Help with Silvertone 1481. Scratchy sound but not the pots!

[Schwebel]

Hello all,

I have a Silvertone 1481 that I have been working on. This is my first tube amp project. I picked it up at a garage sale super cheap with a blown speaker, a rotted output transformer, and no tubes. I put new tubes in it, a bigger Hammond 125 output transformer, new speaker, and a three prong cord. It fired right up but the pots were super scratchy and popped when using them. So I replaced them with high quality pots. Now the amp sounds great but when I move the volume, tone, or the volume on my guitar I get a faint scratchy noise, it doesn't pop anymore, and the scratchy sound isn't near as loud as it used to be but I have no idea what it could be.

Could this be the filter caps? The amp seems to have plenty of volume and is actually really really quiet. Very little hum.

[Enzo]

More likely coupling caps.

[mozz]

As was stated above, the coupling capacitors are most likely bad. When they start leaking, it puts DC voltage on the pots, which they don't like and start to make static.

[Chuck H]

Wait though... The amp doesn't have scratchy pots anymore. The guitar does!

That has to mean there's a ground fault at the input (at least, perhaps the entire chassis and/or 0V) and maybe a bad tube. Or if there's a grid to plate circuit at the input then THAT cap is leaking AND there's a ground fault. But I'm thinking bad tube. I've had bad tubes do some weird things. One of the weirder things was not being able to turn the guitar all the way down and just that one tube caused the problem. No problem with the amp or guitar at all otherwise.

[dkevin]

I had one of the small Silvertone amps that crackled in this same way. I found that the tube socket (preamp) was making intermittent contact with the tube pins. I tried cleaning it multiple times but finally had to replace it and the crackle was no more. Try moving the tube in a gentle circular motion when the amp is on (set at very low volume) and if the amp sputters and pops, you may be looking at a similar problem.

[pdf64]

Try a different 12AX7; some have excessive grid current, which creates Vdc at the grid, leading to scratchiness in any pots that are in the grid circuit.
http://bmamps.com/Schematics/Silvert...rtone_1481.pdf
Alternatively, the 2 12AX7 triode sections are arranged in cascade with no interstage attenuation between them; that can lead to instability / oscillation, from coupling between the 1st stage grid and the 2nd stage plate nodes.
22k grid stoppers mounted at the socket terminals would help to mitigate that.

[Enzo]

He said volume, tone, and volume on guitar. That implies there are noisy controls on both amp and guitar. I still vote couplers. The guitar itself begs for an input cap.

[Chuck H]

You're right. I read it as volume and tone on the guitar. So ALL possibilities still exist. Old amp stuff. Leaky caps and old tubes. Fortunately none of that stuff is TOO damned expensive.

[Enzo]

And generally not a surprise.

[Schwebel]

Update: So I actually had some new orange drop coupling caps hanging around and installed them. It has cured most of the problem. Now it only slightly makes the noise on the amp volume when I go past 75%. Maybe I just have a defective new pot. I might try cleaning it just in case. I think a bad tube or socket might still be present too. Unfortunately I don't have any good spares right now, but I live really close to parts-express so I might drop in sometime this week and get new sockets and a 12ax7 or two. When I moved the tube around a little bit the amp started popping and crackling at idle. Also there are some pretty crudy looking solder joints I might clean up as well.

I think my ultimate goal with this amp is to duplicate it with all new parts while I tweak and learn with the original. I really love the sound of it and would love to use it as my main gigging amp (I am always mic'd and use my amp as a monitor).

[Leo_Gnardo]

When I moved the tube around a little bit the amp started popping and crackling at idle. Also there are some pretty crudy looking solder joints I might clean up as well.

Before you give up on those pre tubes, try cleaning the pins with some #600 super fine sandpaper. Of course I'd be looking for an excuse to visit Parts Excess - see if any of those beautiful models they have in their catalog actually work there.

And of course, freshen up dodgy looking connections. It's not a matter of "how big a solder blob can I make." Removing excess solder & creating a good looking connection, all to the better. Pick up some solder removal braid at PX, and if you don't have some quality 60/40 tin lead solder, get some. You don't need their expensive fancy schmancy silver solder, just some good old Kester.

[Chuck H]

A little Deoxit (no lubricant) on the tube pins scrubbed in a circular motion in the sockets is probably a good idea. Be sure all the solvent has evaporated before firing up the amp.

[dkevin]

As for making a replica of this amp, I made one for my grandson this last Christmas. I bought a ss Radio Shack guitar amp from a local recycler for $5 and gutted it. I built a 1481 in the cabinet (6X4, 6v6 and 12AX7) using an Edcor PT I had and a Magnetic Components Champ OT. It sounded OK with the stock RS speaker but the final touch was a Warehouse G8C. Now that kid can drive his parents crazy and learn to appreciate a good tube overdrive at the same time.

[TimmyP1955]

600 is too course for tube pins. Use Crocus Cloth.

[Leo_Gnardo]

600 is too course for tube pins. Use Crocus Cloth.

Thanks for the suggestion. I remember crocus from hi school metal shop. Well half-remember, had to do some looking-up.

A super fine crocus would be a good solution too. Available in 140 grit up to 2000 & possibly even finer. Not so easy to find in small quantities, goggle sends me right to amazon offering 25 sheets of 800 grit, $24 cheap & free shipping, a lifetime supply. A local machine shop might be cooperative and sell a sheet for a buck or two. Some helpful folks suggest Ace hardware stores carry it but you better ask one of the older guys with grey/white/missing hair, the younger help will send you to the garden center for crocus bulbs.

There's emery cloth too but BUT BUT it's claimed that the grit is conductive, and you don't want to leave conductive dust behind. Crocus grit is supposed to be "rouge" aka iron oxide, presumably not conductive. Still it doesn't hurt to wipe off the glass & pins once you've cleaned the crud off tube pins no matter what you use.

FWIW I'll stick with paper, I can get it in 800, 1000, 1200, 1500, 2000 grit.

There's a method to the madness of using 600. One or two quick swipes, you're down to clean metal. And the surface has been turned into a mini file. As you re insert the tube, it scrapes crud from the socket electrodes, enabling good electrical contact where it's needed. In a perfect world where we all have lots of time and customers have unlimited budgets, sure we'd replace sockets with fresh shiny new ones. In a bid to save time & money, I use this method. Thousands of happy amps & customers, can't all be wrong.