Hot Rod Deville noise

[dpm309]

I have a Hot Rod Deville on my bench that was making a horrendous squealing/motor boarding noise after it was on for a few minutes.  Did not matter if the volume was all the way down or the guitar plugged in.   Ran the amp for a few minutes and the volume dropped considerably and then the noise started.  When I jumpered C36, the volume came back up but the noise continued.   Attached is a picture of what I think is the problem, C36 (22/500V).  You can see some white powdery substance on the + side (I scraped some of it away with a chopstick).  You can also see the jumper cap attached to the - side of the cap that I jumpered to see if this stopped the noise.  The date on the board is 1996 so this amp is at least 15 years old and maybe time for a re-cap.  A couple of the other caps are showing a little bit of the white powder also.  I will go ahead and replace all of the filter caps to see if this does the trick.  I have seen several  posts on this forum regarding this amp and it seems to have a lot of problems with the PCB board and cold solder joints.  It appears that everything else is working fine except for the noise problem after it has warmed up for a few minutes.  Is there anything else I should be looking at?  I haven't pulled the board yet but a visual inspection of the top side does not show any scorch marks.

Thanks

Dan

[plexi50]

I have see the white powder residue before on these caps but that doesnt mean they are bad unless the indicator hole is showing signs of protruding or open. The ribbon cables can cause osscilations according to how they are bent and curved some times. But most of all the solder joints all need to be looked at real good. Take a pic of every suspect looking solder points and open the pic on a computer. You would be surprised what you cant see with the naked eye. This has helped me to quickly find board problems i would never have been able to see with just my eyes. Tube pins of the heaters on the power tubes are a problem at times with the pin sticking up between the solder and board intermitantly making connection.  Amp distorts badly because one of the tubes drop out when filiment looses contact with the heater voltage. Solder all input jacks and look at them good. They may look soldered but they may be loosing contact as well. Also check and resolder all power supply caps. I have seen caps just laying on the board loose and not soldered at all arching over to the board. Thats most always indicated by a thunder and lightning sound if you smack the amp on the top of the cab. If that ever happens shut off amp,take board out and start soldering all the PS caps. There are many things in this amp that can go wrong so check everything from the tube pin soldering to your ground next to the input jacks,etc,etc

One last thing. There are many mods out there for this amp and some of them are overkill with owners trying to get that REAL FENDER CLEAN.  

I happened to get one in this afternoon. The treble peak cap was a 150pf. The mid cap was 153k numbered.  It also had 120pf ceramic caps on the Volume and Drive pots. The result was an amp with so much treble that the owner thought he had microphonic tubes. It was clean alright. So clean it had no definition left. No fullness,no depth. Hope this helps

[stratele52]

Is not a tube making this noise ?

[dpm309]

I don't think it is the tubes.  They all tested out good and when I tap on them, it does not make the noise go away.  As I mentioned earlier, I will examine the PCB when I pull it to change out the filter caps (should be getting those tomorrow).  I have a great magnifying lamp on my bench that allows me to see everything up close on the PCB.  A good magnifying lamp is a must for any amp tech.  I will pay close attention to all of the soldered connections, especially the tubes, ribbon cable connections, and power supply caps.  When I chopsticked the amp, I did not notice any change from any of the components.  Will post the results after I re-cap and examine the PCB.

Dan

[plexi50]

I don't think it is the tubes.  They all tested out good and when I tap on them, it does not make the noise go away.  As I mentioned earlier, I will examine the PCB when I pull it to change out the filter caps (should be getting those tomorrow).  I have a great magnifying lamp on my bench that allows me to see everything up close on the PCB.  A good magnifying lamp is a must for any amp tech.  I will pay close attention to all of the soldered connections, especially the tubes, ribbon cable connections, and power supply caps.  When I chopsticked the amp, I did not notice any change from any of the components.  Will post the results after I re-cap and examine the PCB.

Dan

Check out my latest thread for what i think might just be your problem /

[dpm309]

Well, the amp is still squealing etc after new filter caps.  It ran quiet for a couple of minutes and then started making the squealing noise.  Turned if off and back on and when I tapped on R78 and 79, it started up again and I also notice that one of the power tubes was red-plating, so I shut it down.  I examined all of the ribbon connections for loose solder joints and connectivity as suggested by Plexi and everything checked out.  I will pull the power amp PCB to see if everything looks good there.  Should I be looking at the bias circuit also?  What would cause only one of the power tubes to redplate?

Thanks,

Dan

[plexi50]

Swap the power tubes around and see if the same socket is redplating. If the same socket is redplating with the other tube then you should check the coupling caps and resistors. Replace them if needed. Also retention the power tube sockets and make sure that they are making a tight tube pin connection. A bad or shorted power tube will redplate sometimes. The swapping of your power tubes will help to narrow down the problem

[dpm309]

Plexi, thanks for the tip.  Swapped the power tubes and I am getting redplating on the same tube after the amp ran for about 5 minutes so that should rule out a problem with the socket and associated components.  Initially these tubes checked out fine with my tube tester (B & K 747).  When I let them both warm up for at least 5 minutes, the bad tube reading was in the "?" area.  Also, I put in a good pair of 6l6s to further narrow down the problem and the amp has been running for about 10 minutes with no problem.  Looks like a new set of power tubes are in order.  The original tubes are Sovtek 5881 WXT and I can obtain these locally from Antique Electronics.

Dan

[plexi50]

Next time you test a power tube in your tester flick your finger against the tube as you are in test mode and see if the meter on your tester fluxuates. Shorts dont always show up on my tester so i use this method with power tubes. Basically the tube is coming apart inside or loose weldled points

[dpm309]

Plexi, tried you suggestion and the bad tube definitely showed problems when I tapped it.  The other tube is still OK.  The owner did not want to wait for the Sovteks so I went and purchased a set of JJs and installed and biased them.  The owner very is happy with the way the amp now sounds with new caps and power tubes.  Don't have any other amps on my bench right now so I can get back to finishing my Dual Single Ended amp.  This single ended amp can use a variety of power tubes and can switch between classic Fender tweed sound and an edgier Marshall type sound.  Will post pictures and sound bites when
completed. 

Dan

[plexi50]

Glad you found the problem. Generally tubes are the first thing i check. But latley i have had a case of DA