Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: pbman1953 on February 18, 2016, 06:30:07 pm
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I have a Fender 300 professional tube amp. 6-6550 tubes are used. About 4 years ago I bought from ebay 10 SED winged C 6550's. I installed 6 in the Fender and 4 in another amp. All of them have bee performing perfectly. Last night,while playing, one tube glowed orange and the amp blew a protection fuse and blew the main 10 amp fuse. There are 3 tube protection fuses. One per pair and I tested the tube pair ,from the blown fuse. The glowed one was dead but the other was fine. I replaced the fuse and tried the good one back in the amp and all 5 worked. I left one socket empty.
So my question is what would you guy suggest to replace the bad SED 6550? I could go out and buy a single but I won't know of its tested value and pay $70-100. I really don't want to dish that out . As far as biasing, the amp has a balance and bias section on the back of the amp . I chatted with Eurotubes and they suggested that I send the 5 remaining tubes to them and they would re-create a matched sextet with a JJ 6550. They continued to tell me that once they'd document what I had and if another tube would blow they would simply send me the correct match to my set.
I felt that was kind of cool idea but wanted to run this by the forum.
Thanks
Here is how the amp is biased-
Output tube bias adjustment instructions
1. Turn on the 300T and let it warm up for at least two minutes with the STANDBY switch in the ON position. Make sure the amplifier is connected to a speaker or equivalent load.
2. Remove the bias controls cover box.
3. With a DVM set to it's most sensitive DC voltage scale, measure the voltage between the BIAS 100mV test points, and adjust the BIAS control for a reading of .1 VDC (100mVDC)..
4. Next, connect the DVM to the BALANCE 0V test points, and adjust the BALANCE control for a reading of 0 VDC.
5. Replace the bias controls cover box
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imo running a pp amp with 5 tubes is asking for trouble. I would expect problems with the o/t. For the short term, remove one tube and balance the circuit, so the o/t sees approximately the same signal on each side of the transformer. Will the tone be different, I would suspect so.
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Actually, I just tested the amp to see if the 5th tube was ok then shut off. I never used the amp in a play situation with 5 tubes .
I decided to send the 5 to Eurotubes for a test and to see if he could match up a JJ 6650.