Hi, I have noticed in many schemes the presence of a diode between the grid and the cathode.
I would like to have, if possible, all the information about it, if it is useful and when, if there is a degradation of sound ... etc ..
I also notice that there are those who implement it with a resistor ..
What is resistance needed?
What value should it be put on?
Thanks! I attach some pictures ...
I would like to have, if possible, all the information about it, if it is useful and when, if there is a degradation of sound ... etc ..
I also notice that there are those who implement it with a resistor ..
What is resistance needed?
What value should it be put on?
Thanks! I attach some pictures ...
On the spot correct !
When the circuit is working the diode is blocking and has no affect on the sound, it only comes to play when conditions are dangerous for the tube.
It's needed for any direct-coupled cathode follower to prevent grid-cathode arcing at startup. It does not affect the sound at all.
Cathode-Follower OTL and SIT FETs
Cathode-Follower OTL and SIT FETs
Attachments
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You are correct that it can protect grid-cathode arcing at startup. But there are few million Marshall amps out there without one. Certainly not a high failure rate.
Having said that, one tube that did have a high failure rate in that position was the Sovtek 12AX7LPS. I am thinking the diode would have certainly helped.
The diode has another purpose than just for direct coupled circuits . . .
For RC coupled stages, where the designer of the circuit insists on sub-Hertz frequency response, there will often be a 0.47uF, 1uF, or larger coupling cap from the previous stage's plate, to the grid of the next stage.
A capacitor that has 0 Volts across it, is a dead short when the amp B+ first comes on.
If the tubes are still cold, the previous stages plate load resistor passes exactly the same transient current to the next stage grid . . . Just as if it was a direct coupled stage.
Who ever thought of that?
Who ignored it?
12AX7 grids, for example, are many times thinner than a human hair.
Do not run grid current into a 12AX7 grid, not even transient current.
What is the rated grid current for a 6DJ8?
What is the rated grid current for a 12AX7?
These are small signal tubes, they are not RF Class A2, or RF Class C power tubes.
The 6DJ8 and 12AX7 are not even small RF Oscillator tubes . . . But the 6J6 is (16mA grid current).
Attention to details.
For RC coupled stages, where the designer of the circuit insists on sub-Hertz frequency response, there will often be a 0.47uF, 1uF, or larger coupling cap from the previous stage's plate, to the grid of the next stage.
A capacitor that has 0 Volts across it, is a dead short when the amp B+ first comes on.
If the tubes are still cold, the previous stages plate load resistor passes exactly the same transient current to the next stage grid . . . Just as if it was a direct coupled stage.
Who ever thought of that?
Who ignored it?
12AX7 grids, for example, are many times thinner than a human hair.
Do not run grid current into a 12AX7 grid, not even transient current.
What is the rated grid current for a 6DJ8?
What is the rated grid current for a 12AX7?
These are small signal tubes, they are not RF Class A2, or RF Class C power tubes.
The 6DJ8 and 12AX7 are not even small RF Oscillator tubes . . . But the 6J6 is (16mA grid current).
Attention to details.
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ok, thanks for all the information.
It seems that the light bulb is better than the diode.
i still don't have what light bulb i can use here in europe..in italy.....
And what about the resistance in series with the diode? Because? wh
at value?
Is 1k the value that is always good?
Thanks again
It seems that the light bulb is better than the diode.
i still don't have what light bulb i can use here in europe..in italy.....
And what about the resistance in series with the diode? Because? wh
at value?
Is 1k the value that is always good?
Thanks again
The resistor is only found in guitar amps, to ensure the diode does not make the clipping too 'solid state' when driven positive. You don't need it for hi-fi (it isn't really needed for guitar either, to be honest).
'Light bulb' needs to be a neon lamp:
https://uk.farnell.com/multicomp/mc08310015/neon-lamp-wire-ended-t2/dp/1139256?st=neon lamp
Many Neons, like the NE2, fire at about 90V, and then stabilize at about 60V.
If you think 60V from grid to cathode is OK . . . well then OK.
If not, then Not!
At least, probably not on Audio Tubes.
If the first stage of the DC coupled stage triode goes open, then 60V will always be at the grid with respect to the cathode (not so nice).
You will have to add much to the circuit complexity to get it to keep the grid to cathode restrained to a low + voltage).
Just my opinion.
If you think 60V from grid to cathode is OK . . . well then OK.
If not, then Not!
At least, probably not on Audio Tubes.
If the first stage of the DC coupled stage triode goes open, then 60V will always be at the grid with respect to the cathode (not so nice).
You will have to add much to the circuit complexity to get it to keep the grid to cathode restrained to a low + voltage).
Just my opinion.
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Thanks Merlinb, I'm in Italy and I buy on this site, TME. What specifications should the bulb have ... what important? There are also reds and greens ... So I look for it.
can this be ok?
NEON-3 BRIGHTMASTER - Filament lamp: neon | orange; 90VDC; 65VAC; 0.5mA; O: 4mm; L: 10mm | TME - Electronic components
Thanks Again
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It does not matter, any general purpose neon bulbs will work, they are all basically the same. But why not use a diode?
Why??
60V is fine for standard audio tubes, they don't seem to care until you exceed 100V.
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Why not use a diode? I do not know! I'm going to light bulb! The valve I have to protect is a 6n1p, Vkf = 100v.
I buy this:
NEON-3 BRIGHTMASTER - Filament lamp: neon | orange; 90VDC; 65VAC; 0.5mA; O: 4mm; L: 10mm | TME - Electronic components
Thanks for the advices.
I buy this:
NEON-3 BRIGHTMASTER - Filament lamp: neon | orange; 90VDC; 65VAC; 0.5mA; O: 4mm; L: 10mm | TME - Electronic components
Thanks for the advices.
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