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my bias is running away on one side even with the bias backed all the way off on that side!

Sorry that this question is a double post, but after I posted in 'amps/preamps" I thought, "Wow, tubes is probably the more applicable section." I tried to delete my original post in "amps/preamps", but since I already got a reply, it wouldn't let me. Could a mod delete my firts post in "amps"? Sorry & thanks.

Anyway, This is for a Cary CAD 280SA (V12) which runs with 12 EL34s, separate bias pots for left & right side & a single power supply for the amp.

I have been running two sextets of the reissue4d Mullards that I put in when , in 2017, I took the amp out of a 7 year hibernation. Everything has been pretty good, but recently I felt like the sound was deteriorating, particularly on female vocals. I had also noted that the bias on the right side was moving up or down after listening sessions, so with that in mind, I set about trying to identify a tube going out. Per instructions I received from Cary (back when I took the amp out of hibernation) I left all 6 tubes in the RIGHT side & backed the RIGHT bias all the way off & used the LEFT side to establish a reference of 40 mA bias with one tube, & using that reference on the LEFT side bias pot, I began checking all the other remaining (five) tubes. None from that sextet were reading any way crazy numbers. I then put the 6 tubes I had just checked out, over to the RIGHT side, & I then repeated the procedure with the remaining 6 tubes (again using the reference setting of the LEFT bias pot). Again, no way crazy numbers, & some of the tubes matched up nicely.

I am sorry to make this a protracted tale of woe, but somewhere along the way I dropped my ancient Radio Shack multi meter & it went into the cease to function mode (& I did try new fuses). Last night I finished this job with a cheapo multi meter that I wasn't crazy about that I got free from Harbor Freight. I did install 2 tubes per-side that had matched up real well & set the bias to 90 mA per-side.

Today I went to Lowes & bought a multimeter that I felt better about & decided I would repeat the procedure.

Before I began the procedure, tonight, I checked the bias that I had set on the left & right sides last night. I did note that the left side had remained solid, but the right side had dropped about 15 mA. So I began to repeat the above procedure, again, WITH 6 TUBES IN THE RIGHT SIDE AND THE RIGHT BIAS BACKED ALL THE WAY OFF. As I was working with tubes using the left side to match them with, I noted that the LEDs that indicate bias were ON on the right hand side & the tubes on the right side were glowing & starting to "red-plate". "Not good," I thought, so I turned off the amp. I then confirmed that I did have the bias backed all the way off on the right side (yes, it was), & I switched the multi-meter over to the right hand side, & turned the amp back on. Again, with the bias backed all the way off, the meter started indicating bias that was running away rapidly.

I apologize again for the length of this post, but what I did next to trouble shoot was to remove the right tube fuse & I used the left side to check EVERY tube one at a time to see if there was one tube doing something crazy--& there wasn't.

Thank you for indulging me to this point. I do plan on attempting to contact Cary ASAP, however, before COVID 19, ASAP was difficult. In the mean time, I was wondering if any tube savvy members would read my sad story & say, "A-ha! Your problem is [fill in the blank]."

Anyway, if you were able to stick with my long story & you have any input on something I should look for, I'd love to hear. I can solder & de-solder, & if this was something as basic as a resistor or a diode or something such as that that had fried, I would certainly like to try my hand with this thing as opposed to boxing it up & shipping it to North Carolina (providing that Cary even still services this model).

Thanks again, & once again, sorry for the length of this.

7-5-2019 "Our army manned the air, it rammed the ramparts, it took over the airports, it did everything it had to do, and at Fort McHenry, under the rockets' red glare, it had nothing but victory."

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Topic - my bias is running away on one side even with the bias backed all the way off on that side! - immatthewj 21:13:50 07/22/20 (42)


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