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    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Heater Wiring - the Good the Bad and the Ugly

I suppose anything is possible! Would 6.3V AC blow a 450V electrolytic?

Remote debugging when we don't know what he did, and probably he doesn't know what he did, is not easy.

I don't know the specifics of the amp in question, but if mis-wiring the 6.3V AC also took out the rectifier for the HT, either by a direct short or through some other component breakdown, then the caps might see high voltage AC... for a short while...

Low voltage AC could do the damage but I'd expect it to take longer.
 
gkenne said:
connecting pins 4 & 5 of the EL84 to pins 4 & 5 of the 12AX7s,
That won't work. Most B9A valves have the heater on pins 4 and 5, but some double triodes have a different arrangement involving pin 9 too. For these, such as 12AX7, you connect 4 and 5 together, then wire the heater supply to 4/5 and 9. Or you can use 12.6V on 4 and 5, ignoring pin 9.
 
I've found that on filament windings with no center tap bypassing both sides of the filament line with caps works better to eliminate hum than a balance pot. I used .47uf @ 100V ceramic caps.
 

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Could be a sign of spurious RF oscillation getting propagated along the heater wiring, rather than normal hum.

Not RF, looked at it on a scope. I first tried a 100 ohm balance pot and it worked but still had a little residual hum, using the caps no hum. I also tried connecting the caps with clip leads across different tubes and the result was the same. If it was some spurious pickup the results would have varied moving the clip leads around. Perhaps someone else can try it and see if they get the same results. I first used Illinois Capacitor coupling capacitors and the results were the same, I only went to ceramic caps because they are physically smaller.
 

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I've found that on filament windings with no center tap bypassing both sides of the filament line with caps works better to eliminate hum than a balance pot. I used .47uf @ 100V ceramic caps.

Hello
Thank you very much.
Great idea, I hope these work for the higher power GU50 to.. That is 12.6V but I must have to check the tube data.
I have 12.5V DC also. Is not better if I use DC for the GU50.
Again I have to take a look at the tube data what is the recommendation. Great help, thanks one more time.;)
Greetings Gabor
 
No noise??

Hi Gilles.

I'm getting 3.9mv and 2.9mv 100hz respectively at the terminals of my AC supplied JELabs 300B monoblocks. Nothing fancy here, just AC straight to a wirewound dropping resistor, routed around the outside and with a hum pot. I haven't tweaked it for optimum performance, just made sure the wires were twisted and placed to the outer edge of the chassis. Would be interesting to see what other folks are achieving and what measurement would be considered "No noise". (Now I've measured it I'm going to have to find out why I've got 1 mv difference. Damn!!)
 

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If you wanted to use cathode bias on a DHT, but still use those .47uF bypassing caps for the filaments, where would you put the cathode bias resistor?

--

I should think you would just connect the resistor from filament to ground. But then the .47 caps would act like a bypass and boost at very high frequencies. I'm only familiar with DHT power output tubes and on a power tube you would want a decent value bypass cap. Might be best to use a separate filament transformer in such a case.
 
Hi Gilles.

I'm getting 3.9mv and 2.9mv 100hz respectively at the terminals of my AC supplied JELabs 300B monoblocks. Nothing fancy here, just AC straight to a wirewound dropping resistor, routed around the outside and with a hum pot. I haven't tweaked it for optimum performance, just made sure the wires were twisted and placed to the outer edge of the chassis. Would be interesting to see what other folks are achieving and what measurement would be considered "No noise". (Now I've measured it I'm going to have to find out why I've got 1 mv difference. Damn!!)

hi

The differences may be due to the tubes. The 100Hz stems from harmonic distortion from cathode to anode. The distortion may differ, hence the amplitude of 100Hz. I wrote a bit more about this here:

http://tentlabs.com/Components/Tubeamp/Tubefilament/assets/Heatingmethods.pdf

The amount of DC shift wen driving the tube a bit harder is also an indicator of the 300b linearity, see:

http://ken-gilbert.com/images/pdf/2 non linear stages.pdf

best,

Guido