Yes, it's a digital, line-level signal (input, in blue). The output is yellow. Same in all three pictures.
The 20kHz signal is stronger without a load (no speakers), and it goes away when switching off the amp. It does not change with moving the cables for my probes. It isn't present in the input. The oscilloscope also doesn't have a power supply, just a laptop with USB power. So, logically, I can confidently say it's originating inside the amp, can't I?
Below is an earlier measurement with L and R channels that shows these spikes in both channels.
Switching noise could be, of course.. I recently added another bridge rectifier to the 6.3V AC filament heating circuit, and it's not yet loaded, output wires just dangling. I didn't think it could output 20kHz... but then I come across this thread ... Could that be the source?
The issue with the left channel being a bit stronger was resolved after I calibrated my probes for capacitance.
The 20kHz signal is stronger without a load (no speakers), and it goes away when switching off the amp. It does not change with moving the cables for my probes. It isn't present in the input. The oscilloscope also doesn't have a power supply, just a laptop with USB power. So, logically, I can confidently say it's originating inside the amp, can't I?
Below is an earlier measurement with L and R channels that shows these spikes in both channels.
Switching noise could be, of course.. I recently added another bridge rectifier to the 6.3V AC filament heating circuit, and it's not yet loaded, output wires just dangling. I didn't think it could output 20kHz... but then I come across this thread ... Could that be the source?
The issue with the left channel being a bit stronger was resolved after I calibrated my probes for capacitance.
So, I experimented a bit - it's not the extra bridge rectifier.
Connecting chassis to ground makes the 19.8kHz signal go away; alas, now I have an audible and visible 60Hz hum... 🙂
Connecting chassis to ground makes the 19.8kHz signal go away; alas, now I have an audible and visible 60Hz hum... 🙂
That 60Hz hum was a ground loop, I found the source outside. If I could filter out this 19.8 kHz EMF noise in some other way, that would be nice...
You should always have the chassis safety grounded (protective earth), however you might want to keep the amp negative away from safety ground to prevent ground loops. To keep it safe you could use a 'ground lifter' basically a rectifier bridge and parallel resistor between amp ground and safety ground (chassis) to allow some voltage diff between the grounds, but not enough to make it dangerous. You could also use some small caps from input ground to chassis for RF interference.
OK, I tried a 470p mica cap connecting amp negative to chassis ground, or also input ground to chassis ground, and that seems to solve the ground loop problem but doesn't keep the EMF away.
I did some experiments on this amp today. Wanted to play around with the resistor values for the 'splitter' so I put in some resistors and a trimmer instead of the 30k resistor so I could vary it while measuring distortion.
With equal values (27k), the 2nd and 4th harmonics was significantly higher, while the others seemed to stay more or less the same. With higher resistance the 2nd decreased, and came to a minimum around 31k. I kept increasing it a little bit more, but nothing seemed to happen. 30k was very close to minimum H2.
I also wanted to try different bias to see what happened. The result was not so much. It seemed that H2 increased and H3 decreased a bit and with higher bias (lower resistor values), but not significantly. Since the distortion is H3 dominant, I was hoping to be able to reduce H3. My guess was that the H3 comes from the P/P tubes, but I'm not sure, and I was not able to do much about it.
I also found a problem with one channel on the amp. There were some small 'blips' moving around on the rising flank depending on output level. They are not visible below 4Vpp though (4ohm tap and load). They showed up as pretty nasty high order distortion on the FFT. I found that swapping the output tubes between channels moved this problem. There also seems to be less gain with the problematic tube pair. I ordered 4pcs for 20Eur from Audiophonics (and I had a voucher so I got them almost for free), so I hope I can sort this out by swapping them around and measuring some more.
'blips'
Trimmer+resistors for testing:
With equal values (27k), the 2nd and 4th harmonics was significantly higher, while the others seemed to stay more or less the same. With higher resistance the 2nd decreased, and came to a minimum around 31k. I kept increasing it a little bit more, but nothing seemed to happen. 30k was very close to minimum H2.
I also wanted to try different bias to see what happened. The result was not so much. It seemed that H2 increased and H3 decreased a bit and with higher bias (lower resistor values), but not significantly. Since the distortion is H3 dominant, I was hoping to be able to reduce H3. My guess was that the H3 comes from the P/P tubes, but I'm not sure, and I was not able to do much about it.
I also found a problem with one channel on the amp. There were some small 'blips' moving around on the rising flank depending on output level. They are not visible below 4Vpp though (4ohm tap and load). They showed up as pretty nasty high order distortion on the FFT. I found that swapping the output tubes between channels moved this problem. There also seems to be less gain with the problematic tube pair. I ordered 4pcs for 20Eur from Audiophonics (and I had a voucher so I got them almost for free), so I hope I can sort this out by swapping them around and measuring some more.
'blips'
Trimmer+resistors for testing:
The 27k 30k mismatch has been mentioned previously in the thread, post #16. I remember planning to insert matching resistors, but I think I forgot to do so actually. I did some spice simulations which showed that the 27k 30k should be closer together, post #38.
I had some minor issues with my build that turned out to be the tubes. Got some replacement tubes from ebay and the amp sounds very nice. Also, I replaced some of the hardware, jacks, volume pot etc. right out the bat, as I had better ones.
I had some minor issues with my build that turned out to be the tubes. Got some replacement tubes from ebay and the amp sounds very nice. Also, I replaced some of the hardware, jacks, volume pot etc. right out the bat, as I had better ones.
Do you know why they designed it like this..?If 6F2 is indeed like ECF82 resistor values and operating point iare not right ... not a surprise from a chinese design 😀 .
The cathodyne has unequal 27K and 30K resistors ... distortions guaranteed
They probably ran out of 27 or 30k resistors so just used the nearest! (it would be typical of the Chinese) 😉
I'd suggest that few people would actually adjust the ac signal balance to minimise harmonics, similar to post #166, which goes against the comment "distortions guaranteed" made in #16 and perhaps echoed in #170. Imbalance from the PI triode and the output tubes almost guarantees distortions, so the assertion that high tolerance resistors are needed is a bit cute imho.
Sure, many hi-fi amps dictate matched resistors for a cathodyne, such as in the Williamson, and that certainly minimised differences with new tubes in vintage times. Perhaps that was a past era, and a trimpot should nowadays be de-rigour, along with the expectation that people can and will trim for minimum distortion whenever a PI or output stage tube is changed.
Sure, many hi-fi amps dictate matched resistors for a cathodyne, such as in the Williamson, and that certainly minimised differences with new tubes in vintage times. Perhaps that was a past era, and a trimpot should nowadays be de-rigour, along with the expectation that people can and will trim for minimum distortion whenever a PI or output stage tube is changed.
Nice going! If you're curious it shouldn't be that hard to estimate the balance of the primaries of the OT. Connect your signal generator to the primaries of the OTs and apply a couple of ac volts at different frequencies. An ideal OT would show the same output levels on both sides of the secondaries.
I actually played around with this amp again yesterday. I got the tubes I ordered, but two of them had pins missing.. Anyway, I swapped around output tubes comparing harmonic profiles on L&R channels, and was able to get them very close. The 'bumps' on the trace is gone and gain between channels is also better. I actually ended up using one pair of the original tubes, and one pair of the new tubes, so a mix of 1old + 1new per channel. It seemed the new tubes could go a little bit higher in output before clipping.
I have reported the problem with missing pins to Audiophonics, and have good experiences with them from before, so hopefully I will get a working pair instead.
I also put a 0.1ohm 5W resistor on the filament supply, so now I can run it without the bucking transformer. The resistor gets pretty hot though..
I have reported the problem with missing pins to Audiophonics, and have good experiences with them from before, so hopefully I will get a working pair instead.
I also put a 0.1ohm 5W resistor on the filament supply, so now I can run it without the bucking transformer. The resistor gets pretty hot though..
I've bought this kit in the last few months from Aliexpress for a first valve amp build but am yet to start.
I don't think it's been mentioned before in this thread but on YouTube, "xraytonyb" has an 11 part series based on building this kit point-to-point. The series is titled "Electronics 101½".
He says he didn't choose this kit for any particular reason but simply as an exemplar for demonstrating and explaining valve audio concepts.
While going through the build, where immediately relevant, he digresses into theory with practical "mini-tutorials" that include simple demonstrations and testing.
Overall, as a beginner with a bit of basic knowledge, I found the series very informative and easy to watch.
I purchased this kit after viewing the series.
Of interest to me as an aspiring builder with no more than a multimeter and ancient oscilloscope for test equipment is that he tests an output transformer and concludes that they are of reasonable quality and fit for purpose. In the final video of the series (Number 11) he does a frequency sweep and distortion measurement and then runs the viewer through a couple of simple modifications to improve the overall frequency response and significantly lower the total distortion.
I won't attempt to describe or convey those mods here but suggest that others who are interested in either building the kit or have already built the kit, whether pcb or point-to-point, may find some material of interest in the series and particularly in the final video as mentioned above.
I don't think it's been mentioned before in this thread but on YouTube, "xraytonyb" has an 11 part series based on building this kit point-to-point. The series is titled "Electronics 101½".
He says he didn't choose this kit for any particular reason but simply as an exemplar for demonstrating and explaining valve audio concepts.
While going through the build, where immediately relevant, he digresses into theory with practical "mini-tutorials" that include simple demonstrations and testing.
Overall, as a beginner with a bit of basic knowledge, I found the series very informative and easy to watch.
I purchased this kit after viewing the series.
Of interest to me as an aspiring builder with no more than a multimeter and ancient oscilloscope for test equipment is that he tests an output transformer and concludes that they are of reasonable quality and fit for purpose. In the final video of the series (Number 11) he does a frequency sweep and distortion measurement and then runs the viewer through a couple of simple modifications to improve the overall frequency response and significantly lower the total distortion.
I won't attempt to describe or convey those mods here but suggest that others who are interested in either building the kit or have already built the kit, whether pcb or point-to-point, may find some material of interest in the series and particularly in the final video as mentioned above.