Schematic for a Crate VXF5112 Guitar Amp?

[KMoffett]

Have a Crate VXF5112 with a loud 60Hz hum, that suddenly started, and is unaffected by inputs or gain/volume controls. I don't see (scope) a filtering problem in the power sections. I'd like to isolate amp sections, but it would help the have a schematic, specific to this amp. I can't find it with any Google searches. There are other Crate schematics, but they are too different.

Ken

 

[Harald Kapp]

You could ask the manufacturer.

 

[KMoffett]

Did that. No response. The amp's owner was on hold with them for over 1/2 an hour before he gave up.

Ken

 

[Arouse1973]

Have a Crate VXF5112 with a loud 60Hz hum, that suddenly started, and is unaffected by inputs or gain/volume controls. I don't see (scope) a filtering problem in the power sections. I'd like to isolate amp sections, but it would help the have a schematic, specific to this amp. I can't find it with any Google searches. There are other Crate schematics, but they are too different.

Ken

Hi Ken
60 Hz hum that does not change with volume is normally caused by a ground loop. Have you moved into a different room / house or added any other pieces of equipment to your set up?
Adam

 

[KMoffett]

The hum it there as soon as you switch out of STANDBY...with anything or nothing connected.

Ken

 

[Arouse1973]

Is this a valve amp?
Adam

 

[KMoffett]

Yup! What's available on line:

http://www.musiciansfriend.com/deri...Copy_70DCCA42-D908-4FE9-B078-06927D5E312B.pdf

Ken

 

[Harald Kapp]

Well, that's not a complete schematic, but better than nothing. A tube amplifier can show such a behaviour if the voltage for heating the filaments is not well filtered. Have a look a this voltage with an oscilloscope or, if you don't have one, use a multimeter in AC-millivolt range. If you see hum on this voltage, possibly one of the electrolytic capacitors in the power supply is dead.

 

[Arouse1973]

Well, that's not a complete schematic, but better than nothing. A tube amplifier can show such a behaviour if the voltage for heating the filaments is not well filtered. Have a look a this voltage with an oscilloscope or, if you don't have one, use a multimeter in AC-millivolt range. If you see hum on this voltage, possibly one of the electrolytic capacitors in the power supply is dead.

Correct Harald this is why some people modify the heater voltage to be d.c to remove the Hum.
Adam

 

[KMoffett]

Correct Harald this is why some people modify the heater voltage to be d.c to remove the Hum.
Adam

Heater supply is 6.3VAC with a grounded center tap. 390VDV plate supply has about 10V sawtooth ripple when under load (loud hum). Output plate signals (o-scope) are relatively sinusoidal 60Hz with opposite phases. Makes me think that it's something before their input...like that DSP section in the block diagram.

Just got an email back from the company. After signing away any liability (and my soul ) to them, they are going to send the schematic.

Ken

 

[KMoffett]

Way late in getting back. I got the schematics. Between my vast experience with tube circuits and similar amp circuits on Google Images, I had eliminated most possible problems. I was doing some scope probing with the amp on standby and I noticed that touching pins on one of the two output tubes resulted in a little hum on the speaker, but not on the outer. I had the owner purchase a new pair of output tubes. That solved it. One bad tube, but I still don't know what the problem was with it.

Ken