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by **apurcell22** » Sun Jan 29, 2012 7:24 pm

Hello all,
Just completed a rev C board. When I fired it up, the amp was unstable. Voltage and bias was fluctuating quite a bit once warmed up. I disconnected the feedback and the amp is stable and I am able to get the bias adjusted. I am using the 8 ohm tap, and adjusted to 20.3k. Also using Edcor iron. Anyone have a clue what I did wrong?
Thanks,
Al

by **Quad** » Sun Jan 29, 2012 7:59 pm

Looks like Tom also hit the same issue in his build. You
could probably take a look at his pics for the correct
OPT connections.

http://www.diytube.com/phpBB2/viewtopic ... ead97020d4

by **apurcell22** » Sun Jan 29, 2012 8:12 pm

No, that's not the same issue. My transformers are wired just like Tom's are. If I hade them phased wrong, it would be immediate oscillation, and disconnecting the feedback wouldnt stop it.
Thanks for the assist though,
Al

by **TomMcNally** » Sun Jan 29, 2012 8:44 pm

Hi Al ...

Just to make sure ... you are setting up the bias with the amp
connected to speakers, right? What do you hear? Inputs shorted?

If disconnecting the feedback lead fixes it, it could indeed be a
phasing problem. I don't think rev C and rev D had a reversal.

You could flip the leads and see what happens.

... tom

by **apurcell22** » Sun Jan 29, 2012 9:00 pm

Tom,
Flipping the leads was the first thing I tried. It produced instant oscillation. Yes I am using speakers. With the inputs shorted, all I can hear is a faint hiss.
Thanks
Al

by **apurcell22** » Sun Jan 29, 2012 9:01 pm

Also just a bit of hum with the inputs open.
Al

by **TomMcNally** » Sun Jan 29, 2012 9:12 pm

OK good, I just wanted to make sure I didn't post a picture of
"before" ...

How does the amp sound despite the fluctuations, and how bad
is the bias fluctuating? Do you have one stable digit and the
second and third are varying?

by **apurcell22** » Sun Jan 29, 2012 9:22 pm

I did not play any music through it. I would say it was bouncing between 0.32 an 0.42 v. The last DiyTube st35 I built, the bias was rock steady. Is it possible that it is the tubes?
Thanks,
Al

by **TomMcNally** » Sun Jan 29, 2012 9:41 pm

I'd do two things ...

1) play music and see how it sounds

2) swap the EL-84s around a little and see what happens

Then measure voltages (skip the filaments) and see what's
going on ...

... tom

by **apurcell22** » Sun Jan 29, 2012 11:56 pm

Ok, I dug around and found some more tubes. Now the bias is nice and steady. The amp sounds nice. Thanks for the help. How can I upload pictures?
Thanks again,
Al

by **Shannon Parks** » Mon Jan 30, 2012 5:47 am

What were the tubes you swapped out, Al? Just email me the pix and I can host them.

Shannon

by **apurcell22** » Mon Jan 30, 2012 12:46 pm

Shanon,
The tubes were 6p14p-ev. I put in a pair of amperex on the channel that seemed to fluctuate worst, and the amp stabalized. I guess I will be shopping for tubes.
Thanks for all of your contributions to this hobby,
Al

by **apurcell22** » Mon Jan 30, 2012 12:52 pm

Something else to note, I tried several different 6p14p-ev tubes before I tried the pair of amperex tubes. No combination worked. Some were better than others though.
Al

by **Shannon Parks** » Mon Jan 30, 2012 1:19 pm

I wonder if one of the sockets was a little dodgy? Or maybe just needed "breaking in"?

Shannon

by **apurcell22** » Mon Jan 30, 2012 1:23 pm

I don't think so, as I tried going back to the Russian tubes after the amperex tubes. Same results as before.
Al

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