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

Posted by immatthewj on July 22, 2020 at 21:13:50

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.

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