Silvertone 1335 Constant Noise Hiss and Crackling

[Pinball]

I have a Silvertone 1335 that I cannot eliminate the hiss and crackling noise from. I have changed tubes, cleaned tube sockets, checked grounds and yet I still have this noise.

I first thought it was in the top portion of the amp which contains the first gain stage and volume and tone controls. I pulled that shielded cable from the main chassis and the amp went quiet (made sure to keep a good ground). I connected the oscilloscope lead to that signal path on the main chassis and the exact same noise comes back through the speaker. This was without the inputs and first gain stage attached. That is passed to the main chassis through a shielded rca cable. The B+ and heater to the 12ax7 tube is passed through another cable.


Can leaking DC voltage on coupling caps cause hiss and crackling? I have a little leakage on the preamp stage prior to the power stage. All solder joints look good. All resistors measure fine. I’m at a loss.

[HotBluePlates]

I have a Silvertone 1335 that I cannot eliminate the hiss and crackling noise from. ...

I first thought it was in the top portion of the amp ... I pulled that shielded cable from the main chassis and the amp went quiet (made sure to keep a good ground). ...

You cannot test this amp in that manner.  The heater of the 12AX7 in the preamp chassis is the "cathode resistor" of the output tubes.  When the cable is disconnected, the 6L6s cannot pass current, which is why the noise stopped (because all sound would stop).

You'll need to probe tube grids with your scope, beginning at the input side of the 12AX7 to get an idea of where the noise originates.

[Latole]

Hiss may come from tubes's plates resistors ; values are good , replace them all.
......and from bad filter caps.

Yes bad filter caps can hiss . I fix a amp with this issue.

[Pinball]

I have a Silvertone 1335 that I cannot eliminate the hiss and crackling noise from. ...

I first thought it was in the top portion of the amp ... I pulled that shielded cable from the main chassis and the amp went quiet (made sure to keep a good ground). ...

You cannot test this amp in that manner.  The heater of the 12AX7 in the preamp chassis is the "cathode resistor" of the output tubes.  When the cable is disconnected, the 6L6s cannot pass current, which is why the noise stopped (because all sound would stop).

You'll need to probe tube grids with your scope, beginning at the input side of the 12AX7 to get an idea of where the noise originates.

This amp has two cables. The cable you reference was not unplugged. Only the shielded cable was pulled. I attached a good gnd cable so all I did was pulled the output signal from the 12ax7 in the upper chassis.

[Pinball]

Hiss may come from tubes's plates resistors ; values are good , replace them all.
......and from bad filter caps.

Yes bad filter caps can hiss . I fix a amp with this issue.

All filter caps have been replaced.

[HotBluePlates]

You cannot test this amp in that manner.  The heater of the 12AX7 in the preamp chassis is the "cathode resistor" of the output tubes.  When the cable is disconnected, the 6L6s cannot pass current, which is why the noise stopped (because all sound would stop).

This amp has two cables. The cable you reference was not unplugged. Only the shielded cable was pulled. ...

 

I can only see the schematic, which shows a single 5-pin plug & socket.  Looks like I won't be much help.

[AlNewman]

There is another cable, kind of a different design, I guess it's to unplug the preamp and plug in a different source if needed.

With it unplugged, if the noise stops that means your issue lies in that cable, or any part of the signal before that cable.

Have you cleaned all the jacks and pots?

[Pinball]

There is another cable, kind of a different design, I guess it's to unplug the preamp and plug in a different source if needed.

With it unplugged, if the noise stops that means your issue lies in that cable, or any part of the signal before that cable.

Have you cleaned all the jacks and pots?

Yeah that cable carries the dry signal and ground. I made sure to keep a good ground when I unplugged it.

I did clean and check all jacks and pots. The strange part is when I connect an oscilloscope to the input where I pulled that cable, the same noise comes back. You can hear the same noise in the speaker and see it on the scope.

[SEL49]

The strange part is when I connect an oscilloscope to the input where I pulled that cable, the same noise comes back. You can hear the same noise in the speaker and see it on the scope.

When you disconnect the phono plug from J1, the triode grid no longer has a grid leak path to ground. This may be a factor with your noise. I suggest soldering a 1M resistor across the terminals of J1 to serve as a dedicated grid leak resistor. "IF" this helps the noise issue, you may consider replacing the mixing resistors R43 and R44 with 220K resistors.

It seems that the upper chassis gets it ground connection through the shield of the signal cable. I would provide a dedicated ground cable to bond the upper chassis to the lower chassis.

[AlNewman]

It seems that the upper chassis gets it ground connection through the shield of the signal cable. I would provide a dedicated ground cable to bond the upper chassis to the lower chassis.

This.  Where are you grounding your oscilloscope to when you are testing the input?  Try plugging the cable back in and jumping a separate ground between the 2 chassis.  This should tell you if it's a bad ground in your cable.  If it's still noisy, leave the cable in, or run a dedicated ground, and trace back with your scope till the noise disappears.

Do the pots affect the noise?  There's 2 different channels, which side has an effect, or is it both sides?  These kinds of things will help you determine where to look.

[Pinball]

It seems that the upper chassis gets it ground connection through the shield of the signal cable. I would provide a dedicated ground cable to bond the upper chassis to the lower chassis.

This.  Where are you grounding your oscilloscope to when you are testing the input?  Try plugging the cable back in and jumping a separate ground between the 2 chassis.  This should tell you if it's a bad ground in your cable.  If it's still noisy, leave the cable in, or run a dedicated ground, and trace back with your scope till the noise disappears.

I did try exactly this and added another ground. No difference in noise. Will keep searching.

Do the pots affect the noise?  There's 2 different channels, which side has an effect, or is it both sides?  These kinds of things will help you determine where to look.

[Pinball]

Update***


So finally determined the main noise issue. It was R15 and R16 of the Plate and Cathode of the phase inverter. Replaced those two and the majority of the noise went away. Also replaced resistors at the grid and cathode of the first gain stage. That eliminated the remaining noise.

Thanks for all the help. This circuit is so strange. Especially the 12ax7's getting their filament voltage from the 6L6's. Never seen that before.

[mresistor]

That is interesting   2 x 12AX7 filaments in series coming off of the 6L6 cathodes at 19 volts. 

[rafe]

After changing the the electrolytics on a vintage to old as shit amps. If there are odd noises I check tubes, then resistors. I didn't always do it that way but now I do. If the tubes are good. I always find out of spec. resistors, some way out that usually solves things.... not always but mostly.
I was going to suggest that, but you already found out!