So it looks like I need a cap larger than 33uF for the driver and phase splitter stages.
20Hz signal fed to the amp at near full output 10X probe 20mV/DIV That would be just a little above 400mVPP. I'll find a 450V cap and put it in parallel with the 33uF cap.
Adding another 33uF 450V cap in parallel the signal dropped to about 300mVPP.
Now am I chasing my tail here or does the signal across the filter cap mean the output will be a little lower at 20Hz?
20Hz signal fed to the amp at near full output 10X probe 20mV/DIV That would be just a little above 400mVPP. I'll find a 450V cap and put it in parallel with the 33uF cap.
Adding another 33uF 450V cap in parallel the signal dropped to about 300mVPP.
Now am I chasing my tail here or does the signal across the filter cap mean the output will be a little lower at 20Hz?
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Also at full output into 4 ohms the sinewave doesn't start to show transformer saturation until I get down to 15Hz and the amp seems to be flat to 20Hz.
So should I order a 100uF 450V cap and use in place of the two 33uF caps or leave well enough alone?
So should I order a 100uF 450V cap and use in place of the two 33uF caps or leave well enough alone?
Here's the schematic with the voltages listed. Line voltage kept at 120Vac for all measurements.
There is no ‘signal across the filter cap’. The signal is at the plate, on the other side of the plate resistor from the filter cap, which in fact is not a filter cap but an AC ground for the plate resistor. It’s big enough already.
There is a sinewave of about 200mVPP at 20Hz across the 66uF filter cap. I didn't update the value on the schematic though.
If you connect a dummy load to the speaker output, connect your oscilloscope, then short the input (to ground), do you see 120Hz hum?
If the problem you see is that a 20Hz sine wave doesn't look perfectly clean, that could very well be because of limitations of the output transformer.
If the problem you see is that a 20Hz sine wave doesn't look perfectly clean, that could very well be because of limitations of the output transformer.
Looks very clean like a regular sinewave. Doesn't start looking weird until 15Hz and that's at full undistorted amplifier output into 4 ohms. I suppose the feedback helps with that.
At the 200uF filter cap the ripple voltage is 35mV.
Not sure ultimately how important it is for a mono amplifier, but I like to have zero phase shift at 20Hz and as little phase shift as reasonably possible at 20kHz.
At the 200uF filter cap the ripple voltage is 35mV.
Not sure ultimately how important it is for a mono amplifier, but I like to have zero phase shift at 20Hz and as little phase shift as reasonably possible at 20kHz.
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How about at 5W into 4 ohms? That would be 6.3V peak into 4 ohms (4.45Vrms).
Just about any tube amp is going to deliver less clean power at 20Hz than it will at 1kHz. The OPT is probably rolling off the low end, requiring more negative feedback to correct, which the driver stage may not have the gain to deliver (a LOT of gain may be required). Also, some low frequency rolloff could be due to primary inductance or possibly the OPT is saturating when driving low frequencies to the load. Those are typical tube amp limitations. Besides, how loud are going to play this amplifier? I figure it can't make more than 20Wrms, can it?
Just about any tube amp is going to deliver less clean power at 20Hz than it will at 1kHz. The OPT is probably rolling off the low end, requiring more negative feedback to correct, which the driver stage may not have the gain to deliver (a LOT of gain may be required). Also, some low frequency rolloff could be due to primary inductance or possibly the OPT is saturating when driving low frequencies to the load. Those are typical tube amp limitations. Besides, how loud are going to play this amplifier? I figure it can't make more than 20Wrms, can it?
The amp makes 25 watts into 4 ohms. Now stock it was 20 watts at an unspecified distortion value.
At 20Hz at an output of 25 watts, the sinewave is still clean.
I'll likely never play it at 25 watts, but I cannot anyways as I had to adjust the gain of the amp so the audio would be real low when the Schiit Saga S preamp is set to 0 on the volume control as the relays used for the volume adjustment don't allow the signal to go all the way to zero. So it's likely I can only get maybe 20 watts at full volume control which is likely all the speaker is rated at anyways.
At 20Hz at an output of 25 watts, the sinewave is still clean.
I'll likely never play it at 25 watts, but I cannot anyways as I had to adjust the gain of the amp so the audio would be real low when the Schiit Saga S preamp is set to 0 on the volume control as the relays used for the volume adjustment don't allow the signal to go all the way to zero. So it's likely I can only get maybe 20 watts at full volume control which is likely all the speaker is rated at anyways.
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What I was questioning is if a 200 mVpp signal across a 66uF filter cap would cause any issues.
What I would like to get is something like a North Atlantic 2250 phase angle voltmeter so that I can actually see the phase shift in degrees when testing an amplifier. That would also give me an accurate way to measure AC voltage for calculations such as power and gain.
What I would like to get is something like a North Atlantic 2250 phase angle voltmeter so that I can actually see the phase shift in degrees when testing an amplifier. That would also give me an accurate way to measure AC voltage for calculations such as power and gain.
Yes it's the spot where the 33uF cap is after the 3k resistor. I just didn't update that cap value on the schematic.
With the input shorted, if you look at the output (after the output blocking cap) of the 6U8A pentode (or the grid of the 6U8A triode), do you see any hum/ripple there?
3k ohms and 66uF should make a plenty long enough time constant.
How old are those 33uF capacitors? Are they vintage electrolytics?
3k ohms and 66uF should make a plenty long enough time constant.
How old are those 33uF capacitors? Are they vintage electrolytics?
If there's no noise at the output (which is where it counts) then I'd say the circuit is getting the job done.
3k ohms and 66uF should make a plenty long enough time constant.
How old are those 33uF capacitors? Are they vintage electrolytics?
3k ohms and 66uF should make a plenty long enough time constant.
How old are those 33uF capacitors? Are they vintage electrolytics?
It's impossible to tell until it does. But because there is a clear ripple on the supply cap, and that filter cap is common to both stages, it could be a source of phase shifted FB back to the VA stage. You still don't have the two stages decoupled, they are sharing a common cap, not seperated by another interstage resistor/cap pair ahead of the splitter load resistor.