clevegilbert wrote:So, seems there's folks out there that prefer to strap the pentodes so they function like triodes. What's the expected difference in sound quality? Power output would be lower (right?) and I would think distortion would be lower?
Cleveland,
I think the ST70 has made a fun unit for folks to triode-strap. The resulting output is good enough to drive most systems. Conversely, triode-strapped EL84s are pretty wimpy, and those with efficient enough speakers probably go for SETs instead. Don't recall if I showed the numbers in my earlier article, but expect around 4.5W/channel. Distortion is higher than the stock UL config, but I recall it was mostly 2nd harmonic. As far as sound quality difference, certainly your choice of music and speakers play a factor. It is handy to have a unit with UL taps so you can try both. Personally, I'm quite pleased with the stock ultra-linear circuit and feel it has the best mix of attributes.
In the latest rev of the ST35 board I added some footprints in each of the cathode circuits, presumably for LM317 regulators used as CCS. I built one last week for QC testing of the Rev D and also for playing with triode-strapped configurations. Since Class A operation would be forced, I lowered the B+ down to 270VDC still using the PA774. I did this by cutting the trace to C9 and then jumpering it to C10. I then used the 4H open bracket Hammond choke as a choke input. I paralleled a pair of 75 ohm resistors to set each of the LM317s at 33mA (1.25V/37.5ohms=.0333A). I chose these parameters to ensure staying Class A. I ending up using 6dB negative feedback, which was a 100K resistor. Closed loop gain was like 25dB - about 3dB higher than a stock ST35. The audio analysis isn't anything to write home about. Hard to say what the max usable output is in this config - maybe 3.5W/channel. I need to FFT it, but PP Calc by Glassware says the 3rd and 5th harmonics are wayyyy down there.
BTW, Tube Cad Journal's Push-Pull Calc is awesome (triode and PP only). Download it now for $24 - http://store.yahoo.com/glass-ware/tubjourcomso.html . No affiliation, just a super happy customer!
Shannon
