On Tuesday, 6 November 2012 14:02:15 UTC+11, Phil Allison wrote:
"Phil Allison" ** More testing with the FFT feature on a Rigol scope: 3H is dominant from about 9 watts through to 21 watts. From 1watt to 4 watts, 2H dominates, then 4H, then 3H. If load R is changed to 16 ohms, 2H dominates all the way. 2H cancellation is occurring over a fairly wide range, but it is not complete. I can swap the 6SN7 with almost no change. ** One final test result: 300Bs removed, 10:1 scope probe on grid pin, input 260mV rms to 6SN7. 2H = -24 dB = 6% 3H = -42dB = 0.8 %. .... Phil
Well, with 10:1 probe, presumably you have 2.6Vrms at 6SN7 grid. If the 6SN7 gain is say 17, then its drive to output tubes is 44.2Vrms, ie, 62.5Vpk, or getting close to the dc bias voltage for 300B. The THD you measured in 6SN7 seems about right. It would be interesting to measure the THD of the 300B output stage when driven by a source with less than 0.1% THD which would seem easy to do if you had a spare SS amp laying around and capable of say 28Vrms output and if you had a 1:2 step up tranny giving you 56Vrms drive max to 300B, ie, up to 80Vpk. Without the driver amp disguising the performance of the 300B, you would get to work out if the 2H from 6SN7 is reacting in 300B to increase the 3H of the 300B. My guess is that there would be some intermodulation and increase in 300B 3H. Whether music sounds worse because of IMD products formed because of effective presence of 3H or just 2H us a moot point, and it all depends on the % of whatever THD is present. When I made an SET amp with a pair of parallel 845 in 2008, I needed 105Vrms drive to the 845 grids. So I used 3 x EL84 in parallel with a dc load resistance PLUS a 30H+ choke to prevent whatever other pure dc carrying resistance loading down the EL84, and then I was able to make the 845 grid bias R = 23k, so prevent serious dc generated across Rg if there ever was some "reverse grid current. I recall the THD produced by EL84 in triode was low, but whatever it was, it cancelled whatever 2H the 845 produced, thus helping THD figures. for full details and all schematics,
See
http://www.turneraudio.com.au/monobloc845se55.html
If ever you were worried that 300B grids were fragile, you'd be right. Some would suggest one have cathode biasing always in class A tube amps. One may then connect a silicon diode from grid to cathode so that if ever the driver tube tries to drive the grid positive, the grid cannot go positive, relative to cathode. It measn also that if any arc occurs between anode and grid, and I have recently seen just that, anode then becomes connected to cathode, and fuse may blow or active protect circuit trun off the amp. The down side is that when 300B get tired and old they stop developing their full proper Ek, and then the diodes cause premature limiting of grid swing. NFB causes the diodes to charge up coupling caps even faster at clipping, but then we mustn't let hi-fi amps clip. Usually where you have parallel 300B all with separate cathode biasing networks there will be considerable variation in Ek, perhaps +/- 10V where Ek bias is meant to be say 90Vdc. I get very nervous about amps needing more grid bias than that except in case of transmitting tubes like 845 which are happy with grid bias = -190Vdc, and with Ea at 1,200V, and in a PP amp where 40mAdc idle is just fine with a load of say 12ka-a. In about 2010 I totally rewired a pair of 100W PP Ming Da amps with 845PP. These amps had input tube = SET 6SN7, then 6SN7 as common cathode LTP, then foloowed by pair of 300B as a balanced amp with resistance loading, each with only 7mAdc which was plenty to get the wanted grid swing for 845, about 141Vrms max to each grid.
The 6SN7 would have been better with a CT choke plus R loads to get their Ia higher, but in this case the available B+ was over 1kV, so the 300B could have 50k anode loads from B+ to anode. The Chinese became utterly muddled up as they often do when attempting to make grandiose amps like this, and a complete re-design was needed - long story, no time now to tell it. I discovered that the PP 845 with 3 stage input&driver driver was fabulous sounding, the owner was very happy, and so who needs SET amps? But then life is about appreciating all drinkable wines, seeing beauty in what may not be needed but nevertheless worth having. I have an SET EL34 giving me my kitchen music, seems to sound just fine with an old sensitive speaker. Never mind the old redhead - if she can cook, but of course I am assuming a 300B is a bit temperamental like the old redhead. Having two of them in the same kitchen invites complications.
Patrick Turner.