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Old January 19th 16, 12:42 AM posted to sci.electronics.repair
John Heath John Heath is offline
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First recorded activity by DIYBanter: Dec 2015
Posts: 41
Default Peavey Classic 60 questions

On Sunday, January 17, 2016 at 10:09:24 PM UTC-5, wrote:
"With transistor amps, the same idle current always flows in both output devices cos they are connected in series. "


I have wanted to pound a couple of things into people's heads about this for some time.

What you say is true for idle current, the bias, but if there is an offset the current through the outputs is not equal. With no load it should be no matter what.

Anyway, the sledge hammer is to beat it in that :

1. Adjust the bias first, with no load.

2. Adjust the balance/offset next, also with no load.

Do this after having warmed it up a bit, if you can't, recheck it hot.

If the manufacturer starts this **** about taking out jumpers to measure bias current, **** all that. Just do the math and calculate it from the emitter resistors. Take my word for it.

In fact I can adjust it with no spec. II can just feed it with low level program material and watch the waveform at the collector(s) of the voltage amp(s). The ||_||_ in that waveform, produced when the feedback is taking care of the crossover distortion is the prime test point. Adjust bias to flatten that out, done.

However, going farther, that is biasing it harder, does have some merit. First of all it is not that much, and it also gives the amp more low power damping factor. Believe me I know the difference. I am not talking going into class A here, I think that is stupid. If you are going to go class A go all the way and go SE. Then for the most part you only got even order distortion.


I have a digital AC power meter on my bench to monitor AC power consumption.. It has many uses that I did not think of when buying it. It is useful for eyeballing idling current to see if it is creeping up into thermal runaway. I found it can also be used to set idling current. Adjust the bias pot until ac power starts to change. Then back it off to match the idling power for that type of amp. This saves a lot of time trying to measure the voltage drop across output emitter resisters.