Heater Hum (Resolved)

[MoparWade]

Sorry guys, me again. So, I thought I had found a clever way to run my heater wires but as it turns out I am getting significant hum. I disconnected the transformer heater taps and ran a lantern battery, hum gone. I am new to this so I am not sure if I should be able to hear the PT hum when the heater taps are hooked up but I definitely can. I assume my only choice is to redo the heater wiring using the fly over method? Attached is a picture of how I did my heater wiring.

[PRR]

The blue jumper laying ON one heater lead isn't good.

[MoparWade]

The blue jumper laying ON one heater lead isn't good.

Fair enough but the hum exists before it ever gets there. I have clean signal going in to V1 but after the first stage of amplification it picks up a hum that isn't quite audible but I scoped it and there's a 60hz signal present. After it goes through the second stage of V1 and I probe the plate signal with listening amp it's is very audible. Is it just a flawed wiring technique?

[eleventeen]

Do you have either a "real" (typically green-yellow wire from the tranny led to ground with a solid connection to a transformer mounting bolt OR (actually preferred) the dual 100-ohm resistor from both green 6.3 wires to ground? Almost ALL audio amps---without a CT ground or the synthetic CT---will hum very badly.

[MoparWade]

Do you have either a "real" (typically green-yellow wire from the tranny led to ground with a solid connection to a transformer mounting bolt OR (actually preferred) the dual 100-ohm resistor from both green 6.3 wires to ground? Almost ALL audio amps---without a CT ground or the synthetic CT---will hum very badly.

Yes, I have the synthetic center tap. As recommended by Hoffman I have attached it to the pins on V8 which is closest to the board where the two resistors are located. Could a bad filter cap cause hum that would go away when the heater taps were disconnected?

[sluckey]

I use a very similar heater wiring scheme with good results. I also use the Fender style fly over. And some of my Hammond conversions use a heater scheme that isn't nearly as neat as your scheme. All seem to work just fine. I'll be surprised if changing to the fly over method will be any better.

[HotBluePlates]

What is the amp/circuit? Does the 1st stage where you scope hum entering the system have a cathode bypass cap? Do you have an extra e'lytic cap of 50-250uF (at some low voltage)?

I wouldn't think your heater wiring, or even the blue wire, is to blame for the hum. I routinely do heater wiring which looks "scarier" but is completely quiet.

I suspect heater-to-cathode leakage, or at least we ought to rule it out. If the stage where your scope shows hum entering has a cathode bypass cap (or you can add one), then try tacking in a LOT of uF's... like 100-300uF. If the hum goes away, then it's a tube issue, in the form of heater-to-cathode leakage.

And even if the tube does have that leakage, it may perform just fine in a later stage where signal levels are higher. Sometimes input stages can be very sensitive to leakage and the resulting hum.

[mresistor]

do you have a pic of V1 wiring?

[MoparWade]

What is the amp/circuit? Does the 1st stage where you scope hum entering the system have a cathode bypass cap? Do you have an extra e'lytic cap of 50-250uF (at some low voltage)?

Hoffman AB763 Twin Reverb, yes it has cathode bypass cap and I will have to see what I have for caps to throw at it. I will also try swapping some tubes around and see what effect that has.

do you have a pic of V1 wiring?

Pic of V1-V3 attached.

[sluckey]

I doubt your heater wiring is causing any hum issues in V1.

[mresistor]

looks pretty clean to me ..  so maybe a ground loop issue?

[MoparWade]

Through 200uf at the cathodes on V1 with no change. Swapped tubes to no avail. The hum is affected by volume. As volume goes up the hum gets louder but it also sounds different. Best way to describe it is as if the sound is being created in a pipe and the length of the pipe is being changed or the source of the sound is moving down the pipe.

[HotBluePlates]

Through 200uf at the cathodes on V1 with no change. Swapped tubes to no avail. The hum is affected by volume. ...

If the volume control impacts the hum level, then it is almost certainly happening in the 1st gain stage. That leaves anything from the input jack up to the input of the volume control in the "suspect area".

I see the ground wire running of to the side from that cathode resistor... What input jacks are used and where does that ground go? We may need a photo of the jacks, which also show how you terminated the shield for that shielded input wire.

[MoparWade]

Normal channel inputs. A little hard to get a good photo of this stuff. That ground from the cathode resistor goes to the input jacks. Since the hum goes away with a DC source for heaters and we are confident the heater wires are not the source then is it safe to assume there is some sort of ground loop being created with the artificial center tap?

[MoparWade]

Interesting note: adjustment of the treble pot seems to have the largest effect on hum which is really more of a buzz than a hum if you crank it up. Mid pot has very little effect unless cranked up and bass doesn't seem to effect it. Seems like this is pointing me somewhere but it's pretty late so I'm not going to tinker anymore tonight.

[shooter]

adjustment of the treble pot seems to have the largest effect on hum

you might be looking at dc getting through your signal path.

[MoparWade]

you might be looking at dc getting through your signal path.

Checked for DC volts at all points on my normal channel pots, zero. However, I noticed that when probing the wiper of the master volume I could hear differences in the sound. That is also the other shielded wire. Going to check all my grounding a little later.

[MoparWade]

90% of hum is now gone and 100% gone from normal channel. What's left is probably a cold solder joint. Turns out I had 100k's in my artificial heater tap. Put some 100's in and it's a world of difference.

[eleventeen]

Thank you for the follow up report! Many times a forum member will garner 3 pages of suggestions and pointers only to disappear when the problem gets solved....which it does in 90+% of cases.

[MoparWade]

Thank you for the follow up report! Many times a forum member will garner 3 pages of suggestions and pointers only to disappear when the problem gets solved....which it does in 90+% of cases.

I've noticed that and it drives me crazy.

[HotBluePlates]

90% of hum is now gone and 100% gone from normal channel. ... Turns out I had 100k's in my artificial heater tap. Put some 100's in and it's a world of difference.

Agreed that it is awesome you returned to give input on what solved the issue.

In my case, I am encountering (unexpected) heater-to-cathode leakage on my build. Each of the triode gain stages has a switch to add a higher-value cathode bypass cap to a hard-wired lower-value bypass cap. First tube used in a particular socket gave no hum with the higher total bypass capacitance, but a smooth low hum when set for a lower bypass capacitance.

The hum only there with the smaller bypass capacitance equals heater-to-cathode leakage. I have 3 sleeves (15 tubes) of early-70's RCA tubes, so I went through them to do an in-circuit test for leakage. The hum (for either capacitance) disappeared as soon as a low/no-leakage tube was installed in the socket. To my dismay, 7 of 15 tubes have leakage which give hum in this amp.

A couple tubes of older 60's RCA production have so far been low-leakage and no hum, but I have yet to test/sort all that I have.

I also have a couple of low-level buzz issues to resolve ( think my problem may be similar to yours, but different in a critical way). Once I get those ironed-out, I'll create a thread on the amp, the teething problems & how they were fixed.

[Paul1453]

90% of hum is now gone and 100% gone from normal channel. What's left is probably a cold solder joint. Turns out I had 100k's in my artificial heater tap. Put some 100's in and it's a world of difference.

Whenever I don't recheck my resistor's values with my meter just before soldering them in, this problem seems to rear it's ugly head over and over again.   

I also appreciate the follow up posts that tell us what finally resolves an issue.

I learn many new things watching over your shoulder as you successfully resolve an issue.   

[MoparWade]

I think it's the respectful thing to do and it's a win for everyone. Then maybe someone else can read my post and solve their problem and by acknowledging the help others have given me I stand a better chance of getting help in the future.

[trobbins]

In my case, I am encountering (unexpected) heater-to-cathode leakage on my build. Each of the triode gain stages has a switch to add a higher-value cathode bypass cap to a hard-wired lower-value bypass cap. First tube used in a particular socket gave no hum with the higher total bypass capacitance, but a smooth low hum when set for a lower bypass capacitance.

The hum only there with the smaller bypass capacitance equals heater-to-cathode leakage. I have 3 sleeves (15 tubes) of early-70's RCA tubes, so I went through them to do an in-circuit test for leakage. The hum (for either capacitance) disappeared as soon as a low/no-leakage tube was installed in the socket. To my dismay, 7 of 15 tubes have leakage which give hum in this amp.

A couple tubes of older 60's RCA production have so far been low-leakage and no hum, but I have yet to test/sort all that I have.

Have you tried heater elevation?  That should provide a noticeable increase in heater-cathode resistance for most of those poorer performing tubes, given that you have shown that there is noticeable variation in hum by tube rolling.  I was able to measure quite large changes in heater cathode resistance in 10 samples of old 12AX7, and confirm that resistance was markedly raised by voltage differential.

[HotBluePlates]

Have you tried heater elevation?  That should provide a noticeable increase in heater-cathode resistance for most of those poorer performing tubes, given that you have shown that there is noticeable variation in hum by tube rolling.  I was able to measure quite large changes in heater cathode resistance in 10 samples of old 12AX7, and confirm that resistance was markedly raised by voltage differential.

I don't want to hijack MoparWade's thread more than I already have. 

I may go this route, but have some other minor debugging to do. However, this amp build is short on space, so I'm not certain I have room to add the couple of resistors & wiring needed to elevate the (non-center-tapped winding) heaters in this amp.

Separately, while I have ~50% reject rate on the 70's tubes, the handful (of about another 30) of 60's samples I've tried have all been hum-free. And these are pretty cheap 6BC4's (I paid less than $1/ea, maybe much less). And at 2 for this amp, I think I still have more than a lifetime supply, even rejecting the leaky tubes. So we'll see...