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Re: 200B 5 PA head hum/hiss issue [message #28148 is a reply to message #28094] Fri, 26 March 2021 16:45 Go to previous messageGo to previous message
stevem is currently offline  stevem
Messages: 4739
Registered: June 2004
Location: NY
Senior Member
Ok, here's what I have for you, but first some notes!

1) my wall voltage during these test was 117.5.

2) my dummy load resistor checks out at 4.5 ohms.

3) I had to do these test on my 200B-2 amp ( just 2 channels ) because I forgot that my Brother has my 200B-4 head.
This means that your amp will be drawing a bit more power supply current then mine so your peak power wattage will not be quite what mine was.

2a) my amp still has its original main rail power supply filters from 1969 So who knows maybe your new filters will off set the higher current draw of your amps added two preamp boards and reverb board.
I guess you could unpower two preamp boards if you cared to do you would test out closer to my amp.

At idle I had 39.6 volts on the positive rail and 39.4 on the negative rail.

At idle one 8 volt rail was +8.37 volts, the other rail was - 8.23 volts.

My amps max RMS output voltage was 19.6 for a wattage of 85.3 .
This reading was taken at the very onset of clipping seen on my scope and I found I had better control of that setting point by also adjusting the Treble knob.

At this wattage output my main plus rail was down to 34.1 volts, the negative rail was 34.1 also.

Peak output voltage was 25.16 for a wattage of 140.6 .
At this power level the main plus rail was down to 32.3 and the negative was 32.2.
The regulated power rails where + 8.34 and the other was -8.19.
If your amp test out close to these numbers then your power supply is heathy and we then can look deeper for the source of your issue.

[Updated on: Fri, 26 March 2021 16:48]

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