You are welcome! This thread is long, could you please point me to the post with the schematic that you implemented? Just to make sure I'm looking at the right thing 🙂
Regards,
Jose
Regards,
Jose
Hi Giovanni,
The EF95 is a high frequency tube, used for RF amplification. Some of those tend to become an HF generator depending on the layout etc. There is someting I would do right away, which is always recommended, no matter what: Iinclude grid stoppers, soldered very close to the grid socket pin (in red below). People usually recommend carmon composite resistors for this. Add also the Zobel network to the OPT primary.
Also, I don't think there is positive feedback here, at least at normal frequencies, because the feedback from the EL36 plate goes straight to the EF95 cathode, because of the R-GK/C1 time constant.
Another thing to try removing the EF95, power on the amplifier and see whether there is any noise.
Regards,
Jose
The EF95 is a high frequency tube, used for RF amplification. Some of those tend to become an HF generator depending on the layout etc. There is someting I would do right away, which is always recommended, no matter what: Iinclude grid stoppers, soldered very close to the grid socket pin (in red below). People usually recommend carmon composite resistors for this. Add also the Zobel network to the OPT primary.
Also, I don't think there is positive feedback here, at least at normal frequencies, because the feedback from the EL36 plate goes straight to the EF95 cathode, because of the R-GK/C1 time constant.
Another thing to try removing the EF95, power on the amplifier and see whether there is any noise.
Regards,
Jose
Dear Jose,
thank you very much. I was going to post here, saying that I did read part of that long thread this morning, after which I concluded that I will have a lot of fun on the next days, by experimenting with the grid stoppers, the Zobel network and the feedback. I am happy to know that the feedback is not as I assumed (I am weak about theory ☹).
If I remember correctly, I did already try to remove the driver tube and the noises disappeared, but after finding out the influence of the hands on the output tubes I concentrated my attention to them. By shielding one of the output tubes the amp is really quiet, but this does not exclude that very high frequency resonances could still be there, I think. Thus, better to proceed with the mods suggested.
So, thank you very much! But I will wait for the arriving of the audio waveform generator to see step by step if any changes will affect the response. And will keep you updated.
Kind regards
Giovanni
thank you very much. I was going to post here, saying that I did read part of that long thread this morning, after which I concluded that I will have a lot of fun on the next days, by experimenting with the grid stoppers, the Zobel network and the feedback. I am happy to know that the feedback is not as I assumed (I am weak about theory ☹).
If I remember correctly, I did already try to remove the driver tube and the noises disappeared, but after finding out the influence of the hands on the output tubes I concentrated my attention to them. By shielding one of the output tubes the amp is really quiet, but this does not exclude that very high frequency resonances could still be there, I think. Thus, better to proceed with the mods suggested.
So, thank you very much! But I will wait for the arriving of the audio waveform generator to see step by step if any changes will affect the response. And will keep you updated.
Kind regards
Giovanni
PS: I did read this morning that a further cause could be a single wideband speaker, which is the case here. The amp is driving a pair of Philps 9710 alnico speakers.
No problems, and enjoy the troubleshooting, it can be a lot of fun!
One thing to note about the signal generator: A decent sound card and REW software can be used as a signal gemerator and oscilloscope, and even measure things like noise, harmonic distortion etc.
One thing to note about the signal generator: A decent sound card and REW software can be used as a signal gemerator and oscilloscope, and even measure things like noise, harmonic distortion etc.
