12AX7 questions

[riccoryder]

It might be noise above the audible frequency range from the switching power supply. Turning up the scope gain and looking at what's there with no input signal may help. If it is high frequency, speed up the scope sweep to where you can measure the period of one complete cycle. 1/period = frequency

Some class-D amps also have significant out of band switching noise.

Software can be used along with the sound input on a computer to act as a spectrum analyzer. That can show you distortion products, but the sampling frequency is likely too low to see switching noise. (use care not to overdrive the input)

[DF96]

"Spurious" means something which is false or misleading.

Multiple scope lines often mean hum, which is not synchronised to the signal, but could also mean low frequency oscillation. Thick traces could be noise or HF oscillation. Trouble is, just putting a probe on a circuit can change its operation.

[azvrt]

Thanks guys I'll let my father read this as well.
I guess if the amp sounds sparkling on very revealing high quality recordings then I guess I don't have much of a problem, except for the smaller amp messing up a little on that Dreams bassline and certain piano notes, though I'd need to double check the piano with another amp, who knows it's just reproducing what's there in the recording with great detail. I doubt it however.
And the fact that the weaker amp is more powerful at the very same gain setting than the larger amp is also quite strange. Hopefully ZED will respond to my mail and I'll post the info here.

'Trouble is, just putting a probe on a circuit can change its operation.'
Indeed. I tried another amp of mine and the test did not work, nothing showed up on the scope, so who knows the test I performed with the scope and probe is messing up these particular amplifiers' proper operation.

I should have stayed in school

[azvrt]

I'd like to take this opportunity to thank the boys and girls on the forums for being there for each other, as the reply I got from the manufacturer of these amps wasn't very kind nor helpful.

'Dear Fred,

We do not have schematics for these amplifiers as they were built about 14 years ago.

There are no spare boards available.

You are asking me to diagnose issues from 6,000 miles away based on what you hear, this is simply not possible for me.

I designed and built these 14 years ago and any listening which I did back then I certainly cannot remember.

Regards'

[HawaiiGuy]

I'm thinking that riccoryder is right, those o'scope traces look like you're getting PS noise.

Trace the circuit to see where the tubes are getting their plate voltage from. Try bypassing the plate supply electrolytic cap with a 0.1mfd film cap (with equal or higher voltage rating) in parallel with electrolytic.

[lduarte1973]

Hi , if you like that vintage sound (with rolled off highs) you can try swapping your tubes with Mullard , i have tryed a few tubes on my preamp and the mullards should fit your taste , Brimar are nice too but a bit more sparkly on highs , you might want to have a look at the link there´s a description on how the several brands sound like

http://www.audiotubes.com/12ax7.htm

[DavesNotHere]

change the electrolytic capacitors. the metalized polyester caps are low quality, consider changing those too. as both of these have lowered shelf life since they live in a car. polypropylene caps and low impedence/esr electrolytic caps are preferred replacement. Vishay 715P series poly and Panasonic EEU are some suggestions.

replace the 1/4w carbon film resistors located on the tube riser card as these components are not suitable (they lack temperature stability) I would recommend Vishay/Dale ccf60 series.

[DF96]

How long do metallised polyester caps last? I am not aware of a wear-out mechanism, provided they are not subject to overvoltage spikes.

[SY]

With the thermal and humidity fluctuations one can see in a car, polyester caps can definitely go bad- they have greater temperature sensitivity and far greater moisture pickup than polypropylene. You'll see drifts of value and large increases in tan delta. As well, unless they are foil caps (unlikely) the metallization can corrode, leading to much higher ESR, downward drifts in value, and eventually total failure.

[DF96]

OK. Most of my knowledge on component lifetime comes from vintage radio sites, where very few car systems crop up.