Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jeffs on August 12, 2015, 06:34:01 pm
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A friends 65 Twin Reverb RI had tremolo issues. I took look with the o'scope and the test waveform was not correct (15V swing). Everything else looked ok so I swapped the GT ECC83S out for a GT 12AX7 and the circuit was cured.
Then I looked at the tube spec sheet and see modern 12AX7s are specified at 300V maximum plate voltage (the 1975 RCA manual shows 330V). The schematic test waveform shows 420V on the plate (6) and 13V on the cathode (8) of V5B (LDR driver). This seems to exceed the 300V plate voltage spec considerably.
Could this have led to premature failure of the tube?
Thanks
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Fender has been successfully abusing 12AX7s for over 50 years. I'm not concerned.
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The plates on the ECC83S are about half the area of a more common example of a 12AX7.
Could this cause the tube to fail prematurely or do I need to dig deeper for a problem?
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What problem? I thought you fixed it?
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Wel,l I fixed the first thing I found and stopped looking. If the high plate voltage is not the root cause I should keep looking or it may fail again.
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High B+ on that circuit may be a factor but that circuit has been used for over 50 years. It's a proven reliable circuit. Tubes do fail, many times just getting weaker and weaker. That weak tube may even work just fine in another socket. I would not worry about it. 12AX7s are pretty cheap.
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I agree with everything Sluckey said.
The tube which was there was likely just not strong enough (for whatever reason) for that circuit function. As he said, you could probably pop it into a different socket on the same amp and get acceptable performance.
Then I looked at the tube spec sheet and see modern 12AX7s are specified at 300V maximum plate voltage ...
"300v" is probably conservative, now and in the past. It has mostly to do with the spacing of parts inside the tube, and a little to do with the spacing of the pins outside the tube. Your tube socket and how dirty it is could be a bigger factor in how high a plate voltage you can get away with than even the tube itself.
I've got some McIntosh MC-30's, and there's almost 500v plate-to-cathode on their driver 12AX7's. I think your amp is fine.
Besides, do you think Fender would risk massive warranty returns if the tubes commonly available couldn't handle the amp's operating conditions?
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Designing with negative margins must work in some cases.
I subbed a 12AX7 for a 12DW7 and got stung after several months because the 12AU7 side cathode to heater voltage was exceeded (12AU7 200V v. 12AX7 180V). The follower circuit driving the tone stack would quit after a few minutes operation.
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I agree with everything Sluckey said.
If you want to "fix" what isn't broke, create a B+ tap for the trem tube in the PS that meets your design specs. should be pretty straight forward, or carry an extra amp or a bag of tubes.
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I subbed a 12AX7 for a 12DW7 and got stung after several months because the 12AU7 side cathode to heater voltage was exceeded (12AU7 200V v. 12AX7 180V). The follower circuit driving the tone stack would quit after a few minutes operation.
Was the 12DW7 new manufacture?
The old tubes could handle higher voltages much better in most cases then newly made tubes.
A number of guys here have put a newly made/modern tube in an amp with a cathode follower (CF) driven tone stack (TS) and the tube gave up the ghost in seconds to minutes. They put a NOS tube in and it worked fine.
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I subbed a 12AX7 for a 12DW7 and got stung after several months because the 12AU7 side cathode to heater voltage was exceeded (12AU7 200V v. 12AX7 180V).
I would rather think you stung yourself because a 12AX7 is not a good sub for a 12DW7. A 12DW7 (12AU7 side) makes a much better cathode follower because it can deal with much higher current. I doubt the small difference in cathode to heater rating was a factor.
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1 of the newly made 12DW7's you can get either way, 12AX7, 1,2,3/12AU7, 6,7,8 or 12AU7, 1,2,3/12AX7, 6,7,8, in the pin out.
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I was reviewing Valve Wizard chapter 1.23 and see the difference - maximum voltage (Va0) v. maximum quiescent voltage (Va-max).
http://www.valvewizard.co.uk/Common_Gain_Stage.pdf (http://www.valvewizard.co.uk/Common_Gain_Stage.pdf)
The data sheets typically specify Va-max, but neglect Va0. Instantaneous plate voltage can safely exceed Va-max. The article quotes Va0 = 550V for a 12AX7. This would be the maximum HT supply voltage.
The safe operating area is bounded by the plate power dissipation.
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Agree.
Mac ran 12AX7 far over 300V in some very reliable amps.
AFAICT, when this design pulls up above 400V, the plate current is essentially zero. Then the 550V spec applies.
If the tube is taking "any" current, the neon lamp will be dropping another ~~60V.