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Our Guitar Department sent over a new Fender Super Bassman that was stated to have the power tubes Red-Plating, after being set for the wrong cabinet impedance, and it blew the mains fuse. I never did power it up in the state it came in. Only two of the back row of power tubes had evidence of Red-Plating.
Not one of my favorite amps to service. I pulled it apart, seeing I hadn't yet worked on this amp and the hold-down clamps lacked the hi temp silicone rubber tubing I install on them to add shock-mount protection as well as hold the upper spring in place on the 'hat' so it doesn't fall apart when removing a tube.
I began with the clamps. The springs weren't holding in place when I slipped the 5/32" ID tubing over the spring/clamp fingers, so I installed heat shrink tubing first to hold them, then slid the hi temp tubing on. I then added tubing over the fingers. Placed the measured/slit tubing into the large open hole, instead of that flat rubber disc that Fender used.
Tube 1B had been broken, presumably from the amp having been dropped, and that left it with a frosty-white coating on the inside of the tube glass, and a nasty long crack on one side of the tube glass. The other two rear tubes of that half of the output stage had Red-plated, and evidence of the dark scorching of the plates was present.
I pulled out my Hiwatt DR201 Bass Amp Clone, which I had built earlier this year, where I had added test facilities for testing/matching 6550 and KT88 Power Tubes, and went thru these five tubes along with another new Sovtek 6550 tube the Guitar Dept came up with. These tubes survived, and the match in plate/screen current was within a couple mA of each other, as was that sixth tube. So, I now had a matched set of tube that worked.
No idea if the Power Amp chassis is working, nor if the Auto-Bias circuit worked. I found out this time you could position that T-shaped chassis inside face-out to test the power supply circuit. All measured ok.
I tack-soldered the Cathode Test Probe set that I use when I'm selecting power tubes for the Super Bassman. That let me monitor the cathode current under biasing as well as under drive conditions. The Auto-Bias circuit DID work. It even continued to work switching in and out of Standby, where some of them fail to do so.
The only problem I did run into was the Preamps' Ground Wire mounting screw's threaded #6-32 hole in the chassis was stripped. No access from inside the chassis to use a longer screw to thread thru and hold with a nut, so I ended up re-threading the hole for #8-32
Not one of my favorite amps to service. I pulled it apart, seeing I hadn't yet worked on this amp and the hold-down clamps lacked the hi temp silicone rubber tubing I install on them to add shock-mount protection as well as hold the upper spring in place on the 'hat' so it doesn't fall apart when removing a tube.
I began with the clamps. The springs weren't holding in place when I slipped the 5/32" ID tubing over the spring/clamp fingers, so I installed heat shrink tubing first to hold them, then slid the hi temp tubing on. I then added tubing over the fingers. Placed the measured/slit tubing into the large open hole, instead of that flat rubber disc that Fender used.
Tube 1B had been broken, presumably from the amp having been dropped, and that left it with a frosty-white coating on the inside of the tube glass, and a nasty long crack on one side of the tube glass. The other two rear tubes of that half of the output stage had Red-plated, and evidence of the dark scorching of the plates was present.
I pulled out my Hiwatt DR201 Bass Amp Clone, which I had built earlier this year, where I had added test facilities for testing/matching 6550 and KT88 Power Tubes, and went thru these five tubes along with another new Sovtek 6550 tube the Guitar Dept came up with. These tubes survived, and the match in plate/screen current was within a couple mA of each other, as was that sixth tube. So, I now had a matched set of tube that worked.
No idea if the Power Amp chassis is working, nor if the Auto-Bias circuit worked. I found out this time you could position that T-shaped chassis inside face-out to test the power supply circuit. All measured ok.
I tack-soldered the Cathode Test Probe set that I use when I'm selecting power tubes for the Super Bassman. That let me monitor the cathode current under biasing as well as under drive conditions. The Auto-Bias circuit DID work. It even continued to work switching in and out of Standby, where some of them fail to do so.
The only problem I did run into was the Preamps' Ground Wire mounting screw's threaded #6-32 hole in the chassis was stripped. No access from inside the chassis to use a longer screw to thread thru and hold with a nut, so I ended up re-threading the hole for #8-32
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