16-05-2020, 09:36 PM
Mark Hennessy Wrote:...you could add a control that lets you mix between the "enhanced" sound from the valve amp and the "clean" sound before.... It would give you a convenient front panel control which you could label "Niceness", allowing you to choose your poison, according to the recording...
Amie Wrote:Circling back to Andy's amplifier ideas, that push pull guitar amp that I had played with recently that had the 6BW6 op valves, had a decent sized output transformer and was an "ultralinear" but had a couple of controls that altered the bias on the output stage that were on the front panel, one allowed the amp to run into starved distortion at low level, the other altered the balance a little. They did alter the character of the sound considerably...
I will have to look at the notes I made, however it did have NFB but the thing was quite clever as the "bias" pot's were each ganged with a switch, (just as a volume or tone control would be with a mains switch), the switch would shunt the NFB when you wanted to increase the bias. Same also applied to the AC drive balance, (I will call it), with both controls turned off it operated as a normal UL amplifier.
Looks like the amplifier could easily have satisfied Mark's suggestion as a 'Niceness' control, in fact if you had 3 controls, one for bias, another for feedback, and a third selecting tapping points on the O/P transformer for screen-grid feeds (or even a sort of twin-ganged Variac type thing, for continuously variable from straight pentode to triode operation), there's enough combinations available to suit anybody - a knob for 'Niceness A,' 'Niceness B,' 'Niceness C.'
It's less benefit with beam-tetrodes like 6BW6 than pentodes like EL84, but a side benefit of UL is the added contribution to the output power from the AC component of the screen-grid current.
With feedback control turned back a bit to give less NFB and higher gain, bias control turned up to give more bias hence crossover distortion, and UL tapping appropriately set to give more power and gentle clipping, there'd be soft fuzz, hard fuzz, and plenty of loudness. So, Andy, how about giving it a try and report back?
