peavey clone again

[bluemoon]

so, i still trying to do something with this amp but not much long, shematic from output stage i attached, also with mesuared RMS voltage weighted with 8 Ohm passive resistance. only difference is power supply +-35V insted +-25V as yuo can see on drawing, the peak on diagram is the moment when the 1Vpp sinwave signal is applyed, and after that voltage drops to cca 6.5V on resistor. during this test supply voltage drops from +-35V to +-32V and i think it is minor fall. with scope i monitored a signal on C21 to be sure there is no visible distorsion which can affect the signal waveform on output. so, i'm interested to know, this situation is same on similar original peavey or not. i already built on protoboard an amp with TDA7294, to try to implent it in this amp, but i was supprised with unusual connection of output transistors and power supply to speaker, so i abandoned this idea. if i connect my board with TDA7294 usual way, i must disconnect tube dynamics.
at the end, there is anyone mesuared the nominal output power on peavey bandit and wich results he get
tnx a lot for your time

[Enzo]

Actually the output cconnection is pretty common. Most QSC power amps are wired this way, Many solid state PV amps are, and many other brands use it. One advantage is that if an output device fails, the speakers are not directly conected to a power rail.

OK, so if I read you right, there is a 6VDC offset on the speaker terminals? And power supply sags. I wouldn';t worry about the sag, but where did you reference your meter for these measurements? The rails are not refernced to ground, they are referenced to the speaker. Most times it doesn't matter, but when it does matter, you need to know where the readings come from.

Did BOTH rails say the 3v? or did you check just one? Is it possible that one rail sagged 3v but the other ROSE 3v? Just checking?

Are R71,72 in the speaker return OK?

You have both main rails at +/-35v, but are the +/-15v rails for the op amp U2 OK? There are two zener regulated rails for those - CR8, CR36.

Other than the unfamiliar speaker connection, this is an utterly conventional output stage. Pair of outputs, a pair of drivers, simplest bias string between then, all driven by an op amp.


Now maybe I misinterpreted your scope picture, and that represents 6VAC of signal on the output. In that case, try removing Q10 at the input. A leaky Q10 or faulty signal at its gate could severely limit output.



ANd let us talk about the speaker connection. "Normally" we see the power supply common grounded, the output hot and the speaker between that output and ground. Now imagine we built the amp on a piece of wood, no ground. The speaker would connect between that hot output and the power supply common, right?

When we ground that hot output point instead of the power supply common, the speaker is still between the output and the power supply common. The circuits are the same except we decided to ground the other side of the speaker.

In a really basic sense, imagine a light bulb, a battery, and a switch all wired in a loop - the classic electricity 101 experiment. If we built it in a chassis, we could ground any one point in that circuit. Doesn';t matter which point. That is what we have here: speaker, power supply, drive circuits all in a loop. You are used to seeing the power supply end of the speaker as ground, but they used the other end as ground. otherwise it works the same.

[bluemoon]

mea culpa. yesterday night i realised where i was wrong, tnx to your kind explain, now the picture is complete. i doing repair of electronics about 25years, and the "mainstream" in my job was hi-fi. i measuerd, commonly, amps power to check there is all OK, with same equipment and same way. last night when a tryed to compare input signal vs output signal on resistor, using a same 2 ch scope, and the *** amp starts to "generate" smoke, i was desperate. then a taked a look again to schematic and voila, there are 5 different GNDs with 5 different potentials. that was somethig new for me after 25 years (maybe i'm steel to young). i reapired before guitar and other amps, but not peavey or something with this type of connection. now, with other aproach i get cca 16.5 V RMS on 8 Ohm, and that is cca 34W. the problem is whit some other part of amp, because i get only max 0.5Vpp, cca 0.2V rms, on C21, and i think that is not enough. i apply sinwave to input on front panel, cca 100mV, and with gain it can be aplified nicely to 2Vpp, even on pin 1 of IC U2A is 2Vpp, but on C21 signal is much lower. DC values are all ok. i was to tired to continue last night. so, great tnx again to schow me a right way to solve this problem, if it is can be solved anyway.
best regards