I have a Dean Markley CD60 tube amp. 50W 6L6 output stage. Nice little amp. It had bad filter caps which I fixed, but then I encountered extremely high bias currents in one tube and high current in the other.
I determined the extremely high one was probably bad. It was up at 90mA with the other tube at 55mA. I checked for leakage in a coupling cap and don't find anything wrong.
I put a matched set of tubes in and I can not reduce the bias below 45-55mA in both tubes. I can't just change a resistor value to be able to go more negative, since with the pot all the way down it only goes to -55V or so. I can switch it and get the bias from the end of the transformer instead of the tap, but it seems to me that -55V on the grid should be enough to limit the current?
The plate voltage is around 460V, so 45mA is a pretty hot setting. 460x.045= 21W which is only 70% of 30W. I don't usually set anything to 70%, but this one I may have to.
Perhaps I could get a set of graded tubes that have a low bias current, but I hate to buy tubes I don't need. I plan to sell it and make a profit. At least that's the plan.
Any ideas on what else I can check that might make the bias current too high? Everything I checked seems OK, and it sounds OK.
[/url]CD120 Page 4 by Dennis Kelley, on Flickr[/IMG]
I determined the extremely high one was probably bad. It was up at 90mA with the other tube at 55mA. I checked for leakage in a coupling cap and don't find anything wrong.
I put a matched set of tubes in and I can not reduce the bias below 45-55mA in both tubes. I can't just change a resistor value to be able to go more negative, since with the pot all the way down it only goes to -55V or so. I can switch it and get the bias from the end of the transformer instead of the tap, but it seems to me that -55V on the grid should be enough to limit the current?
The plate voltage is around 460V, so 45mA is a pretty hot setting. 460x.045= 21W which is only 70% of 30W. I don't usually set anything to 70%, but this one I may have to.
Perhaps I could get a set of graded tubes that have a low bias current, but I hate to buy tubes I don't need. I plan to sell it and make a profit. At least that's the plan.
Any ideas on what else I can check that might make the bias current too high? Everything I checked seems OK, and it sounds OK.
I would lift one end of each coupling cap, all at the same time, and see if the bias point is better.
I measured the voltage on both sides of the 220K resistors and there was no drop in voltage, so I doubt there is any leakage through the cap.
Does -55V sound like it is about right for 45mA with a 460V plate?
Does -55V sound like it is about right for 45mA with a 460V plate?
I don't see a data sheet with that operating point on it. Maybe so, if the screen voltage
isn't very much lower than the plate voltage.
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What matters is the screen voltage.
Which in this amp is also ~~460V.
And Mug2, which is about 10 for 6L6.
Over-simplified, 460V/10= 46 "should cut-off the tube".
In fact high-volt guitar amps idle well down in the curve and we should allow some more. But -55 seems ample.
> I don't see a data sheet with that operating point on it.
Use the triode curve. Especially, as in this case, where Vp and Vg2 are very similar (choke or small resistor). But even when they are some different, Vg2 on the triode curve will be very close, Vp matters much less.
6L6GB Tung-Sol curve shows 460V and -45Vg1 gives 50mA. -60V gives 10mA. These way-cold curves are not well controlled and there will be considerable variation. While 6L6GC is tuned to work from/in 6L6GB data/apps, there will be variations from different makers. Still I would expect <50mA.
If the first 100uF cap in the bias supply has quit, voltage will be less than designed. It could fail utterly and still not have gross buzz in bias or the opampery. Worth shunting a good cap across to see if voltage changes.
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Brand new filter caps and bias circuit filter caps.
I will probably just let it go. It just bothers me when you can’t adjust and have to leave the pot all the way over.
I also have one of the tubes which the bias is higher and unstable and tends to drift. I decided it may be a bad socket so I bought replacement sockets. I had a bad socket cause unstable bias, and it ended up burning up the ot before I figured out what it was. But I think the damage was done that time before I saw it. Interstingly it was a Spectra 30, which Dean Markley bought and used their designs and put Dean Markley on them.
On that amp I was probing the bottom of the circuit and all seemed well, but the tube itself was seeing other voltages due to the bad socket.
I will probably just let it go. It just bothers me when you can’t adjust and have to leave the pot all the way over.
I also have one of the tubes which the bias is higher and unstable and tends to drift. I decided it may be a bad socket so I bought replacement sockets. I had a bad socket cause unstable bias, and it ended up burning up the ot before I figured out what it was. But I think the damage was done that time before I saw it. Interstingly it was a Spectra 30, which Dean Markley bought and used their designs and put Dean Markley on them.
On that amp I was probing the bottom of the circuit and all seemed well, but the tube itself was seeing other voltages due to the bad socket.
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