Problems with a 45w Super

[-CB-]

That looks about dead stock original.  The bias supply capacitors are iffy as delivered - being rated to barely cover the voltage, but thats not your problem.

The screen grid resistors are looking uncracked, unburnt... you can never tell on those unless you chopstick them, but they appear to be candidates for being "ok".

Carbon composition resistors are noisy as hell... with thermal noise and with moisture noise.  The crackles you get from some amps are totally due to the carbon comp. resistors being ok, moisture in the carbon.

The big blue tone and coupling caps are the ones I see more problems with than any other.  DC leakage is the deal.  If  the controls are not scratchy, you're "probably" ok.  Or if they're scratchy but clean up ok, and stay cleaned up, ditto, probably ok.  One thing you can see with coupling caps though... is when they leak current in the preamp, from stage to stage, it throws the bias of the next stage all to hell.  The bias on the preamp stages is a very low voltage, cathode biased, class A operation.  It takes just over a few milliamps to throw the preamp out.  Sometimes this can "come and go" with volume shifts.  When the voltage on the preamp grid goes up, its relationship to its cathode changes.  Ordinarily the preamp grid is a few volts less than the cathode, but raising it up will make the preamp behave very badly, sometimes resulting in a runaway tube (glows cherry or orange on the plates - not the heaters).

Your power cord was installed by a hack.

Now we need to see inside the cap pan - the pan with four screws on the bottom of the chassis.  This is what always separates the men from the boys.

Thanks for the pix... so far so good.

 

[Return of Guitlouie]

I almost always feel dumber after having read a post by =CB=. 

 

[-CB-]

I almost always feel dumber after having read a post by =CB=. 

I'm open to answering any question you might have brother.... 

 

[Mor Paul]

Trust me, Louie. I've asked plenty of questions.  :laughing7:

 

[AprioriMark]

I take it I shouldn't lick these?

full size image: http://img4.imageshack.us/img4/8854/markampinsidespart2001.jpg

full size image: http://img137.imageshack.us/img137/1572/markampinsidespart2002.jpg

full size image: http://img94.imageshack.us/img94/8287/markampinsidespart2004.jpg

-Mark

 

[-CB-]

Ok, its beyond time for a recap, and your tech was amiss for not doing it.

Those paper covered (metal on the inside) caps needed to go into the trash probably 10 or more years ago.  You can see the bulge and leakage in the pictures.  Someone replaced ONE cap.  Just replace them all - use nothing else but SPRAGUE TVA ATOM capacitors.  That is what is called for, nothing else or I'll personally wear out a pair of boots on somebody's backside. :toothy10:

While you're at it, replace the two caps on the bias supply with 50uf @100v TVA Atom caps.  You'll have a perfect pi filter then, and have damn near zero bias induced hum.

While you're at it, convert the bias balance control to a bias level control, so that it actually does something.  You use matched tubes at that point.

BTW, some tubes "look burnt" by design, whereas they have a coating on the envelope (glass) that acts as a reflector inside the tube.

Clean the sockets and tighten them.  Clean the pots.  Call it a day and give it some good tubes and you'll have a decent amp that will last for at least another 50 years at that point - except it'll need a recap again in 20 years or so.

 

[-CB-]

I should also say, this is something you can do yourself if you have any soldering skills at all and are willing to follow directions....

 

[AprioriMark]

Ok, its beyond time for a recap, and your tech was amiss for not doing it.

Those paper covered (metal on the inside) caps needed to go into the trash probably 10 or more years ago.  You can see the bulge and leakage in the pictures.  Someone replaced ONE cap.  Just replace them all - use nothing else but SPRAGUE TVA ATOM capacitors.  That is what is called for, nothing else or I'll personally wear out a pair of boots on somebody's backside.

While you're at it, replace the two caps on the bias supply with 50uf @100v TVA Atom caps.  You'll have a perfect pi filter then, and have damn near zero bias induced hum.

While you're at it, convert the bias balance control to a bias level control, so that it actually does something.  You use matched tubes at that point.

BTW, some tubes "look burnt" by design, whereas they have a coating on the envelope (glass) that acts as a reflector inside the tube.

Clean the sockets and tighten them.  Clean the pots.  Call it a day and give it some good tubes and you'll have a decent amp that will last for at least another 50 years at that point - except it'll need a recap again in 20 years or so.

I do get that some tubes have a dark metal coating on the inside.  I promise that the gnarly darkness on the inside of the tube that I replaced wasn't intentional.

Where should I go about looking for the "SPRAGUE TVA ATOM capacitors" and "50uf @100v TVA Atom caps?"  I could google and go for it, but is there somewhere specific I should bookmark?  I'm going to tackle my Bassman 100 after this... I seriously wish I could apprentice somewhere or something like I did for sound engineering back in the day.  Also, what should I be looking for in the paper caps as far as rating?  Is it really common, or do I just read what's on those?

I should also say, this is something you can do yourself if you have any soldering skills at all and are willing to follow directions....

I'm game.  My soldering skills aren't amazing, but I will follow directions like a mofo.

I really appreciate your patience and willingness to be so darned awesome.

-Mark

 

[-CB-]

Sprague TVA ATOM caps are available at www.mouser.com    You want 20uf@500v, 50uf@100v, 70uf@350v

Then you should get a layout for an AB763 Super Reverb, and copy the bias supply.  You'll need a few resistors. 220k, 470, 27k should do it.  You'll need 2 watt carbon film resistors.  PM me... I've got bins and bins of em (think freebies for a SASE).

You'll move a few wires to make the older (better) bias supply.  You'll keep the dual filtration of the newer bias supply. 

When changing the caps, cut the lead close to the cap, then tug on the lead and try and remove it from the board.  A lot of those had extra long leads which were used to thread to the next eyelet.  What you do, is just snip it close at the eylet, suck out the eylet, and insert the new caps lead into the eyelet along with the remnant of the old lead (now serving as an "inter-eyelet" jumper).

MARK all the capacitors for PLUS AND MINUS with a marker pen on the chassis before you take the old ones out.

Observe polarity when you put the new ones in.

When you're done with the service, you'll have a stronger sounding, more punchy attack amp.

You can make up a safe shorting lead, - two INSULATED alligator clips, 1m resistor, and wire.  Clip----wire----1m res----wire----clip.

Place one end on ground FIRST, then clip onto the V shaped 100 k resistor set over on the input side of the amp.  You may get a little spark  when you do this - dont touch the metal lead.  INSULATE the 1m resistor inside some shrink tubing too.  Leave it there, and check for voltage between there and gound.  In a few seconds it should be nil....

THEN make up a 1meg resistor on a short wire and solder it to the place that V forms (there's a wire there too) and solder the free end to ground.  That is a permanent "pull down" resistor.  The 1meg to ground presents a very low drain on the circuit, does not effect it in any discernable way.  What that does is makes the amp safe when you a.) have the power off and b.) have the amp in the "play" mode - not on standby

Once you get it drained, you can work on it just fine, touching anything.  Be sure to CHECK with a voltmeter first, before touching anything the first time - in case the pull down resistor didn't work, or fried open.  Once checked to be safe, you're good to go for the duration of the repair.

 

[AprioriMark]

First of all, thank you a ton.  I can absolutely follow these instructions.  I do have one question:

INSULATE the 1m resistor inside some shrink tubing too.  

I'm assuming that I should do this before clipping the shorting lead to the cap?

-Mark

 

[-CB-]

Yup.