Members Soundstorm Posted October 8, 2012 Members Share Posted October 8, 2012 Some of you may remember a friend of mine's cab had sort of blown my Matamp awhile back. It got hot enough inside the chassis to unsolder 3 of the 4 screen grid resistors. I simply resoldered them, put in an older set of Sovtek EL34's I had lying around (should still have plenty of life in them), and biased them. The previous =C='s I had in there were completely shot, and had all red plated, but by the time I got the amp only one was redplating because there was only one resistor still soldered in. All resistors read 1K evenly (and I did unsolder one side before taking my measurement), so they all seem to be functional 1K, 6 watt resistors. Anyway, everything sounds fine now except that I'm getting a slight hum from the amp even when it's on standby mode. It's always been totally quiet even when on, maybe a slight hiss when cranked (even off standby). My question is what could be causing this? Any ideas? Link to comment Share on other sites More sharing options...
Members guitargod0dmw Posted October 8, 2012 Members Share Posted October 8, 2012 Just to start with the easiest thing first, have you tried a different speaker cable? Link to comment Share on other sites More sharing options...
Members Slaymoar Posted October 8, 2012 Members Share Posted October 8, 2012 Unplug the guitar cable from it, does it still buzz? If yes, then does it sound like AC buzz 60hz? If yes then it could be a bad cap somewhere, either in the power supply, or in the bias section. I would check voltages of each tube, and would be particularly interested in the phase inverter's coupling caps. A tiny leak there can destroy tubes and output transformer with it. Link to comment Share on other sites More sharing options...
Members Soundstorm Posted October 8, 2012 Author Members Share Posted October 8, 2012 The plate voltage read 506 across all tubes. It definitely sounds like an AC, 60 cycle type hum. This amp is 6 months old, btw. I think I'm gonna replace the resistors that were unsoldered just in case there was any damage done to them. Like I said tho, amp still sounds killer. How would a bad cap cause a hum when in standby? Thanks for the advice, mang.Any other ideas? I need to get brand new tubes also, just to rule that one out.Wish I had a tube tester, I have so many tubes lying around. Link to comment Share on other sites More sharing options...
Members Zozobra Posted October 8, 2012 Members Share Posted October 8, 2012 Is the standby a full standby which disconnects the HT? Some only act as inrush limiters. Really badly matched tubes can cause a DC offset in the OT which screws up the common mode rejection and lets hum though. Also check to see if the amp has a humdinger on the heaters. I know some matamps do and if they're set wrong it will pick up hum and rectifier hash. Link to comment Share on other sites More sharing options...
Members isvoid Posted October 8, 2012 Members Share Posted October 8, 2012 ib4 obvious parody thread O.o Link to comment Share on other sites More sharing options...
Members Slaymoar Posted October 8, 2012 Members Share Posted October 8, 2012 The plate voltage read 506 across all tubes. It definitely sounds like an AC, 60 cycle type hum. This amp is 6 months old, btw. I think I'm gonna replace the resistors that were unsoldered just in case there was any damage done to them. Like I said tho, amp still sounds killer. How would a bad cap cause a hum when in standby? Thanks for the advice, mang.Any other ideas?I need to get brand new tubes also, just to rule that one out.Wish I had a tube tester, I have so many tubes lying around. Plate voltage on ALL tubes is 506V? Or just the power tubes? Preamp tubes should not be seeing that much voltage. Link to comment Share on other sites More sharing options...
Members Soundstorm Posted October 9, 2012 Author Members Share Posted October 9, 2012 Is the standby a full standby which disconnects the HT? Some only act as inrush limiters. Really badly matched tubes can cause a DC offset in the OT which screws up the common mode rejection and lets hum though. Also check to see if the amp has a humdinger on the heaters. I know some matamps do and if they're set wrong it will pick up hum and rectifier hash. The tubes don't need to be matched in the newer matamps, they each have their own bias pot. You bring up a good point on the standby switch, and in actuality I think it's called "mute" on the front. I'll look into that. Also, I have no idea what a humdinger is Link to comment Share on other sites More sharing options...
Members Soundstorm Posted October 9, 2012 Author Members Share Posted October 9, 2012 Plate voltage on ALL tubes is 506V? Or just the power tubes? Preamp tubes should not be seeing that much voltage. Just the power tubes. I haven't even looked at the preamp tubes really. I tend to keep my amps out the headshell so I can swap different tubes in and out to compare them. I'm not even sure of the pin functions on a 12ax7 Link to comment Share on other sites More sharing options...
Members Zozobra Posted October 9, 2012 Members Share Posted October 9, 2012 The point remains that you want to have the same DC current through each half of the OT. Granted the tubes need to be real far out of whack for this to be a problem though.A humdinger is where you connect each end of the heaters to a pot with the wiper to ground as the ground reference for the heaters. The idea is that you can balance the reference to cancel noise. Another thing to check is that the PI coupling caps aren't leaking DC in to the power tube grids. This can cause hum and it can also ruin the poweramp! The best way to find and eliminate hum is divide and conquer. Isolate as much of the circuit as possible. I'd start with the poweramp backwards by pulling the PI tube. If it still hums then the problem is after the PI. Excessive ripple in the power supply can also cause trouble. It may be worth registering here: http://music-electronics-forum.comThe trouble shooting section there is full of very knowledgeable techs who can help you out if you can follow their advice. Link to comment Share on other sites More sharing options...
Members Soundstorm Posted October 9, 2012 Author Members Share Posted October 9, 2012 The point remains that you want to have the same DC current through each half of the OT. Granted the tubes need to be real far out of whack for this to be a problem though. A humdinger is where you connect each end of the heaters to a pot with the wiper to ground as the ground reference for the heaters. The idea is that you can balance the reference to cancel noise. Another thing to check is that the PI coupling caps aren't leaking DC in to the power tube grids. This can cause hum and it can also ruin the poweramp! The best way to find and eliminate hum is divide and conquer. Isolate as much of the circuit as possible. I'd start with the poweramp backwards by pulling the PI tube. If it still hums then the problem is after the PI. Excessive ripple in the power supply can also cause trouble. It may be worth registering here: http://music-electronics-forum.com The trouble shooting section there is full of very knowledgeable techs who can help you out if you can follow their advice. Ok. Still very much in the learning process of working on tube amps, but I'll try that forum you mentioned. I think the first thing I'll try is replacing all those resistors because I know they must've come close to burning up if they passed enough current to unsolder themselves. I'll also try to figure out how the standby mode/mute switch is working. As far as a PI tube goes, the amp is set up sort of like the old school oranges in that it has only two preamp tubes, plus the addition of a third which is an extra gain stage when you switch to MV mode. There are literally NO tube amp techs in my area at all. I'm going back to school for Industrial Electronics next semester, my goal is to have a tube amp repair shop on the side of my regular job. I'll have a complete monopoly Link to comment Share on other sites More sharing options...
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