21-10-2023, 08:24 PM
Bias batteries in early 2V sets was nothing to do with cathode circuitry, it was partly to do with with cost of a resistor and that the o/p valve (often the only one with negative grid bias) was colour coded to set the bias. Originally 9V or even 12V packs of B cells tapped every cell (nominal 1.5V). Also being a triode, the anode current would vary as HT ran down. This is a problem for moving iron 2K to 4k cone speakers as the armature would move closer to one pole. There was also a mechanical adjustment, but that was to give more sensitivity for quiet stations. So the grid tap could be reduced as HT aged.
Mallory came up with the idea of a sealed mercury button cell for the O/P pentode (or triode) on later battery or mains sets. A battery of them would be made. However that idea wasn't a success. The cells were used in WII for long shelf life in proximity fuzes in shells and bombs then in early transistor hearing aids. That made them a success. So later he bought a licence from either Eveready (NCC/UC /Ucars) or British Ever Ready (which was founded by US company) for alkaline cells which Ever Ready or Eveready (not sure which) wasn't interested in selling. Mallory marketed those as Duracell, so successfully they renamed the company. Both Ever Ready (Gold Cell) and Eveready (Energiser) belatedly sold them.
Mallory's package technique was later used for Silver Oxide, Alkaline and Lithium cells.
The battery sets got rid of the bias pack when they were Pentode based on o/p and HT voltage not so critical. A resistor was placed between HT- and LT- to develop the negative bias for the o/p Pentode. The other valves use grid leak space charge to bias negative. Interestingly the DK9x or 1L6 L O grid can be DC filtered with 2 x 1M in series and give a negative enough bias for class AB on the DL96 in Push Pull, so some sets don't sacrifice the HT. A 90V layer pack has an endpoint of 54V. But the B7G 1.4 valves will work lower than that, so some sets used 45V (earphones only and 22.5V end point) or 67V in PP3 style packs but longer. The 22.5V in an Avo was used in hearing aids. Older ones used two packs. Later sub-miniature hearing aid valves are 0.7V filament to allow two (the entire amp) off one 1.5V cell and one 22.5V cell.
The only time (and very very rare) that isolated filaments used via a bifilar RF balun (the DC cancels) was the cathode of a grounded grid RF pre-amp. In such case the DC is the regular LT and 0V. I don't know of any cascade amp which would need a floating LT.
A stereo decoder might want a balun or transformer on the cathode, but it would be 19kHz and up, so not big and the filament connections could be bifilar only needing to be a choke at 19kHz to 54kHz (or maybe just 19kHz) and a capacitor to couple. Grundig did a single pentode decoder using also diodes as mixer and audio transformers. It doubled the 19kHz pilot at the cathode to get the 38kHz to mix into the L-R DSBSC and then two transformers.
Better valve amps in radios use both feed back to O/P g2 from a winding on transformer primary and feedback from speaker or extra winding to input. All the cathode resistors are ONLY for grid bias, all decoupled at audio. So that architecture will work with direct filaments. You still need some negative bias from somewhere for the o/p and that is usually (as in all battery valve sets approx 1937 to 1962) a resistor between HT- and 0V carrying entire HT current.
Mallory came up with the idea of a sealed mercury button cell for the O/P pentode (or triode) on later battery or mains sets. A battery of them would be made. However that idea wasn't a success. The cells were used in WII for long shelf life in proximity fuzes in shells and bombs then in early transistor hearing aids. That made them a success. So later he bought a licence from either Eveready (NCC/UC /Ucars) or British Ever Ready (which was founded by US company) for alkaline cells which Ever Ready or Eveready (not sure which) wasn't interested in selling. Mallory marketed those as Duracell, so successfully they renamed the company. Both Ever Ready (Gold Cell) and Eveready (Energiser) belatedly sold them.
Mallory's package technique was later used for Silver Oxide, Alkaline and Lithium cells.
The battery sets got rid of the bias pack when they were Pentode based on o/p and HT voltage not so critical. A resistor was placed between HT- and LT- to develop the negative bias for the o/p Pentode. The other valves use grid leak space charge to bias negative. Interestingly the DK9x or 1L6 L O grid can be DC filtered with 2 x 1M in series and give a negative enough bias for class AB on the DL96 in Push Pull, so some sets don't sacrifice the HT. A 90V layer pack has an endpoint of 54V. But the B7G 1.4 valves will work lower than that, so some sets used 45V (earphones only and 22.5V end point) or 67V in PP3 style packs but longer. The 22.5V in an Avo was used in hearing aids. Older ones used two packs. Later sub-miniature hearing aid valves are 0.7V filament to allow two (the entire amp) off one 1.5V cell and one 22.5V cell.
The only time (and very very rare) that isolated filaments used via a bifilar RF balun (the DC cancels) was the cathode of a grounded grid RF pre-amp. In such case the DC is the regular LT and 0V. I don't know of any cascade amp which would need a floating LT.
A stereo decoder might want a balun or transformer on the cathode, but it would be 19kHz and up, so not big and the filament connections could be bifilar only needing to be a choke at 19kHz to 54kHz (or maybe just 19kHz) and a capacitor to couple. Grundig did a single pentode decoder using also diodes as mixer and audio transformers. It doubled the 19kHz pilot at the cathode to get the 38kHz to mix into the L-R DSBSC and then two transformers.
Better valve amps in radios use both feed back to O/P g2 from a winding on transformer primary and feedback from speaker or extra winding to input. All the cathode resistors are ONLY for grid bias, all decoupled at audio. So that architecture will work with direct filaments. You still need some negative bias from somewhere for the o/p and that is usually (as in all battery valve sets approx 1937 to 1962) a resistor between HT- and 0V carrying entire HT current.
