Silvertone 1484 Restore Buzz and pops

[TurboGuitarMelton]

Hi,
I am restoring an old 1484 twin twelve.


I have replaced all the filter caps, the HV coupling caps (none from the tone stacks), the plate resistors, diodes, power cord.... Tubes are all new production replacements (JJ's)


All voltages check out fine but there are a couple things going on that are stumping me... 


The first is that every 30 seconds or so there is a mild pop that happens. My suspicion is that its the old reverb tank just acting up because its 60 years old but it still happens when the reverb is grounded Any ideas?


The second is the hum level from the circuit. Its more of a buzz and its there when the reverb is switched off. I believe its coming from the reverb recovery on channel 2. When I ground pin 2 of V1 or pin 7 of V2 nothing changes. Grounding Pin 2 of V4 makes the hum stop though! The reverb switch is garbage! It just leaves an antenna for noise! Should I rewire somehow? 


Channel 2 also has a lot of excess noise even when the reverb circuit is grounded. Channel 1 is significantly quieter with its volume maxed. Poor layout?

Edit: just realized there is what looks like a 1M NFB circuit on V1 of channel 1 from plate to grid. That explains the lower volume from that channel. Interesting!

[shooter]

Should I rewire somehow?

you could try a SPDT and connect the "off" position to ground


Identify the BUZZ frequency, that will help.


was the bias cap replaced?


what happens to the pop if you disconnect both reverb and trem, (not just off, remove wires)?

[TurboGuitarMelton]

Identify the BUZZ frequency, that will help.


was the bias cap replaced?


what happens to the pop if you disconnect both reverb and trem, (not just off, remove wires)?

Ill identify the buzz


Yes, bias cap and bias resistor were replaced. I've got a healthy stable -34v on the 6l6 grids also went ahead and replaced the grid 330k resistors


I have tried removing the reverb 12ax7 and the reverb 6fq7 with good results. Most of the noisy hum in the amp goes away.


Ill try disconnecting the reverb and trem tonight

[Latole]

On old amp noise and buzz can come from many places.

I'm sure you know that :

1-Test if heater center tap is working. I, one time see center tap not working. I use 2 X 100 ohs resistors to ground.

2- Is the input jacks's shorting contact to ground are working ?

3- Old tubes' plate resistors may be noisy , put new ones.

4- Schematic show this amp have a 3 prong cable. Right ?

[shooter]

Most of the noisy hum in the amp goes away.

focus on that area then, lead dress, especially if it's an original.
for a test, try separating the reverb return side ground from the send side (if they ground together)
maybe add shielded wire for anything "longish" on the return side

[Latole]

Most of the noisy hum in the amp goes away.

focus on that area then, lead dress, especially if it's an original.
for a test, try separating the reverb return side ground from the send side (if they ground together)
maybe add shielded wire for anything "longish" on the return side

I agree.

[DummyLoad]

Identify the BUZZ frequency, that will help.


was the bias cap replaced?


what happens to the pop if you disconnect both reverb and trem, (not just off, remove wires)?

Ill identify the buzz


Yes, bias cap and bias resistor were replaced. I've got a healthy stable -34v on the 6l6 grids also went ahead and replaced the grid 330k resistors


I have tried removing the reverb 12ax7 and the reverb 6fq7 with good results. Most of the noisy hum in the amp goes away.


Ill try disconnecting the reverb and trem tonight

use your smartphone as a spectrum analyzer.

https://play.google.com/store/apps/details?id=com.raspw.SpectrumAnalyze&hl=en_US

IME the reverb ckts. in that amp are basically garbage. leave the tubes out and/or disconnect. use a pedal, better yet, an external stand-alone reverb unit. that valco reverb tank is joke at best.

--pete

[PRR]

> every 30 seconds or so there is a mild pop that happens. My suspicion is that its the old reverb tank just acting up because its 60 years old

Old knees pop alarmingly. Why would an old tank pop? How would it even know "30 seconds"?

I'd be looking for refrigerator, thermostat, wi-fi jabber, or neighbor's welder.

(I did once work where the wall-power went 113-118-113-118-113-118 all day; oddly this didn't pop my gear.)

[TurboGuitarMelton]

Old knees pop alarmingly. Why would an old tank pop? How would it even know "30 seconds"?

I'd be looking for refrigerator, thermostat, wi-fi jabber, or neighbor's welder.

(I did once work where the wall-power went 113-118-113-118-113-118 all day; oddly this didn't pop my gear.)

  
Great point. Honestly I suspected as much because every time my AC would kick on I would hear it through the amp. I tested the amp in a new location and the pop was gone. This doesn't happen with my other amps. Why this one? It seems very sensitive, does it have something to do with the weird voltage doubling power supply?

IME the reverb ckts. in that amp are basically garbage. leave the tubes out and/or disconnect. use a pedal, better yet, an external stand-alone reverb unit. that valco reverb tank is joke at best.

--pete

Yes the reverb is garbage... When I  take the tubes out the reverb circuit buzz is gone. I think I'm just going to call it with this one and maybe eventually do something different with that rats nest on that side of the amp.
 

On old amp noise and buzz can come from many places.

I'm sure you know that :

1-Test if heater center tap is working. I, one time see center tap not working. I use 2 X 100 ohs resistors to ground.

2- Is the input jacks's shorting contact to ground are working ?

3- Old tubes' plate resistors may be noisy , put new ones.

4- Schematic show this amp have a 3 prong cable. Right ?

1-center tap was good
2-silvertone didnt install shorting jacks... but I shorted the tip and no change
3-I replaced every plate resistor in the amp
4-it didnt have one but I installed one


I honestly think the noisy channel 2 is the long string of wire that travels from one side of the chassis to the other where the reverb switch is AND the fact that the tone stack is right on top of the power transformer. When I short the reverb wire/remove the tubes the amp is much quieter. Most of the critical Ch2 wiring goes right under the filament light, the power transformer, and right near the filter caps... What was silvertone thinking? I guess these were cheap back in 1960 too. I'll have to try a real shielded cable next time I order parts.

[Latole]

2- "  silvertone didnt install shorting jacks... but I shorted the tip and no change "
4-  " 3 prong cable, it didnt have one but I installed one; "
-TurboGuitarM

Schematic you show they have. A Schematic is a amps's update ?
Or wrong schematic ?

Here what it look the right one. : http://schems.com/bmampscom/silvertone/silvertone1484.pdf

Found here: http://www.amprepairparts.com/schematics.htm

[Latole]

Same schematic here;

https://www.schematicsunlimited.com/s/silvertone

[sluckey]

Same schematic here;

https://www.schematicsunlimited.com/s/silvertone

You may consider using Hoffman's tube amp schematic library. Link at the bottom of this page.

[Latole]

Same schematic here;

https://www.schematicsunlimited.com/s/silvertone

You may consider using Hoffman's tube amp schematic library. Link at the bottom of this page.

I alway look there. Silvertone 1884 is not very clean in Hoffman' tube amp library. Somebody use pencil on it .
It same as the first one I post earlier at 2:58.14 AM

[sluckey]

It same as the first one I post earlier at 2:58.14 AM

Yes it is. Hence my post.

[SoundmasterG]

The first is that every 30 seconds or so there is a mild pop that happens. My suspicion is that its the old reverb tank just acting up because its 60 years old but it still happens when the reverb is grounded Any ideas?

The second is the hum level from the circuit. Its more of a buzz and its there when the reverb is switched off. I believe its coming from the reverb recovery on channel 2. When I ground pin 2 of V1 or pin 7 of V2 nothing changes. Grounding Pin 2 of V4 makes the hum stop though! The reverb switch is garbage! It just leaves an antenna for noise! Should I rewire somehow? 

Channel 2 also has a lot of excess noise even when the reverb circuit is grounded. Channel 1 is significantly quieter with its volume maxed. Poor layout?

These amps used 20% tolerance poor quality resistors, and you'll get odd noises that can often be attributed to noisy carbon comps or drifted resistors. I have a 1484 that I've replaced everything except the resistors and it makes a bunch of funny noises. They're harder to change than some amps because Silvertone wrapped the leads completely around and through the terminals.


The layout is extremely poor on these amps and they apparently placed the power transformer in the middle of the chassis for weight reasons. Channel 2 is always going to be noisier as a result, and if you try to increase gain very much the hum level goes up exponentially. As the others have said, the reverb sucks. If it works great...roll with it. If it doesn't don't bother. The stacked doubler circuit won't regulate too well and will cause some sag when the amp is up loud. The small value caps they used in the doubler contribute to the hum and you can reduce some if you upsize those caps. Those are essentially the same as having a 25uF cap in any normal circuit, so you can use 220uF 150V caps in those spots and end up with the equivalent of a 60uF cap which will reduce hum a lot.


If you search out old posts by me and 1484's you can find stuff from at least a decade ago when I modified my brother's 1484. Added a Fender reverb circuit, completely changed the layout, added a lot of gain, etc., and ended up with something pretty cool that took WAY too long to do because of the stupid layout and cramped chassis in those amps. There are some useful tips and suggestions in there if you decide to modify things.

Greg

[scstill]

The layout is extremely poor on these amps and they apparently placed the power transformer in the middle of the chassis for weight reasons. Channel 2 is always going to be noisier as a result, and if you try to increase gain very much the hum level goes up exponentially.

I also have this problem of excess hum on Channel 2. Increases as volume is turned up is gone with volume down. The volume pot also measures at 250k (75% low), so I have ordered replacement pot thinking it might be the culprit. This forum advised that since the hum changes with volume adjustment it should be produced at or before the vol pot.

Also, the noise is gone with guitar plugged in, so I mocked a shunted jack which eliminates the noise when guitar not plugged in. It seems to be a poor grounding problem. Had not considered the closeness of the PT, so if the hum stays after pot replacement it likely is the cause, and I will install shunted jacks in Ch2.

BTW - Ch1 is clean without hum and also measures low on its vol pot.

Update - the new pot is installed. The hum is gone until level 6 then increases as volume is increased. With the old pot it started at level 1, so it is better but still present.

[sluckey]

The volume pot also measures at 250k (75% low), so I have ordered replacement pot thinking it might be the culprit.

I bet when you take that pot out of the circuit it will measure just fine.