Quote:
Originally Posted by Stephie Bench
Hi John,
Very nice. A couple of comments. On the screen stopper, sometimes you can get a "free" improvement in performance by tailoring the value of the stopper resistor. I think, but I am not familiar with that tube, that a slightly larger resistor may improve the distortion, particularly at moderate power levels. The second comment is the cathode bypass cap. Depending on its overall quality, bypasing that cap with another non electrolytic, say 1 or 10uF polyprop will both improve HF performance, and often lower distortion, since it removes the nonlinear effects of the cap.
Thanks for publishing those.
One further note in general note on low cost SE amps. By using a tetrode or pentode connected in triode (or UL) mode, you often get the benefit of a subtly better (more precisely, lower order) plate resistance linearity. This often doesn't particularly show up in static distortion measurements, but under the condition of a reactive load, it produces lower distortion. Nice benefit from using a "cheap" tube. You can ve4ify this by making your static load reactive somewhat, and measuring the distortion in that condition.
Stephie
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Thanx for your comments Stephie. I’ve never tried adjusting the G2 resistor on the OP section. I simply used something less than one K in order that G2 can get what it needs while the plate is heavily conducting. I’ve been primarily interested in avoiding parasitics. G2 needs a lot of current when the plate load line goes up into the knee of its conduction.
But in real life the loadline is for the most part elliptical. And it can & does regularly pass thru a –ve rp region in the lower LH corner of the plate family. So that adjusting the G2 resistor for optimum would be worthwhile. Something to try!
During this project I tried the 6LU8 in straight pentode operation. It was somewhat less than ideal. Not recommended at all. But it was designed as a sweep tube, audio not in mind at all.
On the more recent projects I’ve included in parallel with the PS electrolytics better quality caps of say 0.1 to one microF for the reasons you’ve noted. But I’ve done no serious SE projects since these two.
For the Twin Coupled Amp (ie Norman Crowhurst) projects I put together a simple speaker simulator for stability, THD & IMD tests. I found that to be very useful. Keeps things quiet as well. Attached here.
Not much going on with toobz these daze. It is snowing again here out in the woods NW of Toronto. We had a huge ice storm just before Xmas so lots of trees down. The power was off for three full days. Later today I gotta get on the tractor again. And if Spring ever gets here the chain saw will be very busy!
Cheers, John