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In Reply to: RE: 300B IT 1:1 845, A2 possible? posted by Paul Barker on December 27, 2021 at 23:49:02
Im not up to speed with posting images here. In this distortion chart, if you can see it, guess which power level is in A2!
I think Im allowed one photo a week. So Ill have to explain rather than show pictures that tell the story best. If you operate 801a in A2 the distortion against power output is as it is here up to 6.4 watts. The minute you pass over to A2 distortion skyrockets. This is before you add to these figures the effect dc resistance in coil has on grid current.
Edits: 12/28/21 12/28/21Follow Ups:
"The minute you pass over to A2 distortion skyrockets."
I'm not familiar with the 801A, but my 211-based SETs don't respond like that. The transition from A1 to A2 is very smooth, just a steadily increasing level of distortion (mostly even-order) without any unusual characteristics. Regarding the OP's question, it sounds as though the DCR of the IT might be too high.
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Buy Chinese. Bury freedom.
A 300B has a rP in excess of 1kΩ @ 30ma and then both the primary and secondary DCR needs to be added to those numbers which will take the drive impedance up even higher. I think there is a general ROT that the input impedance of an A2 grid drops like a rock and could very well be equal to the 1500Ω output Z of the driver which would net a 6dB loss in the positive going direction at the crossover point to A2.
Not to sidetrack too much but Douglas (pak protector) has used the drastic grid A2 grid impedance change to advocate for cathode drive where the load impedance even though is low, stays constant. The other interesting thought around this came from Bob "Darling" Danialak where he showed a 0 bias 833 amp at NY Noize. He made the argument that while the A1 vs. A2 grid impedance is radically different... Placing the crossover point between them at 0V keeps things symmetrical and any distortion caused by the crossover happens at all output levels. (He did drive it with a 20Ω source Z) To this day that amp has been the only A2 amp that has really impressed me with its sonics.
dave
Hi Paul,
Thx for response, some tubes like 805 requiring positive grid bias, normally 805 SE amp need NFB to sound normal, but my intention is drive 845 without NFB
Your distortion photo concern me and I do not expect A2 power would be such sky-high distortion
I heard many cases that 845 with beefy driver can push 845 to A2 also maintaining low distortion, something like extra 15watt A2 on top of A1 30watt,total around 45watt, the A2 extra 15watt is supposed to handle instant peak passage
With CF direct couple, I can get A2
But with IT, as stated above, not able to drive 845 into A2, in fact for the webs I did search do not show successful A2 operation, so I am wondering how Cary 805C could use one 845 to put out 50watt RMS(Cary 805C use 300B IT to 845)
I'll have to dig out some old posts but I looked at the 805c circuit which is nothing more than a IT coupled driven by a 300b.
I'm doing something similar with a GM70 and easily getting A2 out of it as well. But, it does have some distortion (on paper). I think the Cary is the same.
I know A2 doesn't always sound bad, but the 801a looks bad. I have an 801a amp that I kept to A1 because of those findings, You get a very nice 6.4 watts easily, I am using interstage coupling.Our UK gathering Nick Gorham and I did a collaberation, he built the driver I brought the power supply and output stage 833a's
Nicks circuit is an adaptation to provide more current, so he put 6BX7 as driver. Its a level shifting circuit , bias adjustment on input valve so whole cirquit must float at input, we used a cap, could use input transformer, with voltage potential difference allowed.
the level shifting circuit is known as ACF-2. Its really mint sounding, don't dismiss it because its not modern and doesnt use niche valves. It sounds good and drives A2 grid current .
https://www.desmith.net/NMdS/Data/955_Augmented_Cathode_Followers.pdf
Edits: 12/28/21
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