Bias issue with Cayin TA-30

66.74.128.150

Posted on March 29, 2012 at 22:30:49

Pat S.
Audiophile

Posts: 347
Location: Westminster, CA
Joined: April 10, 2002

Hi Everyone,

So here's the deal. I installed a new matched quad of 6L6's in my Cayin-TA30 integrated and was in the process of setting the bias for each tube as I usually do. The amp had been warmed up for about 15 min and I had each 6L6 drawing 37ma. Then, I started hearing a some terrible static out of the left speaker and noticed some smoke coming from the power switch. I turned it off immediately.

After a bit I powered the amp back on. Everything seemed fine. But, when I went to check the bias across each biasing resistor (one end of each resistor is connected to pin 8 of the 6L6 socket), only 3 tubes showed a reading on my multimeter. The fourth read "0" across that particular bias resistor.

I'm obviously a novice here. So I'm wondering what exactly happened. Three tubes are biased at 37ma. The other tube I can't get a bias reading on. I have the bias trim pot set to where the other three tubes' trim pots are, hoping that it is in the same range. They sound very good and the tube with no bias reading isn't exhibiting any unusual behavior (such as red glowing plates or noise).

Any idea of what is going on here? And, do you think the trim pot is still active on the tube that is giving no reading across the biasing resistor?

Thanks,

Pat

RIP Dave Brubeck: 1920-2012

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RE: Bias issue with Cayin TA-30

Chris O

06:14:19 03/30/12

5

RE: Bias issue with Cayin TA-30

Pat S.

09:21:02 03/30/12

4

Update...

Pat S.

20:49:55 03/30/12

3

...

Show full thread outline!

RE: Bias issue with Cayin TA-30

Chris O

06:14:19 03/30/12

5

RE: Bias issue with Cayin TA-30

Pat S.

09:21:02 03/30/12

4

Update...

Pat S.

20:49:55 03/30/12

3

RE: Update...

Jim McShane

19:07:56 04/01/12

Thats great

Chris O

06:07:01 03/31/12

RE: Thats great

Pat S.

17:59:31 03/31/12

RE: Bias issue with Cayin TA-30, posted on March 30, 2012 at 06:14:19

Chris O

If you try putting the "dead" tube into another socket to see if it is really dead, which I am sure it is, and the tube is shorted internally you will cause more damage to your amp.

Sometimes, even new tubes fail. Your bias measuring resistor on that socket is probably blown. I doubt that you will get any tube to bias in that socket now. You need to check and/or replace the resistor.

The cause of the failure is most likely the tube itself, the other cause would be if the tube socket is not making good contact with the grid pin and it lost bias.

Where did you purchase the tubes?

By the way, smoke is always bad. It is the sign of a part destroying itself. When you get smoke, it is really unhelpful to try firing it back up again without finding out the fault.

RE: Bias issue with Cayin TA-30, posted on March 30, 2012 at 09:21:02
Pat S. Audiophile Posts: 347 Location: Westminster, CA Joined: April 10, 2002	I purchased the tubes from BnB Tubes. I've gotten tubes from them before and have never had an issue. I'll check the resistor and replace it, if needed. Thanks. RIP Dave Brubeck: 1920-2012

Update..., posted on March 30, 2012 at 20:49:55
Pat S. Audiophile Posts: 347 Location: Westminster, CA Joined: April 10, 2002	It turns out that the 1K resistor across pin 4 and 6 had blown. I replaced it and I can now bias that particular tube again. All 4 tubes are holding steady now. Tested the tubes before I put them back in the amp. All 4 6L6's test okay for shorts, leakage and emissions on my Eico 666 tester. RIP Dave Brubeck: 1920-2012

Thats great, posted on March 31, 2012 at 06:07:01
Chris O	Just keep a close eye on it for a while. Pin 4 is the screen grid, Pin 6 is an unused pin on the tube and that lug on the socket is used as a tie point by the amp maker. Where do the other connection(s) for pin 6 go? For the 1K resistor to blow, your tube drew too much screen grid current.

RE: Thats great, posted on March 31, 2012 at 17:59:31

Pat S.
Audiophile

Posts: 347
Location: Westminster, CA
Joined: April 10, 2002

Chris,

Here is a picture of the inside of an amp like mine. The only real difference between the one in this picture and mine is that mine has 4 bias pots mounted on the circuit board in the center. The one in the pic was modded for external bias. Note the 1K resistors across pin 4 and 6 of each power tube socket. There are also yellow leads coming from one of the lugs on pin 6 of each socket. The 10 OHM biasing resistors are connected to pin 8.

Cheers,

Pat

RIP Dave Brubeck: 1920-2012

RE: Update..., posted on April 1, 2012 at 19:07:56
Jim McShane Dealer Posts: 5910 Location: Chicago Joined: January 13, 2003	Be careful - you may have an intermittent short in that one tube. The 1K resistor (as has been mentioned in other posts) is a screen stopper resistor, and the only way for it to fail would be for the screen to pass too much current. And as you now know if there is no voltage to the screen then the tube won't pass any current. But watch that tube - it may act up again.

Tube DIY Asylum

Bias issue with Cayin TA-30

66.74.128.150