I think I have an issue with my amplifier. I finished it a little over a year ago and have been very happy with it. Yesterday I measured power output and was a bit surprised when it started clipping at 20 W. I measured some voltages from the power supply board today and it looks like that the -V bias voltage is lower (or higher?) than it should be at about -50 V. Looking through my notes from when I built the amplifier it was -92 V.
Any thoughts? Can the low power output be caused by wrong -V bias voltage?
Any thoughts? Can the low power output be caused by wrong -V bias voltage?
Please check phase splitter current as well, for sure the limited negative voltage limits the power.
Thanks! How/where do I measure the phase splitter current?
Btw. I think the -92 V I measured a year ago was with no load.
Btw. I think the -92 V I measured a year ago was with no load.
I meant the current across the leds of the phase splitter, that should be around 2 mA.
You can try a doubler to make the bias voltage more negative.
You can try a doubler to make the bias voltage more negative.
Please:
Either put up an exact schematic of your amplifier,
Or please tell us which of 1405 posts has the exact schematic attached.
Many troubleshooters feel like a blindfolded Archer, who is rotated an unknown number of degrees, and then asked to 'hit the target'.
"All EL34 Baby Huey Amplifiers are created equal, but some Baby Huey Amplifiers are more equal than others" With credit given to George Orwell's book "Animal Farm". Sorry George for the modified quote.
Either put up an exact schematic of your amplifier,
Or please tell us which of 1405 posts has the exact schematic attached.
Many troubleshooters feel like a blindfolded Archer, who is rotated an unknown number of degrees, and then asked to 'hit the target'.
"All EL34 Baby Huey Amplifiers are created equal, but some Baby Huey Amplifiers are more equal than others" With credit given to George Orwell's book "Animal Farm". Sorry George for the modified quote.
Please see attached schematics as found in this post: https://www.diyaudio.com/community/...huey-amp-and-psu-mk2-pcb.368190/#post-6541976 I built the amplifier with the boards from the group buy following these schematics (with some modifications as suggested in this thread).
I assume I need to do something with negative voltage supply?
I assume I need to do something with negative voltage supply?
In this post https://www.diyaudio.com/community/threads/el34-baby-huey-amplifier.326920/post-6156479 R2 is changed to a lower value to increase the negative voltage. Is this something I can do as well?
About led current. Can I measure across R16 on the amplifier board?
About led current. Can I measure across R16 on the amplifier board?
Do not assume anything.
Especially if it once worked properly, to spec, and perfectly; and now it does not.
1. It may never have been fully to specification
2. Some part broke/changed value/died; a bad solder connection, short, etc.
3. Negative voltage supplies that are too low of a voltage, do that because they have something wrong; or because the load on the supply increases.
4. A re-design of a circuit that has a bad part or bad connection does not get fixed just because it gets re-designed.
Fix an amplifier first, then apply any new modifications / improvements.
Especially if it once worked properly, to spec, and perfectly; and now it does not.
1. It may never have been fully to specification
2. Some part broke/changed value/died; a bad solder connection, short, etc.
3. Negative voltage supplies that are too low of a voltage, do that because they have something wrong; or because the load on the supply increases.
4. A re-design of a circuit that has a bad part or bad connection does not get fixed just because it gets re-designed.
Fix an amplifier first, then apply any new modifications / improvements.
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Please, measure the AC voltage of the winding dedicated to bias and powerdrive.
Then measure the DC positive voltage for the powerdrive.
Then measure the DC positive voltage for the powerdrive.
I suspect that the issue has been present since the amplifier was newly built and that I didn't notice it because the amplifier sounded good and that I failed to measure the negative voltage supply under load and also didn't measure the power output.
Yesterday I measured all the voltages from the transformer and from the power supply board. The only issue I noticed was the low negative voltage. Similar problems has been mentioned before in this very long thread. My plan now is as following:
Yesterday I measured all the voltages from the transformer and from the power supply board. The only issue I noticed was the low negative voltage. Similar problems has been mentioned before in this very long thread. My plan now is as following:
- Remove the power supply board and use a 50v transformer that I have for safer testing.
- Test the negative power supply with and without a load to again confirm there is a problem
- Modify the negative power supply according to gingertube's post #739
- Test the negative power supply again with and without a load to again
that R2 value is important.
It needs to be 22K or even less, otherwise the B- supply is reduced too much. This circuit is clamp circuit to adjust to B- Supply such that the output voltage is just less than the lowest dip from the ripple on the raw supply.
Check the raw B- supply first.
If that is sufficient then check R2, The original schematic showed it as 470K, that is no good. 22K is what I used but you can also try 15K or even 10K for a smidge more voltage but reduced ripple reject capability.
Cheers
Ian
Many thanks!
I'll check the raw B- supply and if that is OK try a 22K resistor for R2. Should I leave R1 and R6 as is?
I'll check the raw B- supply and if that is OK try a 22K resistor for R2. Should I leave R1 and R6 as is?
OK. I've adjusted R2 to 21K by adding a resistor in parallel which resulted in a negative voltage of about -70V under load. I also measured all the outputs from the power supply board under load:
+Vdriv 15.4V
-Vbias -70V
+HV 358V
I then adjusted the R16 to 33K (according to a table earlier in this thread). I measured the current through R16 to 1.8 mA. The tubes (KT77) are biased to 45mA.
But this didn't do much for the power output. This is just before it start clipping with a 8R dummy load:
+Vdriv 15.4V
-Vbias -70V
+HV 358V
I then adjusted the R16 to 33K (according to a table earlier in this thread). I measured the current through R16 to 1.8 mA. The tubes (KT77) are biased to 45mA.
But this didn't do much for the power output. This is just before it start clipping with a 8R dummy load:
Any ideas what else I can check? If I've made a mistake on the boards, I've apparently made it on both boards.
I'm using a Toroidy TTG-KT88PP transformers. It says 4k primary impedance. Is that the same as Raa?
Tou should get twice that, approximately.
With 40% UL, plates should go down to 100V in AB1. Double check the shunt feedback resistor’s value. It could limit the power of the amp.
With 40% UL, plates should go down to 100V in AB1. Double check the shunt feedback resistor’s value. It could limit the power of the amp.