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Stixx 29th January 2005 09:24 PM

Siemens C3g headamp problem
Hi all,

recently I finished my first tube project, a headphone amp using Siemens c3g tubes. I already submitted my amp to the photo gallery, but now I also have to make a thread of it.

The amp has started to sound fine, I guess those tubes just needed some burn in. But…from the beginning I had hum on one channel only, and so far I haven’t been able to track the cause down.
When I read on this forum it seems that the hunt for hum is one of the favourite pastimes of tube-diy’ers!

Anyway…I tried a few things and rerouted some wiring, especially the leads from the signal part to ground. I also upgraded the cathode resistors to Caddock ones (and changed the value to 200 from 180) and experienced a major increase in sound quality. Not sure whether I can fully blame the Caddocks, but when this is so they are well worth the money!
Only, the humming on the right channel remained…It is not strong but somehow obnoxious since it seems to blur that channel and sorta pulls the stereo image to the right also.

Doing some measurements (which sounds easy said but was exciting to me… the very first time I was into the guts of a working tube amp…well I survived and didn’t cause a mess.. :D) I recorded the following: psu at the last cap is around 300V and exactly the same for both channels. Before the anode load resistors I am measuring identical voltages also, but after the resistors it reads 209 volts right and 206V left! The anode load are a five pack of 33k resistors and measure in at 6.65k each, absolutely identical! On the cathode I measured 3.07V right and 3.16V left. Must be the tubes then…..well I swapped the c3g left and right, but nothing changed! I also tried another pair….ditto!

I also suspected the psu with them many chokes, but they should all be at 90 degrees to the transfo (see pic). So slowly but steadily I am running out of ideas!

Since there is a lot of experts o this forum (like….real “masters of hum”!!) I am hoping for some input, ideas or advice…I know us bloody newbies have those problems all the time, but hey….I can always learn something!

Thanks in advance,

PS The humming goes up with volume and appears to be 120 cycles….

Attachments following: schematic first

inside amp
this is not the last version of my wiring....but it shows the general layout. Now I also have real shielded wires from the input rca's to the pot, more capacitance on the output and .....

the uninevitable

reaction 30th January 2005 12:13 PM

Try to use AC heating instead of DC - just put two 100-200 ohm/2-3W series resistors in parallel with your AC heaters wires and connect the midlle point to ground.

You can use 100-200 ohm wirewound pot instead of the resistors (middle pin to ground) and you can adjust manually the most musical and humless point. You can adjust the AC heaters voltage to 6.3V adding one or two symmetrical 10-25W wirewound resistors.

Then you can build a voltage divider and make your heaters supply floating towards the main ground.

These two things always help me to solve any hum related problems.

speaker 30th January 2005 10:30 PM

Re: Siemens C3g headamp problem

Originally posted by Stixx

When I read on this forum it seems that the hunt for hum is one of the favourite pastimes of tube-diy’ers!

Sorry to hear of your troubles hum-wise Oliver. I might try physically checking every connection on the "bad" side relative to the "good" side. "Pin 8 goes here on this side & Pin 8 goes there on that side....." Tedious for sure, but you might get lucky and find the problem straight away too.


garbage 31st January 2005 01:28 PM

just wondering...

are the heaters ac or dc?
also, did you ground the metal housing?

just noticed... heaters are dc.
maybe you can ground the metal housing via the center pin of the loctal socket.


Stixx 31st January 2005 05:32 PM


thank you for the input.....but......sockets are grounded via the center pin:(

I' ve checked my wiring a dozen times but maybe I have to check a dozen times more....

richwalters 31st January 2005 09:15 PM

Hi there....I'm not familiar with this tube.......looking at the circuit values it's running with considerabe current... and looking at your wiring arrangement, I wonder if you are getting modulation hum from the hv input rectifier side..........capacitively coupled via the wiring.

You mentioned sound quality with cathode resistor change...this seems like HF....starting up from stray inductance/long leads
In suspect circuits I often put a finger (with rubber glove on) near circuits where hum is originating......if the hum varies with finger placement.. despite all the 6.3V changes; in my case I found the ECC88 tube going into HF oscillation (modulated by the 100Hz ripple) which implies a better layout. You have only one amp..a the AB test is the best...... In many designs the old fashioned screening can often solved the problem.


garbage 31st January 2005 11:35 PM


Originally posted by Stixx

.....but......sockets are grounded via the center pin:(

my c3g are battery bias with 2 NiMh. you might want to try that as a last resort.

Stixx 10th February 2005 11:01 AM

2 Attachment(s)
Well, after trying a lot of changes I finally found out that the cause of the humming right channel is due to a real beginners mistake......orientation of the chokes!
I put them at 90 degrees to the transformer but didn't know that the cores inside are oriented at 90 degrees also so in the end they are parallel to the trans....ouch!
This means a new top-plate or part of it, since I will split the existing one and put the psu on a plate of its own.

To not do more mistakes like that i have attached an illustration of the planned layout....
can the chokes be oriented the same or is it better to also put 'em at 90 degrees to each other?

Thanks in advance,

garbage 10th February 2005 12:39 PM

Re: finally.....

Originally posted by Stixx
can the chokes be oriented the same or is it better to also put 'em at 90 degrees to each other?

hi Oliver

glad that you have traced your hum problem.
my current 300b amp has chokes that are of the same orientation.

there's slight hum of about 10mv at the speaker outputs. the hum does not increase with volume. and it does not seem to be affected by the psu chokes being in the same direction.

since you are rebuilding, might as well move your current psu chokes outboard via some wires, but placed in the same orientation to see if it causes hum for you? at least this is a sure way to tell if it works. ;)

Stixx 10th February 2005 12:50 PM

Good idea before I finalize everything!

Thanks Garbage and good luck with your amp.......:up:

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