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PostPosted: 02 Mar 2016, 17:05 
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Joined: 20 Feb 2014, 10:35
Posts: 59
Location: NZ
Great looking build, nice & clean!
As to the hum, I notice the choke is mounted on the wooden frame, you could try running a wire from the choke frame to ground.
May not be the issue, but won't hurt to try.


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PostPosted: 02 Mar 2016, 18:22 
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Joined: 10 Sep 2015, 13:36
Posts: 33
Chris65 wrote:
Great looking build, nice & clean!
As to the hum, I notice the choke is mounted on the wooden frame, you could try running a wire from the choke frame to ground.
May not be the issue, but won't hurt to try.


Thank you.
I meant to ground that choke but completely forgot until now that you remind me.
At first I did'nt notice the hum until about a week after I built and started using it,
then I turned up the pot to max and there it was.
I'll attach a ground wire to the choke and see what happens.


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PostPosted: 02 Mar 2016, 18:34 
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Joined: 18 Aug 2015, 07:38
Posts: 166
What I have to say is not what you want to hear. I built a tube amplifier recently that had enough gain for a pot but with the pot in the circuit it hummed. Taking the pot out and the hum went away. My problem was much the same as yours, incorrect layout, I think. I hope the choke ground suggestion works. On the build pictures notice all the PS components were kept on one side with the transformer and even a dividing shield separating the two sections. I hope I am wrong but I think this is your problem. I am sure most builders that have built more than one amp have done the same thing. I did not use a divider but instead used a bigger case than specified and keep the transformer and associated parts on one side and the audio parts on the other. You have the inputs and outputs on one side and the audio components on the other side with the transformer and PS components in between. Mine does not hum except maybe a tiny bit when the pot is turned wide open. At any listening setting it is quite with horns. At least the hum is not so bad that you can use it. I love this little preamp with the RCA clear top 12AU7.


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PostPosted: 02 Mar 2016, 19:44 
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Joined: 10 Sep 2015, 13:36
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wdecho wrote:
What I have to say is not what you want to hear. I built a tube amplifier recently that had enough gain for a pot but with the pot in the circuit it hummed. Taking the pot out and the hum went away. My problem was much the same as yours, incorrect layout, I think. I hope the choke ground suggestion works. On the build pictures notice all the PS components were kept on one side with the transformer and even a dividing shield separating the two sections. I hope I am wrong but I think this is your problem. I am sure most builders that have built more than one amp have done the same thing. I did not use a divider but instead used a bigger case than specified and keep the transformer and associated parts on one side and the audio parts on the other. You have the inputs and outputs on one side and the audio components on the other side with the transformer and PS components in between. Mine does not hum except maybe a tiny bit when the pot is turned wide open. At any listening setting it is quite with horns. At least the hum is not so bad that you can use it. I love this little preamp with the RCA clear top 12AU7.


Yes, I was thinking exactly the same thing, layout and separation (shielding) of the components.
I will probably have to make a new top plate with a better layout and shielding. I'll see about that in
the summer.
I will ground the choke but I'm not too confident it will solve the problem.

As I mentioned, the hum only becomes noticeable only as the volume pot approaches maximum.
I get enough sound level with the pot at the 9 o'clock position so the low level hum is not affecting
my listening enjoyment.


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PostPosted: 02 Mar 2016, 23:34 
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Joined: 20 Feb 2014, 10:35
Posts: 59
Location: NZ
Your layout is not too bad & a dividing shield should not be absolutely necessary but may help. This a line pre-amp, not a phono stage which is more susceptible to noise. Yes, the power supply components could be further from the signal section, but another thing I now notice is the wiring for the heater on to the 12A*7 socket - I would twist all those wires tightly together & the wire to pin 9 should pass over the socket.
I can't see what you've done up to & from the rectifier socket, but the wiring should also be tightly twisted together.
For examples see: http://www.valvewizard.co.uk/heater.html

Transformer buzz can also inject a bit of noise, I put rubber washers under the transformer mounting tabs or even isolation mounts sometimes.

Some hum & noise is a part of tube amps, whether it is too much depends on the system & your perception. I have a little hum & tube noise on my build, but I have to go way past normal listening volumes (& I like it reasonably loud when I can), so it does not concern me.


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PostPosted: 03 Mar 2016, 07:01 
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Joined: 18 Aug 2015, 07:38
Posts: 166
The one thing you may want to try is moving the inputs to the other side closer to the 12AU7 tubes. I now try and keep my inputs close to the first tube stage in all my tube amplifier builds. It keeps the leads short. I believe your input wires go right under your transformer and are much too long susceptible to picking up hum. It would only involve drilling 2 holes and you can say the empty holes are for ventilation. I keep my heater wires high above all components and drop them down to their sockets at 90 degrees and do not lay them flat. An old technician trick.

With a well designed PS section and with well designed amplifying circuits they can be as silent as SS. I have some that are dead quite on my 103db horns. With a preamp you have to be more particular and with a phono tube preamp be exceptionally careful. Any noise in the first stage is going to be amplified by each additional stage.


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PostPosted: 03 Mar 2016, 07:54 
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Joined: 10 Sep 2015, 13:36
Posts: 33
Thanks for all your help. I will most likely do a make over in the summer

"Yes, the power supply components could be further from the signal section, but another thing I now notice is the wiring for the heater on to the 12A*7 socket - I would twist all those wires tightly together & the wire to pin 9 should pass over the socket."

Yes, I should have done a better job on the heater wires, that is one of the things I'll have to fix.

"I now try and keep my inputs close to the first tube stage in all my tube amplifier builds. It keeps the leads short."

I considered that but for aesthetic reasons I placed the phono jacks at the rear, I dislike seeing cables sprouting up at the front. I'll probably move them to the front.

Thanks again for your help.


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PostPosted: 03 Mar 2016, 08:05 
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Joined: 20 Feb 2014, 10:35
Posts: 59
Location: NZ
wdecho wrote:
I keep my heater wires high above all components and drop them down to their sockets at 90 degrees and do not lay them flat. An old technician trick.

Can we see how this is done, I don't recall seeing a photo of your build?


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PostPosted: 03 Mar 2016, 11:59 
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Joined: 09 Oct 2012, 19:43
Posts: 338
Location: Vancouver Canada
One suggestion springs to mind. From your pics it looks to me your cables between input / output jacks and the tube are covered in spaghetti. Keeps things neat but not shielded. Due to the jacks being on the rear and the signal wires having to pass all AC in the amp, Maybe replacing the unshielded with shielded cables.
I would also un-solder the heater wires at the sw and twist them together then re-solder to sw. Push the twisted pair as close to the metal plate as possible.


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PostPosted: 03 Mar 2016, 14:29 
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Joined: 10 Sep 2015, 13:36
Posts: 33
laurie54 wrote:
One suggestion springs to mind. From your pics it looks to me your cables between input / output jacks and the tube are covered in spaghetti. Keeps things neat but not shielded. Due to the jacks being on the rear and the signal wires having to pass all AC in the amp, Maybe replacing the unshielded with shielded cables.
I would also un-solder the heater wires at the sw and twist them together then re-solder to sw. Push the twisted pair as close to the metal plate as possible.


Okay thanks. I'll be taking care of all these issues later on, most likely late spring or early summer.
The hum is only noticeable with the vol pot well past the 12 o'clock position and I hardly ever have to
go much beyond the 9 o'clock position before the sound level gets too loud for me.


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