Univox U-1246 problem-Solved!
- Thread starter CJ
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> cathode follower into a paraphase
I do not see a cathode follower or a paraphase.
> got a positive grid voltage at the blue arrow wtf?
Says that point should be +87V. Much variation possible, should work anyway.
Do you have the right image?
I do not see a cathode follower or a paraphase.
> got a positive grid voltage at the blue arrow wtf?
Says that point should be +87V. Much variation possible, should work anyway.
Do you have the right image?
CJ
Well-known member
mind reader, yes there was a 12AX7a in that hole, will work but distortion city as only 1/2 a volt headroom, lowers output power also.
what cathode follower? not a paraphase, what we have is a direct coulpled triode into a split load inverter,
problem was pin one of tube socket not making contact, make money off the rain up here, especially last winter,
had full B+ on plate pin and 77 volts on cathode, tube was cutoff but still had cathode current, must be grid current going thru previous tube plate resistor.
working now, added screen resistors to GE black plate 6l6GC's , have to run 40 ma on each tube to get good sound, hope they hold up with that 700 volts on the plates, they should with only 350 on the screen, print calls out for minus 31 bias and that is what we have,
upgrade 100 K plate R's to 1 watt, 1/2 watt resistors run warm.
amp runs quiet, all grounds go to input jacks which were mounted on painted chassis, so we sand the paint off and put on star washers, same the the spk jacks,
notice the ratio on the output transformer, 10 K pri load, 24;1 for 16 ohm tap, 35:1 for 8 ohm tap, best to run Z switch at 16 ohms and feed an 8 ohm speak, new voltages with the now working V3 circuit,
what cathode follower? not a paraphase, what we have is a direct coulpled triode into a split load inverter,
problem was pin one of tube socket not making contact, make money off the rain up here, especially last winter,
had full B+ on plate pin and 77 volts on cathode, tube was cutoff but still had cathode current, must be grid current going thru previous tube plate resistor.
working now, added screen resistors to GE black plate 6l6GC's , have to run 40 ma on each tube to get good sound, hope they hold up with that 700 volts on the plates, they should with only 350 on the screen, print calls out for minus 31 bias and that is what we have,
upgrade 100 K plate R's to 1 watt, 1/2 watt resistors run warm.
amp runs quiet, all grounds go to input jacks which were mounted on painted chassis, so we sand the paint off and put on star washers, same the the spk jacks,
notice the ratio on the output transformer, 10 K pri load, 24;1 for 16 ohm tap, 35:1 for 8 ohm tap, best to run Z switch at 16 ohms and feed an 8 ohm speak, new voltages with the now working V3 circuit,
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CJ
Well-known member
underbelly of pc board,
board crackles when you solder it, there was a thread about that a ways back, crackle means the traces are in the process of liftoff,
green thing with curled leads is bias diode.
100 K resistors were changed to 1 watt and 8.2 k bias resistor for divider installed to get better range,
board crackles when you solder it, there was a thread about that a ways back, crackle means the traces are in the process of liftoff,
green thing with curled leads is bias diode.
100 K resistors were changed to 1 watt and 8.2 k bias resistor for divider installed to get better range,
Attachments
CJ
Well-known member
CJ
Well-known member
CJ
Well-known member
The 10K load goes-with the 700V B+. You won't safely get more power from higher voltage, you have to suck less current. If we scaled the B+ down to 0.707, like 500V, we could current up 1.414 times, load Z would shift by 0.707/1.414 or half or 5K.... a typical value for 6L6 at 500V.
The lower current at 700V also suggests lower screen voltage so bias/drive is not huge--- hence 350V on screens.
6550 and KT88 have similar suggested conditions, with Vp about twice Vg2. This did not make sense in 5U4 days, but with sillycon diodes the Doubler connection made a LOT of sense and naturally gives a half-V tap.
I'd not fret about good 6L6GC taking 700+V on plates (not screens!). The same guts were sold with a top-hat as TV H-Sweep bottles with very high (pulsed) plate voltages.
The lower current at 700V also suggests lower screen voltage so bias/drive is not huge--- hence 350V on screens.
6550 and KT88 have similar suggested conditions, with Vp about twice Vg2. This did not make sense in 5U4 days, but with sillycon diodes the Doubler connection made a LOT of sense and naturally gives a half-V tap.
I'd not fret about good 6L6GC taking 700+V on plates (not screens!). The same guts were sold with a top-hat as TV H-Sweep bottles with very high (pulsed) plate voltages.
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