I might call this purposeful headroom limiting in the age of unreliable broadcast limiters, especially given that it's open loop gain and difficult to distinguish the transition into distortion. This creates a headroom bottleneck before the program amp, which can stay clean rather than pass on an overload, 2 amps in series farting, etc.
The current draw for both amps is 8mA, there will be about 0.8mA going to each 6AQ6 which will bias each tube at ~1.4V which is what we would expect.
The problem is with the 6C4 which will have ~3.2mA/tube making the bias point on the early model 150 x 0.0032 = 0.48V, or the later model 220 x 0.0032 = 0.7V. These bias points are way too low and will overload with the slightest voltage from the 6AQ6.
I've ever seen this (very high grid resistor and no cathode resistor) in first stage of old guitar amps, described as "ultra-high mu" stage . I can't remember wich amp it was right now.
Lassoharp, did you try the 0.75V bias before settling on 4V?
That keeps the rp low and reduces the output impedance to the lowest possible for that tube/transformer combination without feedback.
Ok, I'm confused here. I'm seeing bias of 0.75 on 150r = about 5ma. For lowest rp of V2, would you not be going up to 9-10ma or higher? Without looking at data sheets I'm guessing 5ma = around 12K for that particular tube, and that it could achieve as low as 8K rp under higher current conditions. If it turns out to actually be 10K effective rp under stock conditions, then wouldn't we still be at slightly less than matching load on V2? (10Krp into 10K plate load// 15K OT pri.) = slight shift towards more distortion relative to higher loading plan.
Ah, yes, 6C4 closer to half 12Au7 than any of the octal equivalents. So that makes sense to use that tube for numerous reasons - size, heater power, and ability to run low supply voltages and have lower current draw at a lower rp than what came before it.
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