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This is the third Matchless DC30 Combo amp that’s come into the shop in the past month….two from an outside client, and this latest one from our rental inventory. This one was shrieking. When I saw that label written on the gaffer’s tape on the cabinet, my mind went to the EF86 Pentode circuit of Ch 2. Powered it up, and sure enough, turn the Ch 2 Volume up, and it’s shrieking…acoustic feedback with that tube making like it’s a microphone too close to the PA System monitor speaker. And, rotating the tone switch, yup…there’s the familiar microphonic behavior of that circuit.
The previous two DC30’s I serviced, one had the Switchcraft L12A input jacks not making good contact on the Normal’s, so they were behaving like ungrounded inputs with nothing plugged in. Not so bad to deal with on Ch 1, but add that to the microphonic EF86 Pentode of Ch 2 and it was nasty. Cleaning the jack’s normals and re-tensioning the contacts was easy enough. I didn’t have any spare EF86 pentodes on hand. I looked thru the 9-pin Pentodes I had salvaged from some old Tektronix gear, but the best candidate didn’t have the same pinout, so I did what I could with the component leads on the shock-mounted tube socket. That particular tube I did manage to reseat a few times, and got it to cease with it’s microphonic behavior….just ended up with the white tube noise. The other amp didn’t have that problem.
But this one, now having ordered a TAD EF86 tube as a spare for the client’s next trip, I at least had that to try. I unplugged what was installed…a J/J EF806 pentode. I plugged in the TAD EF86, and still had the microphonic issues, so I pulled the chassis, remembering there’s little I could do with the stiff component leads of the parts glued together and parked directly above the tube socket itself. I played with the lead dress as best I could, though didn’t make any success.
I did try inserting some 0.031” thick silicon rubber strip, cut to the circumference of the tube, and inserting it into the inside wall of the tube socket which serves as the anchor for the spring-loaded tube shield, but like the previous DC30’s, that tube diameter is ever-so-much larger than the ECC83S tubes, that you can’t insert the EF86 tube with that rubber strip.
I had inserted some thick paper between the tube socket flange and the tube, which seemed to help before. Not this time. I then tried some thin white packing foam I had tucked away, thin enough to cut and fit inside the tube shield to cushion the glass and did just fit. That seemed to help a little, so I put the chassis back into the cabinet.
Just what I was afraid of. When I powered it up and turned Ch 2’s volume up, it was shrieking again. Acoustic feedback....added the speakers into the picture, which I didn't have over on the test stand. I removed the tube shield, and the shrieking stopped. I removed the foam insert, put the shield back into place, and it had still remained stopped. Just had the microphonics to contend with.
I don’t have the schematic for this DC30, and, I was a little reluctant to start reducing the circuit gain to combat these two microphonic tubes. Is the solution having enough of a collection of these pentodes to select thru and find the ones that aren’t microphonic? Any other suggestions?
The previous two DC30’s I serviced, one had the Switchcraft L12A input jacks not making good contact on the Normal’s, so they were behaving like ungrounded inputs with nothing plugged in. Not so bad to deal with on Ch 1, but add that to the microphonic EF86 Pentode of Ch 2 and it was nasty. Cleaning the jack’s normals and re-tensioning the contacts was easy enough. I didn’t have any spare EF86 pentodes on hand. I looked thru the 9-pin Pentodes I had salvaged from some old Tektronix gear, but the best candidate didn’t have the same pinout, so I did what I could with the component leads on the shock-mounted tube socket. That particular tube I did manage to reseat a few times, and got it to cease with it’s microphonic behavior….just ended up with the white tube noise. The other amp didn’t have that problem.
But this one, now having ordered a TAD EF86 tube as a spare for the client’s next trip, I at least had that to try. I unplugged what was installed…a J/J EF806 pentode. I plugged in the TAD EF86, and still had the microphonic issues, so I pulled the chassis, remembering there’s little I could do with the stiff component leads of the parts glued together and parked directly above the tube socket itself. I played with the lead dress as best I could, though didn’t make any success.
I did try inserting some 0.031” thick silicon rubber strip, cut to the circumference of the tube, and inserting it into the inside wall of the tube socket which serves as the anchor for the spring-loaded tube shield, but like the previous DC30’s, that tube diameter is ever-so-much larger than the ECC83S tubes, that you can’t insert the EF86 tube with that rubber strip.
I had inserted some thick paper between the tube socket flange and the tube, which seemed to help before. Not this time. I then tried some thin white packing foam I had tucked away, thin enough to cut and fit inside the tube shield to cushion the glass and did just fit. That seemed to help a little, so I put the chassis back into the cabinet.
Just what I was afraid of. When I powered it up and turned Ch 2’s volume up, it was shrieking again. Acoustic feedback....added the speakers into the picture, which I didn't have over on the test stand. I removed the tube shield, and the shrieking stopped. I removed the foam insert, put the shield back into place, and it had still remained stopped. Just had the microphonics to contend with.
I don’t have the schematic for this DC30, and, I was a little reluctant to start reducing the circuit gain to combat these two microphonic tubes. Is the solution having enough of a collection of these pentodes to select thru and find the ones that aren’t microphonic? Any other suggestions?
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