Ok - so I have a low level hum ; that was previously a high level hum....it was solved by my repairman by dressing and moving an internal heater cable to a new orientation.
Oh and my repairman is from now on going to be ME I think
after my latest experience with help from everyone - thanks!
The cable takes DC heater to V4 across the underside of the amp, and it got moved during a recent repair, then twiddled to a lowest noise location and tied in place.
V4 hums when you touch the glass, which reduces but still hums if you touch the chassis and the glass of the valve. No other valve does this in the amp
I can't help but think the two are related, and this is still introducing some residual hum?
Any ideas?
Oh and my repairman is from now on going to be ME I think
after my latest experience with help from everyone - thanks! The cable takes DC heater to V4 across the underside of the amp, and it got moved during a recent repair, then twiddled to a lowest noise location and tied in place.
V4 hums when you touch the glass, which reduces but still hums if you touch the chassis and the glass of the valve. No other valve does this in the amp
I can't help but think the two are related, and this is still introducing some residual hum?
Any ideas?
Yes you are picking up induced noise from the circuit ,your body acts like a capacitor that's why it reduces when touching the chassis .
I take it changing the valve makes no difference ?
What type is it as a test is to fit a LOW GM ( gain ) valve of the same type in its place to see if the hum reduces otherwise the routing of the cable is suspect .
You could try soldering in a piece of quality co-ax --earthing the shielding ( if possible ) depending on whether one side is earthed as standard or not to see if that helps in place of the re-routed heater cable that would help narrow down the problem .
I take it changing the valve makes no difference ?
What type is it as a test is to fit a LOW GM ( gain ) valve of the same type in its place to see if the hum reduces otherwise the routing of the cable is suspect .
You could try soldering in a piece of quality co-ax --earthing the shielding ( if possible ) depending on whether one side is earthed as standard or not to see if that helps in place of the re-routed heater cable that would help narrow down the problem .
changing the valve makes no difference. This is the circuit, and the next valve in line has the same DC supply in sequence, but does not hum on touch. Could it be as simple as slightly poor contact from this supply somehow - or something like that. Maybe the pin 9 on the 12AX7 valve needs an earth?
https://www.arcdb.ws/Database/SP8/ARC_SP8_manual.pdf
https://www.arcdb.ws/Database/SP8/ARC_SP8_manual.pdf
Pin 9 is the center tap of the heater supply as the valve can operate from either 12.6 volts or 6.3 volts .
If your heater chain is 6.3 volts then pin 9 should be earthed --if it is 12.6 volts then do not earth it directly.
The 12AX7 has a 10 volts 1 watt zenner on the cathodes this goes faulty or overheats --check it out.
If your heater chain is 6.3 volts then pin 9 should be earthed --if it is 12.6 volts then do not earth it directly.
The 12AX7 has a 10 volts 1 watt zenner on the cathodes this goes faulty or overheats --check it out.
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If the tube hums when you touch the glass, could you be coupling the hum to its anode?
If you interrupt V4's heater voltage and substitute another same-type tube's heater in the series connection so that V5 is heated normally, then does touching the glass of V4 still make the hum?
Alternately, if you ground the grid of V4 to the same point to which V4's cathode bias resistor R28 is grounded, then does touching the glass of V4 still make the hum?
Both checks should be done with the shortest possible wires, not with long clip leads.
If you interrupt V4's heater voltage and substitute another same-type tube's heater in the series connection so that V5 is heated normally, then does touching the glass of V4 still make the hum?
Alternately, if you ground the grid of V4 to the same point to which V4's cathode bias resistor R28 is grounded, then does touching the glass of V4 still make the hum?
Both checks should be done with the shortest possible wires, not with long clip leads.
“In my house there's this light switch that doesn't do anything. Every so often I would flick it on and off a few times just to check. Yesterday, I got a call from a woman in Madagascar. She said, 'Cut that out.'”
― Steven Wright
Yeah, several hours of my life went up over "why" one channel does this and not the other, in tube electronics. It became more of an ego thing, regarding my capability of diagnosing and correcting. Guess what? - Inanimate object 1, Joe zero...
What really got me hooked was that it didnt matter which or what tube I put in that side - Oh, this'll be easy...to conquer. Right!
― Steven Wright
Yeah, several hours of my life went up over "why" one channel does this and not the other, in tube electronics. It became more of an ego thing, regarding my capability of diagnosing and correcting. Guess what? - Inanimate object 1, Joe zero...
What really got me hooked was that it didnt matter which or what tube I put in that side - Oh, this'll be easy...to conquer. Right!
Tonescout you do realise that a 12AT7 =ECC81 is an RF TUBE --my circuit of your amp ( I could get to your link ) shows it as a 12AX7 more appropriate to audio, 12AT7 is an VHF double triode used in radio circuits .
Both cathodes pin 3 and pin 8 connect to the zener diode .
REMOVE the 12AT7 --fit a 12AX7.
Both cathodes pin 3 and pin 8 connect to the zener diode .
REMOVE the 12AT7 --fit a 12AX7.
Ok, just DO NOT TOUCH V4 GLASS, period.
What business do you have touching it anyway?
AFAIK left hand holds guitar and fingers frets, right hand picks, strums or fingerpicks.
Reverse for left handed people.
Do you have 3 hands and don´t know what to do with the extra one?
I tried the shielding the apparently noisy cable and it made no difference. It is weird because this valve is amazingly sensitive to noise pick up, yet all other valves are inert. it can even detect my hand near the valve with a buzz/hum sound. I will try the other advice in the next couple of days. Opcom, thanks and f I follow your test advice what will I be able to deduce - what will this tell me?
Opcom has a point in regards to earthing the grid which is the most sensitive part of a tube.
An ECC83/12AX7 operating specs are -
plate=250 volts .
grid=-2 volts ( minus ) .
current -plate=1.2ma.
gain=1.6ma/V.
when 6.3 PARALLEL heaters are used . 12 volt heaters have different specs.
What happens when you put an earthed screening can on it ?
B9A 50MM VENTED SCREENING CANS - Langrex
An ECC83/12AX7 operating specs are -
plate=250 volts .
grid=-2 volts ( minus ) .
current -plate=1.2ma.
gain=1.6ma/V.
when 6.3 PARALLEL heaters are used . 12 volt heaters have different specs.
What happens when you put an earthed screening can on it ?
B9A 50MM VENTED SCREENING CANS - Langrex
I have never tried one, will give it a go. Thanks Langrex are a short drive from me, if the world was not in lockdown again.
My logic was more simplistic, I have two 12 AX7 valves in a row, V4 has a noise on touching it, V5 does not yet they seem to be in a very similar arrangement...therefore something is amiss with V4 in someway.
My logic was more simplistic, I have two 12 AX7 valves in a row, V4 has a noise on touching it, V5 does not yet they seem to be in a very similar arrangement...therefore something is amiss with V4 in someway.
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