Hi there,
Just put together an amplifier, which is working great, giving a healthy 37W at 50Hz into 10 ohms (8 ohm tap) before clipping.
The circuit built is a Radford STA25, built exactly down to the component (with the correct transformers).
However, there is an issue: One channel, as soon as the load is disconnected, instantly oscillates at some ultrasonic frequency. Obviously this is not something I want to keep triggering for the sake of the output stage, but the other channel is dead stable, and this circuit/transformer combination simply shouldn't do this - Radfords are stable with no load.
As far as I can tell, both channels are identical.
The only omission is I've left the heaters floating, but this is simply due to the hurriedness of the construction (though the signal wiring is fairly neat). Grounding the heater winding of the unstable channel does not cure it, unfortunately.
I've applied a signal generator to the primary of both OPTs each in series with its own 1k resistor (75 ohm src) to roughly simulate an imperfect voltage source, and simultaneously scoping the secondaries of each. They both track fairly well in amplitude and very well in phase, diverging slightly above 10khz but until then there is no discernible difference. With both traces overlaid in dual channel mode, it simply looks like one.
I think/HOPE the output transformers aren't the culprit. I have yet to swap them to see if the problem follows them due to the hassle.
As far as I can tell, the primary leads to the OPT don't get excessively close to any sensitive signal circuitry wiring - but they do pass under the PCB where the components reside.
The input stage on the faulty channel begins to roll off at about 10Khz by design, which I have confirmed by measurement, so the driver circuitry doesn't have excessive bandwidth.
All in all I'm quite stumped!
If anyone has any insight into this I would be very appreciative
Just put together an amplifier, which is working great, giving a healthy 37W at 50Hz into 10 ohms (8 ohm tap) before clipping.
The circuit built is a Radford STA25, built exactly down to the component (with the correct transformers).
However, there is an issue: One channel, as soon as the load is disconnected, instantly oscillates at some ultrasonic frequency. Obviously this is not something I want to keep triggering for the sake of the output stage, but the other channel is dead stable, and this circuit/transformer combination simply shouldn't do this - Radfords are stable with no load.
As far as I can tell, both channels are identical.
The only omission is I've left the heaters floating, but this is simply due to the hurriedness of the construction (though the signal wiring is fairly neat). Grounding the heater winding of the unstable channel does not cure it, unfortunately.
I've applied a signal generator to the primary of both OPTs each in series with its own 1k resistor (75 ohm src) to roughly simulate an imperfect voltage source, and simultaneously scoping the secondaries of each. They both track fairly well in amplitude and very well in phase, diverging slightly above 10khz but until then there is no discernible difference. With both traces overlaid in dual channel mode, it simply looks like one.
I think/HOPE the output transformers aren't the culprit. I have yet to swap them to see if the problem follows them due to the hassle.
As far as I can tell, the primary leads to the OPT don't get excessively close to any sensitive signal circuitry wiring - but they do pass under the PCB where the components reside.
The input stage on the faulty channel begins to roll off at about 10Khz by design, which I have confirmed by measurement, so the driver circuitry doesn't have excessive bandwidth.
All in all I'm quite stumped!
If anyone has any insight into this I would be very appreciative
Running with no load connected is a good way to ruin an OPT, so be careful. HF stability is not just determined by the circuit and the components but also by the exact layout of everything. The 'well-behaved' channel might still have an HF peak, but you won't see it unless you plot a wide frequency response. But you should not do this, unless you can afford to buy a new set of OPTs.
The solution is simple: never disconnect the load. If you can't guarantee this, add a Zobel network.
The solution is simple: never disconnect the load. If you can't guarantee this, add a Zobel network.
According to the circuit diagram I found a lucky find, the NFB-circuitry is quite complicated. You could first check that the amount of NFB is same at both sides.
Secondly you could reduce it few dB and see if it helps.
Hi DF96,
The fault was only discovered by chance when my dad changed some speakers over with the amp still on (though without signal). A loud pop resulted when the speakers were connected, along with some cone movement as the circuit re-established DC balance. If it was me I probably would never have discovered it!
For various reasons I can't guarantee a load - I wont necessarily be the only one using it. What happens under heavy signal conditions (with no load) is unavoidable though.
I will look into a zobel. Can you recommend any values? 100nF + 10 ohms sounds about usual...
Ideally I'd like to not mess with the circuit too much - this particular one usually work just fine, although to be fair I really have never been in the habit of running valve amps without a load - this was discovered by accident!
To be fair, the layout is the same as every other STA25, which is a bit disconcerting. I may have just stumbled across an unfortunate combination of factors though...
Sudden HF oscillation with no load could be caused by parasitics in the output stage, not instability in the feedback loop. You might find that there are bursts of oscillation at peak signal too. Have you had a look with a scope?
Anyway, a Zobel on the output may help. Alternatively, snubbers on the OPT primary taps (g2-anode).
Anyway, a Zobel on the output may help. Alternatively, snubbers on the OPT primary taps (g2-anode).
The signal looked very healthy into a (mostly) resistive wirewound dummy load - very clean clipping and no sign of HF bursts.
I have 1K screen resistors directly on the output valve sockets, though the screen stoppers (2.2k) are located on the PCBs, which always worried me slightly.
The first thing you need to determine is if that NL oscillation occurs with the feedback loop open. If it goes away, then your NFB loop is the problem.
I had the same thing happen with a similar design: high gain stage up front (not pent, cascoded triode stage, but the Av was up there in pent territory) and an oscillation around 100KHz. That needed to be fixed before connecting the NFB to make certain a small problem wouldn't become a big one later.
In this case, the fix was a 4K7, C-comp grid stopper. From that schemo, I don't see any grid stopper on that first high gain stage. There is also no screen stopper either. I would add a 1K C-comp screen stopper as well.
I had the same thing happen with a similar design: high gain stage up front (not pent, cascoded triode stage, but the Av was up there in pent territory) and an oscillation around 100KHz. That needed to be fixed before connecting the NFB to make certain a small problem wouldn't become a big one later.
In this case, the fix was a 4K7, C-comp grid stopper. From that schemo, I don't see any grid stopper on that first high gain stage. There is also no screen stopper either. I would add a 1K C-comp screen stopper as well.
Hi Miles, the problem is indeed due to the FB loop, the problem goes away with the first tube pulled.
Regarding grid stoppers on the input valve, wouldn't C6 (500p cap) negate the need for things like grid/screen stoppers? It seems to roll off fairly quickly on its own
I'd be happy to be proved wrong on this however
Regarding grid stoppers on the input valve, wouldn't C6 (500p cap) negate the need for things like grid/screen stoppers? It seems to roll off fairly quickly on its own
I'd be happy to be proved wrong on this however
Argh, annoyingly enough, I've noticed that it IS prone to oscillation with a load - when turning off a burst is seen on the scope for a couple of seconds as the HT collapses. I'm fairly sure it didn't do this before. This is so strange
I didn't see this before, but now it looks like an NFB problem. It's possible that a feedback design can be marginally stable at high open loop gains, yet go unstable at lower open loop gains, as what happens when you power off, and cathodes begin to cool off, and HV decay.
That means inadequate phase margin. May be this is one of those designs that drove up the NFB to ridiculous porportions, and worked just swell with the OEM OPTs, but, like the Williamson, won't work "as is" with substitute OPTs? A lot of designs from the 1950s did that.
"This is so strange
"This demystifies it: Nyquist Stability Criterion.
Yes, oscillation at reduced gain can be a sign of what I think is sometimes called conditional stability (although that term has another meaning too). The plot of loop gain snakes around the (1,0) point but manages to miss it unless the gain is reduced. Unless deliberately introduced (which it sometimes is for special applications), conditional stability is best avoided.
The design uses quite a lot of feedback - 20dB I believe, but the transformers I have, in theory, are the same as the originals.
If the OPT is not the original, then all bets are off as to the original stability components being appropriate for your clone transformers. It might be best to first establish that the FB factor is the same as the original design achieved, and then apply stability networks that are optimized for your particular transformers -- assuming they will remain stable with the original FB factor applied around them.
Dave
Dave
Last edited:
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.