Tube amp mishap: hissing, sudden volume bursts

[roosterfrenzy]

Hey how's it going,

 

Been gigging with a Peavey 6505+ tube amp for about 5 years now and over the last year it's been giving me grief. Thought I should drop a line here before taking it to a shop in case there's a simple remedy or at least an easily identifiable problem.

 

Basically, it's been giving me two problems:

1. A LOT of hissing when turned up, which I'm told is normal but seems excessive in comparison with other amps, and

2. It will change sound and noticeably increase in volume occasionally. The trebles seem to die down and get muffled when this happens. This doesn't happen every time.

 

Let me know if you think you know what the issue may be. It might be worth noting that in the 5 years I've been using it, I haven't replaced any tubes and my technical know-how is pretty limited.

 

In any case, thanks in advance!

[WRGKMC]

Volume jumps can be caused by a number of things. You need to isolate the cause. First things I'd do is try a different speaker cable. Never ever use a guitar cable for a speaker cable. It will fry it from the inside out and become intermittent just like you're having a problem with before it shorts.

Once fried, the cable is toast. you can save the ends but the wire itself needs to be thrown away.

 

Second would be to clean the tube sockets with Non lubricating contact cleaner which contains denatured alcohol. You want to apply it to the tube pins (not the socket itself) when the tube is cold and stick it in the socket when the pins are still wet. Don't just squirt the socket. You'll flood the circuit board if the sockets are PC mount and the long term effect is, the protective film applied to boards is stripped away leaving the solder joings exposed. Alcohol accelerates oxidation and a white film which is conductive can appear between traces and short connections.

 

Next you want to check input and output jacks for cracked solder connections. You can clean them using the same method as above, apply the cleaner to the plug then stick the plug into the jack several times. Don't flood the jack, especially if they are plastic PC mount jacks. If there is a bad solder joint and you flood the jack you clean that bad connection and it may wind up working for a short time. In reality you're only masking the cause and it comes back as soon as the solder oxidizes.

 

If the problem is worn pots its pretty common for them to become crackly and intermittent. You may set the volume to a sensitive spot and then heat or vibration makes it move to a dead spot. You can turn the pot while playing and see if there's any scratchiness. If there is you use a lubricating cleaner designed for pots and give them a squirt form the inside then work them. You should only clean them if thay are failing and only clean the pots that need it. You decrease their lifespan by cleaning.

 

Also tube socket should only have none lubricating cleaner. Tubes get hot and the mineral oil and silicone lubricant of the lubricating pot cleaner cooks and creates a film making the connection worse. Heat also draws in dust through convection air currents which cool the chassis and the sticky baked on lubricant becomes a magnet for dust and dirt making for mad contacts.

 

Likewise, you should never use non lubricating cleaners on pots. If there is no lubricant, the rotor contact will strip through the thin carbon surface in no time and then you'll need to replace them.

 

Lastly it may be time for tube replacement. You may only be having one tube fail and you could swap one till you find the culprit but chances are, the others are likely to follow. You could do just the preamp tubes. They don't require any biasing and they usually cost around $10 each.

 

Power tubes can also cause the exact same problem. I recently changed the tubes in my Fender Bassman and had this issue. It had been 10 years on the Groove tubes in there and I needed to change the preamp tubes because one was going bad. The tubes were still good but you want to change them all when symptoms begin.

 

I first bought a set of Ruby tubes on sale and that started popping and cracking within a minute or two after applying the B+. Powered it down immediately and sent those suckers back. The elements were likely beaten up from being shipped such long distances. They can ship them back to china as far as I'm concerned.

 

Next I tried a set of JJ power tubes. This set didn't create loud pops but you could hear an intermittent crackle where the his level would come up then drop. I again pulled them out.

 

Third set I bought Electro Harmonix tubes because I had good luck with them in my Music Man Head. That wound up being the brass ring. Loud, Bright and no crackle or hiss. The EH tubes are a special build from Russia, either Sovtek or a Yugoslavia build. They are the best tubes I've found so far. I did a comparison of 12 different preamp tubes and the EH blow all the others away for transparency and loudness. Haven't come across a bad one yet. They may not last as long as some other tubes but even when they do get old they sound as good as other tubes do when they are new.

 

If your amp requires the power tubes to be biased you need to allow a burn in period because the Getter inside the tubes is fresh and may still be dispersing and values can change. Usually a 24 hour period gets them to settle down and then the bias settings will remain stable.

 

There can be other things too. I've had many screen resistors go bad because I was running speaker cab impedances that were too high. Many times a solder joint will crack on them because of the heat on the tube sockets. Reflowing the solder is all that's needed.

 

Caps can go bad too. Power supply caps usually cause hum or take out fuses, sometimes other noises. Coupling caps which join amp stages can go bad and restrict the signal. I was having issues with that same bass amp I changed tubes on. The bass channel was intermittent and would jump from a muted to a bright tone. Thought it was the tube but it was still there, I eventually changed a cap in the tone stack and all of a sudden it came back to being fully functional. Hadn't heard the amp sound that good in 20 years (not that I use the bass channel much)

 

Not sure if these are your problems but those are the normal troubleshooting process. If you don't own test tools and know how to use them these steps cover most of the bases a novice can try. Beyond this you'd need an electronic tech to circuit trace the problem to find the exact fault.

[onelife]

The very thing that makes tube amps great is also their weakness.

 

If the noise is affected by the volume control(s) then I would suspect your problem is with one or more of your preamp tubes.

 

I believe the 6505 uses 12AX7 preamp tubes so I would suggest you get a couple of known good ones and sytematically swap them with the ones in your amp. This should help you find the source of the noise.