Amp or tube singing and a bias question

[johnsonad]

The soft start retrofit is in, tested, biased and in the burn in process.  I use sweeps with white noise played in loop to burn in.  The amps are hooked up to my new crossovers to burn both in at once.  Upon first testing them I put sign wave (multiple frequencies) signal through and measured it on a scope to make sure things were working right.  So far everything has tested right on.  Now for the problem.

The right amp is nearly as loud as a small speaker.  I can't tell if it is a microphonic tube or the amp itself as the sound isn't directional enough to tell.  I can hear nearly the full sweep with the higher frequencies the loudest and the white noise.  Could it be a capacitor singing vs. a tube?  Could someone point me in the right direction to trouble shoot this?

Second question for PJ or others on biasing the tubes.  The instruction manual has you measure A2, turn the amp off and wait the 20 minutes the test on the OB side of the 300k resistor.  I didn't wait the time but instead tested A2, calculated the value needed, measured the OB side and adjusted the pot.  Afterward retested A2 and adjusted the same way if needed.  Is there anything wrong with this process?

[Paul Joppa]

Probably the parafeed cap or the output transformer, since those see the highest voltages. Possibly the plate choke. Are the mounting screws tight but not too tight? You can swap the tubes to the other side to check that possibility.

Connecting and disconnecting the meter to points that have 450 volts DC is dangerous. That's why the instructions say to connect the meter before turning the amp on, and disconnect them after it is off and the capacitors have discharged.

[johnsonad]

Thanks Paul, I'll start searching for the source.  Is there anything you can do if it is in the iron or cap?  Really what I mean to ask is will it have an effect on playback?  If not, I can live with it but I'll swap tubes and test the screws for tightness.

Thanks for the advice.  I'm using a long insulated probe on the DMM to touch points with the positive end and have a small clamp for the negative.  I'll be more careful in the future.

[Paul Joppa]

The sound is not unusual, and not usually a problem. If it's a lot louder on one side, it probably indicates at least a difference between components; hard to say a priori whether that's a problem or just normal variation. If it's a capacitor, I know that Clarity Cap has done a bunch of research to develop less-microphonic caps. I suppose others may have done so too, but Clarity Cap has bragged about it rather than keep it as a trade secret.  :^)

[johnsonad]

Thanks Paul, I'm not going to worry about it then.  Those clarity caps you wrote about are quite expensive btw.  I'll try to find happiness with the Auricap that's in it now

[johnsonad]

Paul, another bias question.  Today I rolled in the 396a and it took three tubes to bias up right.  The first I'm not sure what the problem was.  The second, I couldn't turn the pot enough to get the voltage right and the third, the voltage drifted up very slowly for 20 minutes and was a pain to get to settle.  Did I screw up the soft start somehow or are some tubes just harder to bias?

[Paul Joppa]

The drifting tube is probably just the cathode re-forming after a long idle period; that happens sometimes. It should be stable once it's broken in.

The tube that could not be adjusted is most likely worn out. You can't adjust the bias to less than 2.5 volts, and a fresh 5670/396A/2C51 would take 4 volts bias. If it need a lower bias, that means you have to push the tube harder to get enough current, indicating that the emission is weak. It's possible that a few tens of hours running will restore the cathode, if it just needs re-forming as described above, but I suspect it is just worn out. A transconductance tube tester would tell you more about the condition of the tubes; the simper testers are less informative.

[Grainger49]

That is good information to be out there for Paramount owners.