tube hum in a 6SN7 Aikido - is this weird? - diyAudio
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Old 5th March 2007, 03:53 PM   #1
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Default tube hum in a 6SN7 Aikido - is this weird?

In a previous posts (well many actually), I have lamented the hum in my Aikido. More recently, some answers to my posts have suggested that my 6SN7s may be contributing to the problem.
The tubes that I have are:

Sylvania 6SN7GTA
Tung-sol 6SN7GTB
Magnavox 6SN7GTB

I have one pair of each tube. This is where it gets weird......if I use an identical pair (eg Sylvania) as input and output in one side of the Aikido, I get no hum at all. I can do the same for the Tung-sol and the Magnavox. If I mix the tubes, even pairing the Tung-sol and Magnavox (same tube type), I get hum.

So, could identical brand and tube types in input and output be somehow acting synergistically in cancelling the hum?

What is the chance that the hum is being caused by my 6.3VAC heaters, even though I have tightly coiled the AC wiring and smashed it against the corner of the chassis? Should I simply invest in a 12VAC transformer and rectify it to give 12VDC - at least then I could chose to use wither 6V or 12V heaters?

Has anyone else experienced this?

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Old 6th March 2007, 12:05 AM   #2
jayme is offline jayme  United States
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I have only tried EH 6SN7s, but I had several that were too hummy in my Aikido. Of the 6 I own, one is terrible, one is moderate, one has just a tiny bit of hum (and sometimes it is completely quiet...comes and goes), and the rest are dead quiet.

Needless to say, I only use the 3 quiet ones and the one with the tiny amount.

I've got 6.3V AC heaters, with a B+ around 330V and my filaments biased up to 90V (so that I don't exceed heater-to-cathode limits).

I'm running them within plate dissipation spec, as well. 5mA of current per section on the front tubes, 10mA on the back tubes.

I haven't found a reason yet, nor a solution. I'm definitely interested if others have similar experience with 6SN7 aikidos....
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Old 6th March 2007, 03:29 AM   #3
Tweeker is offline Tweeker  United States
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Have you tried using a center tapped (real or artificial) heater feed?
Be sure your foil hat has a good low impedance ground.
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Old 6th March 2007, 04:15 AM   #4
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I got a few more tubes in the mail today. So far, the quietest tubes are my GE and Tung-sol 6SN7GTB tubes. I can even mix them between sockets and channels. I have to try swapping around my Sylvanias.

Tweaker, my heaters are referenced to 1/4 B+ using the 6.3vac transformer's center-tap. John Broskie thinks that maye it is an off-center center-tap, and that I should create an artificial one from the windings using 1% 1K resistors.

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Old 6th March 2007, 10:52 AM   #5
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How are you feeding the B+ to your channels? Have you properly decoupled ?

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Old 6th March 2007, 04:55 PM   #6
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I am using the Aikido Stereo Octal PCB. It uses two capacitors labelled C5 and C6. The values that I am using are:

C5 goes straight from B+ to GND and is 2.2uF 400V Wima polyprop. John Broskie suggests 1-10uF.

C6 goes from B+ through two resistors (R15 = 80.6K or 84.5K - I can't remember; R16 = 100K) to GND. It is 0.1uF 400V Wima polyprop. The recommended part value is 0.1uF

The only other caps are on the signal output. I am using (cr*p, I can't remember the value) Auricap. I think that I set the value to filter below 10Hz. I wired them in according to their red/black leads which correspond to outer and inner windings.

Having messed about last night with different tubes, I realize that tube brand and type play a major role. If I use my Tung-sol 6SN7GTB as input and GE 6SN7GTB as output, I get no discernable 60Hz hum, but I do seem to get a very small amount of higher (120Hz???) hum from my righgt channel. If I swap tubes between channels, the hum moves to the left channel, so it must be tube related. I haven't yet worked out which tube is the culprit yet.

John Broskie implied in an email that I should use DC heaters - at least he assumed that I was using DC. He also said that he thought that 12SN7s would improve the bass quality. So, I am now looking around for a suitable 12 rectification circuit. If I go to 12V, I'll simply replace my 6.3VAC transformer with a 12VAC transformer, as they're not all that expensive.

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Old 6th March 2007, 06:17 PM   #7
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Location: Mt Pleasant Sc
It sounds like you have some heater to cathode leakage in some of the tubes.
I recommend that you put a 100 ohm wirewound pot across the 6.3 v heater winding and feed the center rotor with about 15 to 30 vdc from the ps. Disconnect the center tap of the heater first!
Now you can adjust for min hum.
Try swapping individual tubes and try to see if the hum follows a tube. That is the bad tube.
Hope this helps
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Old 11th March 2007, 04:11 AM   #8
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I think Ed is right. I have had this problem in 6SN7 tubes in my power amp. If there is any heater - cathode leakage, you will get LOTS of hum.
Your problem seems to be with the tubes and not your circuit / wiring as you state that the hum problem goes to where certain tubes are used.
I think that a lot of people (myself included) get new tubes and automatically think that the fault can't be with them because they're new, but sometimes this is not true.
If it ends up being a leakage problem within the tube, you MUST use DC for the heaters and then you will still get some residual noise unless the heater supply is very well regulated. 12SN7's will be easier to regulate as they take half the current of the 6SN7's.
Either way you should be using DC for the heaters in ANY preamp circuit.
Good luck... Daniel
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Old 11th March 2007, 05:43 AM   #9
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Dan, Ed.

Thanks for the replies. I am going to go with DC heaters. I am awaiting the regulated PSU kit from Welborne Labs. I am sure that I could have made this circuit myself, but at this point, it seems that having a printed PCB and kit parts will result in much less stress. Right now, they're having a sale, so I even got 15% off their regular price.

I'll make another posting once I get the DC working.

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Old 11th March 2007, 08:56 AM   #10
Salas is offline Salas  Greece
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My understanding is that the Aikido idea is to cancel out PSU hum actively, based on its top to bottom triodes correlating it destructively. This automatically asks for close to identical triode sections. Hmm a bit optimistic for aligned wires and metal frames in small glass envelopes. It will produce surprises some times. But there must be a solution nearly always. Using a regulated PSU beats the original concept?
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