Some Very Good Advice
Okay so I think most of the problems are solved and I have a really good tech tip for everyone here. Something you should all probably say something like- Wow! Dude, I can't believe you got this. Ready? When you put the tubes into the sockets be sure to check each tube type because sometimes you might have, in the interest of just putting something in a safe place for a moment, you might have put three tubes that all look very similar together but,
Two of them were power tubes and one of them was a Rectifier tube. Hmmm. I thought the amp sounded under powered but figured that to be due to the wrong impedance speaker for temporary use. No, the amp was running single ended with one output tube being a Rectifier. Fortunately I used a ground lift circuit and a half watt resistor opened instead of frying the filament winding.
Thanks everyone for your assistance with this problem. I'm going to post the schematic and voltage table in the morning when I go back and check the power tube voltages again and fix the ground lift on the PT.
The amp sounds real clean and has good low end response. It responded well to just a guitar plugged into the input. But then the guitar has Hum Buckers!
Regards,
Ccat.
Okay so I think most of the problems are solved and I have a really good tech tip for everyone here. Something you should all probably say something like- Wow! Dude, I can't believe you got this. Ready? When you put the tubes into the sockets be sure to check each tube type because sometimes you might have, in the interest of just putting something in a safe place for a moment, you might have put three tubes that all look very similar together but,
Two of them were power tubes and one of them was a Rectifier tube. Hmmm. I thought the amp sounded under powered but figured that to be due to the wrong impedance speaker for temporary use. No, the amp was running single ended with one output tube being a Rectifier. Fortunately I used a ground lift circuit and a half watt resistor opened instead of frying the filament winding.
Thanks everyone for your assistance with this problem. I'm going to post the schematic and voltage table in the morning when I go back and check the power tube voltages again and fix the ground lift on the PT.
The amp sounds real clean and has good low end response. It responded well to just a guitar plugged into the input. But then the guitar has Hum Buckers!
Regards,
Ccat.
Schematic Voltage Levels Questions
Upon further testing I found one of the output tubes was bad. Although the filament lit it was just dead. That combined with inserting a Rectifier tube last night had me spinning in circles but it's resolved now. Sounds loud. I haven't run it full volume yet as I don't want to blow the speakers.
Some issues are as follows:
Faint scratching sound that seems related to a tube socket;
The only other problem is related to the boost. When engaged I get a feedback/oscillation sound similar to crossed output transformer wires. Is the FB signal being injected at the right place or should I remove the 1K to ground?
Are there any opinions regarding the voltage levels on the 6AN8; in regards to the ratios. This is an area I don't understand. What ratios produce the correct drive signal for the output tubes. At some point I'll measure the actual output power and put a scope on the circuit to check for oscillations.
Any suggestions on how to fix the boost?
Ccat.
Upon further testing I found one of the output tubes was bad. Although the filament lit it was just dead. That combined with inserting a Rectifier tube last night had me spinning in circles but it's resolved now. Sounds loud. I haven't run it full volume yet as I don't want to blow the speakers.
Some issues are as follows:
Faint scratching sound that seems related to a tube socket;
The only other problem is related to the boost. When engaged I get a feedback/oscillation sound similar to crossed output transformer wires. Is the FB signal being injected at the right place or should I remove the 1K to ground?
Are there any opinions regarding the voltage levels on the 6AN8; in regards to the ratios. This is an area I don't understand. What ratios produce the correct drive signal for the output tubes. At some point I'll measure the actual output power and put a scope on the circuit to check for oscillations.
Any suggestions on how to fix the boost?
Ccat.
Attachments
Bears Advice on troubleshooting this amp
Bear said in another post: http://www.diyaudio.com/forums/inst...speaker-not-really-420-watts.html#post4166672
No point in guessing.
You have a scope, according to your posts.
Check the new amp into a dummy load (resistor) while looking at it on the scope with a sinewave around 1kHz going in and measure the actual p-p voltage, take 1/2 of the p-p, multiply by 0.707 and use that value to figure the power into the resistor's ohmage value. That is the true power of the amp.
Then you can run into your speakers and with the scope connected note when you are at or near clipping (the same p-p voltage).
In the event that it starts to sound really bad before clipping, and the waveform in the previous test was not distorted, then you may be running out of drive voltage from the previous stage(s). A scope - again with the dummy load connected - will tell you that too.
Then you will know where ur problem is.
Until then, don't guess and muck about.
_-_-
PS. the bias on this amp should be for class AB, so quiescent probably on the order of less than 1 watt. The amp is in the 30 watt range, unless the supply voltage is high enough then the top end is about 50 watts. Regardless, this is a difference of <3dB.
Regarding the 1 watt quiescent dissipation at idle- The only way I can understand this happening with the 410 plate voltage is, the idle bias current must be conducting to ground through the cathode, (I think I just learned something), and not through the plate? Otherwise it would be 17 watts. Is this correct? I measure through a 1 ohm cathode resistor and read in mv.
To calculate idle diss should I measure the voltages from grid to cathode and multiply by bias current?
PPS. The power supply, solid state or vacuum tube, big caps vs. small caps will have very little influence on if the amp can make a reasonable amount of undistorted power... the output iron also will have nil effect unless ur trying to run a low bass signal and seriously saturating the core.
This could be the case. It is an amp that has lots of clean low end. Not sure I agree with the cap statement of non effect as that is the reserve for the output and if it is emptied too low there would not be sufficient current to push the speakers; I would think?
(sounds like the speakers might be overdriven)- paraphrased.
This sounds like a good place to start in the diagnostics and I recognize other have mentioned this direction also. I've used the amp on a regular basis now and it plays well until I crank it. And by that I mean it has to be played at Club levels before it shows up and I don't play that loud at home... Often...
Do the FB circuit values look right for this design?
I should get to this sometime soon,
Thanks all,
Ccat.
Bear said in another post: http://www.diyaudio.com/forums/inst...speaker-not-really-420-watts.html#post4166672
No point in guessing.
You have a scope, according to your posts.
Check the new amp into a dummy load (resistor) while looking at it on the scope with a sinewave around 1kHz going in and measure the actual p-p voltage, take 1/2 of the p-p, multiply by 0.707 and use that value to figure the power into the resistor's ohmage value. That is the true power of the amp.
Then you can run into your speakers and with the scope connected note when you are at or near clipping (the same p-p voltage).
In the event that it starts to sound really bad before clipping, and the waveform in the previous test was not distorted, then you may be running out of drive voltage from the previous stage(s). A scope - again with the dummy load connected - will tell you that too.
Then you will know where ur problem is.
Until then, don't guess and muck about.
_-_-
PS. the bias on this amp should be for class AB, so quiescent probably on the order of less than 1 watt. The amp is in the 30 watt range, unless the supply voltage is high enough then the top end is about 50 watts. Regardless, this is a difference of <3dB.
Regarding the 1 watt quiescent dissipation at idle- The only way I can understand this happening with the 410 plate voltage is, the idle bias current must be conducting to ground through the cathode, (I think I just learned something), and not through the plate? Otherwise it would be 17 watts. Is this correct? I measure through a 1 ohm cathode resistor and read in mv.
To calculate idle diss should I measure the voltages from grid to cathode and multiply by bias current?
PPS. The power supply, solid state or vacuum tube, big caps vs. small caps will have very little influence on if the amp can make a reasonable amount of undistorted power... the output iron also will have nil effect unless ur trying to run a low bass signal and seriously saturating the core.
This could be the case. It is an amp that has lots of clean low end. Not sure I agree with the cap statement of non effect as that is the reserve for the output and if it is emptied too low there would not be sufficient current to push the speakers; I would think?
(sounds like the speakers might be overdriven)- paraphrased.
This sounds like a good place to start in the diagnostics and I recognize other have mentioned this direction also. I've used the amp on a regular basis now and it plays well until I crank it. And by that I mean it has to be played at Club levels before it shows up and I don't play that loud at home... Often...
Do the FB circuit values look right for this design?
I should get to this sometime soon,
Thanks all,
Ccat.
Attachments
Idle dissipation = ohms law applied to the voltage through the cathode resistor.
Regarding the power supply. The signal is AC. It goes from + to - through zero, each cycle. Each cycle depends on frequency. When the signal is at or near zero there is no draw on the PS - the caps recharge. However the average draw over time causes an average drop in rail voltage, and an increase in ripple seen on the B+ line.
Better filtering, bigger power iron reduces this effect. But it has very little effect on the general operation of the amp. It does have effects on how it may sound or what happens when you clip the amp.
That's what you want to look at (scope helps). You'd likely want to know what the amp is doing when you crank it up to the "club levels" that you mentioned. Bet you are clipping it hard. THEN the average level is high and also this draws considerably more current from the PS.
There is no absolute value for feedback. If you increase the feedback to 100%, then you have ~unity gain, or a power buffer. Less feedback = more gain. The role of the feedback is to use "extra" gain to force the output to follow the input more closely (less "distortion). This may or may not be desirable for a musical instrument amp. But keep in mind that if you reduce the feedback, then likely you'd need to decrease the gain at the input stage to compensate.
The amount of feedback applied would be ideally determined by empirical tests relating to distortion, spectra of distortion, gain, and of course how it sounds...
Regarding the power supply. The signal is AC. It goes from + to - through zero, each cycle. Each cycle depends on frequency. When the signal is at or near zero there is no draw on the PS - the caps recharge. However the average draw over time causes an average drop in rail voltage, and an increase in ripple seen on the B+ line.
Better filtering, bigger power iron reduces this effect. But it has very little effect on the general operation of the amp. It does have effects on how it may sound or what happens when you clip the amp.
That's what you want to look at (scope helps). You'd likely want to know what the amp is doing when you crank it up to the "club levels" that you mentioned. Bet you are clipping it hard. THEN the average level is high and also this draws considerably more current from the PS.
There is no absolute value for feedback. If you increase the feedback to 100%, then you have ~unity gain, or a power buffer. Less feedback = more gain. The role of the feedback is to use "extra" gain to force the output to follow the input more closely (less "distortion). This may or may not be desirable for a musical instrument amp. But keep in mind that if you reduce the feedback, then likely you'd need to decrease the gain at the input stage to compensate.
The amount of feedback applied would be ideally determined by empirical tests relating to distortion, spectra of distortion, gain, and of course how it sounds...
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