I have an ST70 that has this squiggly issue when viewed on the scope. The squiggly doesn't stay the same it's all over the place. Oscillation? Seems like some sort of phase issue.
Anybody come across this with an ST70?
New JJ power tubes. The driver 7199's test good for the triode sections but the Pentode section is 4.4mA/V for one and 5mA/V for the other. This is above min. on my hickok but not average per datasheet. I get full output power from the ST70 but this crazy ugly waveform is weird.
All capacitors are original.
YouTube
-bird
Anybody come across this with an ST70?
New JJ power tubes. The driver 7199's test good for the triode sections but the Pentode section is 4.4mA/V for one and 5mA/V for the other. This is above min. on my hickok but not average per datasheet. I get full output power from the ST70 but this crazy ugly waveform is weird.
All capacitors are original.
YouTube
-bird
IMO, there are several trees to bark up. Leaking coupling capacitors? Certainly dried out 'lytics, whose ESR is way high. Carbon composition resistor drift? Unfortunately, the JJ Octal based tubes we get in North America are highly suspect.
The 7199 is a royal PITA. JMO, retain the OEM small signal topology by using a Triode Electronics driver board populated with 2X EF86es and an ECC99. Those types are in production and perform better than the corresponding stuff inside the VERY scarce 7199.
The 7199 is a royal PITA. JMO, retain the OEM small signal topology by using a Triode Electronics driver board populated with 2X EF86es and an ECC99. Those types are in production and perform better than the corresponding stuff inside the VERY scarce 7199.
Seems to be a scope issue.
How does it sound ? What's displayed on a silent signal ?
I tend to agree with you that it's a component that is beyond it's life expectancy. I'm leaning towards power supply caps. I'll put it on the FFT and see what sort of distortion figures I get later this afternoon.
It's funny the last time I saw the same thing it was an old Regency amp that is a copy of the Dynaco circuit. Same thing, those amps had all original caps and the owner didn't want to replace anything that wasn't completely dead. They sounded fine on speakers.
I have looked at the new front end boards, seems like the way to go since the 7199 is extinct.
I am almost positive it's not a scope issue, I test a lot of amplifiers with this scope and I have only seen this issue a couple times, always vintage high power amps with lots of feedback with original caps. Last time I can remember was some Regency mono amps that are the same circuit as the Dynaco circuit (except a 6AN8 driver). I didn't do distortion measurements yet, maybe that will give us more info. If I can talk the owner into replacing the caps we'll see if it improves and I'll be able to update the thread and help anyone that comes across this issue. The Regency amps sounded good on speakers.
What kind of scope is it? Can you delay the trig of next sweep ? This is to make sure only one sweep is seen. I get the impression that the scope triggers
on several different signals ( not only the sine ). Could be signal and a hum.
Another way is to use a single shot trigger and examine the results.
That smaller background trace looks like the trigger., triggering problem?
The sine wave has lots of distortion on it, or looks like it, but again it looks like the scope isn't locking on/triggering correctly. Could you have an intermitant or iffy connection, like a dodgy ground where the OP is failing, then correcting ad infinitum? Just thinking out loud.
Is it doing this on both channels? Check the usual, dirty valve bases/contact's, bias V at the OP valve pins, swap valves.
Andy.
The sine wave has lots of distortion on it, or looks like it, but again it looks like the scope isn't locking on/triggering correctly. Could you have an intermitant or iffy connection, like a dodgy ground where the OP is failing, then correcting ad infinitum? Just thinking out loud.
Is it doing this on both channels? Check the usual, dirty valve bases/contact's, bias V at the OP valve pins, swap valves.
Andy.
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I have a Tektronix 475
Yes both channels are affected.
I didn't think it was the trigger because the trigger light is illuminated and locked. Usually when I have a trigger issue the light doesn't illuminate and lock and the sine wave is doubled and looks exactly like the normal one, this is a really faint squiggly signal but I guess it could be the scope. Just strange I almost never have any trigger issues with other devices I test.
I will hook it all back up later and also measure distortion at various output powers. I'll also try adjusting the trigger.
Yes both channels are affected.
I didn't think it was the trigger because the trigger light is illuminated and locked. Usually when I have a trigger issue the light doesn't illuminate and lock and the sine wave is doubled and looks exactly like the normal one, this is a really faint squiggly signal but I guess it could be the scope. Just strange I almost never have any trigger issues with other devices I test.
I will hook it all back up later and also measure distortion at various output powers. I'll also try adjusting the trigger.
It's not trigger level that could be adjusted, it's delay after a sweep. This to
make one sweep and not a continuous triggering with both the sine and some
random disturbances. It could be hum that is seen. Varying the
frequency of the sine might give a hint what we see.
One sweep, and a longer time. A memory scope would be perfect, but
a 475 is usually bright enough for observation.
Yes, and no....
The "squiggly" is non synchronous with the 1KHz test signal. The scope is triggering on the 1KHz as it should, and it can't trigger on two things at once so the other signal "drifts" through time, unsynchronized. It is probably 120 Hz hum, but it could be some high frequency.
Try removing the 1KHz, adjust the scope to trigger on the "squiggly" then slowly turn up the 1KHz. With a little bit of 1K, it should appear to ride the hum. Turn it up more and the scope will lock on to it and the hum will drift.
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