Where do you get that idea from?
I think you my have mis-read the TIP30A spec. The sat spec is usualyfor Vce and thats what i see when i look at it.
The Vce(sat) has no coloration the regulationon this circuit, but the on voltage of a bipolar transistor, the VBE(on) does.
that voltage will be seen as an additive or subtraction (depending in your circuit) voltage in series with your zener.
So, VBE is the voltage we need to consider here.
http://www.mouser.com/ds/2/68/tip30-845497.pdfhttp://www.onsemi.com/pub/Collateral/TIP29B-D.PDF VBE 1.3 Max (so yes it can be less then 1.3 - best to measure it)
Thanks, guys. Good info. I will calculate the current next time I have time to work on the amp. Some other info based on your input:
1.) R25 *is* at the top end of its tolerance @ 132 ohms.
2.) The zener has been replaced and I got the exact same voltage
3.) When I did temporarily trick the circuit with 2 series diodes, it brought the voltage to -14.9 and both the resistor and transistor ran much cooler.
Not sure what all that means, but I figured you might might know ;-)
Thanks for the help. The amp has preamp problems, too, but I don't even want to get into them until I feel secure that the power amp section is working properly.
Peace,
Bud
The resistor being cooler could simply be that with the voltage rails unbalanced the amplifier circuit could have been conducting with no signal input.
The 4558 op amp that used to bias Q2 and the whole dc coupled amp circuit is driven from the +/-15v. if the +/- 15v rails that power it are not balanced, the bias could be off and one side of your amp would conduct.
looking at the circuit id not be surprised if 1/2 of your power transistors were also getting warm if you had the speaker connected.
I happen to be currently working on a tip122 / tip127 +/- voltage regulator circuit myself with a VBE of 2.5 so it threw me for a second also.
Don't take my word on VBE being part of the equation for your output voltage. .....Measure it yourself. ... measure the actual VBE from Base to Emitter, and the actual voltage across the zener. (or across the 2 diodes and zener if you ahve the 2 diodes in)
Subtract the two and i bet you end up with your output voltage.
This isn't to say there might not be something wrong. Check for excessive idle current with the +/- 15v balanced (close to the same voltage + and -)
measure the voltage across r25 and do some math, see what the current draw is.
Basically though, if the voltage is significantly higher the the expected 10v, then our circuit is drawing more current then it should and you should trouble shoot that.
My belief is still threes not a darn thing wrong. the circuit is working as it should. If so, then leave in your two 1n400x diodes and call it a day or get yourself an appropriately rated voltage zener that takes the VBE into account
they make zeners in 16.1,16.2,16.3 voltage ranges just for this reason.
and for those with VBE of .6 they have 15.6v zeners ... i'm sure you have seen that value, it is popular.
this is where getting out your meter and actually measuring will tell you what you need.
however ...... (diodes)6v+.6v = 1.2v
15v+1.2v= 16.2 (total zener voltage)
-16.2v + 1.3(Vbe) = -14.9V
Curious how the math works out
Good luck, keep us posted
(edit for grimmer)