Thanks for the clarification .... that makes sense.
And PRR, that Fisher is similar to the Encel with the exception that it appears to be a full cathode bias, with bypass cap.
The thing I wonder, is what practical difference is made with the 'combo' bias, with a fixed grid voltage providing about half of the cathode-grid dc voltage, and the cathode circuit resistance providing the rest.
My understanding is that
- 'fixed bias', sometimes also described as being 'Class AB' in the data sheets example setups, provides a less programme dependent bias (more stable) and is generally of a higher efficiency and therefore power.
- 'self bias' or 'cathode bias' does vary the tube's operating conditions more with the higher rms programme and usually results in a lower available power.
Fixed bias sometimes is described as be more 'hifi' and cathode bias more 'compressed', leading some to favour one for hifi application and the other for instrument amplification.
Surely a simplification, but is 'combo' bias the 'best of both worlds' ?
I generally use finals with fixed bias so I can closely achieve my desired operating conditions; cathode bias and combination of both is new to me.
The only other time I've used 'combo' bias it is with my vari-mu limiter builds, especially in the fc660 and it's local derivative, the poorman 660, where one is trying to set operation on a specific part of the remote cuttoff transconductance curve, where it has significant 'bendiness'.
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Just found another popular 6BQ5/EL84 integrated amp using 'fixed + cathode' or 'combo' bias - the Scott 222 series.
- it takes the lowest HV rail, neon bulb regulates it to +48V and uses that as fixed bias to the grids of the finals, which also have a +60V cathode bias circuit; giving a +12Vdc cathode-grid net bias voltage.
- that same +48Vdc neon regulated rail is also used for 4x series heaters of 12V each for the input stages (4x 12AX7)
(the remaining heaters use a 6.3Vac winding with one end connected to the ground line - I don't think I've seen that before; an ac winding seemingly 'ground referenced')
The Scott 222 is a most interesting circuit for anyone interested in an integrated amp design with AX7s and EL84s. There is a write up in Vacuum Tube Valley Issue 13