Reverb Return Hum (was: What fails when operating an amp with no load?)

[spacelabstudio]

So, here's an object lesson on why you shouldn't work on tube amps when you've only had four hours of sleep.  I was doing some work on a Gibson Scout when I turned it on without a load.  I didn't notice until I caught a whiff of ozone and then heard one of the transformers oscillating.  Turned it off, connected the speaker, and now with the volume on 0 it self oscillates loudly at the same frequency I heard coming from the transformer.  High but not dog whistle high.  A few khz, probably.

So my next step (maybe after some sleep) is to figure out which component(s) failed.  What's the most likely to have failed in this scenario.  Is the self oscillation indicative of a particular type of failure?

FWIW, the transformers in this thing are very corroded anyway.  I strongly suspect the output transformer and it would not break my heart to have to replace it.  Except then I have to consider, if I'm going to start throwing iron at it, and it's such a crappily built (but cool sounding) little amp, whether I shouldn't just rebuild in a chassis that is easier to work on with a layout that isn't completely insane.  Choices....

Theories welcome.  I'll probably at least have the OT out of circuit and checked with an ohm meter fairly soon.

Chris

[spacelabstudio]

Sorry, ignore me.  I really should get some sleep.  A few things:

- I forgot that the speaker output jack that *I* installed was shorting if nothing was plugged in.  So I hadn't been operating it with zero load.

- I had also forgotten that just the day before I had found that I could get the amp to self oscillate if I cranked the reverb all the way up and didn't plug in a guitar. 

The latter is related to the reason I was looking at the thing in the first place.  Trying to reduce some hum/buzz coming in via the reverb return circuitry, which is somewhat insane. 

And my story wasn't adding up, anyway.  If the OT or other crucial power amp component was fried how was the oscillation getting to the speaker? 

Anyway, plugged in a guitar and turned down the reverb, sounds fine, except for aforementioned hum/buzz in the reverb.

[stratele52]

Your problem could be a unshielded wire between you reverb out / in . Or bad ground. I don't know if these amp use input /output rca jack ,like Fender.

Also a unshielded wire could be use between pin of V1b and the resistor R12 , C7.  V1b pin can be an antenna if unshielded or "improper" wire is use to connect R12, C7.

[spacelabstudio]

Your problem could be a unshielded wire between you reverb out / in . Or bad ground. I don't know if these amp use input /output rca jack ,like Fender.

Also a unshielded wire could be use between pin of V1b and the resistor R12 , C7.  V1b pin can be an antenna if unshielded or "improper" wire is use to connect R12, C7.

That's a good guess.  R12 and C7 are soldered directly to the V1b grid pin, though.  The cable from the springs to the grid is shielded.  I find that if I wave some metal pliers in the general vicinity of C7, which is a newish looking orange drop, that the hum gets louder so it may still be that that is the source.  For the moment clueless as to what I might do about it.  What is the role of C7, there, btw?

I did get rid of the oscillation issue be rearranging some lead dress, so at least I've made a little forward progress.

Chris

[stratele52]

C7 is a filter. You can remove it an see how reverb work. As you see , lead dress may affect. As your resistor are directly solder to the V1b grid everything look fine. Can you push  R12 close to the chassis ? Same for C7 , if you must leave it in the amp.

Replace the plate resistor R14 and why not R 15 , Plate resistor are noisy when old. You can replace all of your other preamp tubes's plate resistor

Try a new tube.

What appened with foot pedal F3 when you send to the ground ; no  Hum or buzz ?

Also I see on schematic this amps have not filament ground; this is very important for quiet amp.
Make artificial center tap with two 100 ohms 1/2 watt resistor, one end solder each side of the filament , the other end to the ground. Solder them close to the transformer wires

[spacelabstudio]

C7 is a filter. You can remove it an see how reverb work. As you see , lead dress may affect. As your resistor are directly solder to the V1b grid everything look fine. Can you push  R12 close to the chassis ? Same for C7 , if you must leave it in the amp.

Replace the plate resistor R14 and why not R 15 , Plate resistor are noisy when old. You can replace all of your other preamp tubes's plate resistor

Thanks, I'll try all of this stuff later today or tonight, as work and children allow.

Try a new tube.

Did that.

What appened with foot pedal F3 when you send to the ground ; no  Hum or buzz ?

Amp is dead quiet when the reverb is switched off.

Also I see on schematic this amps have not filament ground; this is very important for quiet amp.
Make artificial center tap with two 100 ohms 1/2 watt resistor, one end solder each side of the filament , the other end to the ground. Solder them close to the transformer wires

It's not on the schematic, but there is a wire from one leg of the heater circuit to ground.  I replaced that with an artificial center tap, which made no difference.  I did listen to it with heaters completely floating and that was very noisy, but there is no audible difference between just grounding one leg and doing the artificial center tap thing.

Chris

[spacelabstudio]

Ok, so I've spent an inordinate amount of time on this.  Because I'm stubborn.  I don't really even use reverb very much.  I certainly don't really care whether this amp has usable reverb.  But I hate being stumped.  Anyway, I changed out C7, R12, C8, R13 all with no change in hum.  Tried grounding things in different places, yanking wires further apart from each other, nothing. 

But I got rid of the hum.  I don't understand how, though.  The last change I made, that actually made the hum go away, was I removed C8 (cathode bypass cap for reverb return stage).  I was meaning to replace it with a different capacitor but on a lark, listened to the amp without it first.  No hum.  It's quieter, obviously, but I can crank the knob further to the right and still get as much reverb as I ever use.  (Fender on 2.)  I guess I might see if I have any LEDs in the right ballpark and replace R13 (cathode resistor) with an LED and see if I get the volume back without the hum.

It also just happens that I didn't have a C7 in there when I found this config, so no C7 or C8 currently.

Anyway, glad it's working but completely stumped why getting rid of the cathode bypass cap would get rid of hum. 

[spacelabstudio]

Ok, so I've spent an inordinate amount of time on this.  Because I'm stubborn.  I don't really even use reverb very much.  I certainly don't really care whether this amp has usable reverb.  But I hate being stumped.  Anyway, I changed out C7, R12, C8, R13 all with no change in hum.  Tried grounding things in different places, yanking wires further apart from each other, nothing. 

But I got rid of the hum.  I don't understand how, though.  The last change I made, that actually made the hum go away, was I removed C8 (cathode bypass cap for reverb return stage).  I was meaning to replace it with a different capacitor but on a lark, listened to the amp without it first.  No hum.  It's quieter, obviously, but I can crank the knob further to the right and still get as much reverb as I ever use.  (Fender on 2.)  I guess I might see if I have any LEDs in the right ballpark and replace R13 (cathode resistor) with an LED and see if I get the volume back without the hum.

It also just happens that I didn't have a C7 in there when I found this config, so no C7 or C8 currently.

Anyway, glad it's working but completely stumped why getting rid of the cathode bypass cap would get rid of hum. 

And in the time it took me to write the above post, the hum came back.  Hmmm.

Chris

[HotBluePlates]

Couple things to try:

Unplug the tank. Does this kill the hum? If not, proceed with the following.

Where is R12 grounded? If it is anywhere except the same place R13 finds a ground, move it so that R12 and R13 are grounded at the same place. In Fender amps, that often kills reverb hum. You should be able to replace C8 and still not have hum.

[stratele52]

 It's not on the schematic, but there is a wire from one leg of the heater circuit to ground.  I replaced that with an artificial center tap, which made no difference.  I did listen to it with heaters completely floating and that was very noisy, but there is no audible difference between just grounding one leg and doing the artificial center tap thing.

Chris
_____________________________________________________________________________________
I understand that your transformer heater wires have no center tap to ground and you put on side of the heater to ground .
Don't do that it is a noisy wiring , do what I suggest ; 100 ohms  1/2 watt resistors each side of the heater to ground

[spacelabstudio]

Couple things to try:

Unplug the tank. Does this kill the hum? If not, proceed with the following.

Where is R12 grounded? If it is anywhere except the same place R13 finds a ground, move it so that R12 and R13 are grounded at the same place. In Fender amps, that often kills reverb hum. You should be able to replace C8 and still not have hum.

Got it, finally.  Yeah, naturally I had unplugged the tank in trying to figure things out.  It was not the source of the hum.  R12 and R13 were grounded to the same place.  But I moved where they were grounded to, and that has finally gotten the hum pretty will gone--quieter than the buzz coming in from the guitar pickups.  So I'm calling that fixed.  The point where they were initially grounded ended up grounding to chassis at the inputs.  The point I moved it to grounded to chassis at one of the rivets for the tube socket, which is continuous with the tube shield, if installed.  It was a pain in the butt to get everything rerouted there, but I got all put back together with both C7 and C8.  I went with a 2.2uF for C8 just to try to de-emphasize any low hum that might still be in there.  Sounds great.

I understand that your transformer heater wires have no center tap to ground and you put on side of the heater to ground .

Sorry, I guess I wasn't clear.  I didn't do that.  That's how I found it.

Don't do that it is a noisy wiring , do what I suggest ; 100 ohms  1/2 watt resistors each side of the heater to ground

That's what I did.  I replaced the wire to ground with 2 100R resistors to make an artificial center tap.  FWIW, there was no audible difference in this case. 

Thanks!
Chris

[HotBluePlates]

R12 and R13 were grounded to the same place.  But I moved where they were grounded to, and that has finally gotten the hum pretty will gone--

That's the trick.

The resistor is part of the grid circuit for the reverb return stage. However, most amps ground this resistor where convenient (usually at the jack). The "convenient spot" is never where the tube itself finds its ground, and often hum is the result (due to ground currents through the chassis between where the grid is grounded and where the cathode and filter cap are grounded).

In a new build, you just have to follow best practice and see what you get, even if that means diverging from "vintage practice". In old amps, you simply try whatever works until you find something that you can live with.

Ideally, the filter cap feeding that stage, the cathode resistor and bypass cap, and the 220k grid reference resistor are all grounded at the same spot. That rarely happens in practice, unless you specifically plan for it.

You'll see why planning a layout is all about predicting everything that could possibly turn into a problem, then fixing it before it does.