Twin High (white) Noise

[frankeg]

The 70 SF Twin reverb I'm slowly restoring has a high amount of white noise and a little hum on the vibrato chan. I'm having trouble fixing it. To isolate it, I pulled the normal chan tube and grounded pin 7 of the vib chan. No Noise at all. I took that gnd off and grounded pin 2 - lots of noise... What's in between is the 1st half's cathode circuit which I checked and re-soldered and the plate circuit which includes the tone stack. It certainly seems like poor grounding but nothing I've tried has helped. The pre-amps power supply cap is tied to the brass plate at the normal chan soldered gnd. The 1st pwr supply cap goes to a chassis lug near the pt. The vib chan gnd including vol pot and mid pot are tied to the cathode gnd of the vib chan and a separate wire goes to the normal chan gnd. All the pots are secured to the brass plate with the grip/tooth washers. I separated the vol and mid pot gnds which were tied together at the vol pot into separate wires - no change. I did not try soldering the pot gnds to the pot caseing. I'm also thinking about taking all the gnds (1 wire each) back to the pwr supply gnd. Any thoughts or advice would be much appreciated

[Merlin]

If you have carbon comp resistors in there then it may be worth swapping the first triode's anode resistor for a metal film type.

[eleventeen]

After having just done 3 noisy old Fenders of mine (SF '68 AB763 Deluxe Rev, highly modded ~1970 SF Princeton Rev, and '65 Pro Reverb) I will tell you that replacing all the 100K plate load resistors in the early preamp tubes (at a dead minimum) is now "shop policy" as far as I'm concerned. It's not even worth thinking about if you're wanting to de-noise the thing. If it's open on my bench, as many of those plate resistors get replaced as possible. What I find interesting is that the amp has to be on for about an hour or more before the full before-after noise reduction from replacing (only) these resistors occurs. But it sure does. So you might not notice it so much if you rip and replace those resistors and turn the amp back on while the solder joints are still warm. After it cooks for about an hour though, the difference is ridiculous. If you are on a noise-hunt and you don't like hissy - crackly, replace all those 100Ks. Dougs 3W metal ones worked just great.

[HotBluePlates]

Before you do that...

Do you have a 1/4" plug with the tip shorted to the sleeve? If so, plug it in. If not, use a screwdriver or jumper to temporarily ground the tip of the Vibrato channel input jacks to the chassis.

Tell us what happens.

[frankeg]

I changed all pwr supply caps, cathode caps, coupling caps, tone pots and plate resistors (to metal film) as part of the restoration. I have re-checked those joints and re-soldered some. I didn't change the reverb plate resistors because I ran out but when the 2nd pre-amps pin 7 was grounded it was very quiet which lead me towards the tonestack. I will do the input jack 1/4" plug when I get home and let you know. When I do this, should I leave the 1st pre tube out as it is now or re-insert it? Thanks guys for the help! I'm starting to loose sleep over this....

[frankeg]

Ok, I shorted the vibrato chan input jack with a 1/4" jack which is shorted tip to sleeve and the noise did not change. Still there. Just for my understanding though isn't that the same as grounding pin 2? In the meantime, I'm going to rewire the input, vol and treb pots  using new shielded cable that I have since the originals are still in it. I'm also going to see what happens When I lift those pots from the brass plate... I'll keep plugging and watch for any more suggestions. Thanks, fg

[HotBluePlates]

Yes, sorta. The point is that you can have white noise and hiss from a jack in which the shorting switch is not contacting the hot contact. If that happens, there is hiss and hum with nothing plugged in. If you short at the grid pin, you'll stop the noise, but you won't know why it happened.

It is assumed you have all the tubes in for this test, and that only the volume for the channel under test is turned up. You check both jacks for the given channel to be sure neither is causing the problem. If the jack is bad, you can repair or replace it.

If grounding the grid of the first tube solves the problem, the issue is either the plate load for that triode, the cathode resistor for that triode, or the jack and associated resistors connecting to that triode.

If grounding the grid does not solve the noise, I'd tend to look at the reverb section; make sure the reverb pot is all the way off. In old Fender's they are often hissy if you have a high supply voltage.

[frankeg]

Hey Brian,
    I was stationed at Ft. Meade (414th Signal Co.) for my last year. I was supposed to go to Arizona when I PCS'd out of Sinop Turkey but my mother was ill and I requested the east coast. Anyhow, thanks for your help. I see the difference now in shorting the input. I pretty much have it solved. I changed the remaining 100k res. in the reverb section with ones I ordered from Doug (he's fast), replaced the old coax with new and moved the grounds. Originally the vib chan vol and mid pot were connected to the gnd of the 2nd half triode. I noticed this half shares its cathode with the normal chans 2nd stage. I thought maybe there was too much going there so I moved them to the vib chans 1st half chatode. Much better now but It could have been any of the above cause I didn't check after each change. At any rate thanks again to you and others willing to help. This is my hobby and I'm learning as I go... frankeg