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PostPosted: Mon Sep 02, 2019 3:02 am 
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Capacitors can take a while to break in sometimes. Usually not a problem with new ones, though.

Can you tell if the hum is 60Hz or 120Hz? That could give a clue to where it's coming from. The hum could be coming from an external source, like being too near a fluorescent light or some other source of electromagnetic radiation. The amp might be quieter with a metal cover on the bottom of the chassis.

Same with the ticking noise. That could be external, too. Did you try running the amp in a different room or on a different AC circuit?

5Y3 tubes can't deal with as much filter capacitance as 5AR4s and they aren't as efficient, so if the amp is running well with the 5AR4, stick with it. A choke is for power supply filtering, too. It could further reduce your hum. Vox used them.


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PostPosted: Mon Sep 02, 2019 2:15 pm 
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Ok, I turned the amp on today and the hum is back and unbelievably loud, I think it is definitely 120hz hum as well, it is high pitched and sharp sounding. I can't hear the ticking though, I've unplugged everything in the room besides the amp.


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PostPosted: Mon Sep 02, 2019 6:28 pm 
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120Hz hum means it's coming from the power supply. Check your connections to the filter capacitors. Also it could be a flaky rectifier tube.


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PostPosted: Mon Sep 02, 2019 6:53 pm 
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Here is a vid of the hum/buzz

https://www.youtube.com/watch?v=eCXflOr ... e=youtu.be


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PostPosted: Tue Sep 03, 2019 2:27 pm 
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WHen I measure the 68k resistors on the grids of V1, V1, they show 33k, is this normal?


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PostPosted: Tue Sep 03, 2019 8:08 pm 
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The buzz is highly variable on where the guitar is in relation to the speaker, it can be extremely loud, or just loud.
Are capacitors that exhibit microphonics when I tap on them normal or should they be replaced?
Should I run the coax cables under the board?
Should I install a choke?
Are there instructions for how to install a choke on this amplifier available?


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PostPosted: Wed Sep 04, 2019 12:53 pm 
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A choke won't fix this.... it is not normal. Triple check all your connections in the power supply section against the layout AND the schematic. Guitar placement varies because the hum is there to begin with and it just makes it worse.

coax cables over/under... really doesn't matter that's kinda the point of shielded runs... which leads to another question... are you sure your shields are terminated properly only on one end?

All of your chassis ground connections are good... nice and tight... good solder joints... no paint between the screw and the inside of the chassis?

Which caps specifically are making noise when tapped?


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PostPosted: Wed Sep 04, 2019 1:32 pm 
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Ground connections are all very tight, there is no paint on the screws or chassis. Went over ground lugs with a wire brush to make sure any possible oxidation isn't in the way.

The capacitors that are microphonic are the two 0.22uf above V3 on the layout (C6, C10), and the two 0.01uf in between V3 and V4 (C7, C8) on the layout.

I notice there is scratchiness in the 1M volume potentiometer for the 12AX7 channel coming through the speakers when the standby IS engaged, shouldn't the amp be dead quiet with this standby engaged?

I retraced all connections in the power section and reflowed all solder. Changed out indicator light for a different one (I had substituted a replacement because I damaged the first one).

Changed 5AR4 for a different one. Changed all 12ax7s and EF86.

Still have this really loud hum and rhythmic ticking/thumping.

Should I just take it all apart and rebuild the entire thing? Broken single strand wire somewhere that will be impossible to locate?


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PostPosted: Wed Sep 04, 2019 3:30 pm 
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The microphonic capacitors probably are not a problem. Lots of things in tube circuitry are microphonic. Even patch cords. If all four capacitors do it, it's probably normal. I've never tried tapping the ones on my TC-15 to see if they are microphonic. They wouldn't cause excessive hum, anyway.

I see in the schematic there is a 47k resistor across the Standby switch, so the B+ is never totally off, just reduced in standby.

The 68k resistors at the inputs are normally in parallel, so they would measure about 33k unless patch cords are plugged into both the Hi and Low input jacks. There's a good explanation in the Builder's Guide for how the input jacks work.

A choke would be wired in on the terminal strip where the 1k5 resistor is mounted. It would replace the resistor. But as Darrin said, that's not why your amp is buzzing.

Does the amp still hum excessively with nothing plugged into the inputs? What if the volume controls are at 0?

If you post a couple of pictures showing the interior of the chassis and all your wiring it might help. But right now it sounds like you have a flaky ground someplace.


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PostPosted: Wed Sep 04, 2019 4:00 pm 
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Okay-

Here are some pictures, let me know if you need clearer/more detailed ones-

https://imgur.com/a/faTq6F2

Would using jumpers on the pre-amp tube pins that are 1.5 inches long cause hum? I used some longer ones for ease of installation, would this be a possible source if they are running over heater wires?

Is it possible the 110v taps are interacting with the pilot light wiring and the 0v white and black neutral wiring causing hum?


>Does the amp still hum excessively with nothing plugged into the inputs? What if the volume controls are at 0?

The 12AX7 channel hums quite loud when nothing is plugged in, however it hums much louder when something is plugged in, the hum in both situations is effected by volume.
The EF86 channel does not hum when nothing is plugged in, but exhibits a "white noise" type sound, the white noise sound is effected by volume. This channel has a loud hum when a guitar is plugged in.

I've gone about rechecking voltages and ground points and have not located anything odd except perhaps one point which is low, this is the turret at the very end of the board which has a 330k resistor on it and is connected to pin 6 of the EF86 as well- this reads around 84v when the schematic indicates it should read 131v.


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PostPosted: Thu Sep 05, 2019 1:45 am 
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Thanks for the pictures.

At the ground lug on the chassis near the EF86 input jacks, there should be three wires. One to the input jacks, one to the EF86 and another to the preamp ground bus on the circuit board. I can see the wire to the EF86 in your pictures, but I can't really see the other two. They are there, right? I'm assuming the one to the ground bus on the circuit board connects to the underside of the board.

Can't really see where the ground wire to the 12AX7 input jacks connects to the board, either. It should connect to the ground bus on the board at the nearby 22uF capacitor.

Everything else looks OK. The longer wires shouldn't matter. Try to keep them as far away from heater wires as you can, though. And any wires that cross each other should do so at 90 degrees.

The unused wires on the power transformer are all AC and won't interact with each other. Just keep all the AC wires together, tucked into corners, and well away from any signal wires.

The DC reading at the EF86 that is low might depend on where the Contour switch is set.


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PostPosted: Thu Sep 05, 2019 8:38 am 
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Thanks for taking a look at the pictures.
Yes, there are 3 ground wires going to the preamp ground, and the two grounds that go to the input and volume of the EF86 channel are connected under the board.

As a precaution against one of them perhaps having been broken I've attached a ground to the chassis and then probed the points in which those are attached and get no result there either.
I've chopsticked the wires as far as possible from heater wires and hear no change there either.

Is the scratchy volume pot exhibiting noise even when the stand by is engaged something to look at? Wouldn't that indicate there is something messed up with the way the standby is working or parts connected to it? It makes a scratchy sound for the first 10% of travel or so. I've retraced the wires around there and reflowed solder, nothing seems out of place.
There was one lug on the master cut that had .05v, shouldn't they all be 0?
There was also an increase/change in hum when moving wires running under the 12AX7 inputs, one of the wires connected to the .0015uf, 50pf, or .022uf seemed to be the culprit, the change in hum wasn't huge but was noticeable.

I noticed there are two different ways of constructing the input jack wiring for the head configuration, with one no bus wire going through all tabs on the first (sleeve) section as shown in the layout, and another without the bus wire as shown in the manual, are both of these correct?

Image

Image


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PostPosted: Fri Sep 06, 2019 4:36 pm 
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Do you have an audio probe? Something like this...

http://musicfromouterspace.com/analogsy ... racer.html

They are extremely useful in isolating problem areas. In this case, it may not point you exactly to it because hum is a bugger to track down sometimes but it MIGHT help you isolate the problem area.

Hum like this is usually a problem with bad grounding, usually a component connected where it shouldn't be or bad solder joints. I've heard stories of people checking their circuits over and over and over and over again and not finding their error until like the twelfth time they looked at it.

Sometimes I need to walk away from a project for a week and when I come back to it the problem is right there staring me in the face.


Last edited by dtp on Fri Sep 06, 2019 4:46 pm, edited 1 time in total.

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PostPosted: Fri Sep 06, 2019 4:45 pm 
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OK... spitballing here but it may point you in the right direction....

Remove the 12AX7's from the 12AX7 channel but leave the phase inverter tube in. Does it still hum? Put them back.

Remove the EF86. Does it still hum?

MIGHT point to which channel to focus your search on.

When I built my TC15, I messed-up the contour knob wiring... easy to do that's a complicated piece. Are you sure everything there is OK? No wires touching where they shouldn't? The wires are connected to the the correct lugs on the back of the switch?


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PostPosted: Fri Sep 06, 2019 6:39 pm 
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Good advice. Divide and conquer as they say.

First make sure your power amp doesn't hum. Remove V1, V2, and V3. Ground C7 and C8 if you have to. (the ends marked with an "X" on the schematic).

If that's quiet, remove the ground at C8, add the EF86 and get that channel quiet. Then remove the ground at C7 and add V3 with its volume pot turned down. Finally add V2.

The two input jack wiring diagrams you mentioned are the same. One of them just grounds the switched terminals on the sleeve connections. Not really necessary, but it makes the wiring easier.


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PostPosted: Fri Sep 06, 2019 7:32 pm 
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Another spitball. I overheated the signal wire on a shielded cable and inadvertently grounded it to the shield by melting the internal insulation. Also, maybe to the back of a pot (in your seventh picture)?
Don't know if it applies to your situation. Just a thought.

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PostPosted: Fri Sep 06, 2019 8:50 pm 
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Good eye. The shield could touch the body of the pot in that picture causing a ground loop. That's why I like to put shrink tubing over the shields at both ends - covering it completely at one end, and leaving just enough bare shield at the other end to solder to the lug.

Yes, make sure no uninsulated wires are touching the chassis anywhere, except the ones soldered to the preamp and power ground lugs and the grounds on the speaker jacks.

Another thought - It's OK using three lugs at the power ground since the bolt holding them is tight, but I would solder all four wires to the same lug if I could. Same with the three wires at the preamp ground. It just avoids one more thing that could go wrong.


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PostPosted: Sun Sep 08, 2019 8:44 am 
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Make sure your cell phone not close to the amp. That can cause ticking!
The work looks good so if you still have hum, hit up your geound connections, specially at the PT ground.
The 5Y3 rectifier will work fine , give you a tiny bit less headroom and more sag when playing hard.

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PostPosted: Mon Sep 09, 2019 3:19 pm 
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Thanks for all the responses and input-

The shielded co-ax is not touching the pot-

I will do the procedure for checking power supply with removing tubes/grounds-

There is continuation of funky stuff with that volume potentiometer, the gain of the amp sounds normal/as expected for 90% of the travel, but at the last bit, the amp gets incredibly frazzled sounding, almost like a ring mod or octave fuzz, it definitely shouldn't sound this way...any ideas about how this pot passes sound/crackles through with the stand by engaged (amp should be completely mute)?


Edit:

Ok I turned the amp on with V1, V2, V3 removed and there was no hum.
With V1 removed and V2, V3 installed there was some hum, but this seemed very dependent on where the guitar was in relation to the amp and I could get it pretty quiet in some positions.
With V2, V3 removed and the EF86 in there was minimal hum except when the volume for that channel was at zero it emitted a loud hum. When I turn the volume up on that channel the hum goes away. When I install V2, V3 and remove the EF86 the EF86 channel emits a loud buzz when the volume pot is at zero.
I attached a ground to the ground lug on the pot and then one to the chassis and the hum would go away. Broken strand or possible desolder under the board when installing the components in the turrets?

When I install all preamp tubes and run the EF86 channel, I can hear the hum on the EF86 channel when it is at zero, even when I install a ground from the potentiometer ground lug to chassis ground, IT DOES go away if I press hard on the alligator clip with my finger as in I am the thing that is grounding it/cutting the hum, not the pot ground lug to chassis lug wire. The amp is still noisy/buzzy/humming with that potentiometer ground lug to chassis ground wire installed, and it still makes the frazzled/ringmod/octave fuzz sound when the 12AX7 channel preamp is dimed with the master control engaged.


Edit 2:

Well this is making my brain hurt with how erratic everything is with this amp. I just turned it on again to check the EF86 channel with guitar positioning and it is now NOT humming/buzzing when the volume is at zero, it previously had been doing that 5 minutes ago. The 12AX7 channel however IS, and when I install that EF86 volume pot ground lug to chassis ground wire, I can get the hum to go away simply by putting my finger or hand near the wire, not touching it, if my finger is around an inch from the ground jumper the hum will lessen a large amount.

Edit 3: Ok now the EF86 preamp volume is buzzing with it set at 0% and I can get it to stop by interacting with the ground wire, or knob (?)...I will change out the volume pots and see what happens, maybe the contact inside it is busted? Very weird...all my electronics projects seem to have issues with extreme settings on the pots bringing out all sorts of weirdness. I have this exact issue with a 6G15 I built where it buzzes loudly at 0% on the dwell, and even some pedals I've built where they have issues at min or max settings. :hmmm:


performing the .01uf groundings again-

With v1, v2, v3 removed I can hear static type sounds and some pops/crackles, not super noticeable but there.
Hum increases when ground wire to C8 is removed.
Hum goes away when C7 wire is removed, so with both C7 and C8 connected to ground there is no hum, if I remove one of these and leave the other there is hum. With both grounds removed there is no hum.


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PostPosted: Mon Sep 09, 2019 9:08 pm 
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Problems that come and go or change are usually due to a bad connection someplace. Sometimes a bad part like a flaky pot. If you can reduce the hum by touching things, that indicates something isn't grounded properly.

Check your volume pots to make sure there is 0 ohms resistance between the wiper and chassis ground when the pots are all the way down. Also check to make sure there is 0 ohms between the wiper and the top end of the pot when the pot is dimed.

The good news is your power amp is quiet with its inputs grounded. That means the power amp and the power supply are good.


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