Greetings Friends. I wanted to share the progress I've made with this little amplifier.
This amp started out as a Pentode/UL switchable amp but it was pointed out to me that the PT was under-rated, so I re-wired it as a low-powered Triode amp. I took my operating point right off the EI EL84 Datasheet:
and borrowed heavily from the Marblewood amp's driver stage to come up with this circuit:
The amp starts to clip just below 2 watts with a 1k 500mVRMS input. I've cobbled together a testing rig using REW and have made measurements of the distortion profile of this amp at various output levels. There is a strong 2nd harmonic and relatively low levels of all other harmonics at all output levels.
In the image above the amp is producing 1VRMS at output into an 8-ohm load, that's 1/8w and already producing 3.47% THD.
1/2w
3VRMS, just above 1w. 3rd is starting to rise.
4VRMS, 2 watts.
0dbFS. 2nd harmonic remains dominant.
So. 2 pretty watts. Flipping the switch into U/L gives a slight increase in power while adding more of the other harmonics into the mix. Bass starts to drop off below 100Hz, expected from a guitar amp OPT.
One question this has brought up: is it the 3.5k output transformer that gives this circuit such nice 2nd harmonics? The datasheet shows different ratios of 2nd and 3rd harmonics across varying Load Resistance which gives me the idea that a lower Ra is ideal for tubes wired in Triode. Or is it just the EL84 that behaves this way?
thanks for taking a look!
This amp started out as a Pentode/UL switchable amp but it was pointed out to me that the PT was under-rated, so I re-wired it as a low-powered Triode amp. I took my operating point right off the EI EL84 Datasheet:
and borrowed heavily from the Marblewood amp's driver stage to come up with this circuit:
The amp starts to clip just below 2 watts with a 1k 500mVRMS input. I've cobbled together a testing rig using REW and have made measurements of the distortion profile of this amp at various output levels. There is a strong 2nd harmonic and relatively low levels of all other harmonics at all output levels.
In the image above the amp is producing 1VRMS at output into an 8-ohm load, that's 1/8w and already producing 3.47% THD.
1/2w
3VRMS, just above 1w. 3rd is starting to rise.
4VRMS, 2 watts.
0dbFS. 2nd harmonic remains dominant.
So. 2 pretty watts. Flipping the switch into U/L gives a slight increase in power while adding more of the other harmonics into the mix. Bass starts to drop off below 100Hz, expected from a guitar amp OPT.
One question this has brought up: is it the 3.5k output transformer that gives this circuit such nice 2nd harmonics? The datasheet shows different ratios of 2nd and 3rd harmonics across varying Load Resistance which gives me the idea that a lower Ra is ideal for tubes wired in Triode. Or is it just the EL84 that behaves this way?
thanks for taking a look!
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Trioded EL84 with 3k5 load isn't distortion champion.
IMO 5k OPT is best solution.
IMO 5k OPT is best solution.
Ah. my captions need work. 57% THD is from 0dbFS test signal. Amp clips at 2w, that's the chart where THD is 19.5%. I'm still figuring out how to get usable data with REW. Right now I'm aiming for a target output power instead of dialing in the input test power.
w
w
