Old amplifier circiut

[camt1]

hello, i have been given an old amplifier made by Burdett and Dales of Sidcup in Kent. It proclaims to be 100w sterio,its old at least 15 years. So far i have replaced some burnt out resistors and the mains wiring and it works ok (driving 2 10"subwoofers no problem) .My question is , is what do you guys think i could do to improve this circiut . The only thing that concerns me so far is the output transistors dont seem to get warm at all but the driver trans (Q4 and 5 ) get very hot to the touch (no heat sinks on (Q4.5.6.7) ,could there be a problem with the bias ?.I wondered if maybe it would be worth replacing the transistors and caps for new and better types. I have included the circiut diagram any ideas and thoughts would be welcome.
thanks camt

[Wavebourn]

Your schematic contains errors, mostly emiotters and collectors are swapped on some transistors.

[richie00boy]

Make that most transistors

To be honest, messing about modding stuff like this is generally a waste of time. You'd be better off just re-using key parts like the power supply and heatsinks, case, maybe output transistors, to make a better design. Although having said that, the schematic you've drawn (I can see through the wrong component orientations) is not a really bad design by any means.

[camt1]

sorry corrected drawing my mistakes not used to ms viso

[camt1]

schematic hopefully right this time

[Wavebourn]

Yes, it is right now, and pretty good classical design for 8-16 Ohm load.

[darkfenriz]

This amplifier has no thermal sensing, so works with no questient current in output stage to prevent thermal runaway.
Replace 47R with Vbe multplier circuit mounted on main heatsink and set questient current of output stage to some 15-30mA.
You can also change electrolytic cap in feedback path (22u) to bigger one bypassed with 100n quality cap.

[john_ellis]

You could try putting a Vbe multiplier in place of the 47 ohm bias resistor. You don't say if the drivers are on the same heatsink as the output pair, but I assume they aren't.

There has been a lot of discussion about bias on the Class B thread....

2N3772's are a bit old. They may be slow and you could swap them for a modern pair of MJ21194's or if you are willing to hack the board a little more a complementary pair of MJ21193/4's.

TIP3x's don't make good drivers- they're slow too. So if you want a bit of response try swapping these for an MJE15030/31 pair or similar. And then you could change the VAS and current source to faster devices too...

It all depends how much you want to upgrade!

John

[camt1]

Thanks for the replies , only the output tansistors are on heatsinks (large 4" x 4" one for each transistor ). The drivers have no heatsinks and Q4 and 5 run extreamelly hot while although the others even under hard use seem to stay cold.This seems to be different to every other amp i have used . With no quiscent current bias and operating in class B i assume that the crossover distortion will be high. would i be right in thinking of the following values for the VBE multiplier R1 1K , R2 2K variable pot and a BD139 for the transistor mounted on the heatsink any comments would be very helpfull
thanks camt

[phase_accurate]

It looks as if this amp behaves like an Edwin amp where only the drivers conduct at low output currents. This correlates well with your observation of drivers being hotter than the output devices themselves.

Regards

Charles