Thanks,
Dave
I assume the amp has a push-pull output stage with two 6CA7/EL34 tubes. Try swaping the two tubes and see if the other's plate gets red in that socket. If it does, then there may be a problem with the bias to that tube. Maybe a grid return resistor open or increased in value or perhaps a leaky coupling cap from a previous stage. I believe with a 450 volt supply the bias would be around 38 to 40 volts. Should be able to measure this between the G1 and cathode of each tube.
Radiodoc
***************
:Hello All,
: I put the GZ34 back in and the neon light works great and the amp works. The JJ tubes still have the tiny blue glow inside the square holes in the plate (gas?). Now that I have the higher plate voltage back the plates in one of the tubes lit up bright red. So I shut it off. I've heard that the JJ tubes do that. Can that be controlled by setting the bias higher? It was about 40 volts and I set it to about 30 when the glow occured. Is higher better for bias or lower? I am measuring between the bias pin and the chassis to obtain this voltage.
: The fuse does not open now that I have solved the problem. I believe that the amplifier is now working normally. But I still want to get that voltage corrrect. I still haven't replaced the selenium rectifier yet. This is the bias diode.
: The tubes would not work and I would not hear anything unless the bias C voltage was working correctly. Is it normal for the plates to glow red? I don't think so. This is too hot. It worked better with a 5U4 as the rectifier because the voltages were lower. It would be better to get the amp to work at the higher voltage and take full advantage of the EL34s. So what's next?
:
:Thanks,
:
:Dave
Thanks,
Dave
:Dave,
:
:I assume the amp has a push-pull output stage with two 6CA7/EL34 tubes. Try swaping the two tubes and see if the other's plate gets red in that socket. If it does, then there may be a problem with the bias to that tube. Maybe a grid return resistor open or increased in value or perhaps a leaky coupling cap from a previous stage. I believe with a 450 volt supply the bias would be around 38 to 40 volts. Should be able to measure this between the G1 and cathode of each tube.
:
:Radiodoc
:***************
:
:
::Hello All,
:: I put the GZ34 back in and the neon light works great and the amp works. The JJ tubes still have the tiny blue glow inside the square holes in the plate (gas?). Now that I have the higher plate voltage back the plates in one of the tubes lit up bright red. So I shut it off. I've heard that the JJ tubes do that. Can that be controlled by setting the bias higher? It was about 40 volts and I set it to about 30 when the glow occured. Is higher better for bias or lower? I am measuring between the bias pin and the chassis to obtain this voltage.
:: The fuse does not open now that I have solved the problem. I believe that the amplifier is now working normally. But I still want to get that voltage corrrect. I still haven't replaced the selenium rectifier yet. This is the bias diode.
:: The tubes would not work and I would not hear anything unless the bias C voltage was working correctly. Is it normal for the plates to glow red? I don't think so. This is too hot. It worked better with a 5U4 as the rectifier because the voltages were lower. It would be better to get the amp to work at the higher voltage and take full advantage of the EL34s. So what's next?
::
::Thanks,
::
::Dave
I think I have specifications for the 6CA7/EL34 in pdf format if you would like it. I can email it to you. The plate running red in normal light may indicate a problem with the bias on the tube. Test the bias on each tube from G1 to cathode. It should be around 40 volts on each grid. The selenium may be ok as there is basically not any current drawn from the bias supply and you mentioned that you could get 40 volts or so out of it. There are some amps that the tubes are pushed extremely hard and the plates have a faint glow. However, I would think Heathkit would not do that. Sometimes service data is hard to find. Perhaps so, on here, they don't have it, but I betcha there are some who are looking for it when they have time as I.
Radiodoc
*****************
:Radiodoc,
: There does not seem to be any service data available for this amplifier. It has variable bias controls. It was set to 40 volts and I lowered it thinking it was too high. If 40 is right, then I will return it to 40 and leave it alone. This amp has test jacks to measure the bias as it's adjusted.
: OK, I'll switch the tubes. Doesn't the red plate damage the tube? Wont it melt the glass? It does get rather hot. What about the selenium diode inside? Should it be replaced, and with what value dropping resistor? All the paper and electrolytic capacitors were replaced correctly.
:So maybe the resistors are off value. I can only go by what the resistors say on them as to the value.
: I wish someone had the service data for this amplifier. But nobody seems to have it.
:
:Thanks,
:
:Dave
:
::Dave,
::
::I assume the amp has a push-pull output stage with two 6CA7/EL34 tubes. Try swaping the two tubes and see if the other's plate gets red in that socket. If it does, then there may be a problem with the bias to that tube. Maybe a grid return resistor open or increased in value or perhaps a leaky coupling cap from a previous stage. I believe with a 450 volt supply the bias would be around 38 to 40 volts. Should be able to measure this between the G1 and cathode of each tube.
::
::Radiodoc
::***************
::
::
:::Hello All,
::: I put the GZ34 back in and the neon light works great and the amp works. The JJ tubes still have the tiny blue glow inside the square holes in the plate (gas?). Now that I have the higher plate voltage back the plates in one of the tubes lit up bright red. So I shut it off. I've heard that the JJ tubes do that. Can that be controlled by setting the bias higher? It was about 40 volts and I set it to about 30 when the glow occured. Is higher better for bias or lower? I am measuring between the bias pin and the chassis to obtain this voltage.
::: The fuse does not open now that I have solved the problem. I believe that the amplifier is now working normally. But I still want to get that voltage corrrect. I still haven't replaced the selenium rectifier yet. This is the bias diode.
::: The tubes would not work and I would not hear anything unless the bias C voltage was working correctly. Is it normal for the plates to glow red? I don't think so. This is too hot. It worked better with a 5U4 as the rectifier because the voltages were lower. It would be better to get the amp to work at the higher voltage and take full advantage of the EL34s. So what's next?
:::
:::Thanks,
:::
:::Dave