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MG Head Help!

post #1 of 18
Thread Starter 
As some of you resident electronics guys may be aware, like a few others on this site, I recently modded my MG Head DT by removing the negative feedback wires from the board. The sound improvement is rather dramatic. However, I've been having a problem with a buzzing (not a hum or a hiss) that is predominantly located in the left channel. After checking everything over (playing around inside......I have no clue what I'm doing), this is where I'm at:
OK...after further research into the buzzing matter, I think I've discovered something, but I have no idea what it means. With the amp upside down and the bottom plate removed, and my CD3000s plugged in, the hum is present UNTIL I put my fingers near the two red wires leading from the front switch. As I approach the wires (very carefully I might add), the buzz lessens and then disappears entirely when I touch the wires (either one or both, it doesn't seem to matter.) As soon as I let go and pull away, the buzz reappears. Also, if I pull the rectangular switch housing out of the front of the chassis, the buzz gets much louder. The buzz at times is centered in my head, at other times it's only in the left channel. I'm assuming that my body is acting as a ground somehow and that is what is reducing the buzz. Touching the wires leads to total silence. I removed the ground from the rear and scraped some more paint off the chassis to make sure there was a good connection. No improvement. If anyone has any ideas how to permanently remove this buzz it would be greatly appreciated. I really don't want to have to stand there with my fingers on the wires while I'm listening to music...........but if I have to...... Any help is appreciated.
post #2 of 18
sounds like a grounding problem .Not being real familiar with this amp,are the wires in question the hi/lo Z switch wires ?
If so,maybe you could get someone to replace them with shielded wires ,with the shield connected at ONE END only.Otherwise you could possibly set up another problem-the dreaded ground loop.

Option #2 would be to disconnect the wires,slip a ferrite bead on each and re-attache them to the switch

option #3-if there is enough slack try moving the wires around for least amount of noise

Hope this helps man
post #3 of 18
I believe the wires he is talking about are the wires to the front power switch. They run along the side of the PC board near the left channel output tube. I suggest trying to move these wires as far away as possible from any circuitry. This may be difficult since there is not much slack. These wires are supposed to be twisted. It seemed suspicious to me all along that these power wires would be routed so close to the circuitry.
post #4 of 18
Thread Starter 
Quote:
These wires are supposed to be twisted.
Well, they are definitely NOT twisted together. But I did notice that the closer they are to each other, the lower the buzz.
post #5 of 18
It may not help. but I suggest disconnecting these wires from the power switch (they should just pull off) and twisting them if there is enough length. Also, try to move them away from the circuitry. I think this is a long shot, though.

I'm also wondering if somehow when you disconnected the feedback that now some wires are acting like an antenna. Try moving any disconnected wires around to see if this has any effect.
post #6 of 18
Thread Starter 
Thanks for the suggestions, Rick and Kim.......I'll try the wire twisting tonight. I've already played with the wires I disconnected (they are now heavily covered in electrical tap), and there is no change in the buzz no matter how I move them around. I think, as you suggest, the answer to my problem lies with those two red wires from the power switch.
Another question......without a signal, I notice that turning up the volume contro to about 10-12 o'clock completely eliminates the buzz. Removal of the negative feedback wires has resulted in a dramatic increase in the gain of this amp. Since the music is painfully loud at this setting, is there any way to somehow leave the volume knob at high position and install a volume control/attenuator between the 1/4 inch headphone jack and the headphones? Would a simple in-line headphone control from Radio Shack be sufficient? Would there be any sonic drawbacks to this approach? Again, thanks for any help/suggestions you can offer in this matter.
post #7 of 18
I have the OTL version. When I turn off the NFB (same thing as removing the wire I believe) I get the buzz in my left ear as well. I might have to mess around with it as well.
post #8 of 18
Thread Starter 
Success! .................almost.................I think.
Well I twisted the 2 red wires together (I couldn't get them off the switch, so I just popped the swith box out and turned it a few times, which twisted the wires in the process.) The buzz, with the ultra sensitive CD3000s is still very slightly there, but nowhere near the level it was at. It is also consistently centered, not left channel only. With music playing, even during quiet passages, it is basically inaudible. Through the Senns and AKGs, it is dead silent at all times. I just hope it remains this way, because the sound out of this thing is incredible. I'm in the process of acquiring an Earmax Pro and I'll be anxious to see how it compares to the modded MG Head. Thanks to all who helped me out with their suggestions............I love this place!
post #9 of 18
To drop the sensitivity of the amp you can add additional resistance in line with the pot by soldering good-quality resistors between the input wires and the pot. But, many people say this reduces dynamics.

The red wires to the switch are carrying household AC current at line level right underneath the left output tube socket and some of the left channel signal traces.

You have finally improved your amp enough that the noise floor has been pushed low enough that you notice this grunge. Congrats, you now know what 60-cycle AC noise sounds like, and you'll always be able to identify it in the future.

Twisting the wires was the right thing to reduce the electrical field from the wires, but you can do better if you want. With low-impedence cans, you might hear the buzz again. I know I did, until I shielded the wires.

Unplug, allow caps to drain, take all usual safety precautions & don't even crack open a tube amp in the first place unless you know how not to get killed.

Get some 18 gauge twisted pair cable with a braid shield (handmade electronics sells it for $2 a foot if you can't find something cheaper locally).

Unsolder the red wires from the switch but be careful, because this kind of switch is REALLY easy to melt. Cut the shield back just enough to get two tails on the twisted pair to solder onto the switch blades. You want the shield to cover as much of the twisted pair as possible. BUT it's a good idea to put a little heat shrink on the cut end of the shield, so you don't run a risk that the shield braid will touch the blades of the switch and pick up household current. Again, be careful soldering the switch; it melts easily.

On the other end, tease some wire out of the shield and solder a regular insulated wire tail on to it -- fairly small guage is fine. Fold it back and heat-shrink the cut end of the shield again as above. Solder the twisted pair to the board to replace the red wires, then solder the wire tail from the shield back to the ground hole on the board back by the filter caps.

If you make additional improvements you may notice the buzz comes back. This is because the filament feeds are also unshielded and carrying AC at high current. So, rip 'em out and replace with shielded wire (including the feeds between the tube sockets). I used Canare Star Quad (cheap from Markertek) because I had it around and it's very flexible.
post #10 of 18
Thread Starter 
Thanks for the fix, Videoshielded. Unfortunately, I think my soldering skills have deteriorated over time. The only soldering I've done in the last 30 years involved de-soldering the two green negative feedback wires. I'm gonna have to get up the courage for this project. Would Radio Shack carry the things I need?
Thanks again.
post #11 of 18
They'll have the heat shrink for sure. Don't know about the cable; you might have to order it through their website.

Fear not -- your soldering skills are still up to the job, I'm sure.
post #12 of 18
the inline resistor suggestion is a good one,however i would not permanately add it to the amp.Instead I would build a simple attenuator circuit in an external box that would allow it to be switched out of the signal chain.
Total parts count would be
1-the enclosure
2-two pairs of RCA jacks
3-four resistors
4-DPDT switch
5-wire

Going a step along would be to use a rotary switch with several discrete attenuation steps for use with headphones of different sensitivities.You really want to use the amp volume control at the mid to upper range of the rotation due to the notorious problem of poor interchannel tracking at the extreme low end position of most volume controls.
post #13 of 18
An easier way to shield the wires mentioned above might be to wrap metal foil tape around each pair. This may be enough, but connecting the metal tape to ground by soldering a wire to it might be necessary. You can find metal tape at some hardware or electrical supply stores. You would have to keep the tape away from other connections so it doesn't cause shorts. I would use double-sided adhesive foam tape to hold the wires in place against the side of the chassis.

With respect to reducing the gain, another solution would be to add a resistor between the the output (wiper) connection of the pot and ground (maybe 4.7K to 10K). This would lower the gain without adding resistance between the input and the grids of the 12AX7s. This also doesn't require disconnecting any wires
post #14 of 18
I've been quite annoyed by the electrical buzz on the left channel until lastnite. Took one of your advice (not quite sure who, may be videoshielded) and moved the wires as far away from the board as possible, then taped them to the side wall. The sound is perfect now, absolutely no buzz, hum or hiss.
post #15 of 18
I did the same thing, except I gave them a small twist as well. It got rid of most of the hum/buzz. Very happy.
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