I want do check my amp to determine if I'm getting HF oscillations in the power amp section. I have a BK 20 mhz scope but also could pull out a 60 mhz Tek if need be.
I've noticed what appears to be dashed lines on the lower sweep rates. By dashed lines that is to say, the trace of the 1 khz input signal is faintly broken up into dashed lines, perhaps 20 dash per cycle... This is not affecting (effecting?) the tone but I'd rather not have it. Perhaps due to the grid stoppers on the power tubes having full length leads to the grid connection; but I want to understand the diagnostic process also for future reference.
Any suggestions on the settings and what I'd be looking for? I do know the traces would be higher frequencies but don't know what sort of range or the best sweep settings.
Regards to all,
Ccat.
I've noticed what appears to be dashed lines on the lower sweep rates. By dashed lines that is to say, the trace of the 1 khz input signal is faintly broken up into dashed lines, perhaps 20 dash per cycle... This is not affecting (effecting?) the tone but I'd rather not have it. Perhaps due to the grid stoppers on the power tubes having full length leads to the grid connection; but I want to understand the diagnostic process also for future reference.
Any suggestions on the settings and what I'd be looking for? I do know the traces would be higher frequencies but don't know what sort of range or the best sweep settings.
Regards to all,
Ccat.
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If you had high frequency oscillations it would be obvious with noise on the sine wave output.
I bought an old Maplin 225WRMS amplifier module off ebay with parts missing.
I bought in new 2N3055 and MJ2955 and after fitting them the amplifier had oscillations on the output. It was in the MHZ region. I had to increase the VAS capacitor to kill the oscillations. I checked the new transistors on a semiconductor analyser and they had massive gain compared with the original transistor spec.
This is why care has to be taken updating or replacing transistor types with higher gain and/or bandwidth.
I wish I had taken a photo of the scope now to show you.
It showed up on my scope as a burst of oscillation on the peaks of the sine wave.
I bought an old Maplin 225WRMS amplifier module off ebay with parts missing.
I bought in new 2N3055 and MJ2955 and after fitting them the amplifier had oscillations on the output. It was in the MHZ region. I had to increase the VAS capacitor to kill the oscillations. I checked the new transistors on a semiconductor analyser and they had massive gain compared with the original transistor spec.
This is why care has to be taken updating or replacing transistor types with higher gain and/or bandwidth.
I wish I had taken a photo of the scope now to show you.
It showed up on my scope as a burst of oscillation on the peaks of the sine wave.
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