crackling through tube amp

[Pickngrin]

I have an old Fender Musicmaster Bass amp that's been acting funny lately (I don't play it very often). I hear a crackling through the amp, even when there is nothing connected to the input, and there is a lack of treble. However, when I was testing it out the other day, I realized that it is an intermittent problem; it seemed to transition to normal functioning.
What should I be looking for as possible culprits? Bad tube (the three tubes are glowing orange)? Bad capacitors?
Thanks

[Spudman]

Tubes are always an easy first place to start. Try rotating them to other sockets or swap them out for a set that you know is good to see if the problem resolves.

If nothing changes then you'll need to go to step two.

[Pickngrin]

Thanks, Spud. I don't have any extra tubes to try. That said, I know nothing about buying tubes -- the different brands and esoterica makes it overwhelming. Any recommendations for a good site to order from?

[Spudman]

Before you order tubes just try switching which sockets they are in to see if that affects your noise issue. Obviously pre-amp tubes go only in pre-amp sockets and power tubes only in power tube sockets.

[Pickngrin]

This amp has just one 12AT7 preamp tube and two 6AQ5A power amp tubes. I'll try switching the two 6AQ5As.

[kiteman]

This amp has just one 12AT7 preamp tube and two 6AQ5A power amp tubes. I'll try switching the two 6AQ5As.

Try tapping on the tubes with a pencil while it's on. If there's a bad tube it'll show up on tapping.

[ZMAN]

Sometimes just taking them out and reseating them will get rid of any crud that has accumulated on the contacts. Try tapping them with the rubber on the end of a pencil. It it crackles or you hear a tinny sound you should replace them.
Tubes are funny. I had a new amp use two the first year and I have other tube amps for years and no problem.

[tunghaichuan]

I would suspect that the crackling problem is the tubes. It is the easiest thing to check first. Cleaning the tube pins and tube socket contacts is a good idea as well.

If you swap out the tubes and clean the sockets/pins and the problem continues it could be a conductive circuit board. Sometimes the board can retain moisture, especially in humid climates and cause it to go conductive. The only way to cure this is to remove the board from the circuit and bake it in an oven with low heat to evaporate the moisture.

Something else to consider is that the amp uses a transformer for a phase splitter. This function is usually accomplished by using a tube, but in the case of your amp, there is a third transformer in the circuit that actually serves as the phase splitter. It's not likely that this is the problem, but I thought you should be aware of it.

Fender made two versions of this amp: the earlier one had 6AQ5 tubes like yours. A later version had 6V6GTA tubes. The 6AQ5 is a 7-pin mini based version of the 6V6 tube. The 6AQ5 is not made anymore, so you'll have to buy NOS.

The good news is that your amp is cathode bias, so you really don't need to worry about setting bias when you chance out the tubes.

6AQ5s are still relatively inexpensive:

Tube Store (http://search.store.yahoo.net/cgi-bin/nsearch?catalog=thetubestore&query=6005&.autodone=http://thetubestore.com/)

Tube Depot (http://search.store.yahoo.net/cgi-bin/nsearch?catalog=yhst-8476489043850&query=6005&.autodone=http://edit.store.yahoo.com/RT/NEWEDIT.yhst-8476489043850/6a31ae1ded2d/nsearch.html)

They don't even have to be matched.

The 6005 is an American industrial version of the 6AQ5. The Sylvania 6005 sounds pretty good, much better than the GE 6AQ5A.

The preamp tube should be a 12AX7, not a 12AT7. If you want a lower gain preamp tube, go with a 12AY7, 6072, or 5751.

If swapping and cleaning doesn't fix the problem, take it to a good tech. The amp should probably be checked over and have routing maintenance.

Good luck.

[Pickngrin]

Thanks a bunch for the tips. I tapped the preamp tube with a pencil and sure enough, it crackled like crazy. Neither of the other tubes did. I will pick up a new preamp tube...
thanks again
UPDATE: as it turned out, I happened to have an extra tube (the only extra tube I've ever owned in my life)...an ECC8 stamped with a Fender logo. Problem solved..no more crackling!