Hi, I state that I am a beginner. 
I have an ear834.
Polarized V1 stage with Rk 1,8k / Vk 0,9v / 0,50mA.
Polarized v2 stage with Rk 2.0k / Vk 1.1v / 0.55mA.
On V2 I tried various IR leds and diodes, the best options for the same Vk are IR leds or 2 x 1n4148. All good..the preamp sounds happily.
I then tried to put a CV (led or diodes) on the V1 stage ..... but nothing.
Does not work.
I assumed that perhaps little current was circulating, so I gave more current to the IR LED (and diodes) taking it from the B + (285v), through a 220k R, about 1.3mA. Still not working
Maybe the valve is swinging?
Here's what I see.
Yellow is the incoming signal.
For now I have tried 2 x1n4148 and IR LEDs. Same result..a kind of square wave.
But on V2 everything works smoothly.
I specify that my ear is modified. the active riaa also includes the tube V1 (as audioresearch sp6).
Thanks for any help!
I have an ear834.
Polarized V1 stage with Rk 1,8k / Vk 0,9v / 0,50mA.
Polarized v2 stage with Rk 2.0k / Vk 1.1v / 0.55mA.
On V2 I tried various IR leds and diodes, the best options for the same Vk are IR leds or 2 x 1n4148. All good..the preamp sounds happily.
I then tried to put a CV (led or diodes) on the V1 stage ..... but nothing.
Does not work.
I assumed that perhaps little current was circulating, so I gave more current to the IR LED (and diodes) taking it from the B + (285v), through a 220k R, about 1.3mA. Still not working
Maybe the valve is swinging?
Here's what I see.
Yellow is the incoming signal.
For now I have tried 2 x1n4148 and IR LEDs. Same result..a kind of square wave.
But on V2 everything works smoothly.
I specify that my ear is modified. the active riaa also includes the tube V1 (as audioresearch sp6).
Thanks for any help!
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Well ... EAR834 Schematic is a pretty simple circuit.
Very likely that the forward biased diode (whether that would be an infrared LED, a 1N4007 or most-anything else in the low VF type) is backwards.
Diodes (as opposed to LEDs) can tolerate surprisingly high reverse voltage, so won't necessarily conduct.
In any case, even if the diode presents itself as a dead short (i.e. a wire), the very low current flow would still result in output, albeit likely quite compromised.
The oscillogram and FFT spectrum if I am to take them as representing the squarish wave as "the problem", are from where exactly? Please look at the aforementioned linkie, and reference that.
GoatGuy
Very likely that the forward biased diode (whether that would be an infrared LED, a 1N4007 or most-anything else in the low VF type) is backwards.
Diodes (as opposed to LEDs) can tolerate surprisingly high reverse voltage, so won't necessarily conduct.
In any case, even if the diode presents itself as a dead short (i.e. a wire), the very low current flow would still result in output, albeit likely quite compromised.
The oscillogram and FFT spectrum if I am to take them as representing the squarish wave as "the problem", are from where exactly? Please look at the aforementioned linkie, and reference that.
GoatGuy
Hi Rongon, I've read a lot of your posts and discussions here on DIY.
Very useful!
I suppose the led is on correct side because I have already done the work on the v2 stage.
Led anode (long pin) connected to valve cathode.
Cathode led connected to GND.
The 220k resistor which takes the (additional) current from the B+ is connected to the valve cathode.
It takes current directly on the decoupling capacitor pin.
I don't know why on V2 everything goes smoothly on V1, no.
PS: the stage V2 work without addittional current from B+.
Very useful!
I suppose the led is on correct side because I have already done the work on the v2 stage.
Led anode (long pin) connected to valve cathode.
Cathode led connected to GND.
The 220k resistor which takes the (additional) current from the B+ is connected to the valve cathode.
It takes current directly on the decoupling capacitor pin.
I don't know why on V2 everything goes smoothly on V1, no.
PS: the stage V2 work without addittional current from B+.
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Hi
...the images are screenshots of my usb oscilloscope.
Taken on the RCA output of the first channel where I tried to put the led on V1.
The (IR) led on v2 is already working on both channels.
thx
You should measure what is the voltage drop on an IR led at 0,5mA ...
I bet is not near that 0,9V with cathode resistor , that's why is not working right . At 0,5mA I'm not sure if the LED is slightly ON to emit light , I wouldn't use it anyway for 12AX7 a low current valve .
The same for 2 normal diodes in series ... the voltage is about 1,2V greater than 0,9V so the bias is not right for that anode resistor .
The anode voltage should be also measured in both cases .
I bet is not near that 0,9V with cathode resistor , that's why is not working right . At 0,5mA I'm not sure if the LED is slightly ON to emit light , I wouldn't use it anyway for 12AX7 a low current valve .
The same for 2 normal diodes in series ... the voltage is about 1,2V greater than 0,9V so the bias is not right for that anode resistor .
The anode voltage should be also measured in both cases .
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The V2 stage however works perfectly with both IR LEDs and 2 x 1n4148, same voltage (1.1v). But I chose the solution with 2 x 1n4148 for low current reasons .. as you say.
Anyway I wonder why in V1 it doesn't work.
Ok, thanks, I get it. It's probably clipping.
I can still have advantages to keep the bias to led (or diodes) on V2, right?
I could try a CCS on V1 ... CCS and CV can give me the same advantages on V1 (Excluding the gain which is already at maximum ...)?
Or even the CCS (on v1) won't work for the same clipping reason?
thanks
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Thanks, I can measure them in the meantime on V2, being that the stage seems to run smoothly.
What should I check with the voltage values?
Do I measure them directly on the anode pin?
Thanks if you explain it to me.
Yes directly on the anode pin . Voltages should be about the same with cathode resistor and LED , if not other changes must be done
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Always talking about V2.
When I put the led ir and then after the 2 x 1n4148 (both solutions gave the same Vk of the original 2k..Rk resistor) I measured both before and after the grid voltage at the CF input and it was practically identical.
I'll do the same with the anode voltage, But I suppose it is almost identical.
I will update!
Thanks again.
When I put the led ir and then after the 2 x 1n4148 (both solutions gave the same Vk of the original 2k..Rk resistor) I measured both before and after the grid voltage at the CF input and it was practically identical.
I'll do the same with the anode voltage, But I suppose it is almost identical.
I will update!
Thanks again.
Now here it is time to go to bed, I will post the schematic with voltages (I measure them again) and updated component values.
Thank you so much. : P
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