I fired up my GU50 PP pentode amp for the first time, hooked up to a dummy load. I heard a strange high pitched 'gargling' noise coming from the OPT. I've used this OPT several times and never encountered this. Should I be as alarmed as I am?
Thanks.
Thanks.
Your output tubes may be driving the OPT harder than in previous amplifiers. I've encountered "singing" output transformers due to core magnetostriction. This becomes especially apparent when the amp is driven with a square wave signal for transient analysis. Even varnished transformers can sing a little under this treatment.
I have also had something like this happen when an amp oscillates because of reversed leads from an output transformer
Thanks guys, here's more info: there was no signal present.
I've worked on the phase splitter earlier, without the power tubes. I suspect that part is oscillating. So far I've been busy getting the voltages right.
There is no feedback arrangement present yet.
I had turned the bias voltage all the way down ( -50V) before I put in the power tubes.
I'll remove the power tubes and do some scoping tomorrow.
I've been meaning to start a thread about the amp build, but I'm a little unorganized..
I've worked on the phase splitter earlier, without the power tubes. I suspect that part is oscillating. So far I've been busy getting the voltages right.
There is no feedback arrangement present yet.
I had turned the bias voltage all the way down ( -50V) before I put in the power tubes.
I'll remove the power tubes and do some scoping tomorrow.
I've been meaning to start a thread about the amp build, but I'm a little unorganized..
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I never power up an amp for the first time without a scope on the output. Oscillation can be really hard on the amplifier, and it's very easy to get the phasing on an OPT backwards.
Wondering if anyone can recommend a thread or a link to have a primer on using an oscilloscope. I realise that if I am going to be able to debug issues, or verify optimum settings, then an oscilloscope is invaluable.
Price is not really an issue - when for £50 you can be up and running with a respectable USB based scope, which is probably all I really need.
Problem is - even though I can see the benefits of having a scope, I do not really know how to incorporate a scope into my way of working.
Price is not really an issue - when for £50 you can be up and running with a respectable USB based scope, which is probably all I really need.
Problem is - even though I can see the benefits of having a scope, I do not really know how to incorporate a scope into my way of working.
The differential stage after the concertina PS is seriously oscillating. I should have put a bet on it, I knew what was coming.. Because I have the tube sockets on a separate board* with leads going to a mounting strip with the resistors and coupling caps. I didn't have much room on the breadboard, so those leads became quite long and entangled.. Uh-oh no-no... I'm making the leads much shorter and straight now.
It was oscillating at a few hundred Hertz at a serious voltage, no wonder the OPT started protesting.
* grid stoppers mounted directly on the socket
It was oscillating at a few hundred Hertz at a serious voltage, no wonder the OPT started protesting.
* grid stoppers mounted directly on the socket
There must be videos about it on YouTube. Repair videos are also very interesting.
Just a small quibble, relating to another poster's oscilloscope question. Wiring lead lengths of a few inches can't really cause oscillations at a few hundred Hertz, although you may (or may not!) see the actual oscillation manifest this way.
Parasitic oscillation at 100 MHz and above is quite easy for vacuum valves, and can't be seen at all on inexpensive USB scopes. They're often very hard to see on conventional CRT scopes. Still great tools, but parasitics are sneaky.
When a high frequency oscillation manifests at a lower frequency, it's classically called "squeeging". The oscillation drives the valve(s) out of oscillating range, they recover, oscillate again, repeat. If this happens, we can sometimes see (or even hear, hence the name) the repeat rate, but we can't really count on it happening in all cases.
All good fortune,
Chris
Thanks Chris!
I used a small auto range scope, and at first it really couldn't decide what range to pick. So that makes sense.
I used a small auto range scope, and at first it really couldn't decide what range to pick. So that makes sense.
Over at Audiokarma there's a good thread on "Oscilloscope basics." It's for analogue scopes but it will introduce you to the basics that apply to any scope. Picoscope also has a lot of video tutorials. The Picoscope 2204 is a good basic USB scope and the free software is good. If you're going to be dealing with feedback amplifiers, I strongly recommend the 2206B since it's built-in signal generator goes up to 1MHz, which is very useful for checking for super-sonic oscillations and stability. It's a bit pricier but worth the money. There's also a third-party app that generates Bode plots and lets you see the full frequency response and accompanying phase shifts and margins.