Problem with guitar amplifier

wiseco16 · 20th September 2004, 10:05 PM

Howdy fellas - Im havin trouble with my guitar amplifier. Its an older Randall Century 170II SS. It is a 2 channel amp (clean & overdrive), sharing an EQ.

My problem with the amp is on the distortion channel. Getting any volume requires the volume and gain pots to be maxed out on this channel. Although the sound is there, it is very weak, and not overdriven at all.

After mailing Randall several times, i finally got a schematic.

Schematic

- I have replaced the (2) 4558 op amps.
- The gain and volume pots on the dist channel seem to be working fine.

I am clueless as to what may have happened. If anyone can help me out, i'd appreciate it.

rcavictim · 20th September 2004, 11:47 PM

Hmmm I think I see two problems. First the speaker isn't connected. Second and more serious is that your guitar amp's tubes are all missing.

Some time back manufacturers made a few guitar amps but forgot to put the tubes in them. I hear musicians didn't care for them that much. This sounds like perhaps an opportunity to upgrade?

wiseco16 · 21st September 2004, 04:55 AM

You are right about the tubes missing. My mesa/boogie awaits me once i graduate college (hence the solid state amp) cause im broke.

In the meantime i was going to hopefully try? to fix this beast, cause its all i have to practice with.

Once again, any ideas would be appreciated.

djk · 21st September 2004, 12:18 PM

The J-fet has a 10K resistor to set bias and is bypassed with 10µf TO GET GAIN.

If this cap is dried up the amp will have very low gain.

wiseco16 · 21st September 2004, 02:19 PM

hey thanks for the suggestion, i see what your getting at. Im gonna pick up this cap tonight and change it I'll let you know how it goes.

wiseco16 · 21st September 2004, 11:28 PM

Just got done installing the 10 uf cap on the fet with the 10k biasing. Volume and gain pot on the dist. channel still must be cranked to get any sound out of the amp.

Any one else have any ideas? Im not sure, could it be possible for that fet to be bad, or would that result in no sound at all?

20th September 2004, 10:05 PM	#1
wiseco16 diyAudio Member Join Date: Mar 2004 Location: Pittsburgh	Problem with guitar amplifier Howdy fellas - Im havin trouble with my guitar amplifier. Its an older Randall Century 170II SS. It is a 2 channel amp (clean & overdrive), sharing an EQ. My problem with the amp is on the distortion channel. Getting any volume requires the volume and gain pots to be maxed out on this channel. Although the sound is there, it is very weak, and not overdriven at all. After mailing Randall several times, i finally got a schematic. Schematic - I have replaced the (2) 4558 op amps. - The gain and volume pots on the dist channel seem to be working fine. I am clueless as to what may have happened. If anyone can help me out, i'd appreciate it.

20th September 2004, 11:47 PM	#2
rcavictim diyAudio Member Join Date: Jun 2004 Location: Cool end of a soldering iron NW of Toronto	Hmmm I think I see two problems. First the speaker isn't connected. Second and more serious is that your guitar amp's tubes are all missing. Some time back manufacturers made a few guitar amps but forgot to put the tubes in them. I hear musicians didn't care for them that much. This sounds like perhaps an opportunity to upgrade? __________________ I.Q.Test. Have you ever purchased a recreational snowmobile?

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21st September 2004, 04:55 AM	#3
wiseco16 diyAudio Member Join Date: Mar 2004 Location: Pittsburgh	You are right about the tubes missing. My mesa/boogie awaits me once i graduate college (hence the solid state amp) cause im broke. In the meantime i was going to hopefully try? to fix this beast, cause its all i have to practice with. Once again, any ideas would be appreciated.

21st September 2004, 12:18 PM	#4
djk diyAudio Member Join Date: Feb 2001 Location: USA	The J-fet has a 10K resistor to set bias and is bypassed with 10µf TO GET GAIN. If this cap is dried up the amp will have very low gain.

21st September 2004, 02:19 PM	#5
wiseco16 diyAudio Member Join Date: Mar 2004 Location: Pittsburgh	hey thanks for the suggestion, i see what your getting at. Im gonna pick up this cap tonight and change it I'll let you know how it goes.

21st September 2004, 11:28 PM	#6
wiseco16 diyAudio Member Join Date: Mar 2004 Location: Pittsburgh	Just got done installing the 10 uf cap on the fet with the 10k biasing. Volume and gain pot on the dist. channel still must be cranked to get any sound out of the amp. Any one else have any ideas? Im not sure, could it be possible for that fet to be bad, or would that result in no sound at all?

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