Heater wire placement preference

[Underwood]

 I think most people recommend above the the pins. I am trying to track down a 60 cycle hum and was wondering if anyone recommends redoing the heater wires. It is a 1960 concert amp and mine are tucked under the chassic lip and then go under not over the other wires to the pins. I will also be checking to make sure they are all in phase. I found some on my super reverb that were not in phase. I guess it happened a lot.

[eleventeen]

Whether by personal pain, anecdote, or research, the benefits of "scenic" heater wire placement are IMO largely overstated. Phasing, in particular, has been shown to be a nothingburger. "Under" versus "over" other wires means nothing; the case can be made that where wires cross other [signal] wires, one wants 90 degree angles as much as possible. That much makes sense.



While it is certainly possible that your heater wires are inducing some hum, it would be the 3rd or 4th potential culprit on my list.


If you want to definitively eliminate heater wire dress as your hum-bandit, I would do 3 things:


1: Remove 12A_7 tubes, one by one, starting at the power tube end and again, from the preamp end. Take notes.
2: Cut the heater chain after the 6L6's so that the internal PT powers the 6L6 tubes but no others. Get a 6V lantern battery (or, a high-quality DC supply of sufficient ampacity = no of tubes * .3 amps each) and power the preamp tubes from an unassailable clean, clean DC source.
3: Consider replacing the factory 6.3 volt winding CT (which you remove from ground and tape up the end) with the synthetic CT made up from 2 qty 100 ohm resistors to ground. Or, use a 250 ohm pot, ground the wiper, and see if adjusting the pot yields anything.

[Underwood]

I did remove the phase inverter and powered up the amp and the hum was gone. I then replaced it with another tube and the hum is back. What does that tell me?

[eleventeen]

It tells you that the hum originates before the phase inverter, which should be zero surprise. It tells you utterly zero as to whether said hum is heater-related.


You really have to keep going with this exercise because of the nature of the push-pull circuit (the 6L6's and arguably, the PI) has a hum-cancelling characteristic. If you had hum coming from the output stage (the 6L6's) it would probably be a roar, it would not be subtle. Very rarely do we find issues with the PI.


What you are trying to isolate here is the >>source<< of the hum. If it is occurring in a stage where *after* that stage, the next tube produces voltage gain, the gain-producing stage could be doing its intended job (gain) just fine. The overall job of the amp is to take 1/4 volt out of your guitar and turn it into "big" volts and "big" amps such that volts * amps = watts can wiggle your speaker.


The most likely thing is that your early preamp stages have some hum and it is being dutifully amplified by the rest of the amp. That's true of almost all "too much hum" situations.

[eleventeen]

This is going to be somewhat tricky and maybe a lot less useful on this amp, 6G12.


Because, in a blackface amp, if we start with the input jacks for one channel; we feed one half of one tube which equals one triode. The output of that triode then feeds the tone controls for that channel and returns to the second half of that same tube. That means, if we pull out that tube, we are disabling ONLY that one channel and leaving the other channel unchanged.


In *this* amp, one channel feeds 1/2 of the first tube and the second channel feeds the other half of THAT SAME TUBE. [All 7025/12AX7 triodes are identical]  Both inputs go through one tube. Then,  the outputs of those first triodes drive the tonestacks, come back to the second tube, but again, they share that tube.

This is a significantly less intelligent way to do things, because if V1 goes out, you lose BOTH channels. If V2 goes out, you lose both channels. In a blackface, one or the other channel would stay working. It may not be your reverb/trem channel, but you would still have an amp.


But it also affects how useful the tube-pulling exercise will be. Nevertheless, you should do it anyway. As you work your way toward the small tube farthest from the power tubes, you are looking for the one that causes the hum that bugs you. The middle 7025 is the trem oscillator and the 7025 two positions away from the 6L6's is also part of the trem in the is VERY unusual trem ckt. I would not expect the trem to be making your hum (IOW, counting from the first small tube, I would not expect V3 nor V4 to be the problem children, but it's not impossible)

[eleventeen]

Incidentally, before you go nuts (because you mentioned "buzz") are you sure your hum is not being induced from lighting dimmers or some appliance running somewhere else in your house? Just asking.

[MakerDP]

Incidentally, before you go nuts (because you mentioned "buzz") are you sure your hum is not being induced from lighting dimmers or some appliance running somewhere else in your house? Just asking.

Or too close to computer monitors or fluorescent lights?

[Underwood]

I will try the pulling of the tubes. I have also ordered a new power cord. The 16 gauge I put in was too big and the wires were too thick. It was difficult to fit the white wire and also the transformer wire to the terminal. I think I have a bad connection there. Would that make a hum noise?

[trobbins]

So the first simple test is to reduce the two volume pots to zero, and check if you have any hum.  Then raise each one separately to max, and check.  That will narrow down the circuit region to focus on.

[kagliostro]

May be I've read the tread too quickly but

my council is you can help yourself pulling tubes as told and using a simple audio test probe

you can find it here

http://el34world.com/Hoffman/tools.htm

(as amp you can use a PC amplified speaker)

K

[Underwood]

I took out tubes one by one and here's what happened. V5 phase inverter no hum, but no signal at all. V4 no hum but no signal. V3 still a signal and playable but hum is back and no tremolo. V2 hum is there but no signal not playable. V1 hum is there no signal not playable. What does that tell me?

[shooter]

Are you using the "listening device" K talked about or just using a signal source and pulling tubes, listening to the amp?  Also are you leaving the tubes out as you go, or putting them back in? Re-read eleventeen's posts
he has about all the usual suspects listed.  The oddity to me

V3 still a signal and playable

You shouldn't get signal through when the tubes out, but I didn't look at the schematic to see what V3 is

[2deaf]

From the schematic and layout, it looks like V3 is half the vibrato and V4 is the other half.  V1 has the first stage of both both channels with a shared cathode.  V2 has the second stage  of both channels with a shared cathode.  If this is the case and you are applying a signal to the vibrato channel only, the hum source would be narrowed-down to V4 and/or the associated circuit and hardware.   

[trobbins]

V4 no hum but no signal.

Did you try that with the remaining functioning vol pot at zero, and max, to check if that circuitry is all fine?

The problem with pulling V2 is that supply 'A' hum can still inject in to signal path. 

A much easier test is just to min the volume pots and listen for hum when all valves are in.  If no hum noticeable in speakers, then unplug all inputs and repeat test for hum when each vol pot is increased.

You may just have a guitar lead problem.

Hum could also come from the tremolo oscillator stage, so putting the Intensity pot to min will identify if the hum originates from that part of the circuitry.

[jjasilli]

Incidentally, before you go nuts (because you mentioned "buzz") are you sure your hum is not being induced from lighting dimmers or some appliance running somewhere else in your house? Just asking.

I got into this amp building / rebuilding thing because of hum in my Hot Rod Deluxe.  Now I'm scores of amps down the road.  But it turned out the hum was caused by the "smart" lighting rheostat in my ceiling fan.       So, I got inside all these damn amps for nothing!        But now I'm hooked.   

[jojokeo]

Humm has a different sound and meaning to each one of us. There's a myriad of possibilities and like throwing darts in the dark. One more type like the ceiling fan, florescent lighting, appliances, monitors, etc. is simply sitting straight on right in front of the amp with your guitar. As if you're playing at it. Now back away and/or turn 90º to the side. You'll hear noise level changes just doing this. Hopefully this isn't it (but just wanted to mention it). Do you have shorting input jacks? Without anything plugged in and the input shorted is it quiet? Then plug in only the guitar chord, is it back now? When troubleshooting this stuff, NEVER skip any steps. Always start at the beginning of the circuit.

Without knowing the amp or layout, it could simply be transformer placement, almost anything... I haven't seen if the heater wiring has a center tap or not? Is it hooked up to the power tube's cathodes? Have you tried elevating DC? Again, just throwing a few of many darts out there to help you think of and try stuff.

[Underwood]

I ordered a new power cord and am waiting for that to come before I mess around with it again. It may have been the giant hunk of solder connecting the white wire and black transformer wire to the small terminal on the ground switch. The noise/hum is not that bad, but I will try to explain it correctly.
As soon as you turn the amp on from standby it sounds like you have it cranked up to ten. You do not have to be plugged in, and the volume is on zero. It does not get any worse when you plugin or turn the volume up. So it is completely playable. The only thing is playing soft quieter things during recording would be too noisy.