allen Accomplice that had hum from the reverb. Try disconnecting the 'verb and see if the noise stops.
David Allen said this about it:
I happened to get one in this week from June of 2000. It had more hum in the reverb than I cared for. One thing I did that worked pretty well was to replace that 750 ohm 3W (1K green in my photo) resistor with my small TC choke mounted inside the chassis just to the left of the MV pot. That reduced the hum significantly so I took it a step further. I moved the red lead from the reverb transformer from the noisier "screen grid supply" at pin 6 of the power tubes and reconnected to the next power supply node where the 100K and 82K plate load resistors for the phase inverter get their power. It was easier to connect there than at the end of the middle filter cap. The hum in the reverb is gone.

I did the second part of this and it helped a lot.

Mike

[benabloom]

I just unplugged the reverb tank and it made no difference. Thanks for the suggestion.

I also checked to seeif my Multimeter will test for frequency and it does not. The Hum seems to sound between an A and a B if played on the low E string (5th or 7th fret respectively) not sure if this helps.

What next?

B

[WaltC]

"The Hum seems to sound between an A and a B if played on the low E string (5th or 7th fret respectively) not sure if this helps."

Sounds more like a 120 Hz hum (power supply related maybe). You can always take it to a tech, or give Dave A. a call. Since tube swaps/replacement didn't work, my guess would be filter cap related. You say that it is worse when the MV is "all the way up" and that's fully "CC" (counter-clockwise?). That seems odd to me because my Old Flame was off when CC and all the way on when fully clockwise.

Be really careful! Filter caps and B+ voltage work means dealing w/ over 400V of DC, and that can kill you.

first check all your ground connections, if the filter caps are not properly grounded (loose solder joint, broken wire, etc.) then they don't filter.

Does the sound still come through on top of the hum? Does the signal volume go up and down properly with the setting of the master? one thing you can try is to start pulling tubes (starting with the very first pre-amp tube, usually the farthest on the right looking at the back of the amp) and see when the noise stops. The noise could be occurring at any stage and just become more noticeable when the over-all volume goes up.

Let us know what you find out...

[benabloom]

"The Hum seems to sound between an A and a B if played on the low E string (5th or 7th fret respectively) not sure if this helps."

Sounds more like a 120 Hz hum (power supply related maybe). You can always take it to a tech, or give Dave A. a call. Since tube swaps/replacement didn't work, my guess would be filter cap related. You say that it is worse when the MV is "all the way up" and that's fully "CC" (counter-clockwise?). That seems odd to me because my Old Flame was off when CC and all the way on when fully clockwise.

Be really careful! Filter caps and B+ voltage work means dealing w/ over 400V of DC, and that can kill you.

first check all your ground connections, if the filter caps are not properly grounded (loose solder joint, broken wire, etc.) then they don't filter.

Does the sound still come through on top of the hum? Does the signal volume go up and down properly with the setting of the master? one thing you can try is to start pulling tubes (starting with the very first pre-amp tube, usually the farthest on the right looking at the back of the amp) and see when the noise stops. The noise could be occurring at any stage and just become more noticeable when the over-all volume goes up.

Let us know what you find out...

Yes the signal volume does go up and down properly when I turn up the preamp volume (knob closest to input). The only thing that increases or decreases the hum is turning the MV. Clockwise is louder in my case (sorry for the confusion) and All the way up is fully clockwise. I pulled all tubes to no avail, also, when the preamp volume is fully CC or off, the symptoms of the hum prevail as I turn up the MV.

I will pull the chasis (after discharging the caps:) and see what I can see with my eyes and I'll try to post some pics!

[WaltC]

When I suggested pulling the pre-amp and PI tubes I meant with the amp on and a signal listening for the effect on the hum. You can (to be safe) switch the amp on, verify the hum, switch the standby off, pull the first pre-amp tube, switch the standby back on, and check for the hum. Hum still there? Then turn the standby off, pull the next pre-amp tube, turn the standby on and check for the hum again. Repeat until the hum disappears or you run out of tubes <G>.

That said (and on further thought) it may be that you have a leaky coupling cap feeding the MV and PI tube. The first two coupling caps (leading to the MV gang pot) are connected to B+ and are used to block the DC and let the signal through. The second set of coupling caps (between the MV pots and the power tube's grids) are to isolate the MV pots from the DC bias voltage (negative) that is delivered to the power tube grids along w/ the signal. If any of them "leak" dc voltage, and the dc voltage has ripple present, the you could get hum that way too.

Harder to check unless you really know what you're doing and have a DMM or capacitor tester capable of checking for DC leakage, but you can unsolder the lead to each cap individually to check if that has any effect. Again BE CAREFUL! -50 to +400+ Volts DC on there.

[benabloom]

Ok, I pulled the Chasis and visually inspected the circuit. Nothing apears to be broken off or loose. All Caps look unmolested and not VISUALLY damaged.
The Hum does not seem to be interactive in ANY way with music being played out of the amp. Its like a Backround HUM that is always there and gets louder or softer as I turn the MV up and down. The music I am playing also gets louder and softer so the relationship between the 2 does not change if that makes sense.

The ground lug of the PT is tight, and all ground contact appear to be intact. The amp was built in 2001 so the caps are from then...not too old.

The only thing that looks a little suspect is the Foil/shielding on the underside of the TOP of the headcase has a 4 inch section that looks like it was ripped out in some way. Could this be part of the problem? I also now wonder if the power at my house is causing or contributing to the hum... Could the messed up shielding/foil be letting AC hum get through?

While I understand the basics , know how to solder/ unsolder, discharge the caps, bias my amp etc... I don't really know where to start or what to test. I have a DMM, however need to be walked through the steps.

How can I tell using my MM if a cap is leaky?

I took pictures and can Post pics of anything that might help. I have a good tech that I can take the amp too if need be, but have 2 weeks of shows ahead of me and really want to use this amp.

[WaltC]

Some DMMs have a capacitance test feature, but I've not had much luck with them. There are (old school) dedicated capacitor test meters, and I've built a kit to detect ESR (leakage) and use it often. You can test, if you're careful, for DC voltage across the coupling caps. You'll have (depending on the location in the circuit) from 200VDC or so up to over 400VDC on one side of the coupling cap and should have (theoretically) zero VDC on the other. Small leakage (in mili-volts) is normal, nothing higher than 1% or so though.

If you're getting a ground loop hum through the power plug in the house, then none of this matters however<G>. The torn shielding shouldn't be the problem, but make sure none of the conductive material can sag down into the chassis when installed in the cab, you don't need periodic shorts and excitement!

You can always try the amp at a friends house or other location just to eliminate that variable.

If no joy, then it's off to the tech...

[benabloom]

Ok I have been gigging with this amp despite the hum issue, because I haven't had time to deal with it... Now another symptom had creeped up..

When I am playing at stage volume and lay into the notes on the low E string the notes mush out and are softer than the rest of the notes...

I emailed David Allen and he thinks that I have a bad Filter Cap...

I purchased new Filter Caps to install (Sprague Atoms 1 40u/500v, and 3 20u/500v as per the schematic). I am handy with a soldering Iron , this is a turret board , and I have a multi meter, however I have never doen a filter cap replacement before, so can you guys give me any advice before I get started? (obviously discharge the caps)

Should I replace 1 by one and see of the symptoms are corrected, or just replace them all and then check?

http://i340.photobucket.com/albums/o339/Horseradishtree/IMG_1834.jpg
http://i340.photobucket.com/albums/o339/Horseradishtree/IMG_1835.jpg
http://i340.photobucket.com/albums/o339/Horseradishtree/IMG_1843.jpg
http://i340.photobucket.com/albums/o339/Horseradishtree/IMG_1834.jpg
http://i340.photobucket.com/albums/o339/Horseradishtree/IMG_1845.jpg



There are 3 filter caps in a row (20/500v) and there is one filter cap that goes directly form the DC power switch to ground (40/500v). Which one would be the most likely culprit if that is even possible to know and how can I try and trouble shoot as to which one is bad if I don't have a capacitence tester?

Thanks,

Ben

[jay42]

Swap out the 40uF first. There's no logic beyond that. Do all three of the 20uF's if the 40uF doesn't do it. I suppose before any of this, you should read the voltages at each B+ node and make sure they're appropriate. I had an amp with a pi filter bias circuit. The second electrolytic was bad and while the voltage wasn't collapsed, it was low and the output tubes were red-plating.

[benabloom]

Thanks!

Last night, I decided to swap power tubes one last time just to make sure it was not tube related before I go swapping Electrolytics (don't want to do the work and risk messing the amp up more if it's not cap related to begin with...) After all, the amp was built in 2001.

Put in the other set of JJ 6l6's I had recieved as a part of a Quad. The 1st pair was the ones in the amp while these symptoms were occuring..
As soon as I took the amp off standby BAM blew the mains fuse. Looking at one of the new tubes revealed that it had gone white. Bad tube right out of the box... since this was part of a quad.... I got more suspiceous..

I put in a new fuse, and installed a matched set of 7581a tubes that I had. Took it off standby and everything is stable. Biased it up to about 35ma and played some licks...( sounds sweet:)

The Hum has improved, however it IS still there. Now the Hum comes on strong when the Master Volume is turned to about 3 oclock. Its hardly noticeable until 3, but gets louder after that on the MV dial. Overall, it seems to have come down in overall volume (the hum that is). It is in fact a 120hz hum as verified by my Iphone's freq analyzer app.

Am I crazy ? Maybe the Hum I am hearing is normal for this amp, and it was compounded by faulty tubes (the JJs only had a few months on them, however they replaced JJs that had been in there for over a year)

What amount of Hum when this amp is turned on Full tilt with no guitar plugged in is normal?

While the amp was live, I chop sticked my way arround inside the amp tapping on all the electrolytics and there contact points and everything seemed really stable.

Should I still go ahead and try to swap the Electrolytics ? I am not against bringing it to a tech, I just am short on downtime between gigs.

[Prairie Dawg]

Buy em all and change them out at one schwack. While you're at it, change the preamp cathode electrolytics and whatever is in the bias system. I personally would not trifle with the Sprague atoms. I'd get myself some good Xicons or Nichicons or whatever Teddy Weber's got. No point in doing the labor four times.

That way you've got a baseline of known good components to work from if you don't find it on the first go round. After this you can do a voltage map and make sure you've got B+ in all the right places.

[tmac]

My guess is that it's 60 hz & after doing all of this you'll still have some hum when the master is maxed (or close to max), just the nature of these amps with this particular layout & grounding scheme (star grounding if I recall). The PPI master volume filters a lot of the hum until you turn it way up and you notice it's there. You may be able to reduce it somewhat by reworking the grounding but it's a very tricky thing. You can search for some grounding method articles like the "Larry grounding" and Doug Hoffman has some good info on grounding http://www.el34world.com/charts/grounds.htm

[jzucker]

all old flames hum when you turn up the master.

[benabloom]

all old flames hum when you turn up the master.


Thanks for commenting here. That is good to know. Maybe I am chasing my tail then...

The weird thing is that it goes from ...virtually no hum to ...oh yeah its humming at one spot on the dial (now 3pm) and then gets louder from there..

Dave Allen emailed me back and suggested taking AC reading on all of the + sides of the Caps while in operation . He gave me some #s to look for see below...

"Normally, I would expect around 3V at the 40uf and about 250mV, 20mV and 5mV at the 20uf caps. Readings significantly higher might indicate a bad filter cap. This is a dangerous test so get a tech or know what you are doing."

[jzucker]

Thanks for commenting here. That is good to know. Maybe I am chasing my tail then...

The weird thing is that it goes from ...virtually no hum to ...oh yeah its humming at one spot on the dial (now 3pm) and then gets louder from there..


I can't imagine it needing new electrolytic caps already. Mine had the same behavior as yours. My dual showman reverb is much quieter surprisingly.

[jay42]

"Normally, I would expect around 3V at the 40uf and about 250mV, 20mV and 5mV at the 20uf caps. Readings significantly higher might indicate a bad filter cap."After looking at voltages, you might tack in one of the 20uF caps across the 40uF. You can go 60uF with a GZ-34/5AR4 rectifier. If it makes an audible difference, then basic filtering is probably the culprit. If not, report back...