If you can calculate the primary impedence, hopefully it will be 5k anode to anode (2.5k each side) - if the amp was designed properly in the first place. If the voltage on the anode of the output 6L6s is about 270 volts, which at 285 volts on the centre tap might be right (my fault on a previous posting) and the bias resistor is 125 ohms then they are operating in 'text book' A1 mode, giving 18.5w at 4% THD. Is there a resistor from g2 on the 6L6s to the HT supply? The screen grids should not exceed 270 volts either.
Operating the 6L6s in triode mode without changing the cathode resistors will be making them perform on an unlinear part of their characteristic, so distortion is possible. Also remember that your 6L6s are not matched, and differences in their characteristics might be more obvious in triode mode. Greater drive will be required from your phase splitter for triode mode and as said, the output tranny's primary impedence will need to be higher - as a quick experiment try a 8 ohm speaker across the 4 ohm taps to see if the reflected impedence gives a better match for triode operation (triode op requiring 8k anode to anode).
Operating the 6L6s in triode mode without changing the cathode resistors will be making them perform on an unlinear part of their characteristic, so distortion is possible. Also remember that your 6L6s are not matched, and differences in their characteristics might be more obvious in triode mode. Greater drive will be required from your phase splitter for triode mode and as said, the output tranny's primary impedence will need to be higher - as a quick experiment try a 8 ohm speaker across the 4 ohm taps to see if the reflected impedence gives a better match for triode operation (triode op requiring 8k anode to anode).
EC8010:
I have measured the transformer as you suggest (using my CD player with a disc of test tones as a signal source). The transformer primary impedence measures between 4500 and 4800 ohms; I got similar results with several of the secondaries.
Bournville:
There originally was a 2.5k resistor to g2; when I measured it earlier, it was at about 265V.
I have measured the transformer as you suggest (using my CD player with a disc of test tones as a signal source). The transformer primary impedence measures between 4500 and 4800 ohms; I got similar results with several of the secondaries.
Bournville:
There originally was a 2.5k resistor to g2; when I measured it earlier, it was at about 265V.
Yep,
Your results on the transformer impedence and the voltages point to pretty standard A1 operation - if there are other windings on the secondary which also seem to have a high impedence this is nothing to worry about. Many PA amps have a high imp secondary (sometimes called a '70 volt line') to operate speakers well away from the amp while cutting losses in long cables which would be pronounced with low imp. The remote speakers would have their own transformers.
If you've removed the g2 resistor for triode operation its not a problem so long as the voltage does not exceed that of the anode, and for optimum triode operation the anode voltage of pp 6L6s is 325 volts, so you are running well within limits.
Your results on the transformer impedence and the voltages point to pretty standard A1 operation - if there are other windings on the secondary which also seem to have a high impedence this is nothing to worry about. Many PA amps have a high imp secondary (sometimes called a '70 volt line') to operate speakers well away from the amp while cutting losses in long cables which would be pronounced with low imp. The remote speakers would have their own transformers.
If you've removed the g2 resistor for triode operation its not a problem so long as the voltage does not exceed that of the anode, and for optimum triode operation the anode voltage of pp 6L6s is 325 volts, so you are running well within limits.
A good demonstration of why I need practice in building tube amps:
I earlier reported that, after switching to triode operation, the amp was distorting horribly and not working well. It turns out I had connected g2 to the cathode instead of the anode ... oops! This explains why it wasn't working very well...
The amp is back to working in triode mode with g2 connected to the anode by a 100 ohm resistor. Without the resistor, the amp immediately goes into some sort of oscillation, doing nothing but switching on and off 60hz hum. Unfortunately, the only 100 ohm resistors that I have are a pair of 100W wirewounds that I have to attach externally with alligator clips.
I earlier reported that, after switching to triode operation, the amp was distorting horribly and not working well. It turns out I had connected g2 to the cathode instead of the anode ... oops! This explains why it wasn't working very well...
The amp is back to working in triode mode with g2 connected to the anode by a 100 ohm resistor. Without the resistor, the amp immediately goes into some sort of oscillation, doing nothing but switching on and off 60hz hum. Unfortunately, the only 100 ohm resistors that I have are a pair of 100W wirewounds that I have to attach externally with alligator clips.
... I would just add to what has already been said that given your 6L6s are unmatched and the output tranny is not matched for ideal triode operation, experimenting with changing component values to achieve stability is par for the course.
If there are any NFB arrangements running from the secondary of the output tranny the conditions for this will have changed also when you change the output stage to triode operation. Since one of the functions of NFB is to reduce THD in tetrode/pentode output stages, a condition not affecting triodes, it could be argued that NFB is unnecessary, although removing it completely may result in instability. You might also want to keep it to try to help overcome the limitations of the small output transformer.
If there are any NFB arrangements running from the secondary of the output tranny the conditions for this will have changed also when you change the output stage to triode operation. Since one of the functions of NFB is to reduce THD in tetrode/pentode output stages, a condition not affecting triodes, it could be argued that NFB is unnecessary, although removing it completely may result in instability. You might also want to keep it to try to help overcome the limitations of the small output transformer.
Cathode feedback
One possibility that you might like to try is cathode feedback. Since your output transformer is tapped to give 4 ohm and 15 ohm matching, it follows that the 4 ohm tap is actually the voltage centre tap. Thus, what you could do is to connect as shown in the diagram. If you match to a 15 ohm loudspeaker, it won't see an DC, but if you use the 4 ohm tap, it will see a very small amount of DC (probably about 100mV). I used this technique very successfully on a single-ended amplifier.
One possibility that you might like to try is cathode feedback. Since your output transformer is tapped to give 4 ohm and 15 ohm matching, it follows that the 4 ohm tap is actually the voltage centre tap. Thus, what you could do is to connect as shown in the diagram. If you match to a 15 ohm loudspeaker, it won't see an DC, but if you use the 4 ohm tap, it will see a very small amount of DC (probably about 100mV). I used this technique very successfully on a single-ended amplifier.
Attachments
What now? (Phono/Preamp stage)
Over the last couple of days, I have gone through and replaced all the old resistors and caps with new components. This has made a **huge** difference in the noise level -- with the old parts, the amplifier had an objectionably high full-frequency-range noise level at regular listening levels. Now, turned up to full gain, far beyond the clipping point with signal present, I have to put my ear right up to the speaker to hear any noise. The only problem remaining is a little hum that was previously completely masked by the random noise, but this is also inaudible at regular listening levels. I had expected more noise to be inherent in the tubes and the design itself; I am surprised how much of a difference parts replacement has made.
Now that I have the output stage working well, I return to the question of how to implement a phono stage w/RIAA compensation. Any suggestions?
Over the last couple of days, I have gone through and replaced all the old resistors and caps with new components. This has made a **huge** difference in the noise level -- with the old parts, the amplifier had an objectionably high full-frequency-range noise level at regular listening levels. Now, turned up to full gain, far beyond the clipping point with signal present, I have to put my ear right up to the speaker to hear any noise. The only problem remaining is a little hum that was previously completely masked by the random noise, but this is also inaudible at regular listening levels. I had expected more noise to be inherent in the tubes and the design itself; I am surprised how much of a difference parts replacement has made.
Now that I have the output stage working well, I return to the question of how to implement a phono stage w/RIAA compensation. Any suggestions?
EC8010:
The tubes I have left:
6SC7, 6SJ7, and 6SL7 as driver for 6L6's. I would like to keep the tubes/bases that I have, so long as "quite a challenge" doesn't mean "absolutely impossible."
ShiFtY:
I am already using star grounding (instead of the convoluted-path-through-steel-chassis grounding method originally in the amp). I just improved on the hum even more by switching to DC for the heater supply.
The tubes I have left:
6SC7, 6SJ7, and 6SL7 as driver for 6L6's. I would like to keep the tubes/bases that I have, so long as "quite a challenge" doesn't mean "absolutely impossible."
ShiFtY:
I am already using star grounding (instead of the convoluted-path-through-steel-chassis grounding method originally in the amp). I just improved on the hum even more by switching to DC for the heater supply.
Lots of questions.
Do you mean that the 6SL7 is the driver for the 6L6s? And is it the phase-splitter? If so, is it DC-coupled to the previous valve? What I'm really getting at is, what's left after essential power amplifier valves have been used? Can you draw a diagram of what you currently have? And can you measure the sensitivity at the input of the power amplifier section?
Do you mean that the 6SL7 is the driver for the 6L6s? And is it the phase-splitter? If so, is it DC-coupled to the previous valve? What I'm really getting at is, what's left after essential power amplifier valves have been used? Can you draw a diagram of what you currently have? And can you measure the sensitivity at the input of the power amplifier section?
sorry about the delay in replying; I have been somewhat busy the last few weeks.
The 6SL7 is the driver and phase splitter for the 6L6's; I think it is DC coupled to the previous stage. The exact nature of the connection between the input stages and the 6SL7 is hard to determine, because this is where all the wires go to the volume/tone controls; these are all fixed behind the front panel which is annoying to remove to see the connections. If necessary, I could remove all the knobs and hard-to-get-at screws to trace this part of the circuit. However, the present state of the circuit shouldn't matter too much, since it will probably need to be completely changed to accomodate RIAA equalization.
If I get a chance to later today, I will try to measure the sensitivity of the output section.
The 6SL7 is the driver and phase splitter for the 6L6's; I think it is DC coupled to the previous stage. The exact nature of the connection between the input stages and the 6SL7 is hard to determine, because this is where all the wires go to the volume/tone controls; these are all fixed behind the front panel which is annoying to remove to see the connections. If necessary, I could remove all the knobs and hard-to-get-at screws to trace this part of the circuit. However, the present state of the circuit shouldn't matter too much, since it will probably need to be completely changed to accomodate RIAA equalization.
If I get a chance to later today, I will try to measure the sensitivity of the output section.
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