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previous: dale Re: Marshall Hum -- 4/21/2000 10:59 PM

Marshall (and other) Hum Sources

4/22/2000 3:54 PM
Ray Ivers	Marshall (and other) Hum Sources Dale, The wireless unit probably had quite a low output impedance. This would affect your amp very much the same as if you plugged in your (non-Les Paul) guitar and turned the volume down to almost zero. The 2210 and 2205 would be especially affected by this - unlike most Marshalls or guitar amps in general, they have NO series resistance between the input jack and the first gain stage's grid, so whatever impedance/resistance is present at the input will be felt directly by the tube. This problem sounds like either an unshielded wire to the first tube's grid (some 2210's have it, some use just a plain wire, and some ground the shield to the plate of the tube, which is good for stability but lousy for hum shielding, I believe), poor lead-dress at the first stage (pull those green wires AWAY from their buddies!) or a ground-related problem at the input jack or input stage cathode resistor (these two grounds should be connected together). If the hum was occurring at a low-impedance 'node', like the top of an unbypassed cathode resistor or cathode-follower output, then you would have to suspect a high-current hum source like a power-supply-related ground loop or heater voltage bleed, but since the wireless unit made such a big difference it sounds like the hum was being injected at the tube's grid. The most important thing with hum is - what is the pitch of the hum? If it's 60 Hz hum (roughly the open low E on the guitar - is that correct?) and sounds really smooth and pure, it's most probably related to the parts of the amp's circuitry that uses 60 Hz - the heaters and the heater center-tap ground, the power transformer primary, (and the field it throws - ever see an amp hum on standby, with the output tubes out?), AC-operated relays, and GROUND-LOOP HUM FROM OTHER EQUIPMENT. If it's 120 Hz hum, it's probably something to do with the DC power supplies in the amp, starting from the power transformer hi-voltage secondary center-tap ground (if used), and moving on to every filter cap and its ground, and also (sometimes I tend to forget this, and it's especially important in the 2205/2210) any other DC voltage derived from the power transformer. In the 2205/2210, the input jack serves to ground out the rectified DC used for the channel-switching circuitry - this is why the red gain-channel LED goes out when you unplug your guitar. There's some real potential for hum bleed here, and in fact everywhere where the channel-switching circuit's tentacles reach (and they reach a fair number of places). If the low-voltage DC to the switching circuitry in this amp develops a hum, it will inject it all over the place, and unless you know where to look, the amp will seem to be humming from multiple, unrelated sources. I had a Two-Rock amp on the bench recently, and I got to talk to the designer (I forget his name). The owner had complained of hum, and I think I knew what it was, only because I had encountered it in my own designs. The relays! I feel that if you're going to use a DC supply for practically anything other than the output tubes' plate supply, it should be regulated, and relays are no exception. They are a finely-wound coil of wire around a magnetic core, right next to a switch (possibly switching a high-impedance, low-level signal at a hi-gain part of the circuit). The designer was pretty laid-back about this, so in keeping with the mood I handed this $4,100 amp back to its owner in a laid-back kind of way. Fixing it would have voided the warranty, and without the factory's OK there was no way I was going to do that to my buddy. Anyway, I like the 2205/2210 (I've owned one, and now own two 4212 50-watt combos), but yes - they have been known to hum on occasion. The distortion circuit in these amps (diode-bridged diode bridge! with bounding resistor and cap) can get metalhead gain levels out of two gain stages, and lets the semi-clean pick attack punch through the distortion - I've used it to mod a number of amps when I only had two gain stages available, and the customers have loved it. Ray

4/22/2000 3:54 PM

Ray Ivers

Marshall (and other) Hum Sources

Dale,

The wireless unit probably had quite a low output impedance. This would affect your amp very much the same as if you plugged in your (non-Les Paul) guitar and turned the volume down to almost zero. The 2210 and 2205 would be especially affected by this - unlike most Marshalls or guitar amps in general, they have NO series resistance between the input jack and the first gain stage's grid, so whatever impedance/resistance is present at the input will be felt directly by the tube.

This problem sounds like either an unshielded wire to the first tube's grid (some 2210's have it, some use just a plain wire, and some ground the shield to the plate of the tube, which is good for stability but lousy for hum shielding, I believe), poor lead-dress at the first stage (pull those green wires AWAY from their buddies!) or a ground-related problem at the input jack or input stage cathode resistor (these two grounds should be connected together). If the hum was occurring at a low-impedance 'node', like the top of an unbypassed cathode resistor or cathode-follower output, then you would have to suspect a high-current hum source like a power-supply-related ground loop or heater voltage bleed, but since the wireless unit made such a big difference it sounds like the hum was being injected at the tube's grid.

The most important thing with hum is - what is the pitch of the hum? If it's 60 Hz hum (roughly the open low E on the guitar - is that correct?) and sounds really smooth and pure, it's most probably related to the parts of the amp's circuitry that uses 60 Hz - the heaters and the heater center-tap ground, the power transformer primary, (and the field it throws - ever see an amp hum on standby, with the output tubes out?), AC-operated relays, and GROUND-LOOP HUM FROM OTHER EQUIPMENT. If it's 120 Hz hum, it's probably something to do with the DC power supplies in the amp, starting from the power transformer hi-voltage secondary center-tap ground (if used), and moving on to every filter cap and its ground, and also (sometimes I tend to forget this, and it's especially important in the 2205/2210) any other DC voltage derived from the power transformer. In the 2205/2210, the input jack serves to ground out the rectified DC used for the channel-switching circuitry - this is why the red gain-channel LED goes out when you unplug your guitar. There's some real potential for hum bleed here, and in fact everywhere where the channel-switching circuit's tentacles reach (and they reach a fair number of places). If the low-voltage DC to the switching circuitry in this amp develops a hum, it will inject it all over the place, and unless you know where to look, the amp will seem to be humming from multiple, unrelated sources.

I had a Two-Rock amp on the bench recently, and I got to talk to the designer (I forget his name). The owner had complained of hum, and I think I knew what it was, only because I had encountered it in my own designs. The relays! I feel that if you're going to use a DC supply for practically anything other than the output tubes' plate supply, it should be regulated, and relays are no exception. They are a finely-wound coil of wire around a magnetic core, right next to a switch (possibly switching a high-impedance, low-level signal at a hi-gain part of the circuit). The designer was pretty laid-back about this, so in keeping with the mood I handed this $4,100 amp back to its owner in a laid-back kind of way. Fixing it would have voided the warranty, and without the factory's OK there was no way I was going to do that to my buddy.

Anyway, I like the 2205/2210 (I've owned one, and now own two 4212 50-watt combos), but yes - they have been known to hum on occasion. The distortion circuit in these amps (diode-bridged diode bridge! with bounding resistor and cap) can get metalhead gain levels out of two gain stages, and lets the semi-clean pick attack punch through the distortion - I've used it to mod a number of amps when I only had two gain stages available, and the customers have loved it.

Ray

Replies:

anonymous hi ray,while we're ... -- 4/22/2000 5:49 PM
anonymous opps, forgot to mention---there are... -- 4/22/2000 6:02 PM