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Got my MG Head DT but it has a problem...

post #1 of 9
Thread Starter 
I got the unit yesterday and have been testing it for quite some time but it developed a slight buzz in the left channel. I've tried a combination of power conditioning/changing interconnects and even reversing the left/right channels to see if the source was the problem. The source seems to be fine and the problem persists even if I hook up some speakers to the MG Head DT.

Joseph Lau recommended changing the tubes and I may try this. Has anyone got any other ideas?

Tubes in their at the moment:
12AX7 - EI
2x 6BQ5 - Sovtek
post #2 of 9
On one occasion when I owned that amp it all of a sudden developed a similar problem as you describe. It turned out to be hologen lamp with a dimmer in the another room. Every time it was turned on the amp displayed the hum. I also had a problem with Svetlana 12AX7's which I was using as a replacement for the stock 12AX7 tube (JJ) that came with the amp. Everytime I installed it a low level hum could be heard in one channel. I was even sent a replacement but that displayed the same symptom. So indeed it could be a bad 12AX7 tube. A good (low cost) 12AX7 replacement would be the Sovtek 12AX7LPS which worked fine in that amp.
post #3 of 9
Thread Starter 
Flashbak,

I have halogen lamps in my house but I tried switching off all lights in the room plus closing the door. It didn't work.

I even switched houses and tried a different power point and power conditioner but no luck either.

I'll try tube rolling as my last resort I think i'll use an Electroharmonix 12AX7.

The weird thing is when the power is off the hum is still there. Could it be because of the Rega Planet? It has no grounding pin on the 3 Pin IEC.
post #4 of 9
In additon of what flashbak said, try changing the tubes, i.e. swapping the left and right channel tubes. It will give you an idea whether it is one of the 6BQ5 tubes
post #5 of 9
Quote:
Originally posted by evilcthul
The weird thing is when the power is off the hum is still there.
Um, que? That should not be the case...it may not be the amp...you sleeping in a vibrating bed? In a garbage truck?
post #6 of 9
Thread Starter 
The funny thing is when the power point is off buzz goes away. When the power point is on the buzz is there. This happened on two seperate power points in different houses and one of the power points had a line conditioner attached...
post #7 of 9
Thread Starter 
Flashbak,

I have a question for you. Was the hum only on the left channel of the amp or across both channels?

Also, my problem is a very low pitched hum kind of sounds like a"burrrrr..." the sort you can hear from a high voltage substation. It's very soft but it can be heard during some parts of the music. The hum is definitely there but I guess my last resort is tube rolling the 12AX7. It could be transformer hum but I doubt it. Putting my ear to the unit all I hear is the gas sound from the tube.

Did the Sovtek kill the problem? Or was it just less susceptible to the interference.

Also, one more question. There are two Sovtek AX7s...
http://www.retrosound.com.au/Valves.html

12AX7LP or 12AX7WA... Which one did you use?

Argh another question came up:

Does the hum get louder or softer if you move the volume up/down?
post #8 of 9
When I first got my MG Head, I had similar problems.. a low frequency hum and a hum in a channel (left I think). Well its been long since, I tube rolled and the unit burned in and I do not get hums (just noise). Maybe time is the answer.

Also, try using different ICs... is the hum still there when the ICs are removed? I get hums when power cords are too close to the ICs as well... try playing with positions. Maybe you need some EMI suppresion like ferrite cores (oh oh, incoming Jude!).

My current tubes are (just fyi):
EI El84s
Svetlana 12AX7
post #9 of 9
evilcthul,
On both occasions (hologen lamp & Sovtek 12AX7) the hum was confined to only one channel of the amp. With the bad tube both channels had the hum but it was significantly louder in one then the other. Actually, I could only hear it at lower volumes levels since a volume increase would cause the music to mask the hum. It was the 12AX7LPS Sovtek (Hi-Gain,Low Noise, Long Plate Spiral Filament Tube). The spiral filament helps to reduce noise.
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