Problems with a 45w Super

[AprioriMark]

Ok, so.  My Super has 2 6L6/5881 Sovteks and a 5U4 (as well as a few AX7s or whatever).  I recently replaced the 6L6s (which made it work again), but whenever I *peak* the input at a certain level, the volume drops until I whack lightly on the chassis above the power tubes.

Any ideas what this could be?  I'm a newb when it comes to this, but I'm willing to learn.

-Mark

 

[Mor Paul]

No clue. Tubes and solder joints are my first guess.

 

[jimh]

Ok, so.  My Super has 2 6L6/5881 Sovteks and a 5U4 (as well as a few AX7s or whatever).  I recently replaced the 6L6s (which made it work again), but whenever I *peak* the input at a certain level, the volume drops until I whack lightly on the chassis above the power tubes.

Any ideas what this could be?  I'm a newb when it comes to this, but I'm willing to learn.

-Mark

You say the volume drops.....  Does it cut out completely, or just get a bit quieter?
Initially, I'd say bad connections on the tube pins, but it could be a dodgy ground somewhere??
Are you sure it definitely only comes back when you knock it?  ie, even if you left it for 10 minutes, still switched on, but without touching it, the volume would'nt come back.

If this is the case then I'd say you have a mechanical problem somewhere. Loose connection, dry joint or such like.
Are the tubes (all the tubes, not just the 6L6s) properly seated in their sockets?

Have you re-biased the 6L6s since you replaced them?  Strictly speaking you should always do this for any new set of tubes (only applies to power tubes), even if you've replaced them with like for like, and even if you've gone for the same brand.  Could be that you are running the tubes too hot, and there is something moving due to heat??

 

[jackthehack]

You talking about one of these? What vintage? Last service?

 

[Patrick from Davis]

Two things that are great to have are electronics cleaner and acid free flux.  While the following may not be what is wrong with your amp, it is good to know.  Take the electronics cleaner and spray the tube pins, insert and take out.  Wipe them off with a paper towel.  Repeat two more times.  It will clean up the contacts, and help if there is anything to be gained there.  It also works really well on input jacks and knobs/pots that are scratchy but not insanely scratchy.  The Acid free flux is great for when you have a bad solder joint.  Use a toothpick to put a small amount on the solder joint.  Heat it up with the soldering iron and re flow it.  Hard to describe, "reflow," but easy enough to tell when you do it.  Those two things will help out with a lot of problems in amps.  Bad solder joints often appear dull and gray compared to the more silvery of a good joint.  Solder will dull with age, but a bad joint looks different.  Good luck.
Patrick

 

[mayfly]

Another thing that it could be are loose tube sockets.  It's possible to re-tension them.

In any case, it's something mechanical - loose connection somewhere.

 

[-CB-]

Preamp tube is numero uno.  Next thing that crossed my brain - having seen a lot of it - is bad electrolytic capacitor in the power supply, or bias supply.  They'll do mystery stuff when dried out, and whacking things get 'em behaving for a little while.  Third thought was preamp tube and/or socket.  Fourth thought was simple input jack issues - lack of tension or build up of crud on the spring piece.  Next thought - bad joint, not from the factory but from previous repair (or attempt at repair).  After that, cracked screen grid resistor where its barely making contact.  After that, cracked coupling capacitor, where its barely making contact.

Thing to do here, because of the age of the amp, is determine how well its been serviced, and when.  If its needing service, do that first.  Then you can go through and chopstick the parts and see if it cuts out.

I'd sure like to see inside the cap pan on that amp... also like to see the top side of the output sockets and the bias supply.

Part of the servicing should be cleaning and tightening of all the socket pins.  That is a must.  Replace sockets that wont hold tension on the pin.  Clean all the pots.  Clean the jacks, replace any that are "sprung".  After that, its an "as needed" thing.  Gonna give you a heads up though - MANY, nearly all of the amps before 1965ish are having leakage problems in the coupling caps - where they leak DC current.  Ideally, no DC current should pass those caps, but its common to get .001 or .0015 amps.  On old amplifiers, you see even .003 or more and they still work well, but the tone controls get scratchy due to the DC on the pots.  I do a leakage test on these, especially when the pots are scratchy and dont "clean up".  Its almost always the DC leakage.  I've seen a number of amps that have had the controls replaced only to have the scratch return after short use.  Its DC leakage.

The leakdown test requires a live amp, and the non B+ side of the capacitor lifted, and jumpered to ground with a 1meg resistor.  For the test to be valid, there MUST be a resistor.  You cannot do the test unless you lift the cap and series it to ground through the resistor.  You can then read the milliamps directly by inserting the meter in series with the resistor (or doing a few voltage drop tests and subtracting).

Anyway... thats where my brain goes  on this

 

[AprioriMark]

Thanks for all the input, everyone.

You say the volume drops.....  Does it cut out completely, or just get a bit quieter?
Initially, I'd say bad connections on the tube pins, but it could be a dodgy ground somewhere??
Are you sure it definitely only comes back when you knock it?  ie, even if you left it for 10 minutes, still switched on, but without touching it, the volume would'nt come back.

The volume doesn't completely cut out, and if I leave it alone, it will come back.  It definitely cuts out once I hit a certain level of volume in playing.

Preamp tube is numero uno.  Next thing that crossed my brain - having seen a lot of it - is bad electrolytic capacitor in the power supply, or bias supply.  They'll do mystery stuff when dried out, and whacking things get 'em behaving for a little while.

I think I have some preamps tubes in my Bassman 100 that I can steal to test.  Given that the reason that I started replacing tubes in the first place was that one day, I turned the amp on and it started making a messed up cascading humming sound.  I turned it off and let it sit until I felt like checking it out.  I saw that the tube I replaced was burned looking, so I replaced both 6L6s.  I'm not sure exactly what year this one is.  It's 45w, 4x10, silverface with the black back panel.  Someone at one point mod'd it to include a preamp out.  The guy I had service it when I bought it said it was well done.

If it's not the preamp tubes, I think I'm going to buy a guide to tube amps and follow all of you folks' advice about what to look for.  I really need to have a better understanding of this stuff.  Thanks again, all of you.

-Mark

-Mark

 

[-CB-]

pictures would help....

 

[AprioriMark]

What do I take pictures of?  Talk to me like I'm... slow... when it comes to this.  I know what the power tubes, transformer and preamp tubes look like, and that's about it.

-Mark

 

[AprioriMark]

Ok, here are some.  No idea if this helps.

-Mark

 

[jackthehack]

That appears to be a mainly stock early '70's Super Reverb.

If memory serves, there are 4 screws going through the top of that amp holding the chassis in place; the pic CB was talking about involves removing those and pulling the chassis to take a shot of it from the bottom, the opposite side from where the tubes insert.

I see from your profile that you're in the vicinity of Warmoth, if you don't have a lot of experience working on tube amps, I'm sure someone could recommend a shop in your area to get the amp checked out.

Do have any info on the service history on the amp? What exactly happened in sequence to put the amp in a dead state that required replacing the 6L6's?

 

[AprioriMark]

That appears to be a mainly stock early '70's Super Reverb.

If memory serves, there are 4 screws going through the top of that amp holding the chassis in place; the pic CB was talking about involves removing those and pulling the chassis to take a shot of it from the bottom, the opposite side from where the tubes insert.

Thanks, I'll do that tomorrow.  I figured that might be what he meant after I posted those.  I'd have to wake my wife to get the camera, and well... hahaha

I see from your profile that you're in the vicinity of Warmoth, if you don't have a lot of experience working on tube amps, I'm sure someone could recommend a shop in your area to get the amp checked out.

I actually talked to a great fellow that a Warmoth employee suggested, and if I can't figure it out, I'll have him save me.  I just really want to learn a little here.

Do have any info on the service history on the amp? What exactly happened in sequence to put the amp in a dead state that required replacing the 6L6's?

The service history on the amp is that I took it to a guy who everyone said was the man back when I got the amp in 1995.  I haven;t had it serviced since, and I've probably played a few hundred shows on it.  At one point, I switched to using a 4x10 cab that was pretty much 2 2x10s, hooked "stereo" from a JCM800 and my bassman 100.  The amp got light use (church) for the last 5years or so, and one day I powered it up and it made the aforementioned cascading loud buzz.  That's what led me to replacing the tubes.

-Mark

 

[jimh]

If you're getting a loud buzz, then I'd say you need to check out the power supply and filter caps.  They are the ones underneath that long low can by the preamp tubes and choke.

Once again, pics will help.

 

[Return of Guitlouie]

And be careful inside that amp.  There's dangerous stuff in there if'n you don't know too much about what you are looking at.

 

[Superlizard]

And be careful inside that amp.  There's dangerous stuff in there...

*nods*

...like spiders.

 

[jimh]

And be careful inside that amp.  There's dangerous stuff in there...

*nods*

...like spiders.

Nah..... You only get spiders in the Australian and Brazilian Supers.  :icon_tongue:

 

[-CB-]

What do I take pictures of?  Talk to me like I'm... slow... when it comes to this.  I know what the power tubes, transformer and preamp tubes look like, and that's about it.

-Mark

I want to see inside the cap pan on the bottom of the chassis.  I want to see the bias supply.  I want to see the output tube sockets from the component side, I want to see the preamp section too now that its a silverface.

 

[-CB-]

Mid 70's from those CTS ... ahem... alnico?? speakers.  Dated Nov 1974.  If the speakers are original, it "may" have hit the store by Christmas 74, but more likely sometime in mid to late January - given the CTS to Fender trip, assembly, warehousing, order fulfillment and shipping to the dealer.

 

[AprioriMark]

Hope these help, =CB=.  Thanks a ton!

link to full size image: http://img145.imageshack.us/img145/386/markampinsides007.jpg

link to full size image: http://img109.imageshack.us/img109/302/markampinsides008.jpg

link to full size image: http://img121.imageshack.us/img121/8670/markampinsides009.jpg

link to full size image: http://img121.imageshack.us/img121/3779/markampinsides010.jpg

-Mark