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Bias problems
Posted:
Fri Feb 15, 2008 5:45 pmby klee
Just installed the Poseidon boards in my MK 3's. One amp I can adjust the bias down to 700mv as listed in the instructions. The other amp however I can only get down to 780mv. I am using a matched quad of KT88. Any suggestions?
Posted:
Fri Feb 15, 2008 6:04 pmby Shannon Parks
Could you post all your voltages (like from the voltage sheet)?
voltage
Posted:
Fri Feb 15, 2008 8:06 pmby klee
V1 (1)165 (3)1.15 (4)2.9 (9)3.1
V2 (1)201 (3)8.6 (4)3.4 (5)3.4 (6)196 (8)8.6 (9)3.1
V3,4 (1)792mv (2)3.5 (3)508 (4)511 (5)67.5 (6)67.5 (7)3.2 (8)792mv
V5 (2)526 (4)6 (6)6 (8)525
(D)2.2 (E)2.2 (F)62 (G)62 (H)65.5 (J)297 (K)453
Re: voltage
Posted:
Fri Feb 15, 2008 11:03 pmby Ty_Bower
klee wrote:V3,4 (1)792mv (2)3.5 (3)508 (4)511 (5)67.5 (6)67.5 (7)3.2 (8)792mv
I hope you mean -67.5 volts on pins 5 and 6.
You might try replacing the 4.7k resistor in the bias supply circuit with something a little smaller (the original MkIII schematic shows a 1k part here). That should allow you to put just a little more bias on the grid which will in turn bring the idle current a little lower. Perhaps your particular KT88 tubes are very eager to conduct.
Also check the voltage in the bias circuit right after the diode (where the diode meets the 100 uF cap and the 4.7k resistor). You should have between -75 and -80 volts with respect to chassis ground.
bias
Posted:
Sat Feb 16, 2008 12:13 amby klee
Yes the voltage after the diode is -76.I'll try replacing the 4.7k rsistor with the original 1k.
Posted:
Sat Feb 16, 2008 8:25 amby Shannon Parks
Your voltages look great. Ty's advice is right on. I think these are some hot rod tubes. What are they?
bias
Posted:
Sat Feb 16, 2008 2:22 pmby klee
Changed the4.7k back to the 1k and the bias set is working. Thanks. The KT88's are Winged C.
Posted:
Sat Feb 16, 2008 10:14 pmby crispycircuit
I had the same problem... I just got my amps up and going and changed both resistors to bias my quad of EH 6550s. I also like the 12BH7a for driver tube. It added more top end extension and opened the sound stage a little...... I'll add pics later....
Posted:
Sun Feb 17, 2008 1:27 pmby klee
12BH7 instead of 12AX7-sounds interesting. Did you make any other changes?
Posted:
Sun Feb 17, 2008 3:40 pmby crispycircuit
That's what fun about this tube swap. NO CHANGES! It's been reviewed on this forum many times before. Try the search mode for more info. A few other members have had the same results which I feel was an improvement...
Posted:
Sun Feb 17, 2008 3:44 pmby crispycircuit
We're talking about the swapping the 12AU7 for the 12BH7..... Driver tube... It located next to the Bias Balance Adjustment Pot (cermet)....
Posted:
Sun Feb 17, 2008 8:05 pmby Ty_Bower
There are many alternate tubes which can be tried, many of which are virtually drop-in replacements. Instead of the 12AX7, you could try an ECC83, ECC803, 7025, or 5751. All are pin compatible, and should have similar characteristics to the 12AX7 (except the 5751, which has less gain). Instead of the 12AU7, you could try an ECC82, ECC802, 5814, or 12BH7.
The 5751 / 12BH7 appears to be a popular combination.
coupla more...
Posted:
Tue Feb 19, 2008 8:36 amby EWBrown
5963s and 6189s are also good 12AU7 substitutes.
/ed B in NH
Re: coupla more...
Posted:
Wed Feb 20, 2008 7:30 pmby Shannon Parks
EWBrown wrote:5963s and 6189s are also good 12AU7 substitutes.
These are two of my favorite ham fest tubes. Plenty of them out there!