Fender Dual Showman Reverb amp. Noisey

[Razorc195]

I have a Dual Showman Reverb and 2X15 cab with JBL F130 speakers, it is quiet when turned on, but gets noisy after a few minutes.

I'ts not the guitar (volume off, and guitar independant) and I turn down the volume(s) on the amp, and the noise persists. I does it in both channels no matter which input is used. Goes away with standby..... Its not too loud and you can play over it....

One thing though, even with Reverb and Vebrato Off, you can lightly tap on the top of the amp and get the noise to cease momentarilly.

Also you can hear the tapping of the amp top throu the speakers, almost like, but not quite like the reverby noise associated with banging any reverb around when the amp is moved, like the springs rattle....any ideas...

[vinman753]

hi,
It could be a loose or microphonic tube. Make sure all the tubes are seated into the sockets properly and if that doesn't do it, then tap each tube lightly with a chopstick or the tip of a pencil and see if you here the tap from the tube through the speakers.

[Chuck H]

Just to clarify... The preamp tubes will make a small audible sound when tapped. Your looking for a louder "ping" or even a "thump", often with a little static noise in the background. Excessive microphonics is what your looking for. Most tubes are microphonic to some small degree normally.

Your problem does sound like a bad tube/s. But you may also be in need of a cap job. Old filters will often hum more and more as an amp warms up. And if your amp hasn't had new caps it's overdue by more than a decade.

Chuck

[Razorc195]

Found it....



Hi;

Yep, that was it, preamp tubes in #2 and #4 positions were microphonic, so both channels had it...Thanks Vinman and Chuck.

Also, when I tapped the 6L6 outputs the #7 and #8 positions tubes exhibit the static heard when playing base notes. Not sure why this happens. I'll change the tubes and see if the noise goes away, or not....If not maybe the filter caps as Chuck suggested. I have a line out to the guy I got the amp from to find out when/if its had the capicatiors replaced...

Thanks

Razor

[Albert Kreuzer]

Sometimes a bad power tube will make that kind of noise. Also check the sockets for dirty contacts.

Cheers,
Albert

[Chuck H]

+1

Chuck

[Razorc195]

dirty socketts

Thanks...pulling the tube and cleaning the pins I can do. but other than just pushing the tube in and out a few times, not sure how to clean the sockets....?

[Razorc195]

One other thing. the previous owner has some different tubes in the amp from what is called out in the fender schematic.

They had 12AX7's in place of the 7025's and 6L6EH's in place of the 6L6GC's. Does anyone know what the differences are, if any???

Any one got a good source for 7025's other than Antique Electronics supply, I already have them on file....?

[52 Bill]

They had 12AX7's in place of the 7025's and 6L6EH's in place of the 6L6GC's. Does anyone know what the differences are, if any???

The 7025 is a industrial number for the 12AX7 tube. Back in the day, they were selected as low noise 12AX7s. So using the 12AX7s are fine.

The 6L6EH is an Electro-Harmonix version of the 6L6GC. Traditionally the GC stood for Glass Case. Sovtek differentiates the Sovtek, EH, Mullard, TungSol, etc. tubes with their own letter suffixes. They are probably explained on their website. Again the 6L6EH should be fine in the amp.

[Enzo]

Bill, you just ran into a pet peeve of mine. Aspen Pittman used to trumpet the GC means Glass COntainer story, but that is not the case.

6L6 was a metal tube. They brought out a glass version, the 6L6G. In that case, the G does stand for glass. Then they came out with an improved version, the 6L6GA. Later came the 6L6GB. The final updated tube was the 6L6GC. G means glass, but C doesn't mean container, it means the third upgrade.

I often wondered: if GC meant glass container, then did GB mean glass bulb? And GA meant glass...ah...can;t think of anything.

Most all tubes you see today are 6L6GC. All the other letters are manufacturer IDs, and they usually then omit the GC part. Ruby, bless their hearts still call one of their tubes the 6L6GCSTRM. SO to put maybe too fine a point on it, the EH in 6L6EH is the brand, while 6L6 is the type. (GC understood) 6L6 is what works in your amp. EH or JJ or whatever suffix is a brand preference. Your 6L6EH is a 6L6GC already.

And Bill covered the small tubes. 12AX7, 7025, ECC83 are all the same tube.

[Razorc195]

Latest update on the showman

Thanks to everyone for all the great information. I tried a little trick. Moved the microphonic tube in v4 to v5 (reverb circuit) and replaced the v2 with a ran in 12AX7 that I had in my stash. Problem solved. Reseated the noisy 6L6's and play'ed the hell out of the amp tonight. No more crackel noise. I did order some more tubes just to have around though.

From all this I was thinking of making a mod to the amp to bring the testpoints for the power tubes out the back (lots of room on a 69) and add replace the resistors with pots, bring those to the back also, to allow changing the power tubes (rebias) without having to pull the chasis to rebias...Any thoughts from the community on this approach? Or would this be a waste of time? if these models don't care about the tubes bias (would be strange i admit). I realize that this would make the amp non original. But, more gig friendly.... Maybe the test points alone would give enough advanced warning to change the tubes prior to leaving home...comments please.

One other thing. Has anyone ever plugged one of these into Twin Reverb type 12" speakers. I have a Solid State (100watt)Stage Lead II that I hardly use and thought about making a smaller cab arangement for small venues...would that work, just make a plug/cable to hook up the speakers...???

[52 Bill]

Maybe the test points alone would give enough advanced warning to change the tubes prior to leaving home...comments please.

How often do you really need to check the bias of the amp? While the current draw will change over the life of the tubes, I don't know if that will be a sign of upcoming tube failure.

Enzo:
Welcome back! I know that I've seen them, but I don't usually remember the GA and GB versions of the 6L6, but you are correct as usual.

[Razorc195]

Hi;

Thats right. I guess the only value would be in being able to quickly change a bad tube, and the pots would replace the need to change biasing resistors on the sockets... It was just a thought. I was wondering if the extra hardware and replacing the resistors with pot's would adversely affect the amp? Thoughts?

[darrellcheng]

I have done this using a disconnected Accessory AC outlet in the back of a bassman, where I placed a 1 ohm resistor between the two prongs, and wired the cathode of one of the power tubes to one side of the resistor and the other side to ground. Then just stick your probes in the prongs and you are all set! I leave the bias pot underneath, I can still adjust with a short screwdriver.