Speaking of unusual heater circuits ...
I've just overhauled a late 60s japanese stereo integrated tube amp with pp el84 finals..
The heater supply for the phono preamp section is the cathode current return from the finals .. and thence to providing bias at the finals' grids !
ie. el84 cathodes -> some resistance -> phono section AX7 heaters (series heaters) -> el84 grids
It all works out well - the EL84 cathodes are around +20V @ 60mA for each pp pair - the stereo pp pairs are connected together, after some rheostat adjustment, giving a total of 120mA current at +20V or so.
That feeds a pair AX7s with dc (under) heating (series tubes with series heaters) for stereo phono preamping....
with around +10V@120mA for each AX7 tube then to ground ... *also* then with a connection of the second AX7 heater (+10Vdc) parallel sent to the grids of the pp EL84 stereo pair ... as a 'fixed' bias (each with 250K resistances grid to ground)
Quite roundabout, for sure - but one does wind up with combo fixed/self bias on the EL84s of around +11Vdc cathode-to-grid *and* dc heaters for each of the phono AX7s of +10Vdc at 120mA.
I was amazed but it does make sense when one one wants underpowered dc heaters on phono preamp section without the cost of an additional heater winding on the traffo plus rectifying/filtering
The el84s are thus biased very hot, at +11Vdc cathode-to-grid, tending towards the class A pp mode - not too efficient, resulting in some 10Wrms into 8ohms each channel at 3% THD.
It surely does sound good - at lower power of 5Wrms or so, THD drops to around 0.5%.
Those crazy 60s Japanese hifi tube designer gurus for low cost solutions!
...
I recapped where necessary, rebiased for lower sensitivity on the aux inputs and got the hell out!
It was way too sensitive to use with a modern computer audio interface, so I set up the 'low sensitivity' aux inputs to handle around 8Vpp for max power ... I left the tuner/tape and phono mag/crystal, mic, tapehd inputs as they were.
One could rework that whole 'high gain' section to be a killer guitar pre, but I resisted and left the old RIAA, NIB weird feedback eq'ing as is
This 'Encel' el84 amp featured an elaborate speaker-with-feedback-coil capability, called 'Motion Feed Back' and a complicated 'Damping' control to boot ..... I replaced all that with a simple variable nfb circuit.
Replaced the original tube set for some fresh ehAY7 (preamp), jj5751 (cathodyne) and russkie 6P14P ev finals ....
The amp was running the original australian made 'milliwatt el84' finals at some high supply .. like +370V plate, +320V screens
Amazingly, these tubes, circa late 60s tested very well!
I replaced with some nos 70s 6P14P ev .. which are a good sounding and cost effective rugged el84 sub ... they are rated for 400V plate, 300V screen with 14W max plate dissipation .. a bit of a 'super el84/6bq5'.
I dropped the screen supply to 300V at first, but then returned it to 320V ... for a little more power at 3% .... the robust russkie has a generous screen dissipation and seems to have no trouble with the higher than spec supply.
The bias setup doesn't lend itself to tinkering
I tried with no success .. always seemed to 'equilibriate' back to +11Vdc cathode-grid but with higher THD, so I gave up!
...
Happy with the result - it's equally at home with low and high senstivity inputs with a lot of adjustability in between.
I'm using it as a stereo FX amp with hifi speakers, but it really sounded good and loud! on a 12" jensen alnico cab too, with a DI box on some guitar 8)
It's super quiet ... absolutely no hum even at full power .. about the best hum performance I've come across in a setup with significant gain - it has those old style steel alloy close fit shield/heatsinks on the phono AX7s.
Quite a feat really, considering it has a voltage doubling HV supply circuit, which usually give more ripple, *and* the ancient main resevoir caps showed no sign of degradation ... I subbed with new JJ multicaps with no measurable improvement, so I kept the originals.
I did replace all the bias electrolytics and all the coupling caps in the cathodyne. A lot of the old carbon comp resistors were all over the place, so replaced them as required too.