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My friends DRRI has no bass from channel 2 but amplifies the treble. Channel 1 works fine with full bass. So the power amp section and speaker are fine. Tried jiggling the tubes, swithching the vibrato and reverb off and on, disconnecting the footswitch, jiggling the input jack, plugging in jack 2 and still no bass. Turning the bass tone knob up sounds like it's trying to work but something is bleeding off the bass. Also jiggled the reverb in and out lines while playing a chord and no effect. I don't think it's a tube problem as it amplifies the treble. The amp is 2 years old. I've not cracked it open yet as I was hoping to get some thoughts on this first. Thanks in advance for any help you might offer.
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Off the top of my head, iounds like the bass pot is broken.
You won't know until you open it up and... have fun removing the pot so you can check it out of circuit. -
Sorry I took so long getting back. Thanks Bruce for your thoughts. I mulled it over a few days and today my friend brought the amp over and we cracked it open. Had fun removing the bass pot. Fun because it's a multiprong pot with seven prongs. Tested the pot and it was fine. Then we gave the whole amp a look over and could see nothing askew. Then I plugged it in and saw that a 6v6 did not light up. I pulled it out and put it in another amp and sure enough it was dead. So we put the pot back which was easy and sealed her up. My friend is off to the store to buy another. I wondered why he reported the amp worked fine in the normal channel. Hmmm? Any way, he told me it was the second time he had to replaqce this tube. I'm aware of this problem to be somewhat common on DRRI's. I'm wondering if any one has any thoughts on how to fix the tube burning problem? ThanksComment
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Well, for starters, you don't usually replace a single power tube in an AB1 amp, you replace the set of tubes. It is possible that the tube that replaced the failed tube last time was drawing excessive current and failed before the older tube because of this. It's also possible that you have a bad bias feed or screen resistor or a cold solder joint in one of those circuits on the failure prone side.
Chuck"Take two placebos, works twice as well." Enzo
"Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas
"If you're not interested in opinions and the experience of others, why even start a thread?
You can't just expect consent." HelmholtzComment
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Thank you Chuck for your thoughts. A pair of power tubes seems the smart way to go.Comment
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Sounds similiar to what mine did. I caught mine before it could take a tube down but it was red plating one side. Took it in to an authorized Fender service tech and according to them there was a cracked solder somewhere in the bias circuit. Fixed it under warranty no charge to me. You might want to check that out as Chuck H indicates. Good luck. Nice amp.Comment
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OK... the bad news is the power tube will have nothing to do with the lack of bass in one channel over the other but it is surprising you didn't notice there was a dead tube right away.
The good news is you found it has a bad tube and are getting that resolved.
I hope the lack of bass in one channel was pilot error and there is nothing really wrong.Comment
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I heard they use cheap caps in DRRI and they should be changed.Take it to my guy for $400.Comment
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What caps specifically are you referring to? Filter caps? $400 for a cap job?"In theory, there is no difference between theory and practice. In practice there is."
- Yogi BerraComment
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I just had my 64 Bandmaster recaped at Kendricks It cost $400. but it is so perfect.It blows my Plexi Marshall away.But in your case the filter caps that Fender used are cheap A good up grade would to change them.I had a problem with my marshall sounding weak I had put new tubes in One was bad.One of you screen caps may be bad.One time some beer and dirt got inside and made a bad connection.Comment
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Back Again.
So it's been about 8 weeks and the amp has blown another power tube. A fresh pair were installed 8 weeks ago. This is the 3rd set on a not so old amp. So I'm wondering what I should look for besides bad solder joints. Bad caps? That's wierd. Thanks for the help.Comment
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Is the tube that blows always from the same socket? Could just be bad tubes, but check the connections, esp pin 5, at the sockets and the PCB. Maybe it's losing bias. I'd make sure the pins are tight and clean. Check the solder connections around R60 and R61 too."In theory, there is no difference between theory and practice. In practice there is."
- Yogi BerraComment
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Different tube
It's not the same tube that blew last time.Comment
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If the amp is biased hot and you run it cranked I would say you need to experiment with different tube brands to find a more durable power tube. Have you ever checked the bias on the amp? It could still be a cold solder joint or some other bad contact too. Just really intermittant. That it's the OTHER tube that blew this time tells me that there's a problem with overall stress on the power tubes. Bias, intermittant short/open circuit, wrong speaker load, cheap tubes/amp cranked, something like that.
Chuck"Take two placebos, works twice as well." Enzo
"Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas
"If you're not interested in opinions and the experience of others, why even start a thread?
You can't just expect consent." HelmholtzComment
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Blown Tube
I would go to index or ENGINE ROOM are more guys that do amp builds there.I have the same problem with preamp tubes too.I bought a crummy little tube tester and found that 1/2 of All my tubes suck.But now the good tubes are all in matched quads and pairs.with out a scope its hard to say....
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