Hi everyone,
Forgive me for the rather simple question, but can anyone point out the grid stopper resistor on this schematic? I'm trying to extend my tube sockets from a PCB to fit an enclosure and the advice I was given was to solder the grid stoppers directly to the socket. I attached the schematic and the reference document I was looking at.
Any help is much appreciated. Thank you
Forgive me for the rather simple question, but can anyone point out the grid stopper resistor on this schematic? I'm trying to extend my tube sockets from a PCB to fit an enclosure and the advice I was given was to solder the grid stoppers directly to the socket. I attached the schematic and the reference document I was looking at.
Any help is much appreciated. Thank you
Attachments
There aren't any on that circuit, but if you're going to extend valve sockets they should probably be added.
The first schematic is a Guitar amp, not a Hi Fi amp.
And that 100k resistor is Not a grid stopper resistor.
A true grid stopper is always connected directly to the grid, with Nothing else connected directly to the grid.
A typical grid stopper of 1k on a 12AX7 would connect directly to the grid; and the other end of the grid stopper resistor would connect to both the grid leak resistor and also to the coupling cap.
The second schematic does not have any grid stopper resistors. If you disconnect the wire from the grids, and connect a resistor between that wire and the grid, that is a grid stopper. The end of the grid stopper resistor needs to be connected right up at the socket tab that is the grid tab.
Using a long wire to connect from the grid stopper resistor to the grid will defeat the purpose of a grid stopper.
I hope that explains it.
And that 100k resistor is Not a grid stopper resistor.
A true grid stopper is always connected directly to the grid, with Nothing else connected directly to the grid.
A typical grid stopper of 1k on a 12AX7 would connect directly to the grid; and the other end of the grid stopper resistor would connect to both the grid leak resistor and also to the coupling cap.
The second schematic does not have any grid stopper resistors. If you disconnect the wire from the grids, and connect a resistor between that wire and the grid, that is a grid stopper. The end of the grid stopper resistor needs to be connected right up at the socket tab that is the grid tab.
Using a long wire to connect from the grid stopper resistor to the grid will defeat the purpose of a grid stopper.
I hope that explains it.
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Thank you for all the replies! I figured even though I am building a HiFi amp and not a guitar amp the theory would be relatively the same. Would a 100k resistor be suitable?
Yes, if you're OK with high cutoff frequency of 8 kHz - they do it in overdiven guitar amps for a reasons completely unrelated to HiFi ones.
Something like 1k would be more appropriate.
Perfect, I read through that article and it suggests something low like 1K for the preamp stage and suggests something like a 5.6K for the power tubes.
I am using a 12AX7 for the preamp stage and two EL34's for the power stages. Does that look acceptable?
Thank you all once again for all of the helpful advice
I am using a 12AX7 for the preamp stage and two EL34's for the power stages. Does that look acceptable?
Thank you all once again for all of the helpful advice
I read that the 12AX7 can take anything up to 68K at which point the high frequency response is becoming curtailed.
1k is probably an acceptable figure. If the amp suffers high frequency oscillation, you can increase the value.
1k is probably an acceptable figure. If the amp suffers high frequency oscillation, you can increase the value.
I find it odd that guitar amps don't commonly use grid stoppers, as a musician myself there is nothing more annoying than picking up radio transmissions on your amp while trying to play. This is common problem in guitar amps.
Anyone know why this is kind of standard in guitar amps? It can't be a cost issue.
Another thing I have wondered, is I see in overdrive pedals where the cathode resistor on the input tube is like a 30K pot and used to adjust the gain / create overdrive distortion. I can see why we don't want to create distortion on HiFi equipment, but at the same time wouldn't that be good to have a way to adjust the gain? I think you could use a pot that would operate over a range that is still not creating overdrive.
Anyone know why this is kind of standard in guitar amps? It can't be a cost issue.
Another thing I have wondered, is I see in overdrive pedals where the cathode resistor on the input tube is like a 30K pot and used to adjust the gain / create overdrive distortion. I can see why we don't want to create distortion on HiFi equipment, but at the same time wouldn't that be good to have a way to adjust the gain? I think you could use a pot that would operate over a range that is still not creating overdrive.
A real gain pot should have 60 dB of range.
You will not get 60 dB of range by putting a pot in the cathode circuit, unless you want to have lots of distortion at some of the "gain" settings.
Good for guitar, bad for Hi Fi.
"Two Different Worlds, We Live In Two Different Worlds" . . .
If you put a pot there, just call it a distortion pot.
You will not get 60 dB of range by putting a pot in the cathode circuit, unless you want to have lots of distortion at some of the "gain" settings.
Good for guitar, bad for Hi Fi.
"Two Different Worlds, We Live In Two Different Worlds" . . .
If you put a pot there, just call it a distortion pot.
So you couldn't use it as a Mu pot in a Hi-Fi environment?
Even if you just wanted a little control over it?
Even if you just wanted a little control over it?
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Blaxshep,
Are we talking Hi Fi?
Are we talking about having 2 gain controls, or just one gain control?
(2 gain controls so that you can get enough range because the cathode circuit pot does not have enough range of gain that is also low distortion).
And 2 gain controls . . . which one do you adjust first or last?
Confusion?
Look at one of the most linear triodes out there, the 6SN7.
If you cover the whole range of 6SN7 Mu (u), by varying the current from near zero to near maximum, you will find other quantities varying, such as plate resistance, rp; and also transconductance, Gm.
Just try and check the linearity of the 6SN7 when you go to those extremes, just to vary Mu (u) enough to get 60 dB of gain range.
I probably lied a little, if you only have 60dB of range in a gain control, even that is not enough.
Go ahead and do the experiment, vary Mu (u) on the input tube of your Hi Fi, and see if it works well, > 60 dB of range, and low distortion at all settings.
Are we talking Hi Fi?
Are we talking about having 2 gain controls, or just one gain control?
(2 gain controls so that you can get enough range because the cathode circuit pot does not have enough range of gain that is also low distortion).
And 2 gain controls . . . which one do you adjust first or last?
Confusion?
Look at one of the most linear triodes out there, the 6SN7.
If you cover the whole range of 6SN7 Mu (u), by varying the current from near zero to near maximum, you will find other quantities varying, such as plate resistance, rp; and also transconductance, Gm.
Just try and check the linearity of the 6SN7 when you go to those extremes, just to vary Mu (u) enough to get 60 dB of gain range.
I probably lied a little, if you only have 60dB of range in a gain control, even that is not enough.
Go ahead and do the experiment, vary Mu (u) on the input tube of your Hi Fi, and see if it works well, > 60 dB of range, and low distortion at all settings.
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Just to rectify this: grid stoppers do not prevent pickup of radio signals. Their purpose is to stop self oscillation due to inevitable parasitic capacitances becoming part of a positive feedback loop.
Good guitar amps do NOT pick up radio.
Either you have not-good amps or you live too lose to the transmitter. (I used to.)
The 34k (not 68k!) grid resistor is very common in Fenders and copies (sometimes mis-copied as 68k). Against the ~~100pFd of a hi-Mu grid this gives a -3dB point like 49kHz, so no harm to guitar and >20dB reduction of the low AM broadcast band.
Either you have not-good amps or you live too lose to the transmitter. (I used to.)
The 34k (not 68k!) grid resistor is very common in Fenders and copies (sometimes mis-copied as 68k). Against the ~~100pFd of a hi-Mu grid this gives a -3dB point like 49kHz, so no harm to guitar and >20dB reduction of the low AM broadcast band.