Acoustic 370 slight distortion (at all levels) and bias?

[teemuk]

If it's a crossover issue you actually want to scope the output at very low output signal level. This way the ratio of crossover region vs. overall signal is higher.

Then again, I listened the clip, couldn't really distinct anything similar to crossover distortion, loaded it the clip to wavefile editor and still couldn't distinct anything similar to crossover distortion.

In fact, IMO, it sounded pretty much fines, aside the odd "wobbly" high frequency note that seems to ring in the signal a moment before it fades out. Was that what you're referring to..?

[johnk_10]

yep, the slight distortion at the end of the note is what i'm talking about. it's actually there all the time but the distortion fades out at a different rate. my 320 had the exact same problem, but once i set its bias to spec, it works perfectly now.

[Jazz P Bass]

The AGC circuit, is that S301 on the power amp?

[johnk_10]

S301 is the momentary switch for it.
i believe that the actual circuit is Q315 and a few other components.

[johnk_10]

i replaced Q15 but it was exactly the same afterwards.

after replacing the 3H inductor in my 320 head today, i checked it's AC current and confirmed again that it is drawing 45-47 watts at idle, so i tacked a 2.2K resistor in parallel with R319 to lower the 370's bias voltage and it went from pulling 26.8 watts without it, to 42 watts with it. the distortion is completely gone now, so i do think it was bias issue afterall. R304 is trimmed to the exact middle voltage (42.7) of the 85.4 volts from the main supply. all of the output transistors are just barely warm at idle, but Q11 is running slightly warmer than the rest of them. for some reason unbeknownst to me (??!!), this amp was running too cold, and i'd be very interested to know if anyone else knows or can measure how many watts their 370 is pulling at idle.

btw, the amp sounds better than ever right now (bigger, fuller and rounder, with clearer highs), and i really liked it before i got into trying to solve this stupid distortion thang.

[johnk_10]

i ended up installing a 3/4 watt 15 turn trimmer bias pot to replace R319 so i could precisely set the bias and adjust to the point where the distortions just disappears. right now that seems to be where the amp is pulling 39 watts @ 120.1VAC.
here's a pic of the bias pot (the blue trimmer in the middle of the pic). i used a dab of hot glue underneath it to secure it so it wouldn't be dangling around in there when i was adjusting it.

[johnk_10]

BTW, in case anyone is interested, i did finally find out what the factory spec regarding how many watts @ 120VAC that a 370 is supposed to draw at idle, and that spec is 50 watts, so since my distortion problem was solved at it being adjusted to 39 watts, i'm happy.