14-07-2017, 10:53 AM
(This post was last modified: 14-07-2017, 10:55 AM by Diabolical Artificer.)
After 6 days of fun and games with the amp giving me loads of gyp I put things back as they were apart from the OPT primary being swapped over that is, all is well. Turns out moving a few grounds and grid resistor ground terminations effect's the stability of the amp considerably.
So amp working like a charm again, stable with NFB applied. Gave it a quick test, and got 0.8% THD at 50w. The amp also now kicks out 162w before onset of clipping, though I don't leave it at that for long as the OPT is rated at 120w.
So I go from the sublime (0.8% THD at 50w for a valve amp) to the ridiculous ( 40v P-P 20khz oscillating hissy fits) all by moving a few grounds 6" or so. Video here - https://youtu.be/S0Qhjzewr-4
A few days later ... Amp now working like a good un. Frequency response is flat from 15hz to 6khz where it rolls off gently to a 3dB point at 20hz. THD is 0.3% at 50w rising to 0.8% at 112w. I have plenty of gain left; with a 1.3v RMS IP I have 136w before clipping. So can lose a few more DB on NFB, but think I'll stick there for now as any improvement in THD is negligable, unhearable and I'll probably run into instability again.
I'm going to struggle measuring THD lower than this as my Marconi TF2331 is at it's limit. My signal source is also a bit high THD wise, over 1v.
So, all that's left to do is attach another protection device that will turn off the amp if the OP stage EL34's Ik goes over 100mA. I've breadboarded a circuit, just have to test. I also have to put a clipping indicator LED driver on the same PCB together with a VU driver. Am in the process of trying out a few circuits. See att. I tried out the ESP one, but didn't like it. The first attached works well. The second is about the same as the first but has more adjustment, IE it can take an IP up to the 6v (max before clipping) that is present at the ECC83. That's probably due to the 220k IP R. I ditched the one that grounds the non inverting IP and just grounded pin 3. This circuit works ok but does odd things like the meter needle dips past 0 on the scale before going up. I'll make a few changes and see.
A.
So amp working like a charm again, stable with NFB applied. Gave it a quick test, and got 0.8% THD at 50w. The amp also now kicks out 162w before onset of clipping, though I don't leave it at that for long as the OPT is rated at 120w.
So I go from the sublime (0.8% THD at 50w for a valve amp) to the ridiculous ( 40v P-P 20khz oscillating hissy fits) all by moving a few grounds 6" or so. Video here - https://youtu.be/S0Qhjzewr-4
A few days later ... Amp now working like a good un. Frequency response is flat from 15hz to 6khz where it rolls off gently to a 3dB point at 20hz. THD is 0.3% at 50w rising to 0.8% at 112w. I have plenty of gain left; with a 1.3v RMS IP I have 136w before clipping. So can lose a few more DB on NFB, but think I'll stick there for now as any improvement in THD is negligable, unhearable and I'll probably run into instability again.
I'm going to struggle measuring THD lower than this as my Marconi TF2331 is at it's limit. My signal source is also a bit high THD wise, over 1v.
So, all that's left to do is attach another protection device that will turn off the amp if the OP stage EL34's Ik goes over 100mA. I've breadboarded a circuit, just have to test. I also have to put a clipping indicator LED driver on the same PCB together with a VU driver. Am in the process of trying out a few circuits. See att. I tried out the ESP one, but didn't like it. The first attached works well. The second is about the same as the first but has more adjustment, IE it can take an IP up to the 6v (max before clipping) that is present at the ECC83. That's probably due to the 220k IP R. I ditched the one that grounds the non inverting IP and just grounded pin 3. This circuit works ok but does odd things like the meter needle dips past 0 on the scale before going up. I'll make a few changes and see.
A.
