Quicky - magic eye between preamp and power stage

Simpleton · 10th May 2007, 08:40 AM

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

Tubes4e4 · 10th May 2007, 09:48 AM

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....

Simpleton · 10th May 2007, 11:15 AM

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!

Tubes4e4 · 10th May 2007, 11:57 AM

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

Simpleton · 10th May 2007, 12:13 PM

Thanks, you've been of great help

kevinkr · 10th May 2007, 03:58 PM

You could just tap the signal from the plate of your output stage through a resistive attenuator, properly chosen values will have negligible effect on the output stage, saving complexity, expense, and the need for an extra stage.

Wavebourn · 10th May 2007, 04:05 PM

TL071 would fit well. But you should remember of transients, and protect it's input by diodes to + and - power sources of the opamp.

Also, you may use double opamp, the second one as a linear detector so your magic eye will start reacting on smaller signals.

Wavebourn · 10th May 2007, 04:06 PM

Quote:

Originally posted by kevinkr
You could just tap the signal from the plate of your output stage through a resistive attenuator, properly chosen values will have negligible effect on the output stage, saving complexity, expense, and the need for an extra stage.

Not enough of voltage swing to drive the eye.

kevinkr · 10th May 2007, 04:12 PM

Quote:

Originally posted by Wavebourn

Not enough of voltage swing to drive the eye.

What?? On the plate of an EL84??? The amplifier in question has an EL84 SE output stage which is what I was referring to - guess I should have made that more clear.

Wavebourn · 10th May 2007, 04:15 PM

Quote:

Originally posted by kevinkr

What?? On the plate of an EL84???

What? 100+ volts output from preamp? and EL84 in it?

10th May 2007, 08:40 AM	#1
Simpleton diyAudio Member Join Date: Jan 2005 Location: Lisboa	Quicky - magic eye between preamp and power stage 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 __________________ "There's something organic about a happy cloud of electrons" - Neutron Bob

10th May 2007, 09:48 AM	#2
Tubes4e4 diyAudio Member Join Date: Jul 2005 Location: Leverkusen	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.... __________________ If in doubt, just measure.

10th May 2007, 11:15 AM	#3
Simpleton diyAudio Member Join Date: Jan 2005 Location: Lisboa	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! __________________ "There's something organic about a happy cloud of electrons" - Neutron Bob

10th May 2007, 11:57 AM	#4
Tubes4e4 diyAudio Member Join Date: Jul 2005 Location: Leverkusen	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 __________________ If in doubt, just measure.

10th May 2007, 12:13 PM	#5
Simpleton diyAudio Member Join Date: Jan 2005 Location: Lisboa	Thanks, you've been of great help __________________ "There's something organic about a happy cloud of electrons" - Neutron Bob

10th May 2007, 03:58 PM	#6
kevinkr diyAudio Moderator Join Date: Sep 2004 Location: Boston, Massachusetts Blog Entries: 6	You could just tap the signal from the plate of your output stage through a resistive attenuator, properly chosen values will have negligible effect on the output stage, saving complexity, expense, and the need for an extra stage. __________________ www.kta-hifi.net

10th May 2007, 04:05 PM	#7
Wavebourn diyAudio Member Join Date: Sep 2006 Location: Pleasant Hill, CA	TL071 would fit well. But you should remember of transients, and protect it's input by diodes to + and - power sources of the opamp. Also, you may use double opamp, the second one as a linear detector so your magic eye will start reacting on smaller signals. __________________ The devil is not so terrible as his mathematical model! Wavebourn: We Create Creativity!

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