Debugging a nice 60/120 hum in a reverb circuit

[pdf64]

As a sanity check, if you've got a footswitch plugged in for the trem/reverb, unplug it; the reverb switch cable requires very good screening, and routing away from hum sources

The DrZ EZG50 (think of a single channel no trem SR AB763 with a type 4 master) has a excellent reverb but only has a short tank (accutronics 8AB3A1B). I'll never buy a long tank again!
The relevance here being that it's much easier to find a happy place for a short tank.

[Leo_Gnardo]

The DrZ EZG50 (think of a single channel no trem SR AB763 with a type 4 master) has a excellent reverb but only has a short tank (accutronics 8AB3A1B). I'll never buy a long tank again!
The relevance here being that it's much easier to find a happy place for a short tank.

"If it sounds good, it IS good" applies here. In some cases I've had to try every tank I have in stock, and it shouldn't be surprising that sometimes it's a short tank that wins the prize. Wish there was a way to predict which would work best in an application but - there isn't.

One nice thing about the CE long tanks intended as direct replacements for old Fender amps, tubesandmore has them available with short, medium and long decay times. I find short decay is the best choice, the reverb doesn't hang in there forever and create the sonic illusion you're playing in a huge arena, otoh it goes on long enough to sound authentic. Now please don't buy 'em all up and leave none available for me.

[Helmholtz]

I can't remember which brand I ended up with in my last reverb build, but I had to do something pretty unconventional to mitigate power transformer induced hum. And it's not as if I was in tight quarters either. This was a full size 2X12 combo cab. Anyway, I tried positioning the pan, jacks facing front AND back, all over the floor of the amp with no joy. The only orientation that would silence the hum was with the tank bottom facing the inside of the lower rear baffle, jacks up. So that's how I mounted it. I had to take some additional measures against acoustic vibration because that panel is prone to do that. The amp is a little more tender than average to reverb crash when moving it while it's on. NBD though because at least it doesn't hum.

Hum is induced in the output transducer (receiver) coil. The better versions use a 3-limb closed core like the one shown at the bottom of the article below:
https://www.amplifiedparts.com/tech-...d-and-compared

Closed cores are far less sensitive to magnetic AC field pick-up.

The steel can shields against outer magnetic and electric fields but leaves the bottom side open/ unshielded. I always wondered why nobody uses or offers a steel bottom plate to close the can/tank and keep interference fields completely out. This should make it mostly independent of positioning/orientation.

[Chuck H]

Thank you for that piece of info. I don't have the amp here to see what sort of core structure the tanks output transducer uses, but I'll surely check that out if/when I get it on the bench again. It would be interesting (in a 'ah ha, you're a cheap bastard' sort of way) if I found that the simpler two leg, open core were used on that tank.

[Justin Thomas]

I don't get what's happening to reverb tanks anymore at all... How is this crap getting to the parts vendors without someone at the design level picking up on it?

WHY THE HELL ARE YOU STILL USING THAT TECHNOLOGY FROM THE 1940s WHEN OUR DIGITAL SILLYCOOTIE WHIZMOTRONS WILL DO THE EXACT SAME THING? And doesn't waste all that extra metal, and doesn't crash when you kick it... See? Good for the environment AND solves the noise pollution problems! Sucks to your spring reverb, because our potato chip centipede reverb does exactly the same thing! And who wants Spring Reverb anyway, when you can have hall and plate? Damn Luddites, ah tell ya what...

(Sarcasm)

Jusrin

[J M Fahey]

Please post here the schematic you are using.

If something you did does not match the original to a t, please state so.

You do NOT need an isolation transformer for your scope in this case, because your amp is transformer powered.

Please scope the output and show us both the "no hum, only hiss reverb on 0" image, of course with no signal input, volume and tone controls set to 0, reverb on 0 (it should look like "grass") and with same scope settings, rising reverb to half ("5") and full ("10").

Thanks.

PS: it would be great if scope setting/scale can be shown, so we can compare , say, "20mVpp Hiss" to, say, "150mVpp Hum/Buzz"

So we also have a before/after reference point to know how much we "improve" results doing different things.

[Chuck H]

WHY THE HELL ARE YOU STILL USING THAT TECHNOLOGY FROM THE 1940s WHEN OUR DIGITAL SILLYCOOTIE WHIZMOTRONS WILL DO THE EXACT SAME THING? And doesn't waste all that extra metal, and doesn't crash when you kick it... See? Good for the environment AND solves the noise pollution problems! Sucks to your spring reverb, because our potato chip centipede reverb does exactly the same thing! And who wants Spring Reverb anyway, when you can have hall and plate? Damn Luddites, ah tell ya what...

(Sarcasm)

Jusrin

Well... Not entirely sarcastic. I've had enough frustration with spring reverbs that I've considered going to the "brick" type products. In fact the ONLY reason I don't is that most people interested in tube amps (which is what "I" do) don't like silichicken reverbs on principal (though what principal is never well defined).

[Justin Thomas]

To be a bit more honest...
In a modern high-gain or otherwise "not-vintage-Fender-or-Ampeg" a Fender or Ampeg reverb is pretty overkill. The Ampeg especially; never turned that thing above 5. Generally if I'm playing a rock or hard rock amp, I'd rather have fine control over a narrower range than full-on surf at my fingertips.

But if you're talking vintage stuff, there is no substitute. And I appreciate the individual charm that comes with each amp's particular circuit. Parts drift, etc. I know they all have the same "circuit," but dang that Dual Showman Reverb had the best reverb I ever heard. So if you're looking for the exact same tone all the time from every single amp, sure, use a modeler, a pedal, whatever newfangled contraptions they use nowadays. Myself? I'd rather ave the mystery, charm, and individuality of each particular amp... But I'm a dinosaur.

Justin

[J M Fahey]

FWIW the Belton Brick sounds very good.
I was *amazed* to learn they don´t use a DSP with some complex algorithm but a trio of very cheesy "karaoke approved" PT2399.
Yet they are clever and use them very well.

[Tom Phillips]

Mike,

After reviewing the complete thread again I suggest that you try a few troubleshooting tests to isolate the potential problem. Following are the suggested steps. Please do them in the order listed and report the results for each test.

1. As pdf64 asked in post #16, tell us if you have a footswitch plugged in. If so then unplug it and tell us if there is any change in the hum issue.
   Next...
2. Unplug the reverb tank output cable at the tank end. Leave the cable attached at the chassis end. Any change? If so what?
   Next...
3. Unplug the dangling tank output cable from the chassis. Any Change?

The purpose of this exercise is to determine if the excessive hum is originating before or in the reverb recovery gain stage. What you discover will determine the next troubleshooting tests.

It’s also helpful to know if the hum is 60Hz or 120 Hz. You can determine that with your scope or you can use your guitar. The B flat on the 5^th string at the 1^st fret is 116.5Hz. The B on the 5^th string at the 2nd fret is 123.5Hz. If the hum note lies between those notes then you know the hum is predominantly 120 Hz. You should be able to hear the beat note. If the hum is 120 Hz then we will assume it is power supply related. If it is 60 Hz then we will assume it is heater circuit, line circuit or power transformer field related. I said “predominantly” because, if you measure with sensitive instrumentation there is always a mix of the two frequencies present. (or 50Hz/100Hz in countries that use a 50Hz grid) 60 Hz hum is not always caused by poor wiring or strong stray fields. It can originate inside a tube if the construction of the tube is faulty. Therefore, an easy test is to try a different tube. In your situation the most likely culprit would be the reverb recovery circuit tube which is the fourth tube from the pre-amp end in a Deluxe Reverb AB763. I’d try that if the hum remains after you have done 1,2 & 3 above.

Cheers,
Tom

[mikepukmel]

Ok some info: If I take the tank side of the output cable off the tank (cable from tank to reverb recovery) gets noisier, sounds like a guitar cable plugged into the amp but not into the guitar. But fundamental freq hum about the same. If I unplug the cable completely (cable from reverb tank out to amp reverb recovery), the hum is about the same. Increases when reverb pot is increased.

No footswitch.

I tuned my open E string down about 4 fret steps to match the tones fundamental. Its pretty low.

To Juan: my hand mod schematic below. I will scope the output, but I have to dig out the scope and hook it up, need to clear out space on my desktop.

...




A little more info: I did the cable in/out tests with the tank sitting outside the amp, farthest from the transformers.

[mikepukmel]

The additional cap is as close to the turret board as I could get. Its mounted to 2 3 lug terminal strips. The resistor is closer to the board. The + side of the cap faces the output tubes. I ran a wire from there, close to the chassis under the board to where the original junction was located, that feeds the reverb driver tube.



When I did this, the capacitor 'can' was full, has 2 x 22uf 500v and 3 x 16 uf 450 caps. I recently bought a 30uf 500v to replce the 2 x 22uf (a little closer to stock specs re capacitance), so I can move the cap that is inside the chassis next to the transformer inside the 'cap can', if that might help.

[mikepukmel]

Just looked it up; the low E is 82hz? No kidding? I would never have thougt the fundamental was that low. Then, the hum fundamental is probably 60.

[Leo_Gnardo]

Just looked it up; the low E is 82hz? No kidding? I would never have thought the fundamental was that low. Then, the hum fundamental is probably 60.

Ordinarily the hum fundamental is double the line frequency due to rectification. 120 Hz in US/Canada & other 60 Hz countries, 100 Hz just about everywhere else where 50 Hz line is standard.

[mikepukmel]

Ordinarily the hum fundamental is double the line frequency due to rectification. 120 Hz in US/Canada & other 60 Hz countries, 100 Hz just about everywhere else where 50 Hz line is standard.

Thanks Leo. Then it can't be 60. Oh boy, where is this hum coming from? I'll dig out the scope (the high school football team is coming over tomorrow to help me carry it from the storage shelf to my desk), I'll try to probe and get a shot of the trace.