Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: bullkelp on April 11, 2025, 12:32:43 pm
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I have a fairly original 62 vibrolux 6g11
I have reolaced the original oxford with a celestion type A. It has a replacement OT (mercury tone clone).
The electrolytics and filter caps have been replaced recently.
Up to about 4 on the volume it is a superb sweet fender brownface tone, but very clean. Beyond 5 where I expected to find smooth drive out of the amp im getting a sort of unpleasant raspy, clipping, poppy breakup. All most like a blown speaker but its not.
Whats the solution to this? Is it remediable?
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What's the timing for when this started happening: has it always sounded that way, or is it a new behavior that came after making changes?
I built a 6G2 Princeton, and it likewise has an unpleasant, raspy overdrive sound that I've been trying to address, so I'm interested in whether you find a solution with your amp.
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Ive only had the amp a few months and its always been this way.
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I owned a '63 6G9-b, which I think is pretty much the same amp. I think the overdriven/breakup tones from that amp was smoother than from most blackface amps I've owned.
Maybe a bad solder joint somewhere that reveals itself with more volume/vibration?
/Max
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Are all tubes and the 6L6s are good ? Matched 10 ma or less ?
What is the bias?
6L6 plate volt :
" cathode current on each tube:
Let say ; 365 volts X 0.042 A ( or 42 ma ) = 15,33 watts for each tube
Volts X current = watts
For a 25 watts tube I will not go over 15 watts
A correctly-biased amps will sound clean and tight at moderate volume, then at higher volumes break up and distort musically."
A correctly biased amp will run efficiently and maximizes the life of your tubes.
Under-biased ( hot )amps will lack punch and the tubes will run noticeably hotter shortening their lifespan
Over-biased (cold ) amps will sound thin and brittle, with the tube running too cool for proper performance.
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Check the resistors values at the output of phase inverter circuit ; the 82k and the 100k
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A number of scattergun thoughts
Are there any modifications to the 6L6 circuit? (like g1-k caps? and if so, are these leaky?)
What plate voltages have you got on the LTP?
Check for DC-leakage on coupling caps between LTP and 6L6s
Check the 0.1uF decoupling cap on the non-inverting grid of the LTP for DC leakage
Maybe scope V1b and V2b when you get above 4. Check that the cathode bypass cap for this stage is good.
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I have a fairly original 62 vibrolux 6g11
Check for DC-leakage on coupling caps between LTP and 6L6s
Check the 0.1uF decoupling cap on the non-inverting grid of the LTP for DC leakage
Yellow Astron caps are notorious for leaking DC volts, and this amp is the right age to be to not have reached the change-over to the blue molded coupling caps. I echo the suggestion to measure carefully for leaking DC volts.