oscilloscope testing output transformer

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oscilloscope testing output transformer

Postby ioginy » Tue Aug 21, 2012 5:45 pm

Good day all.

I recently picked up an old oscilloscope that I thought would be a handy tool for trouble shooting my amp projects. It is a Telequipment Serviscope s51-a. It works like a charm and is in wonderful working condition. I have run a few tests with and such, but now it comes down to using it properly.

I'm working a power amp at the moment using an HH scott UL output transformer. The layout can be found on this schematic

http://pacifictv.ca/schematics/scott210c.jpg

I measured resistance between taps to double check what the schematic is indicating. As far as i could tell everything checked out.

Having an oscilloscope now, I thought there might be some better way of testing the transformer, or the amp in general to find out where the problem is. I have never used an oscilloscope before, and was hoping someone could give me a little run down on what magic can be done with this tool to give me some insight into if my OPT is wired wrong, or merely bunk.

Thanks in advance.

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Never mind. I got some sound coming out of the power amp now. Sadly it is very distorted and ugly and when I turn the volume all the way up, the sound cuts out completely. I think I may have something wrong in the initial gain stage before the PI. If anyone has any ideas, i'd love to hear them, but if not I'll pick away at it and see what I can figure out.

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I did some voltage checks and my idle voltages are as follows.

bias : -47.4
6l6 plates : 489
6l6 screens : 487
6sl7 PI plates : 485
6sl7 PI Cathodes : 8.1
12at7 driver plate : 229
12at7 plate cathode : 4.12
12at7 CF line out Plate : 417
12at7 CG line out cathode : 126.4

I haven't taught myself how to do load lines yet, but I think that is my next step. It seems to me like the cathode voltage on my driver is far too high.
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Re: oscilloscope testing output transformer

Postby Gingertube » Wed Dec 05, 2012 10:00 pm

Can you post a schematic of what you built - it is obviously not the same as the Scott.
It is impossible to comment without that.

When hooking oscilloscopes up to valve amps be aware of the maximum input voltage specification for your oscilloscope(usually printed on the panel next to the probe connector)

The peak DC + signal of the point you are monitoring must not exceed the maximum input voltage spec.

Also be aware that in a push pull amp the output tube anodes can swing to 2 x the B+ voltage.

The way to hook up to this point safely is to use X10 or X100 probes to divide the voltage down before it gets to the oscilloscope input.

Cheers,
Ian
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