Silvertone twin twelve hums on channel two only

[dcline]

Hello,
I'm restoring an old Silvertone twin twelve 1484 head.
I've replaced all the electrolytic caps accept the multi can filter cap, all new tubes, I've replaced the cord with a heavy duty grounded cord, I've replaced the paper .5 uF cap in the tremolo section.
Channel one sound's pretty good with not too much noise.
But Channel two hums pretty bad with no instrument plugged in. Plugging the instrument in reduces some of the hum but something just is not right. I've compared it to my other twin twelve head and it does not have this problem.
It appears that Channel two is the only channel with reverb and tremolo.
The reverb is working but weak. The tremolo works too.
I've tried changing the polarity of the power cord, didn't matter.
I've checked the multi can filter cap with my cap analyzer and it checks out good.
All diodes check out good.
I've checked for open resistors. There aren't any.
I've gone through it over and over looking for a bad or missing ground and can't seem to find any.
Tried disconnecting the reverb tank. Didn't matter.
I'm thinking maybe a bad coupling cap? What would be the best way to check for such a problem? They don't register on my cap analyzer.
I have many hours into trying to find the problem.
I'm up for any suggestions anyone may have.
Thank you in advance

[Garydean]

you may have done all this already....but...

Could be a ground issue at the 2nd input jack. Your volume control should be increasing the hum as well right? Try removing the jack completely and get rid of all the corrosion in that area. Check all soldering points related to that jack. Also move some wires around with a chop stick to see if the wire placement is causing the hum. Sometimes new tubes are faulty. Swap out the preamp tubes with different ones or remove and replace preamp tubes to see if the hum goes away. Corroded or loose pin sockets could be a problem. Wiggle the preamp tubes with a chopstick to see if it changes You should have a ground wire attached to the PT lug, clean that contact as well.

Environment issues could be a problem. Are you playing too close? Turn off all the lights...still an issue? Dirty AC? Try the amp in a different wall socket.

[dcline]

???

Thank you for the reply.
I have tried disconnecting the input jack and it did nothing. The volume control does increase the hum with no instrument plugged in. But only in channel two. You can crank number one and the noise is as expected. Not bad at all for a vintage amp.
I've checked all solder points, and I've tried moving wires around. The hum seems a bit strong for it to be that.
I just tried swapping the tubes around and no difference. All tube pins, jacks and pots have been cleaned very well and are getting good continuity readings.
It's not an environment issue because I plug in my other twin twelve into the same power and speaker and it sounds fine.
I am getting strange ohm reading on the ground of the first jack on channel two when the wire is hooked up. All other grounds referenced to chassis including all other input jacks get a reading of 0 or sometimes .19 ohms which seems normal. But the first jack of channel 2 reads .41 ohms. But when I disconnect it, I still get the hum. Very strange. The second jack of the number two channel gets the normal reading. Both jacks are connected by their own 68k resistor then straight to the grid of the pre-amp tube by a shielded cable and the ground is good on that cable.
This is what makes me think a possible bad coupling cap somewhere? But I hate to start pulling all the ceramic and film caps just to check them on my bench meter for values and make a mess of the amp. I'm looking for another method to trace out the problem.
Thanks very much again,
David

[Garydean]

I'm still suspecting that input jack. That jack needs to have a solid contact to ground. The problem may be before the volume control. Something is wrong prior to the vol control as you said the hum increases as you turn up the vol. does the bass control alter the hum? If so it's between there and the jack.

There should be no DC on the other side of the coupling caps. Check with a DMM. One side of the first 12AX7 could be bad too. I keep saying that because I went through 4 new tubes before I got a quiet one once.

good luck

[52 Bill]

As Gary points out, if the hum goes away when you turn down the volume control, the problem is somewhere before the control.

Does this amp have shorting jacks on the inputs, or like a lot of the old Silvertones does it have open circuit jacks? If they are shorting jacks, be sure that the grounding contacts are making contact.

You mentioned that there is a shielded cable from the jacks to the input grid. Are both ends of the shielding grounded? If they are, you could have a ground loop there. Do you have a grid resistor that references the grid to ground installed?

[dcline]

///????

I suspected the ground on the jack too. I tried grounding it with a jumper and it made no difference. And as I said, disconnecting the jack from the circuit and it made no difference either. About all turning up the bass does is add bottom end to the hum. It does not increase or decrease the volume of the hum at all.
I've ruled out the tubes. Like I said, I have a perfectly working identical head. I've been swapping tubes between heads. It's not the tubes.
I will check for DC on the either end of the caps. Great idea!
My bench meter measures capacitance. You're suppose to use short leads with the cap removed to get an accurate reading. But I did some comparisons between amps with the long leads and the tubes pulled. There are some major differences from the good amp to the bad in a few of the .022 uF caps. Way below tolerance. A couple are barely reading. I bought this problem amp on ebay with no tubes and the power cord cut. I have no history of what was done to the amp. Is it possible that a serge of some sort could have fried a few of the .022 uF film caps?
I ordered some orange drops and I'm gonna try replacing them.
The amp has non-shorting jacks. I've considered replacing with shorting jacks.
The shielded cables are only grounded at one end.
And yes there are resistors going from grid to ground.

I just did that DC check on some of the caps and one of the suspected .022 uF caps is reading voltage on both sides.
I think we may be on to something.

I'm a gearsluts member and not one reply on that forum yet.
I must say, I like this forum and all of your assistance is very appreciated.
Thanks again,
David

[Stingrey]

Hum in channel 2

David,

I'm having the same problem with hum in channel two and cannot figure it out.

Did you ever resolve your hum problem?

[dcline]

not completely fixed

Not completely.
By replacing the shielded cable from the grid to the grid resistors at the jack and moving it around a bit, it seems to be a bit better.
I've been all over it with a scope and the hum seems to be in the area of the first pre-amp tube and the input jacks.
I've replaced the jacks, grid resistors , tubes, tried grounding everything... not to mention that I've also replaced all the caps and out of tolerance resistors.
The amp sounds great but the hum in that channel is still there.
It seems like it's coming from the tube socket but I can see nothing visibly wrong with it. there no arcing or cracks that I can see. It looks like new!
I wonder...
Can a tube socket go bad and still look fine and have continuity?
I've done a lot of amp repair and this is the first one to completely stump me.

[Mars Amp Repair]

From the Schemo,
the fundamental difference between channel 1 & channel 2 seems to be the Reverb loop. I know you said you unplugged the reverb tank & had no difference, but does the hum remain the same when you turn down the reverb level? The reverb level in this amp controls the driver to the reverb, but not the return.

I'm thinking removing the reverb tubes (6CG7 & 12AX7) might be a good way to either isolate or eliminate that ckt from the mix.

If this isolates your hum to the reverb ckt, then you could just remove the 'input' side of the reverb tank & see if the hum is possibly being generated by the tank drive side.

Unplugging the reverb usually creates squealing, oscillation, & more noise since it is such a high gain ckt, so I'm not certain that is really a conclusive test.

glen

[Stingrey]

More on 1484 hum

As far as preamp tube sockets, I did work on a Kalamazoo that had the same kind of very annoying hum...much simpler circuit. Tried everything. Get this, when I sprayed tuner cleaner on the solder side of the wafer tube socket, it got quiet. After it dried out, the hum was back. Ended up changing the preamp tube socket and viola' the problem was gone.

The 1484 has higher quality sockets, but may try that soon.

Also the metal wafer on the can caps can contribute to a ground loop (doesn't seem to help the 1484). I always change them to phenolic, or replace with stand alone caps. Has made significant difference on other amps.

The 1484 sounds like it has an open input cord even when the tip is grounded (like a Fender)...I don't get it yet...will go stare at a schematic later. I have heard this same thing on several 1484s, so it is a generic problem, if not a design problem.

On one of them I actually converted it to a SINGLE POINT GROUND system..still limited improvement.

This is a real puzzler...

[Stingrey]

1484 hum

also, I do hear much of the same sound in both channels, not just ch2

[oc disorder]

Hi I noticed that at the beginning you say replaced all the caps "except the multi can filter cap".... Without removing it from the circuit try soldering a suitable equivalent electrolytic across the "sections" that deal with filteringin the pre-amp (+reverb) and see if that helps...
I know you tested it but maybe its breaking down under load my 2c worth...

[scole]

my 1484 also has hum, when i turn the reverb on it goes away, the amp tech who worked on it says it opens another ground but he didnt know why the hum was there. i hope you can figure this out.

[soundmasterg]

Due to the layout of the 1484 and 1485 with the power transformer right behind the input to channel 2, channel 2 will always have more hum than channel 1, and sometimes it can be quite obnoxious, though if it sounds like you've got a cord plugged in with your finger on the tip of the cord, then that is too loud. They used poor quality input jacks....try some switchcraft that ground with nothing plugged in instead.

I modded my brother's 1484 and completely changed both preamp channels, including the layout, added Fender reverb circuit and tank, built a new head box to house it all, added an EF86, increased gain, etc. I improved grounding as much as I could, twisted wires, etc. I experimented at work with inserting a grounding steel shield in between the inputs of channel 2 and the rest of the circuitry and the hum level did go down noticeably. Elevating the filaments can help sometimes too depending on the amp. Increasing gain makes a noticeable and poor difference in the hum level, though by rerouting some things and shortening grid leads or using shielded cable, I was able to reduce a lot of it. I couldn't use the steel shield as there just wasn't room for it. I did replace the can cap on the three I worked on and used the custom ones from Vibroworld, though I don't think Zack is in business anymore so that option is SOL.

Greg

[Sweetfinger]

Something way back in my memory is bubbling up.. I had a Twin 12 that hummed because the reverb spring had popped off its little posts, I had to open the tank and put it back together.