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Old 23rd June 2006, 07:22 PM   #11
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Thanks for the analysis AndrewT!

This citrcuit was designed around 1979 and uses old semiconducters which is probably why they run them close to their limits to achieve the output power (over 200w into 8 ohm). I am concerned about their voltage ratings too , although this amplifier has been in use since about 1979 and has been unmodified as far as i can tell i.e. it has never blown up before.

The diodes in the bias network are mounted on a transistor heatsink i think, the diagrm shows them in a dotted box.

As the THD of the amplifier at its rated output is no more than 0.05% so the bias issues seem to have been accounted for.

No the amp doesnt have a 220/240v switch, it specifies it can be run at between 220 and 240.

Thanks a lot
Craig
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Old 23rd June 2006, 09:41 PM   #12
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Yes again the proof of the pudding is in the eating. Now you have set bias I would just forget about it all
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Old 24th June 2006, 08:47 AM   #13
AndrewT is offline AndrewT  Scotland
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Hi,
200W into 8ohm is a lot less stressful on output and driver stage than 350W into 4ohms and much less stressfull than 450W into an even lower load.

BTW 200 into 8ohms only requires a Vpk into load of 56Vpk. Most amps of about this power would run on +-65V to +-70V.

Have you tried measuring the supply rails when the amp is delivering significant power into a high resistance (8ohm)? I wonder what the 71V means beside the smoothing caps.

An amp that has survived unrepaired for over 25 years indicates a well balanced choice of components.
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Old 24th June 2006, 09:24 AM   #14
lndm is offline lndm  Australia
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Quote:
Originally posted by richie00boy
Yes again the proof of the pudding is in the eating. Now you have set bias I would just forget about it all
Amps that drift until thermal equilibrium is achieved, can be reliable. I have built amps like this and would periodically measure the bias current when warm. Even with moderate variations in ambient temperature, the bias current was predictable.

The circuit simplicity was not necessarily a bad thing IMO. If anything I think you should simply ensure a consistent airflow through the amp.
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Old 25th June 2006, 08:30 AM   #15
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Thanks for replies everyone.

AndrewT,
I havent measured supplies under load yet, i wouldnt expect them to dip much, the transformer is massive (1.3KVA) plus 36000uf of capacitance.

I am waiting for another DIY member (thejohn) to verify my finding with his lab 600 amplifier which he has run for some time.
I would agree that if the amp hasnt been repaired ever in 25 years its got to be doing somthing right!


Indm,
I have set the bias to a steady 6.5mA after 20 mins of stabilising time, this way i think it should hit around 10mA when the case heats up. Although i will have to check this.

A fan would certainly help keep the transistors inside cooler, but i wonder if it would keep the bias from stabilising? When i biased this thing my window was open and the occasional draft would disturb the bias.

Regards
Craig
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Old 25th June 2006, 09:52 AM   #16
AndrewT is offline AndrewT  Scotland
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Hi,
you have either got to prove that the devices will continue to be reliable at +-85V
or
reduce the rail voltage to the manufacturer's specified +-75V.

I would not support the thought of leave well alone, partly for the reasons given earlier but mostly to find out why the voltage is apparently 13% above specification and it's effect on dissipation abilities, warm up time, ultimate device quiescent temperatures, device peak temperatures, and variation of all of these with changes in ambient temperature (room and case).

BTW. 36mF is very low for the current ability of this amp.
Using information from other designers and findings of my own I always recommend a minimum of 2mF per Apk of output current.
On that basis your 18mF can support a maximum of 4.5Apk per channel. This equates to a minimum load impedance of about 13r. If I were designing this amp for 4ohm duty I would be using +-40mF per channel (a total of 160mF not 36mF) and I would probably try increasing to +-60mF to listen for any improvements before releasing build information.
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Old 25th June 2006, 10:58 AM   #17
lndm is offline lndm  Australia
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If I remember correctly, a fan is not a complete substitute for a heatsink, but is still a good thing. A fan does reduce sensitivity to ambient conditions, stabilising the airflow by outweighing natural air movements.

I used to use a couple of computer fans running on maybe 6 or 7 volts so they were more quiet.
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Old 25th June 2006, 12:54 PM   #18
thejohn is offline thejohn  South Africa
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Craig,
So I took the cover off my 600 today to see if I could verify your findings so far...
With no load connected and gains set all the way down as per the service manual, voltmeter on R634 (one of the 0.47ohm output device emitter resistors), switch on and....
Pretty much 0mv! It then climbed slowly to 8mv after about 10min and settled there. TR605 and 606 heatsinks barely warm.
Powered down, connected load, on again, play some music, amp nice and warm, feel TR506 and 606 again... warm not hot (maybe 40something deg?)
Disconnect load, gains down, check bias, amp still nice and warm... still steady at 8mv.
My mains voltage today is sitting at 224v. We are meant to have 230 but in reality it varies between 220 and 240!
My rail voltages, measured on the main psu caps are +-74v.
Temperature today here is about 24deg. In summer we have regular 30deg+ ambient, I haven't checked bias under these conditions...
Hope this helps...
John
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Old 25th June 2006, 01:02 PM   #19
thejohn is offline thejohn  South Africa
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oh yes, for AndrewT... The bias diode strings (D606 and 651) are in flat packages with a mounting slot. One is mounted on the same bolt that holds driver (TR609) to it's heatsink and the other is bolted to the main heatsink.
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Old 25th June 2006, 03:08 PM   #20
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AndrewT,
I have decided to run my amp off off 220v from a variac permanently to avoid trouble, i like to feel safe when listening to music. It kind of ruins the experience if all you can think of is transistors getting hotter and hotter then boom!

Thejohn, thanks for checking your amp for me.
The fact thast your heatsinks are only warm concerns me, i have checked the voltage measurements around these transistors and they are exactly as the schematic specifies... as are all other voltages marked for measurement?

I dont understand how they could heat up so much if yours are not
Do dying transistors get hotter? i have no idea, but the PNp and the NPN get equally hot on each channel.

Im going to have a serious probe after the football and see if i can find a problem.

Thanks
Craig
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