Thanks Martin,
Had another good go at this today. I had swapped over the preamp tubes for some good Nos ones I have and I redid the voltage chart and all the sinewave reading as attached. the sine readings are a lot better now. Most of the assymetric clipping has resolved so I think there might have been an issue with the PI tube. The max power still falls short though the amp sounds great.
Not sure what you mean about driving the grids to 0v. i checked teh DC at the Grids at Idle, max clean and heavily clipped a it didn't change much. certainly not 0v if anything the neg volts went up a bit. Its in attached. probably means I don't understand what you mean.
best
Steve
Help with a new Build ODS
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Re: Help with a new Build ODS
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martin manning
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Re: Help with a new Build ODS
Yep.Wardy wrote: ↑Mon Sep 15, 2025 6:04 pm Had another good go at this today. I had swapped over the preamp tubes for some good Nos ones I have and I redid the voltage chart and all the sinewave reading as attached. the sine readings are a lot better now. Most of the assymetric clipping has resolved so I think there might have been an issue with the PI tube.
I'm referring to the signal level at the power tube grids. If the peak voltage on the waveform can hit zero volts DC, that is as much clean power as you can expect to get. The 12AX7 PI doesn't have the current sinking capability to drive the grids above zero volts, so if you increase the signal further the waveform will be clipped.Wardy wrote: ↑Mon Sep 15, 2025 6:04 pm Not sure what you mean about driving the grids to 0v. i checked teh DC at the Grids at Idle, max clean and heavily clipped a it didn't change much. certainly not 0v if anything the neg volts went up a bit. Its in attached. probably means I don't understand what you mean.
Re: Help with a new Build ODS
Thanks Martin,
I hooked up the scope to the grids on the power tubes and another probe on the output. I've got a Siglent 1202xe and with all the measures switched on I couldn't see anything moving toward zero as I increased the power. which means I probably need a different setting somewhere to see it. I did notice that the output at the speaker clipped before the grid. I tried a few different power valves with some interesting results.
I had a different pair of Tung Sol 7581a so put them in and got much better readings, got them in the same batch as the other two. I recorded the pk to pk voltage at the grid just before clipping and also what the total rms output was although when maxed out at the grid the signal would always be clipped at the final output.
Also measured the new total poss clean output at the speaker. I think I'm in pretty good shape now. All readings into a 4 Ohm load and all sets biased to 70%. The Tung Sol are rated at 35W and according to Watford Valves the Philips can do 30w. I measured both grids on V5&6 but almost no difference.
Tung sol 7581a --- Grid Pk Pk = 84v Clipped output = 13.9vrms and max clean 13.2vrms. bias 70% 35w
Jan Phillips 6L6WGB. Bias 70% 30W--- Grid Pk Pk 98v Clipped output 14vrms. Max clean 13.5vrms
JJ 6L6gc bias 70% 30w. Grid Pk Pk 90v - Clipped output 14.5vrms max clean output 13.85vrms
All the tubes sounded fine and I left the last ones in. I then reset the PI balance using the ampeg method
So now getting about 48 clean watts so I'm calling it done.
Once again many thanks to you and all the others to chipped in to help, I'm learning but still very new to all this and I couldn't do it without the help I get here and on EL34. I think the last set of sine readings taken throughout the amp represent the thing in proper order so hopefully they might assist someone else tracking down any issues.
I'm getting back to my bass preamp build now
I hooked up the scope to the grids on the power tubes and another probe on the output. I've got a Siglent 1202xe and with all the measures switched on I couldn't see anything moving toward zero as I increased the power. which means I probably need a different setting somewhere to see it. I did notice that the output at the speaker clipped before the grid. I tried a few different power valves with some interesting results.
I had a different pair of Tung Sol 7581a so put them in and got much better readings, got them in the same batch as the other two. I recorded the pk to pk voltage at the grid just before clipping and also what the total rms output was although when maxed out at the grid the signal would always be clipped at the final output.
Also measured the new total poss clean output at the speaker. I think I'm in pretty good shape now. All readings into a 4 Ohm load and all sets biased to 70%. The Tung Sol are rated at 35W and according to Watford Valves the Philips can do 30w. I measured both grids on V5&6 but almost no difference.
Tung sol 7581a --- Grid Pk Pk = 84v Clipped output = 13.9vrms and max clean 13.2vrms. bias 70% 35w
Jan Phillips 6L6WGB. Bias 70% 30W--- Grid Pk Pk 98v Clipped output 14vrms. Max clean 13.5vrms
JJ 6L6gc bias 70% 30w. Grid Pk Pk 90v - Clipped output 14.5vrms max clean output 13.85vrms
All the tubes sounded fine and I left the last ones in. I then reset the PI balance using the ampeg method
So now getting about 48 clean watts so I'm calling it done.
Once again many thanks to you and all the others to chipped in to help, I'm learning but still very new to all this and I couldn't do it without the help I get here and on EL34. I think the last set of sine readings taken throughout the amp represent the thing in proper order so hopefully they might assist someone else tracking down any issues.
I'm getting back to my bass preamp build now