• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

What causes my 0D3 to extinguish?

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.)
 

Attachments

  • 300b assault.pdf
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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.
 

TonyTecson

diyAudio Moderator Emeritus
2003-05-29 2:57 am
Maybunga, Pasig City
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.

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.:D
 
arnoldc said:


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

Cheers.


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
 
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.
 
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.


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

take your schematic and write voltages on it