Resistor (to act like a fuse) for heaters center tap grounding

mroboto

Super Member
Hello,

Situation: This is for a pair of 6SN7 heaters, the center tap of the 3.15V supply

First, the heaters center tap was elevated, I removed the elevation and I connected it to ground via a 120 ohms 2 watts resistor.

But I got a slight hum, so I ordered lower values resistors to have a better grounding for the heaters center tap.

I also grounded the chassis with the 3rd prong of AC power socket, I don't know if this is or could be the culprit for the hum, but first, I think about my bad grounded heaters center tap.

I will soon receive:

4* 0.01 ohm 1 watt Vishay Dale
2* 75 ohms 2 watts Panasonic Metal oxide
and a lot of 0.12 ohm 1 watt Generic Metal oxide

What is the best choice of one or a combinaison of those resistors for this situation ?

Or any other value ?

Thanks for your help.
 
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I lifted the ground from the AC power cord 3rd prong and I still have a slight hum that seems to be the same, when I put my ear close to the woofer (90dB sensitivity woofer).

I hope to be able to keep it grounded by the 3rd prong, because otherwise, the chassis is only grounded by the RCA input jacks.
 
The heater supply should have no current flow to ground, so the value of the resistor is not going to be critical. Honestly you could tie it direct to ground and it won't matter.

Why did you un-elevate the heater supply? If it was quieter that way, I'd put it back the way it was. Often heater supplies are elevated to control heater-cathode voltage differences. The greater the difference, the more chance for hum. It also tends to cause damage in the heater insulation if the voltage difference is too great.
 
I tied the transformer heaters center tap directly to ground, and I still have that slight hum.

I will try to elevate it as it was first..

It was elevated from the B+ cap with a 120K resistor and a 46K as bleeder for the 22uf elevation cap.
 
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I re elevated it, and I still got that small level of hum.

Appart of removing the cathode feedback, my other mod is to remove the 100 ohm resistor and connect the 2.7K cathode resistor directly to ground, and an addition of a 4.7uf 100V CBB film bipolar cap in parallel with the first stage cathode cap and resistor, don't know if my hum could be related to that film bipolar bypass cap, I think I must remove it to see.

ps: I also added bypass caps to 2st stage cathode cap and other power caps.
 
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I removed them, still have little hum, and I am about to give up.

I don't remember hearing that hum before, but it is very slight, maybe it was there at some point and I didn't hear it..
 
If you removed the negative feedback, the gain of the amplifier will increase significantly. It may just be picking up noise from imperfect lead dress or a lot of other things that it wouldn't have with feedback just because of all the gain.

Why remove the feedback? Its a good thing in terms of amplifier performance.
 
Yes, the gain increased a lot, before, I turned volume to 10 o'clock , now at 8 o'clock it's enough.

There is sometimes benifits to remove the feedback from what I read.

From what I think, I was better to not add a bypass film cap in parallel with the cathode capacitor without discarding the feedback circuit, and I wanted to add this film cap.

Maybe it's one faultly KT88 tube ? The last faultly Shuguang KT88 for the first hours was working OK, and after some time was caused ticking noises in both channels, replacing them with Genalex Gold Lion reissues fixed the ticking noise issue, I didn't notice hum in the beginning, but now...

Like every time I do a mod on my new amp, I have tubes problems that makes me undo my work only to realise after that it was a tube related issue.

Maybe tommorow I will try the Shuguangs to see if the ticking noise come back and the hum is gone.

The AC cabling is well made, twisted by the factory.
 
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I did try the Shuguangs and I still got hum, so, the Genalex reissues tubes are OK.

As previously said, the gain increased and he probably pick up more noise.

Would replacing the 1st stage cathode resistor for a higher value be a good idea ?

An approximately twice the value resistor like 5.6K can reduce the bias, the gain, and the hum considerably ?
 
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Looking at the schematic, my concern is the 120v on the second cathode. This violates the 100VDC max. spec. for H-K voltage and can cause a steadily increasing hum. I would rig a voltage divider off the B+ to float the CT on about 60 Vdc. But add a 0.1uF film cap across the 60v. leg to Ground. This should shunt any residual hum to Ground. I've done this and it worked.
 
The capacitor in place was a 100V , instead of 120V or more , so I assume there was less than 100V, seeing the schematic, I replaced it for a 200V, I already added a decoupling cap , a 4.7uf 450V CBB, thanks for your suggestion.
 
Tubes can be noisy. If your heater taps are elevated doubtful if that center tap is the problem. Having a scope would be helpful to determine if transformer or power supply hum. Transformer hum is 60hz and power supply hum 120hz. There are youtube videos that one can hear the difference. Many times the hum is caused by input wires and their placement. Be sure input wires or cables are no where near AC sources. With 90db speakers you should not have hum from say 3 ft. Ground input to see if that makes a difference. Persevere and you will find a cure. I have moved input wires a tiny bit and cured hum on many builds of mine. Shielded input wires are desirable.
 
Hum can be caused in many different ways - I'm not convinced it's the 3.15V heater supply which is causing it here.
If you removed the negative feedback, the gain of the amplifier will increase significantly. It may just be picking up noise from imperfect lead dress or a lot of other things that it wouldn't have with feedback just because of all the gain.
Yes, I agree, the hum could be coming from anywhere and the extra gain has just made it easier to hear.
 
The 5.6K resistors for my first stage 6SN7 cathode are fine, just the gain it should.

Tthe hum was the fault of the non center tapped KT88 6.3V heater tap, I created a virtual center tap trying some 180 ohms resistors I had in hands, the hum can still be heard at 8 inches from the woofers, so I ordered 100 ohms 1w resistors, and I also ordered 500 ohms 2W 10 turns hum balance pots in case the 100 ohms resistors not good enough..

Also, I have 0.01 ohm 1W resistors for the 3.15 V center tap to ground.
 
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