Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: frank57 on August 14, 2012, 01:45:18 pm
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On the old Hiwatt lead 30 el84 amp they used 22k here.
What effect does that have?
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What effect does that have?
They're the output tube grid stop R's. They put them there to help stabilize the PA stage of the amp. They should be mounted right on the tube socket grid pin with as little as possible of the R's lead wire left.
You can increase/decrease the value to keep the amp stable. The higher you go you start to roll off a little treble. They also can/will stop the output tubes grids from over loading, causing nasty sounding distortion. IIRC, guys sometimes go as high as 100K.
Brad :icon_biggrin:
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Is higher better for the power tubes?
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Is higher better for the power tubes?
I would say no, not if you don't need it.
Brad :icon_biggrin:
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EL-84's sometimes sound brash & glassy. Large grid stopper values can tame this issue, if it is an issue in a particular amp. There is an upper limit to the combined values of the grid stopper + grid leak resistors, which are in series with the tube's grid G1 to ground. This upper limit value is specified in the tube's data sheet (and differs depending upon bias method - fixed vs. cathode).
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I think I've seen as much as 56-100k there on some EL84 amps.
The resistor interacts with the input capacitance of the tube. The pair form a low-pass filter (or you could say "high cut"), and typically seek to reduce response above the audio range to prevent oscillation, especially at low radio frequencies.
Used judiciously, you could make them bigger than typical to shave the very high end of the audio range.
But there's a problem: there is a maximum resistance that you should use between the grid and ground (or grid and bias for a fixed-bias amp). The Mullard EL84 data sheet says 300k max.
The grid stopper resistor (10kΩ in your schematic) should be added to the grid reference resistors (47kΩ in your example; sometimes these are called "bias feed" resistors in a fixed-bias amp). So your schematic indicates a total of 57kΩ, you could raise the grid stoppers to 250kΩ without risk to the tube.
Whether it cuts too much treble for your taste is another matter.