The Fischer [stereo pre / Amp, 7868 tubes]

[petemoore]

Seems to work but...
I'm looking for 'the Fischer' schematic.
Preamps are done with 12ax7's, need new sockets.
The amp uses 7868 tubes which I've found EH spares for, they seem expensive though.
Mainly right now just searching for the schematic and tube data sheets.
35 tube watts per channel has it's allure at this time
Are there some equivalent output tubes I might want to look for?
Any fairly close substitutes or tubes I could convert to for the output?

[petemoore]

Whoops...model # is KX-100

[RWood]

If you search for Fisher, not Fischer, you should get more hits.

[petemoore]

yup

I figured that one out, have been searching Fisher, will continue...
I Signed up at HiFi Engine and now have the schematic !
Personal TDSL shows the 7868, as I understand there are a few 'substitutes' that could be 'wrangled' in there [change the sockets and or wiring / bias resistors]. EH has new 7868's for sale though.
It needs a major overhaul, could use new pre-amp sockets, output sockets look a slight bit baked but still feel tight, re-filter-cap job,..other than that the one channel is completely 'off', the other channel sounds quite powerful.
I'll test the output transformer on the dead side, problemly a tube.

[petemoore]

AC input voltage

I'm wondering what AC input voltage the PS was designed for [?], whether the redplating is caused by such...over-voltage [?], and whether a Zener Diode to drop the plate voltage/current would fix it [?].
Some older ones [like this one ??] were designed for AC supply of 110vac, and developed hot running/bias problems when 'modern' voltage of 120vac started getting applied.
Searching for reviews of the new EH-7868's, they are available...

[RWood]

Redplating is caused by too much plate disspation, and part of the formula is voltage but the other part is current.

Is this a cathode biased amp? If so you could increase the value of the cathode resistor to aleviate the redplating. Max dissipation for a 7868 is 19 watts, 70% is 13.3.

Btw, the 7591 may not be pin or base equivalent, but it is otherwise similar in operation to the 7868. Prices are very close, though, although the 7591s were used in more amplifiers and it's easier to find used pulls. They do require octal sockets.

Good luck.

[petemoore]

Well, I'm trying to figure out how the heater supply for 6 of the tubes [filaments are in parallel] makes '42 V' when connected to the cathodes.
Also, 4 filaments are in series to ground...
There's an inductor at the end of one 6-heater string which connects the 'yellow' wire from the PT [heater supply coil] to the 7868 cathodes, also to the filament string for the remaining tube heaters [4, in series].
So it doesn't appear to be cathode biased, there is a bias knob between two resistors [33k and a 680ohm]. These bias knob/resistors are across the 4 seriesed filaments.
I don't know, but wonder if this is 'fixed bias'.
Perhaps simply replacing all the filtercaps and tubes, then going over all the connections/resistances [make sure it's wired right] would bring the tubes to a safe bias point?