Home
AudioAsylum Trader
Tubes Asylum

Questions about tubes and gear that glows. FAQ

For Sale Ads

FAQ / News / Events

 

Use this form to submit comments directly to the Asylum moderators for this forum. We're particularly interested in truly outstanding posts that might be added to our FAQs.

You may also use this form to provide feedback or to call attention to messages that may be in violation of our content rules.

You must login to use this feature.

Inmate Login


Login to access features only available to registered Asylum Inmates.
    By default, logging in will set a session cookie that disappears when you close your browser. Clicking on the 'Remember my Moniker & Password' below will cause a permanent 'Login Cookie' to be set.

Moniker/Username:

The Name that you picked or by default, your email.
Forgot Moniker?

 
 

Examples "Rapper", "Bob W", "joe@aol.com".

Password:    

Forgot Password?

 Remember my Moniker & Password ( What's this?)

If you don't have an Asylum Account, you can create one by clicking Here.

Our privacy policy can be reviewed by clicking Here.

Inmate Comments

From:  
Your Email:  
Subject:  

Message Comments

   

Original Message

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.