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Post subject: Twin Reverb Noise
Posted: Thu Sep 04, 2014 3:47 pm
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Hello all,

First let me apologize for failing to thank BMW and aclempoppi for their suggestions on the reverb squeal problem I was having with this amp earlier. I posted the reverb problem under "68 Twin Reverb Noise", and when I fixed the problem, I posted the results under that topic and then started to explain this new noise without taking the time to thank those who took the time to help me out. I got so wrapped up in the new noise problem that I forgot the manners my pappy drilled into my head. I since removed this new noise message from that post and replaced it with a thank you.

So,
After fixing the reverb noise issue on my AB763 Twin Reverb amp now has a noise on both channels kind of like the noise from a radio that's in between stations. It's not a hum, it's more of a static or hiss. I know that's not very technical but…. The noise starts to get really noticeable when the volume gets to about 2.5.

I swapped out all the tubes. The noise was still there.

Pulling one tube at a time out had no effect on the noise except when V4 was pulled, the vibrato channel was quiet but the normal channel still had the noise.
I isolated the reverb half of V4 from the circuit. The noise was still there on both channels.

I thought maybe the noise was coming from somewhere that was common to both inputs so I temporarily replaced the .01 and .001 caps in the PI circuit with no luck.

I've run out of ideas. Could TR3 or the OT make this type of noise? I thought they made more of a humming sound when they were bad.

Any suggestions of what else to try?

Thanks


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Post subject: Re: Twin Reverb Noise
Posted: Thu Sep 04, 2014 6:21 pm
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Maybe the wires from the jack to the input section is picking up RF noise. Try re-routing these wires and see if that helps. You may need to go to shielded wires, if the amp doesn't already have them.


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Post subject: Re: Twin Reverb Noise
Posted: Sat Sep 06, 2014 6:22 pm
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Bob, How old are the filter caps and bias cap? Had the same year amp on the bench last month. It also had a constant static condition. Turned out to be a weak power tube. Did a V6 through V10 retube and bias. Then, the normal maintenance-cleaning(including retensioning the tube sockets)and all was good. The last culprit I'd suspect would be the OT. Unless, it's seen a ton of humidity, mis-use, and no use for a long period of time(many years). Otherwise, those trannies were built like tanks!!! Art

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Post subject: Re: Twin Reverb Noise
Posted: Sun Sep 07, 2014 6:46 am
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Thanks for the input Art & BMW,
I have swapped out the tubes with no luck.
I did a filter & bias cap change recently.

This all started when 3 rectifier diodes shorted. I turned the amp on and it was humming like a power transformer on a utility pole.
Since I fixed that, I replaced the dual bypass cap shared by V4 & V5 which was causing more hum.

Now that all of that is fixed, I'm still stuck with this hissing on both inputs that increases with the volume.

I have some caps on order to replace the other dual Bypass caps and also to replace if necessary, any coupling caps that may be leaky.

I'm also going to try moving some wires around as BMW suggested. thay may have gotten moved when I changed the 2-prong plug to a 3-prong.
Bob


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Post subject: Re: Twin Reverb Noise
Posted: Fri Sep 26, 2014 12:46 pm
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Noise problem solved. :D :D

It turned out to be the .1 coupling caps between the PI and 6L6's. Replaced them with Malory 150's and that did the trick. No more hiss.

I hadn't done this before because the noise got louder as the volume control was increased, and thought this was an indication that the problem was in front of the volume control. Guess I was wrong.

By the way, are the blue molded caps I replaced important to the tone of the amp.

Thanks to all for your suggestions.
Bob


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Post subject: Re: Twin Reverb Noise
Posted: Sat Sep 27, 2014 8:01 pm
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Glad to hear you found the culprits, Bob!!! The blue caps are choice, when they do their job. I use Alessandro PIO in my amps. But there are probably better caps. The 150's should do a fine job. JMHO, Art

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