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Post subject: Help please with a 65 Twin Re-issue making some odd noise.
Posted: Thu Apr 04, 2013 8:20 pm
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Just purchased off Craigslist, 65 Twin re-issue. I knew the Vibrato did not work. Took it home and replaced a blown-white top V5 and Vibrato is all good. After playing a few times now under real test conditions the amp has a buzz on low notes and it trails with a not always gradual filtering-his-fizz sound along with the great sounding clean tone. The amp came with a full set of JJ tubes that supposedly only have 15 hours on them. Maybe the seller meant 1500 ha! Who knows if it was biased. Maybe the seller bought a retube kit and never biased it and broke V5 when swapping tubes. All I do know is that its not usable as it is. I also plugged into another cab and set the load at 8 ohms and tried pulling the outer then the inner power tubes to see if one was bad. Still broke. Looked inside, nothing burnt or broken to the eye. Very clean amp and very clean inside. Thus, a shinny turd, ha! Hate to buy a bias probe when its probably something else but may have to. I do not like shops and do not trust them with my gear and cash but may have to. I have done basic stuff with amps...20 tube changes, replace blown screen grid resisters, resolder joints, basic mods to 5 watter amps. Never done a cap job but this one has the older brown-yellow speakers so maybe it needs one. I know amps can kill, but basically a DIY looking for advice if I should start replacing parts or take it in? I was really looking forward to this amp and I cannot push it onto someone else like it is, that is no cool! Any help or guidance appreciated


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Post subject: Re: Help please with a 65 Twin Re-issue making some odd nois
Posted: Thu Apr 04, 2013 10:31 pm
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Sounds like one or both of the speakers is frazzled to me.

Try connecting the amp to a 4-ohm load (with all power tubes installed) and see if the symptom persists.

HTH

Arjay

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Post subject: Re: Help please with a 65 Twin Re-issue making some odd nois
Posted: Fri Apr 05, 2013 6:10 am
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It does it when plugged onto the amps speakers and also when I plug the amp into a Marshall 4-12 cabinet set at 4 ohms. I was thinking a speaker issue too, but it does it with an extension cab in place unfortunately.


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Post subject: Re: Help please with a 65 Twin Re-issue making some odd nois
Posted: Fri Apr 05, 2013 3:51 pm
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It might just be tube rattle. Easy enough to use a known-good 12AX7 preamp tube and start swapping it into each preamp position to see, V1 is the most likely, and use a 12AT7 for the PI. Not likely the reverb driver (12AT7), but turning the reverb down can help determine whether it is or not. Bias probes are relatively inexpensive, and a wise investment for any tube amp owner. Even output tubes can develop rattle, and if changing output tubes, it is important to check the bias.

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Post subject: Re: Help please with a 65 Twin Re-issue making some odd nois
Posted: Fri Apr 05, 2013 10:19 pm
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I took 2 knew good preamp tubes alreadyand swapped each tube out. No difference. It has a little tube rattle but thats not a surprise in a combo amp with 12 inch speakers. I believe the sound its making it not tube rattle, its something different. I went ahead an ordered a Bias Probe. Should be hear early next week, Would it possibly help if i used electronics cleaner and went thru the amp? Is there a particular brand of cleaner to use, or should I just get whatever Radio Shack has?


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Post subject: Re: Help please with a 65 Twin Re-issue making some odd nois
Posted: Fri Apr 05, 2013 10:26 pm
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Get some deoxit, I think most of us use it.

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Post subject: Re: Help please with a 65 Twin Re-issue making some odd nois
Posted: Sat Apr 06, 2013 2:40 am
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" a buzz on low notes and it trails with a not always gradual filtering-his-fizz sound "

Ok, after ruling-out speakers and tubes --- this does sound like filter cap issue. Have you checked under the "doghouse?"


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Post subject: Re: Help please with a 65 Twin Re-issue making some odd nois
Posted: Sat Apr 06, 2013 11:01 pm
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By the Doghouse I am guessing that is the metal cover on the chassis that removes to show the caps? I removed it and it looks good to my eye. Nothing looked burnt. I did not touch the caps as my limited knowledge is that they can still hold a charge and have to be drained to be safe. Not sure if they burn like a resistor when bad or if they just need to be tested. The amp was made in February of 1999. So, 14 years old. It was played but overall above average clean inside and out.

I have preamp tubes from another amp. I took tubes and twice swapped out the tube in each section, no fix. I ordered a bias probe to check each power tube and should be here early next week. My next thought was even though my knowledge is limited I thought it could be caps dried up. Not sure if they look bad when gone bad or dried up or if the only way to know is to test or replace them and see?

This evening I took the chassis out and removed the tubes and knobs and used electronics cleaner over the entire amps parts and circuitry. I took the board out with the knobs and cleaned it all and looked at the whole amp. The normal channel treble knob was scratchy, so I cleaned it good to see if I can avoid replacing it. No burnt pieces noted on the underside of that board when I took it out. The only thing that was maybe not correct to my eye was on the main board there were two 1/4 inch plugs on the main board that were not covered with plastic and they were touching metal to metal, by one it said STBY and PG17 I think, STBY means standby I am guessing. I moved them apart so they do not touch anymore. Cannot fire it up as everyone is sleeping. Will post after I do to see if the buzz-noise is still there and will post again after the bias probe gets here. Thanks!


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Post subject: Re: Help please with a 65 Twin Re-issue making some odd nois
Posted: Sun Apr 07, 2013 1:15 am
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Sometimes filter caps can look ok, and not filter well. If the amp is 14-15 years old, you may need new caps. Depends on usage and brand of cap.

Not sure what you mean by PG17. You have any photos? Or can you place it using the service manual's layout?

Thanks! Good luck with the amp.


http://www.webphix.com/schematic%20heav ... manual.pdf


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Post subject: Re: Help please with a 65 Twin Re-issue making some odd nois
Posted: Sun Apr 07, 2013 9:08 pm
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I could not tell by the schematic and its back in the cab so no picture for now. If the sound comes back I will post some pictures.

I tried the amp today after the internal cleaning and other stuff I did yesterday. I let it warm up good by sitting in standby for an hour then played it for 30 minutes. It sounded perfectly fine. No buzz no odd filtering noises. Now before it would start up after about 10-15 minutes sometimes. Not out of the woods yet as it will need more than just one play for a half hour to convince me that its good now, but so far so good. I will post back if it goes back to the buzzing etc. Thanks!


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Post subject: Re: Help please with a 65 Twin Re-issue making some odd nois
Posted: Mon Apr 08, 2013 9:42 pm
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Played it twice and so far no more bad sounds. Who knows maybe it went away, time will tell. The bias probe came in the mail today. I checked the bias of each tube. Pretty well matched set as they were all within 1- 1.5 mA. Unfortunately they came in set at 18-19.5 mA per tube ha! A little cold I think. My guess is the seller of the amp bought a tube set and replaced the tubes without biasing the amp or the store who he said he paid the tube swap for did the same. I reset the bias at about 40 mA per tube and all sounds well so far


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Post subject: Re: Help please with a 65 Twin Re-issue making some odd nois
Posted: Mon Apr 08, 2013 9:44 pm
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Did you measure the plate voltage before resetting the idle bias?

Arjay

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Post subject: Re: Help please with a 65 Twin Re-issue making some odd nois
Posted: Mon Apr 08, 2013 10:00 pm
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Retroverbial wrote:
Did you measure the plate voltage before resetting the idle bias?

Arjay


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Post subject: Re: Help please with a 65 Twin Re-issue making some odd nois
Posted: Tue Apr 09, 2013 7:44 pm
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No. I did not check the plate voltage at all. I usually follow the fact that these modern production amps run about the same plate voltage and are very consistent from amp to amp. I usually use the basic info from Eurotubes and where they find the plate voltage range to be for amps they have tested. In the past I questioned if they were correct and used to check the plate voltage on all the amps before I retubed them and it was always within 10 volts of the average reading of what they said it would be. To me that makes a very small change in mA when adjusting rhe bias. To my ear I would not be able to tell and it will be out of crossover distortion and right in the range of where the tubes should be biased at give or take a milliamp or 2. I already had the chassis in the cab and did not want to take it out again just to measure plate voltage. I may check it if I need to take the chassis out again. Do any of you who have reissue Twins find the plate voltage to vary so much between amps that it really would make a big difference in where to set the bias at? Thanks!


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Post subject: Re: Help please with a 65 Twin Re-issue making some odd nois
Posted: Tue Apr 09, 2013 9:44 pm
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I've noted plate-voltage variances of between 10 to 14 VDC against an "official" schematic spec of 442 VDC for the handful of reissues I've serviced over the years. My own re-issue TR (1999 model) measures 448 VDC, which is twelve volts shy of the 460 VDC specified for the original hand-wired AB763 circuit. It's risky IMO to set the idle bias without knowing that data, especially when installing expensive glass like the NOS blackplate RCA's I have in mine.

Arjay

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