Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: centervolume on September 11, 2025, 09:36:25 pm
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Hello
My first experience with grid leak bias setup and it’s working great for guitar, very earthy tone close to the v front super / dual professional.
However plugging in harmonica and hitting a note cupped through the shure bullet style mics creates an unusual effect where there’s no leading attack to the envelope just quiet and then the amplified note starts to gradually emerge.
It’s as if the force of the signal into the tube grid momentarily overpowers the tubes operation and it takes a moment to recover.
I’d like to understand this and to know if this is a matter of input resistor value mismatched with impedance from the mic, or if this is a grid bias operation effect?
Any help appreciated as always
CV
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Is this different than the sag seen in tube rectified amps?
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Need a schematic of your circuit to evaluate
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It’s as if the force of the signal into the tube grid momentarily overpowers the tubes operation and it takes a moment to recover.
That big harp signal causes a bias shift which takes a bit of time to recover. You have two identical preamps. Convert one to cathode bias and see if that is better for your harp. Look at the 5D5 but use an 820Ω cathode resistor for one tube and leave the other tube grid leak biased. That allows you to easily compare.
https://el34world.com/charts/Schematics/files/Fender/Fender_pro_5d5.pdf
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My first experience with grid leak bias setup ...
... plugging in harmonica ... there’s no leading attack to the envelope just quiet and then the amplified note starts to gradually emerge. ...
What SEL49 said.
This is why you sometimes hear, "Grid-leak bias circuits don't work well when hit with large signals, or guitars with boost pedals, etc."
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Is this different than the sag seen in tube rectified amps?
It would be a very extreme version of sag. The way I understand sag is to compare a silicone plug in the rectifier compared to something like a 5U4? The diodes seem faster and more punchy. I’m not sure if this is swamping? It just gets really bogged down to the point where transitioning from note to note quickly is pointless.if the notes are quieter I think the effect moderates.
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Need a schematic of your circuit to evaluate
Ok it’s been a while since I visited this site here’s a link (will try attachment)
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It’s as if the force of the signal into the tube grid momentarily overpowers the tubes operation and it takes a moment to recover.
That big harp signal causes a bias shift which takes a bit of time to recover. You have two identical preamps. Convert one to cathode bias and see if that is better for your harp. Look at the 5D5 but use an 820Ω cathode resistor for one tube and leave the other tube grid leak biased. That allows you to easily compare.
https://el34world.com/charts/Schematics/files/Fender/Fender_pro_5d5.pdf
Oh yes, thank you very much. I was thinking about if that could be done how difficult it might be etc. this is a model with the nine pin preamp instead of the octal. Will refer to 5D as you mentioned… This way I could have the best of both. Will definitely try this.
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If you’re using one of the 6SC7s for harp, it will work better if you change the bias mechanism to cathode bias on that tube. Try 820R to 1k.
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Is this different than the sag seen in tube rectified amps?
It would be a very extreme version of sag. The way I understand sag is to compare a silicone plug in the rectifier compared to something like a 5U4? The diodes seem faster and more punchy. ...
"Sag" is usually caused when power output tubes draw high current (when you play very loudly), and resistance/impedance in the power supply causes the power supply's voltage to drop momentarily. When the output tube screen voltage drops as a result, it constrains the plate current the output tubes can pull, and reduces output power. So "sag" is a voltage-shift containing output power, that gives the effect of volume-compression.
- Play the same amp quietly, and you'll wonder where is all that "sag" people talk about...
The grid-leak bias method causes the input capacitor to store a charge, that tends to bias the tube away from max plate current.
- The catch is that a large input signal causes grid-current, which charges the input-cap more, and shifts the input tube's bias further away from max plate current and towards current-cutoff.
- Depending on the supply voltage, it might only take 4-7v on the input-cap to completely shut off the input tube.
- Hit this tube with a really large input signal, and the input-cap can be charged beyond 7v. It might take a second or more for the input-cap to drain off enough signal to allow the input tube to pass current again & therefore pass audio.
The sonic effect of "compression" is similar with Sag and an over-driven grid-leak input stage, but the result can be more impactful in the grid-leak stage.