Hi there fellas!
It's been a while....
Sorry to bother you guys, but I don't know how to go about this.
I'm building the Simple EL84 amp from DIYparadise. I also have an EM800 magic eye that I would like to add to the circuit. At 250V the grid voltage is -10V for maximum deflection of the bar, perfect fit if I use the signal from the preamp stage of this amp.
Thing is: I usually couple my level meters at the line out of the source, in a simple grounded cathode auto-bias configuration, i.e., 1M resistor from grid to ground and cathode resistor of Vgbias/Ia. It's pretty easy to use and build and also, a simple night-light can be made this way by replacing the cathode resistor by an LDR (been there, done that).
How do I connect the grid to the preamp without affecting the signal? one 1M resistor in series with the grid and another going to ground?
Thank you
It's been a while....
Sorry to bother you guys, but I don't know how to go about this.
I'm building the Simple EL84 amp from DIYparadise. I also have an EM800 magic eye that I would like to add to the circuit. At 250V the grid voltage is -10V for maximum deflection of the bar, perfect fit if I use the signal from the preamp stage of this amp.
Thing is: I usually couple my level meters at the line out of the source, in a simple grounded cathode auto-bias configuration, i.e., 1M resistor from grid to ground and cathode resistor of Vgbias/Ia. It's pretty easy to use and build and also, a simple night-light can be made this way by replacing the cathode resistor by an LDR (been there, done that).
How do I connect the grid to the preamp without affecting the signal? one 1M resistor in series with the grid and another going to ground?
Thank you
Hi,
I see no chance at all to drive EM800 (alone) from a preamp output, since to get the -10VDC for "full" reading, you will need 10VACrms at least... even more than that, since the RC time constant (between signal rectification and EM800 control grid) will average even more, and the EM800 grid leak (usually the R in the RC filter) is a voltage divider, too ...
Best bet is to insert another simple gain stage between pre-out and the EM800 circuitry. This also will buffer the meter circuit from the pre-out. A stage amplification factor of A=10 will do; you can get that from one ECC82/12AU7 section in grounded cathode, w/o using cathode bypass resistor.
Regards,
Tom
EDIT: Instead of a tubed gain/buffer stage, you always could use BF245C or something like that instead. I don´t think anyone would have a problem with solid state at _that_ position. Would give multi-MegaOhm Zin, too - simply forget about any backlash on pre-out or signal....
I see no chance at all to drive EM800 (alone) from a preamp output, since to get the -10VDC for "full" reading, you will need 10VACrms at least... even more than that, since the RC time constant (between signal rectification and EM800 control grid) will average even more, and the EM800 grid leak (usually the R in the RC filter) is a voltage divider, too ...
Best bet is to insert another simple gain stage between pre-out and the EM800 circuitry. This also will buffer the meter circuit from the pre-out. A stage amplification factor of A=10 will do; you can get that from one ECC82/12AU7 section in grounded cathode, w/o using cathode bypass resistor.
Regards,
Tom
EDIT: Instead of a tubed gain/buffer stage, you always could use BF245C or something like that instead. I don´t think anyone would have a problem with solid state at _that_ position. Would give multi-MegaOhm Zin, too - simply forget about any backlash on pre-out or signal....
Yeah, I don't bother about ss in there, I was even thinking about a TL071 op-amp as a buffer. Can you tell me how to make that one out using that BF245? I know nothing about ss, only a bit about op-amps.
You see, I get full signal input to the valve, no rectification is done, like on an regular valve amplifying stage.
Thanks!
You see, I get full signal input to the valve, no rectification is done, like on an regular valve amplifying stage.
Thanks!
Hi Simpleton,
have a look at this universal magic band/eye/whatelse module, found at Jogis Röhrenbude to get an example how to use f.e BF245 or similar in this context.
Note, the BF245 buffer/driver part runs from 27V, obtained by a suitable Zener from a suitable B+ feed. You only must adjust the dropping resistor(s) to your available B+.
Note on EM800 (I am using it myself in an amp): No need to run it at the maximum 240V mentioned in the large/extensive TFK application report, just use, say, maximum 220VDC at fluorescent band anode pin, which is bright enough and will prolong fluorescent band life....
Regards,
Tom
have a look at this universal magic band/eye/whatelse module, found at Jogis Röhrenbude to get an example how to use f.e BF245 or similar in this context.
Note, the BF245 buffer/driver part runs from 27V, obtained by a suitable Zener from a suitable B+ feed. You only must adjust the dropping resistor(s) to your available B+.
Note on EM800 (I am using it myself in an amp): No need to run it at the maximum 240V mentioned in the large/extensive TFK application report, just use, say, maximum 220VDC at fluorescent band anode pin, which is bright enough and will prolong fluorescent band life....
Regards,
Tom
Simpleton said:Hi there fellas!
It's been a while....
Sorry to bother you guys, but I don't know how to go about this.
I'm building the Simple EL84 amp from DIYparadise. I also have an EM800 magic eye that I would like to add to the circuit. At 250V the grid voltage is -10V for maximum deflection of the bar, perfect fit if I use the signal from the preamp stage of this amp.
Thing is: I usually couple my level meters at the line out of the source, in a simple grounded cathode auto-bias configuration, i.e., 1M resistor from grid to ground and cathode resistor of Vgbias/Ia. It's pretty easy to use and build and also, a simple night-light can be made this way by replacing the cathode resistor by an LDR (been there, done that).
How do I connect the grid to the preamp without affecting the signal? one 1M resistor in series with the grid and another going to ground?
Thank you
Hey Wavebourn,
He's building a power amplifier, and I assume he could connect the EM800 to the output stage EL84 plate rather than what he is referring to as the pre-amp stage. It was just an idea to get the voltage swing required without an additional stage.
Kevinkr understood you right: the preamp is the output stage!
In such case I would avoid an internal triode biasing it deeply and connecting it's anode to the plate of the output toob of your amp through a resistor with diode in series and a capacitor to ground and resistor to B+. I used such trick with EM81, but it was driven by a high voltage transistor.
In such case I would avoid an internal triode biasing it deeply and connecting it's anode to the plate of the output toob of your amp through a resistor with diode in series and a capacitor to ground and resistor to B+. I used such trick with EM81, but it was driven by a high voltage transistor.
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