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What causes my 0D3 to extinguish?

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arnoldc

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Joined 2003
RoadToNirvana.net
#1
I brought my latest 300B amp to a friend who has an ARC SP-8 preamp. We hooked up my amp, but every time he switches on the MUTE lever, the 0D3 on my amp extinguishes its purple glow.

Tonight, it happened that I turned off my preamp before the amp, and I saw the 0D3 extinguish its glow again.

What's going on?

(There was a thread on my 300B amp, but can't find it now. Current schematic attached. The series resistor to the 0D3 is 5k6.)
 

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#4
The direct coupling to your input tube's grid can turn your input tube into a shunt regulator if there is any DC on the input. I would add a resistor from the grid to ground. Your input impedance is very high and any leakage in the tube, or the preamp will affect the bias on the input tube, possibly causing it to draw far more current than you intend.
 
#9
arnoldc said:
Ok, fair enough.

But what's exactly happening? If I'm playing music, how loud or dynamic it is, the 0D3 is fine.
Click to expand...

hmmm.....only way i could think of is the mute circuit on your friend's preamp forces 0 volts on your input tube grid making it conduct heavily and starving your OD3 to cause it to eaxtinguish....try a coupling cap on your amp input.😀
 
#12
arnoldc said:


This didn't cure the problem, but I'm leaving it in the circuit anyway.

Cheers.
Click to expand...


try to recreate that problem , and measure current (observe voltage at resistor preceeding OD3) of input stage(s) during that test .

one possibility is that OD3 is simply starving , other possibility is that you have gassy input tube , which have some grid leak current .

try to decrease slightly that series resistor
 
#13
Oh, I did something else..

1. I plugged shorting RCA into the inputs
2. I turned on the amp
3. The 0D3 glows after a couple of seconds (using 5AR4 rectifier)
4. The 0D3 glow starts dying like a candle after a few more seconds
5. Remove the shorting plugs, and 0D3 keeps on glowing

I think this has got to do with what Tony posted...

The voltage across the 0D3 is 153V when everything's ok.
 
#14
arnoldc said:
Oh, I did something else..

1. I plugged shorting RCA into the inputs
2. I turned on the amp
3. The 0D3 glows after a couple of seconds (using 5AR4 rectifier)
4. The 0D3 glow starts dying like a candle after a few more seconds
5. Remove the shorting plugs, and 0D3 keeps on glowing

I think this has got to do with what Tony posted...

The voltage across the 0D3 is 153V when everything's ok.
Click to expand...


as I can see from your schmtc - grid is intended to be on gnd potential.... ?

take your schematic and write voltages on it
 
#15
Try 100K from the orange LED to B+
Or replace the LED by a resistor. LED in cathode and a coil in anode is not a good solution it terms of stability of working point. LED in cathode is great when a tube is loaded by resistor or CCS, in case of a transformer load resistor in cathode is preferable.
 
#20
I was able to finally replace the LED with 120R bypassed with 100uF.

When I measured the voltage at the 396A cathode, it now reads 1.6V.

I plugged it in to my system, but seconds after it played music, the right channel emitted a popping noise, and the 03D is blinking in symphony. I immediately turned it off.

Any thoughts?
 
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