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hkoetz 4th September 2005 07:25 PM

help, my amp humz
 
2 Attachment(s)
My recently finished tube amp hums. You can see the schematic below. I used star grounding. There are two toroid power transformers.
Things I tried:
Unplugging the amp from the mains: the hum stops (while it plays on for 10 seconds).
Removing the driver tubes: the hum continues.
Adding extra caps in the power supply (1500 uF): the hum continues
Grounding each of the heater leads in turn: the hum continues
Grounding the bias resistor and cap on the EL34's (thus removing the NFB); the hum continues

What else is there to try? Help is much appreciated!

Alastair E 4th September 2005 07:55 PM

Heater supply?
 
Have you tried a positive bias of say 50V on the heater supply to the EL34 valve?--There may also be a leak from heater to cathode in the EL34--Try a new valve.

Have you scoped the supply rails to the amp? What filtering components have you, L/C/R? what values of cap/choke? SE designs have little PSRR and ripple on the supply will cause hum. Does anything you have tried reduced the hum?

Im currently struggling with hum on a breadboard 811A P-P, but thats because Im using DHT's with AC heaters, so I can sympathise!............:smash:

richwalters 4th September 2005 08:34 PM

The grid leak 680K on EL34 seems high although the orig EL34 spec mentions 700K for this condition........However with todays tubes in class A this high value may have to be reduced.

Short of ideas....Have you wired the valve holder correctly..........as others mentioned a direct heater/cath short is a poss, but a rare one. In my 40 years with tubes this used to be fairly common problem afflicted to TV's..(bad design and over-running ).....but I've never come across this in an amp.

Apart from the obvious mentioned, a badly wound power toroid can spew out magnetic fields far worse imaginable than an E/I core can...........how's your amp's mechanical layout ? Have you checked the obvious i.e a shorting turn isn't made with the centre screw of a toroid , i.e touching a lid/cover etc ?

Remember nearly all SE output trannies are E/I constructed with a gap so the core isn't a continuous path....if mounted close to an toroid this opens the poss for stray magnetic entry.
You don't mention the actual hum level ?

A trick if toroid is suspect ...... add extra length wires to a toroid and turn it whilst circuit is operating. Mains Safety i.e insulating wires, connections etc is obvious.
If the gap of an SE output is closest to the chassis...(esecially steel chassis)....re-orientate, so gap is at side/top. This will avoid circulating magnetic currents going through each leg of gap.

Give it a whirl and let us know

richj

hkoetz 4th September 2005 09:25 PM

help, my amp humz
 
There is a blue glow deep inside both EL34's. Is this OK?

Alastair E 4th September 2005 10:35 PM

Glowing blue
 
Most output types give a blue glow within the electrode structure--Quite normal--Some even make the glass glow blue--KT66 popular for this!

So long as there isnt a very bright blue/white glow which means gassy valve you are OK.

check the electrodes through the little hole in the anode--they shouldnt be glowing red-orange. A VERY slight colour on the anode red is OK, but means you are getting quite close to its maximum anode dissapation.--Check Anode currents and voltage accross valve, calculate anode diss.

TheGatesOfFate 4th September 2005 11:02 PM

Probably it is a ground Loop. Check for find it.

SY 4th September 2005 11:04 PM

Quote:

Unplugging the amp from the mains: the hum stops (while it plays on for 10 seconds).
There's possibly a clue. You've got something (probably one of the power or filament transformers, maybe heater wiring) radiating a field, and something is picking it up. Do you have any layout drawings or photos?

tubelab.com 5th September 2005 02:36 AM

Does the amp hum from the second it is turned on or does the hum appear as the amp warms up. If the hum appears before the tubes warm up it is likely coupling from the power transformer into the output transformers. I had this problem in my Lexan amp until I rotated the power transformer 90 degrees.

Does the amp hum with no output tubes? If so there is likely a transformer coupling problem, or a wiring error.

Do both channels hum equally. If not swap the output tubes and see if the hum follows the tubes.

Does the hum sound like a clean 60 Hz (or 50Hz) sine wave. If so it is likely from the filament circuit.

Does the hum sound like 120Hz (or 100Hz) with higher order harmonics (a raspy or buzzy sound). If so it is likely from the rectifier.

If all else fails rig up a switch in series with the transformer center tap (like a standby switch). When you turn this switch off the amp will continue to run off the charged caps for a few seconds. If it hums for those few seconds the hum is not from the rectifier circuit. If the hum disapears before the music dies (keep the volume low) you have a rectifier problem. Those large caps with a solid state rectifier causes large peak current, which can cause noise and hum that can get into low level circuits. If this is the case try reducing the value of the first cap, or putting small resistors in series with each diode, try 10 ohm 1 watt.

hkoetz 5th September 2005 07:17 PM

Help, my amp humz!
 
The problem is now half solved. As there were some funny noises along with the hum, I reversed the output transformer secondary leads of which one leads to the bias resistor of the EL34 (NFB) and the other to ground. Whoa, the hum reduced considerably! I guess my NFB was PFB.

For the remaining humm:

Quote:

If all else fails rig up a switch in series with the transformer center tap (like a standby switch). When you turn this switch off the amp will continue to run off the charged caps for a few seconds. If it hums for those few seconds the hum is not from the rectifier circuit. If the hum disapears before the music dies (keep the volume low) you have a rectifier problem. Those large caps with a solid state rectifier causes large peak current, which can cause noise and hum that can get into low level circuits. If this is the case try reducing the value of the first cap, or putting small resistors in series with each diode, try 10 ohm 1 watt.
The hum reduced when I disconnected the B+, so I think tubelab may have a point here. I'll buy some resistors and implement tomorrow.

To be continued


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