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Chasing hum in a new tube amp

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OK, took the plunge, and built my first DIY tube amp, a direct coupled Darling. Some trouble at first (bad soldering), but got it working with a little help (thanks, Bob).

Sounds great, except for the hum. I searched here and elsewhere, but I can't find any reference to hum the way mine presents.

It is variable with the volume pot, but in what I guess is an unusual way. The hum starts at about 5-10% of full volume, peaks at about 30%, falls off to about 45-50%, and is dead silent above that. So if I play it full on, no hum. Seems equal in both channels (maybe slightly louder right than left).

I don't know how to describe the frequency, except to say its coming out of the mid, not the woofer.

Can anyone help a fella out? Thanks in advance...


Bill
 
It could be several things...

Most likely may be noise being capacitively coupled into the input wiring between the volume control and the first tube's grid. When the volume control is mid-span, the source impedance driving the input is at it's highest, so the wiring is more susceptable to picking up noise. When it's all the way down, it's almost grounded; all the way up, it's connected to the source, which (probably) has a low source resistance.

You usually see this if the input pot is 100k or higher (not if it's 10k, for example).

The coupling can be inside the tube itself (from the filament). In this case usually elevating the filament to some DC voltage (50V or so?) usually cures it.

I've also seen this behaviour if the input tube is oscillating (which sometimes manifests itself as a hum sound). Same idea... when there's a high impedance to ground on the grid, it is more likely to oscillate. A grid stopper resistor in series right at the pin usually cures this.

Another possible cure is to ground the pot case - sometimes that helps too.

Pete
 
I've also seen this behaviour if the input tube is oscillating (which sometimes manifests itself as a hum sound). Same idea... when there's a high impedance to ground on the grid, it is more likely to oscillate. A grid stopper resistor in series right at the pin usually cures this.

I have also found this. If the anode load resistors are not straight onto the valve base pin (ie flying leads) , this can also produce oscillation. A small value resistor on the pin will usually solve this. When I had this issue with a 5687 it manifested as a hum coupled off the heaters.

Shoog
 
I don't see how you guys can diagnose this problem without looking at a schematic or a picture ... pmillett and Shoog suggest adding parts !! ?? !! (Note that the Darling already has some resistors in front of the grids ( http://www.diyparadise.com/Darling.html )

In the absence a picture or diagram of the circuit, I would have guessed the need to fix any and all cold solder joints as bwarden already admits to several = " ... Some trouble at first (bad soldering) ...".

pmillett: " ... Another possible cure is to ground the pot case - sometimes that helps too. ..."

... but I would make sure all the solder joints are good before adding any parts or "grounding" any cases ... ;)
 
I have made no definate advise, simply my experience of what has caused a similar issue for me in the past. These are basic tools in the debugging armoury and can very easily be tried and rejected without causing to much hassle.

He said he had checked for cold solders so I had to assume this was not the issue.

Calm down.

Shoog
 
FastEddy said:
In the absence a picture or diagram of the circuit, I would have guessed the need to fix any and all cold solder joints as bwarden already admits to several = " ... Some trouble at first (bad soldering) ...".

Can you propose a scenario where a bad solder joint would cause the problem reported? Keep in mind that the amp is perfectly functional, and sounds good, except for the odd hum only at mid-volume?

I couldn't think of one. That's why I suggested other things to look at.

Pete
 
Soldering was one heater connection, and a binding post. Continuity of everything checks point to point, no visible cold solder joints.

Schematic attached.
 

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I've seen the complaint often enough! Ground the body of the volume control, install shielded wiring from the input connector to the control and from the control to the input of the amplifier itself and *definitely* install the grid stop resistors as previously advised. Make sure that they are as close to the tube socket as possible.
 
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Hi FastEddy,
I was going to reply the same way as our previous posters. That is until I read your post.

The reason these replies were made was that each poster has experience with this problem. The cause for this is very clear and has been correctly pointed out a few times.

I did repair one amplifier that had the same symptom. In this case, everything was properly grounded. The screened input lead ran parallel and against an AC heater lead for a couple inches. Moving the lead fixed the problem.

-Chris ;)
 
Input pot is 100k. The top plate of the amp is aluminum sheet, and the pot is fastened directly to it, so would that make it sufficiently grounded?

Shielded wiring went in upfront, as that seemed a general recommendation here and elsewhere.

What is a good value to start with for the grid stop resistor? Mounted directly to the grid pin, correct?
 
Chris-

Yes, the pot is grounded.

Another piece to the puzzle - I swapped the 8532/6J4 input tubes for another set that appears to be a different vintage (well, different boxes, at least), and voila! no hum.

So that makes it a tube thing?

Pete-

Is this maybe what you meant about it being inside the tube? I didn't follow your 'elevating the filament voltage' idea. Can you elaborate? Run the heater DC up to 50V? Sort of shock treatment for tubes?

Thanks again to all for their help.

Bill
 
Chris-

More yet, or at least a difference. Way less volume. Have to turn up the volume pot significantly farther (30% instead of 15%) to get the same output. Same source and music.

Different tubes make that much difference, or is this part of the short issue?

So to check it cold, I need to look for continuity between a heater pin and the cathode, correct?

Thanks again,

Bill
 
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Hi Bill,
Substitution is the best way to check for this. Try another tube also.

You can use the "bad" ones to experiment with.
Different tubes make that much difference, or is this part of the short issue?
For leakage - sure. Clean the bottom of the tube to make sure there isn't a conductive path there. Just in case.
So to check it cold, I need to look for continuity between a heater pin and the cathode, correct?
Yes, but shorts like that normally show up when the cathode is hot.

-Chris
 
First offending tube:

Pin 3 to pin 4 (across heater): 2.2 ohms
Pin 3 to pin 1,5, or 6 (grid): 14.24M ohms
Nothing to the cathode or anode

Second:

heater: 2.2 ohms
heater to grid 22.65M ohms
zip elsewhere

So, they're toast. And shorted to the grid. So the grid would have had way more voltage, and that's why they were louder?
 
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Hi Bill,
Try and give those tube bases a thorough cleaning before condemning them. You may need to use alcohol, keep it off the glass, use a toothbrush and really scrub it.

-Chris

Edit: Clean your sockets with alcohol and a toothbrush while you are at it. Let everything sit for an hour to fully dry out. You still may have a heater - cathode short that only shows up when the heater is hot.
 
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