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previous: Randy I have several champs and vibrocham... -- 5/15/2000 9:53 PM

Re: Low volume Champ

5/16/2000 11:30 PM
MBSetzer	Re: Low volume Champ Have you already removed the tubes and measured the actual values of the resistors, caps, and pots to see if any are defective? With the amp unplugged and filters drained of course. Those that can not be individually measured without unsoldering can still be tested in combination with the other parts soldered to them by comparison with one of the good amps. Even the transformers can be checked for gross problems by checking the ohms across the primary, across the secondary, and comparing to the good amp. Also see if one of the windings is shorted to ground or each other by checking all combinations & versus ground. Another simple thing easy to overlook is the tension of the contacts for the tube pins. These can be bent in SLIGHTLY with a dental pick for both octal & 9-pin sockets, of course this could cause electrocution if you forget to drain the filter caps first! If you've reached the point where you feel like replacing everything until it works, it would probably be more efficient instead to troubleshoot with a signal generator and ocilloscope. There is likely to be only one problem now, if you find it and replace the bad component or solder joint you win by only having to find one problem. If you replace more than one thing it is very likely that more than one problem will develop simply due to changing too many things at once. If you replace everything the odds of success decrease dramatically. You could simulate the signal generator using your guitar, but it is hard to be consistent with its output level, you really have to work at it. I would use an electronic keyboard like a Casio and set its headphone output volume low so the champ output is about the same as when you plug in the guitar instead. Instead of a scope, you can use the AC voltage measurement setting of your DMM. You won't be able to see the waveform but at least you will get amplitude readings of your signal at various points in the circuit. Most inexpensive DMMs are not true RMS but using the same DMM on both amps will be as ideal a comparison anyway. This would not be so bad since you have a good champ to compare it to, actually, I would not only check the bad amp against the schematic for proper voltages, but then check it also at every possible trouble point (way more than the ones where voltages are documented on the schematic) and compare to the good amp. If that doesn't turn up any differences (under static/no-signal conditions), then input a signal and trace it through the signal chain. Easier with a scope, but tape down middle C on the keyboard. Set the amps with the bass & treble all the way up, if vibrochamp, remove the second 12AX7 and turn the intensity & speed all the way down. Adjust the volume for easy listening, about 5 on the dial. Be very careful taking measurements on the live amp. Use great patience and forethought. I think it is safest to use probes with insulated alligator clips, attach them to the node with the amp off & drained, then plug it in & fire it back up for each reading. Check the AC MV into the first section of the preamp tube (pin 2), and then the AC Volts out of the same triode (pin 1). You are testing the AC signal from your guitar (keyboard/signal generator) before and after amplification by the first section of the tube. You should see similar amplification between the good & bad amp. If not, you most likely have found a problem in the first triode stage. It could even be as simple as corrosion on the input jack. Wouldn't you feel dumb if is was just the jack and you didn't find out until lots of original parts were replaced? The amp would still not work until you did the complete logical troubleshooting after that. Also minor corrosion on the speaker jack (or terminal lugs if not soldered or poorly joined) amounting to an ohm or two would rob 25percent to 50percent of the output power even if the rest of the amp is perfect. On the first 12AX7 the cathode bypass caps are subject to failure with age that can result in low output symptoms. These are polarized, which must be observed if replacing. The Volts AC (signal) output from the first triode should then pass through the 250pf treble cap, and into the top of the 1M volume pot. Make sure the vol pot is well grounded, heck make sure all grounds in the amp are good If everything is close to the same between good & bad amps so far, then see what the AC signal voltage is on pin 7 (the second triode section input) of the same first tube. The pin 7 voltage is completely dependent on the setting of the volume knob, if there is relative similarity (meaning both amps are good so far), it would now be a good time to adjust the volume controls until the same voltage is seen on pin 7 for each of the good & bad amps, about 0.5VAC. The resulting volume knob settings should be similar provided both amps still have very close to the intended 1meg fixed resistance across the antique pot, and the same working taper as the wiper travels, also the knobs must both have the setscrews tightened when the 10 is perfectly at the top of the dial. Now see if the outputs of the second triode section (pin 6) are similar. They would only be EXACTLY the same if both tubes were operating at the exact same B+ on their plates, same working resistance of their 100K plate resistors, etc. and perfectly matched tubes, so don't expect perfection, even with the exact same input voltage. I am keeping in mind you are reporting a MAJOR difference between good & bad amps. This 0.5VAC input signal should be significantly amplified by the second triode section of this first tube, then output from its pin 6, measure & compare these. If everything agrees so far you have eliminated the preamp from suspicion by logical troubleshooting The output from pin 6 of the preamp 12AX7 will pass through the 0.02mf coupling cap on its way to the pin 5 input of the 6V6GT. If you are measuring a similar AC signal voltage input to the 6V6 on each amp, you should then be hearing about the same output volume from each amp, provided their speakers are the same. Any time you get annoyed with the test tone through the speaker, you could instead substitute a dummy load resistor, 4ohms/10Watt ceramic will get hot but work, use the same kind on each amp. If you have properly eliminated from suspicion the tubes, schematic voltages, component values, connection integrity, preamp, speaker, etc., and everything else surrounding the 6V6, then about all that is left is the 25mf/25VDC cathode bypass cap on the 6V6 and the dreaded output transformer. IMHO the 25mf/25VDC component is under-rated by design defect, I would always replace it with at least a 50VDC component. The amp will also work just fine without this cap at all, it will just have less output. This may be your only problem to begin with. The value IMHO is also a little too many MF for an 8inch speaker, a smaller MF will not let the bass be as flabby, so one of my best champs has a 15mf/400VDC Fastcap from Angela instead of the lower quality original component. The Angela caps are not electrolytic so there is not a polarity concern when using these. the bypass caps can be checked by exchanging between amps, but I would always hesitate to even temporarily remove a good component from a good amp. Now the output of the 6V6 is probably best not measured for AC signal voltage output unless your DMM is truly known to handle AC readings riding on this much DC voltage! Why fry the meter, for some it is their only test instrument? If the right DC voltages are there, and the tube is good, the 470ohm cathode resistor is good, its bypass cap replaced or proven, the only thing left to suspect is the output transformer, I would purchase or borrow a replacement for testing before I would remove one from a good amp. You DID replace all the filter cans already to make sure the power supply is like new before trying to restore the signal integrity While we're at it I am also assuming you have insured that the power supply dropping resistors, 1Kohm and 10Kohm are not defective. Another potential problem area is the 2700ohm Negative Feed Back (NFB) resistor. If this has drifted downward in value over the years it will reduce volume, an upward drift will increase volume (beautifully IMHO). These should have all been verified before trying to match signals between amps. Replacing everything is the process of tearing your hair out to preserve your sanity. Might as well instead tear your hair out while finding the bad component without unsoldering any of the good ones Mentally the result could be the same either way, at least your amp will be better off though with as few unneeded disassemblies as possible. If any frustration results, keep in mind you were originally prepared to expend much more effort & money than you will end up doing. Of course your problem could be with the vibrato tube area and instantly be cured as soon as you remove the vib tube, your signal does not pass through the vib 12AX7, so troubleshooting this would be lots different but still best done after making sure the amp works like a champ, then make it work like a vibrochamp Hope this helps, Mike

5/16/2000 11:30 PM

MBSetzer

Re: Low volume Champ

Have you already removed the tubes and measured the actual values of the resistors, caps, and pots to see if any are defective? With the amp unplugged and filters drained of course. Those that can not be individually measured without unsoldering can still be tested in combination with the other parts soldered to them by comparison with one of the good amps. Even the transformers can be checked for gross problems by checking the ohms across the primary, across the secondary, and comparing to the good amp. Also see if one of the windings is shorted to ground or each other by checking all combinations & versus ground. Another simple thing easy to overlook is the tension of the contacts for the tube pins. These can be bent in SLIGHTLY with a dental pick for both octal & 9-pin sockets, of course this could cause electrocution if you forget to drain the filter caps first!

If you've reached the point where you feel like replacing everything until it works, it would probably be more efficient instead to troubleshoot with a signal generator and ocilloscope. There is likely to be only one problem now, if you find it and replace the bad component or solder joint you win by only having to find one problem. If you replace more than one thing it is very likely that more than one problem will develop simply due to changing too many things at once. If you replace everything the odds of success decrease dramatically.

You could simulate the signal generator using your guitar, but it is hard to be consistent with its output level, you really have to work at it. I would use an electronic keyboard like a Casio and set its headphone output volume low so the champ output is about the same as when you plug in the guitar instead.

Instead of a scope, you can use the AC voltage measurement setting of your DMM. You won't be able to see the waveform but at least you will get amplitude readings of your signal at various points in the circuit. Most inexpensive DMMs are not true RMS but using the same DMM on both amps will be as ideal a comparison anyway.

This would not be so bad since you have a good champ to compare it to, actually, I would not only check the bad amp against the schematic for proper voltages, but then check it also at every possible trouble point (way more than the ones where voltages are documented on the schematic) and compare to the good amp. If that doesn't turn up any differences (under static/no-signal conditions), then input a signal and trace it through the signal chain.

Easier with a scope, but tape down middle C on the keyboard. Set the amps with the bass & treble all the way up, if vibrochamp, remove the second 12AX7 and turn the intensity & speed all the way down. Adjust the volume for easy listening, about 5 on the dial.

Be very careful taking measurements on the live amp. Use great patience and forethought. I think it is safest to use probes with insulated alligator clips, attach them to the node with the amp off & drained, then plug it in & fire it back up for each reading.

Check the AC MV into the first section of the preamp tube (pin 2), and then the AC Volts out of the same triode (pin 1). You are testing the AC signal from your guitar (keyboard/signal generator) before and after amplification by the first section of the tube. You should see similar amplification between the good & bad amp. If not, you most likely have found a problem in the first triode stage. It could even be as simple as corrosion on the input jack. Wouldn't you feel dumb if is was just the jack and you didn't find out until lots of original parts were replaced? The amp would still not work until you did the complete logical troubleshooting after that. Also minor corrosion on the speaker jack (or terminal lugs if not soldered or poorly joined) amounting to an ohm or two would rob 25percent to 50percent of the output power even if the rest of the amp is perfect. On the first 12AX7 the cathode bypass caps are subject to failure with age that can result in low output symptoms. These are polarized, which must be observed if replacing.

The Volts AC (signal) output from the first triode should then pass through the 250pf treble cap, and into the top of the 1M volume pot. Make sure the vol pot is well grounded, heck make sure all grounds in the amp are good :-)

If everything is close to the same between good & bad amps so far, then see what the AC signal voltage is on pin 7 (the second triode section input) of the same first tube. The pin 7 voltage is completely dependent on the setting of the volume knob, if there is relative similarity (meaning both amps are good so far), it would now be a good time to adjust the volume controls until the same voltage is seen on pin 7 for each of the good & bad amps, about 0.5VAC. The resulting volume knob settings should be similar provided both amps still have very close to the intended 1meg fixed resistance across the antique pot, and the same working taper as the wiper travels, also the knobs must both have the setscrews tightened when the *10* is perfectly at the top of the dial.

Now see if the outputs of the second triode section (pin 6) are similar. They would only be EXACTLY the same if both tubes were operating at the exact same B+ on their plates, same working resistance of their 100K plate resistors, etc. and perfectly matched tubes, so don't expect perfection, even with the exact same input voltage. I am keeping in mind you are reporting a MAJOR difference between good & bad amps. This 0.5VAC input signal should be significantly amplified by the second triode section of this first tube, then output from its pin 6, measure & compare these. If everything agrees so far you have eliminated the preamp from suspicion by logical troubleshooting :-)

The output from pin 6 of the preamp 12AX7 will pass through the 0.02mf coupling cap on its way to the pin 5 input of the 6V6GT. If you are measuring a similar AC signal voltage input to the 6V6 on each amp, you should then be hearing about the same output volume from each amp, provided their speakers are the same. Any time you get annoyed with the test tone through the speaker, you could instead substitute a dummy load resistor, 4ohms/10Watt ceramic will get hot but work, use the same kind on each amp.

If you have properly eliminated from suspicion the tubes, schematic voltages, component values, connection integrity, preamp, speaker, etc., and everything else surrounding the 6V6, then about all that is left is the 25mf/25VDC cathode bypass cap on the 6V6 and the dreaded output transformer.

IMHO the 25mf/25VDC component is under-rated by design defect, I would always replace it with at least a 50VDC component. The amp will also work just fine without this cap at all, it will just have less output. This may be your only problem to begin with. The value IMHO is also a little too many MF for an 8inch speaker, a smaller MF will not let the bass be as flabby, so one of my best champs has a 15mf/400VDC Fastcap from Angela instead of the lower quality original component. The Angela caps are not electrolytic so there is not a polarity concern when using these.

the bypass caps can be checked by exchanging between amps, but I would always hesitate to even temporarily remove a good component from a good amp.

Now the output of the 6V6 is probably best not measured for AC signal voltage output unless your DMM is truly known to handle AC readings riding on this much DC voltage! Why fry the meter, for some it is their only test instrument? If the right DC voltages are there, and the tube is good, the 470ohm cathode resistor is good, its bypass cap replaced or proven, the only thing left to suspect is the output transformer, I would purchase or borrow a replacement for testing before I would remove one from a good amp. You DID replace all the filter cans already to make sure the power supply is like new before trying to restore the signal integrity ;-)

While we're at it I am also assuming you have insured that the power supply dropping resistors, 1Kohm and 10Kohm are not defective.

Another potential problem area is the 2700ohm Negative Feed Back (NFB) resistor. If this has drifted downward in value over the years it will reduce volume, an upward drift will increase volume (beautifully IMHO). These should have all been verified before trying to match signals between amps.

Replacing everything is the process of tearing your hair out to preserve your sanity. Might as well instead tear your hair out while finding the bad component without unsoldering any of the good ones ;-)

Mentally the result could be the same either way, at least your amp will be better off though with as few unneeded disassemblies as possible. If any frustration results, keep in mind you were originally prepared to expend much more effort & money than you will end up doing.

Of course your problem could be with the vibrato tube area and instantly be cured as soon as you remove the vib tube, your signal does not pass through the vib 12AX7, so troubleshooting this would be lots different but still best done after making sure the amp works like a champ, then make it work like a vibrochamp ;-)

Hope this helps,
Mike

Replies:

Randy Thanks Mike, for taking the time.... -- 5/17/2000 10:32 AM