Hello,
I've had this Jolida 502 BRC integrated amp for about 8 years now and it's been generally trouble free until now.
The issue I am noticing is when I set the bias voltage on the 6550's, one of the
tubes appear to drop voltage by about 50% after about 5-10 minutes of warm up. I've swapped tubes around and the issue follows that socket. So it does not appear to be the tube itself.
Strangely- if I use the EZ-Bias feature on this amp, the indicator lights will light up around 500mv, and the tube/socket in questions indicator will not go out when the voltage drops, as indicated by my multimeter, in the ports for taking such readings.
I probably wouldn't have noticed this, except a tube in a different channel got to turning red, which seemed to be what was causing the amp to shut down. It's been removed and it's not shutting down now. I want to say it's been 4-5 years since I've checked the bias, and things started heating up in a new cabinet that didn't have as good ventilation. That's when it all began.
I've managed the ventilation, removed that tube, but now there is this voltage drop in an unrelated channel.
Seems to sound fine.
Haven't opened it yet. Any idea what to check?
Thanks in advance!
I believe this is likely close:
I've had this Jolida 502 BRC integrated amp for about 8 years now and it's been generally trouble free until now.
The issue I am noticing is when I set the bias voltage on the 6550's, one of the
tubes appear to drop voltage by about 50% after about 5-10 minutes of warm up. I've swapped tubes around and the issue follows that socket. So it does not appear to be the tube itself.
Strangely- if I use the EZ-Bias feature on this amp, the indicator lights will light up around 500mv, and the tube/socket in questions indicator will not go out when the voltage drops, as indicated by my multimeter, in the ports for taking such readings.
I probably wouldn't have noticed this, except a tube in a different channel got to turning red, which seemed to be what was causing the amp to shut down. It's been removed and it's not shutting down now. I want to say it's been 4-5 years since I've checked the bias, and things started heating up in a new cabinet that didn't have as good ventilation. That's when it all began.
I've managed the ventilation, removed that tube, but now there is this voltage drop in an unrelated channel.
Seems to sound fine.
Haven't opened it yet. Any idea what to check?
Thanks in advance!
I believe this is likely close:
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Pure_Brew,
The 6550 operation with fixed adjustable bias has a maximum specified grid resistor of 50k Ohms. You have over 100k Ohms (>100k), no matter where you turn the bias pot. That can cause thermal run-away (Red Plate).
The KT88 operation with fixed adjustable bias has a maximum grid resistor of 100k Ohms. You have over 100k Ohms (>100k), no matter where you turn the bias pot. That can cause thermal run-away (Red Plate). But I would say that the KT88, though technically not at spec, would be more likely to survive versus the 6550.
The EL34 operation with fixed adjustable bias has a maximum grid resistor of 500k.
I Red Plated a 6550 once. The grid resistor exceeded 50k. I caught it in time, heard distortion, saw the red, and turned it off. With a 50k grid resistor it was fine.
Also, make sure that the 0.22 630V caps are not leaky (at the potential voltage they are used at). I have a method to check cap leakage at voltage.
The 6550 operation with fixed adjustable bias has a maximum specified grid resistor of 50k Ohms. You have over 100k Ohms (>100k), no matter where you turn the bias pot. That can cause thermal run-away (Red Plate).
The KT88 operation with fixed adjustable bias has a maximum grid resistor of 100k Ohms. You have over 100k Ohms (>100k), no matter where you turn the bias pot. That can cause thermal run-away (Red Plate). But I would say that the KT88, though technically not at spec, would be more likely to survive versus the 6550.
The EL34 operation with fixed adjustable bias has a maximum grid resistor of 500k.
I Red Plated a 6550 once. The grid resistor exceeded 50k. I caught it in time, heard distortion, saw the red, and turned it off. With a 50k grid resistor it was fine.
Also, make sure that the 0.22 630V caps are not leaky (at the potential voltage they are used at). I have a method to check cap leakage at voltage.
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Thank you for the reply. I will check the resistors for specification.
What is your method for checking cap leakage?
Best Regards
What is your method for checking cap leakage?
Best Regards
1. For example, suppose a working amplifier has the driver tube plate at +200V. The driver plate is RC coupled to the output tube grid. The coupling cap is 0.2 uF. The output tube is self biased (cathode bias resistor to ground). And the output tube has a grid resistor of 100k to ground.
The numbers I use, for voltage and resistance, are just for illustration. You will have to use judgement for your particular amp and parts.
Disconnect negative feedback so the amp will not oscillate (non feedback amps are even easier, nothing to disconnect).
Disconnect the coupling cap from the output tube 100k resistor and output tube grid (the grid is still 100k to ground). Measure the voltage at the junction of the 100k and the grid. Suppose it is 25mV. OK, the grid current is 0.25uA. Check versus tube specification.
Reconnect the coupling cap to the 100k and grid. Measure the voltage at that point. Suppose it is 100mV . . . you can see that means the cap is leaky. Replace. I have seen old caps at a volt or more, Wow! replace.
Now re-connect the amp negative feedback, and restore to normal operation.
You can apply these ideas for many amps, merely by measuring the voltage on the grid.
2. If the amp has fixed bias, and negative feedback: That means the output tube grid will be at a negative voltage.
Disconnect the negative feedback. Disconnect the coupling capacitor, and connect it to a 100k Ohm resistor to ground. Measure the voltage across the 100k resistor.
If the capacitor is good, reconnect it to the grid. If the cap is leaky, replace it.
Reconnect the negative feedback and restore the amp to normal operation.
3. Your judgement is necessary as to how much leakage you might accept. Most of my amps have about 5mV to 10mV on the grid. But if you are not sure, you can replace the cap, and measure again.
I generally use a 600V coupling cap. My 350V supplies go to over 450V (and the plates go to over 450V) before the tubes warm up. So I will not use a 400V coupling cap there, because it will have more than 400V across it until the amp warms up. With(-) fixed bias supply on the grid, the voltage across the coupling cap is even more.
The numbers I use, for voltage and resistance, are just for illustration. You will have to use judgement for your particular amp and parts.
Disconnect negative feedback so the amp will not oscillate (non feedback amps are even easier, nothing to disconnect).
Disconnect the coupling cap from the output tube 100k resistor and output tube grid (the grid is still 100k to ground). Measure the voltage at the junction of the 100k and the grid. Suppose it is 25mV. OK, the grid current is 0.25uA. Check versus tube specification.
Reconnect the coupling cap to the 100k and grid. Measure the voltage at that point. Suppose it is 100mV . . . you can see that means the cap is leaky. Replace. I have seen old caps at a volt or more, Wow! replace.
Now re-connect the amp negative feedback, and restore to normal operation.
You can apply these ideas for many amps, merely by measuring the voltage on the grid.
2. If the amp has fixed bias, and negative feedback: That means the output tube grid will be at a negative voltage.
Disconnect the negative feedback. Disconnect the coupling capacitor, and connect it to a 100k Ohm resistor to ground. Measure the voltage across the 100k resistor.
If the capacitor is good, reconnect it to the grid. If the cap is leaky, replace it.
Reconnect the negative feedback and restore the amp to normal operation.
3. Your judgement is necessary as to how much leakage you might accept. Most of my amps have about 5mV to 10mV on the grid. But if you are not sure, you can replace the cap, and measure again.
I generally use a 600V coupling cap. My 350V supplies go to over 450V (and the plates go to over 450V) before the tubes warm up. So I will not use a 400V coupling cap there, because it will have more than 400V across it until the amp warms up. With(-) fixed bias supply on the grid, the voltage across the coupling cap is even more.
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