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Audio Research D70 mark1 + schematic

Hallo ! I´m trying to repair a D70 from 1983 . It has a 6DJ8
which determine the "workingpoint" for itself and the following two 6FQ7 tubes for each channel . This poweramp has no solid state parts and the cathodes of the 6DJ8 have two 1,5k resistors connected to each cathode ...... not 110 ohm and 511 ohm as mark 2 . I would very much like to find a schematic for this version of the D70 . ( The two 6FQ7 tube
in each channel have just two 6K powerresistors from their cathodes to earth ...... no other resistors as in version mark2 ) Regards Stewen
stewen1
stewen1 OP
9 posts
 
Thanks for Your interesting and valuable answer ! I am used
to measure high voltages as daily working with guitaramps .
Another thing that puzzles me : both schematics I have show
R31 and R32 ( anoderesistors for the last 6FQ7 tube ), 62kohms. The amp I have here , has 33 kohms and it really seems that they are factorymounted !!!! They are 5Watt each.
Wouldn´t that give a higher anodevoltage ? and also a bit higher anodecurrent ? But perhaps lower swing ??
Thanks for some comments. Stewen
hifigeek1
770 posts
 
V6-7 plates should have 240Vdc on them. Measure across each resistor to make sure they are 64Kohms. Do they say 64Kohms on them? I believe they were black Dale wirewound resistors. They should be the same on both channels. Hopefully someone didn't replace that A.C balance trim pot and change it's value which should be 10Kohms.
stewen1 OP
9 posts
 
Thanks ! Well , these resistors are green , 3% , and have 33K written on them . ( total four resistors ). I´ll check
the voltages You mention . Another question : if I can found
new Cornell/Duplier to replace the large filtercaps ( 800 uF/
450 VDC ) , will that be OK in this amp ? I can arrange four
470uF/450 V and get 2 of 940 uF/450 V ..... I mean , is this
make ( Corn./Dubl.) of enough quality ? Stewen
hifigeek1
770 posts
 
CDE's are fine provided the meet the capacitance and voltage requirements.
stewen1 OP
9 posts
 
Now this D70 has got new filtercaps ! One thing I notice is that if the wipers on the biaspots should lose their contact
completely or partial there will be a currentrush in the powertubes . Judging from the position that these wipers have now with 65 mA through the new 6550:s ( Tungsol reissue ) it
is possible to add a resistor to each biaspot .... between the wiper and the side of the pot that have -47 VDC . I think this
resistor can have 120K or a bit less resistance. Kevin O´Connor describes this security - resistor in one of his books
about guitaramps . If the wiper should lose contact full bias-voltage will turn down ( choke ) the 65 mA current to a lower value ...... Wouldn´t it be possible to add this "mod " to a D70 ???? Thanks for some comment. Stewen
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