Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: fdesalvo on April 25, 2017, 02:15:55 pm
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Hi, gang.
Long time! I've got a scratch-built AB763 (single/normal channel, 2x6l6) that I decided to finish after setting it aside about 4 years ago. It's fully functional and sounds nice, but not sure it's optimal.
The PI plates are sitting @ 310/305 volts under load. V1 is @ 245/241. The 6l6s are @ 445v. I've double checked the resistors/wiring in the PI and all is well.
Has anyone seen this before?
Hope all's well with you and yours ~
fd
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That's higher than I've seen. Are you using a 12AT7 for the PI? Are the resistors in the grid/cathode circuit the correct values? How much voltage is on the power supply node that feeds the PI tube?
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That's higher than I've seen. Are you using a 12AT7 for the PI? Are the resistors in the grid/cathode circuit the correct values? How much voltage is on the power supply node that feeds the PI tube?
Hey, I've got a 12AX7 in there at the moment. The resistors are of correct value and there is 438vdc @ that node. Good to see you're still around, my friend!
Edit - with a 12AT7 the PI's plate volts dive down to 235/238. Node voltage stays the same. V1's plates move up to the high 290s. Sweet mercy.
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The AT is a considerable fatter tube than the AX, sucks harder, pulls plates down, is more able to smack the 6L6es.
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The AT shall stay!
On another note, there's a large discrepancy between measuring the 6l6(wgbs) plate pins to ground and the reading I'm getting from the Eurotubes bias probe that sits between the tube itself and the socket.
The probe tells me the plates are at 408vdc (and 35ma plate current - this amp's cathode biased). The DMM tells me the plates are sitting @ 445vdc - that's a ~40v gulf (cathode voltage?). Which do I believe? Scandalous!
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> Which do I believe?
My head is 9 feet off the ground. Yes, the floor here is 4 feet up. Am I 9 foot or 5 foot?
The "cathode bias" is a new revelation. But as a dart-toss I'd guess G2 is 440V so expect G1 to be near 0.6*(440/10) or say 30V. Other thoughts suggest the bias is way off, may have to be -40V to get where you are at. So yes: 445V to the plates, cathodes standing on a 40V stool, is 408V plate to cathode.
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:)
Converted her back to fixed bias and all is well sitting at 70%. While so doing, I discovered a nasty hum @ 120hz. Turned out to be an ailing bias cap.
Cheers!