5F6a build, low pitched hum

[Bieworm]

hi guys

I just finished a 5F6a build. It has also the 6G16 tremolo and the idea was to build a lower power Bassman. For that I used a 640VCT@300ma power transformer, 5U4GB rectifier, 5881 power tubes and the Classictone OT for a Vibrolux.
I mus say it sounds really nice and practically noise free. There is this low pitch hum that seems to come from the transformers, but it's audible through the speakers. When I (carefully) hold my ear over the open chassis I can hear the same hum mechanically.
What's the best way to determine if it's the PT or OT? I doubt it's the porximity of the transformers towards eachother being the problem.. I've seen builds where they are a lot closer to each other.
Here's my layout and schematic. The amp is built exactly like the layout. (for the nitpickers: I ofcourse added heater wiring)

[Bieworm]

Some pics

[Bieworm]

More pics

[tubenit]

I had a hum once that was "mechanical" like that and it ended up being the choke. So, you might want to rule that out also. I substituted a resistor for the choke and it resolved the issue completely.

Can you feel one of the transformers vibrating?  Can you tell where on the chassis the hum is the loudest?

Looks like you did a nice job on the build looking at the photos.

With respect, Tubenit

[bmccowan]

I had a similar hum in a Grommes HiFi monoblock. I was able to determine it was the PT by holding it tight. But the sound was also coming faintly through that speaker channel. I redid the physical mounts on the PT. The hum went away both mechanical and through the speakers. Since all tubes are somewhat microphonic, I theorized that the vibration was being picked up by a tube or other component.
I agree on the nice build! I have a question, why don't you use the top hole in the turrets for the component leads? It's so much easier to change a component as you tweak the circuit, or if a component is faulty. If done correctly the connections are totally secure, and solder does not escape out the bottom. (those are the two reasons I have heard for not using the hole.)

[Bieworm]

Thanks. About the turrets, that’s how it was instructed on reputable soldering technique tutorials. I only fo the top mount when it is too hard or too populated. The mechanical connection is the best way.

I can hear the hum especially at the OT and PT. The PT is mounted to a metal plate because the standard cutout on the chassis blank was too large. I’ll look into those tips you guys gave me
I always put foam tape between the transformers and chassis though

[sluckey]

Nice build. I can really appreciate your patience working inside that tweed style chassis. You have it packed neatly and tightly!

I've attached a simple mod to your bias switch for your consideration. The mod allows you to have tremolo in cathode bias or fixed bias mode.

You may be able to locate the humming transformer with a screwdriver. Put the end of the handle firmly to your ear and put the end of the blade firmly on the transformer. The screwdriver kinda acts like a headphone amplifier straight to your ear. Probe around for the loudest hum.

[Bieworm]

Nice build. I can really appreciate your patience working inside that tweed style chassis. You have it packed neatly and tightly!

I've attached a simple mod to your bias switch for your consideration. The mod allows you to have tremolo in cathode bias or fixed bias mode.

You may be able to locate the humming transformer with a screwdriver. Put the end of the handle firmly to your ear and put the end of the blade firmly on the transformer. The screwdriver kinda acts like a headphone amplifier straight to your ear. Probe around for the loudest hum.

Well thanks! The build took some planning and anticipating because Iwired and populated the entire turretboard before putting it in.

[Bieworm]

Also thx for the mod idea Steve. I will certainly try it with my next 6V6 build. I think the cathode biased 6L6/5881 won’t wiggle much at 80% bias. But with 6V6 that will be a great mod

[PRR]

....locate the humming transformer with a screwdriver. Put the end of the handle firmly to your ear and put the end of the blade firmly on the transformer. The screwdriver kinda acts like a headphone amplifier straight to your ear. ....

Stethoscope.

The doctor can hear if your heart, lungs, gall, knees, or valve lifters are rackety by moving the stethoscope around.

Not quite as easy on a metal chassis as on squishy flesh.

[sluckey]

I learned that trick from an auto mechanic instructor when I was in trade school. I've also used it to tell which pine tree the pine beetles were eating up.

[shooter]

my Uncle worked on Mack's, stopped by one day n said "I hear a tap'n noise"  he takes out a 2 foot screw driver, walks it down the heads, tells me #3 valves are rattl'n.  after work he brought over a Chilton's(sp?).  Him n dad drank beer while I learned "how to find valuable information in a book" and get real oily 


never thought about in with my Pines, thanks for the tip!

[dwinstonwood]

Looking at that long, slender chassis, I wonder if it could be resonant.

When the chassis is bolted down tight to a rigid, wooden cabinet does the hum decrease?

[Bieworm]

it's definitely the power transformer. I tightened the bolts but there was only a small improvement. The hum isn't really loud or annoying, bet I'd like it better if it were gone.
Although the heater wiring is done really neatly and no DC wires in proximity or whatever... When I read the voltages on each side to ground I read 3.06 an 3.10V respectively. This is a 6.3VCT winding , so the CT is directly soldered to power amp ground. The sad part is I don't have any room for a humdinger pot. But would it be worth a shot? The balance is not that much off to cause hum, or is it?
The chassis is not resonant as far as I can check.
When I pull V1 and V2 the hum is still there. I seriously doubt it's circuit related. It's obvious 50Hz hum coming from the PT sneaking into the signal path. Grounds are rock solid too. And I don't think there is a ground connection in the wrong spot. Bias trem circuit grounds are soldered to the power amp buss.

[swingarm]

This might be a little more extreme than what you are experiencing but it was interesting and might be helpful to someone who salvages/recycles  an old pt that has noise from the start.


How to fix Radio tube amp Transformer Hum Buzz Vibration D-lab Electronics

[bmccowan]

Good video - but I wonder if Terry's insurance agent knows that he is posting videos where he drinks a huge glass of wine and then sticks his hands in powered-up amps. I'm not the safety police, but damn, Terry, that's foolish.

[swingarm]

I hear ya, but, personal responsibility rules 

[bmccowan]

It should, but unfortunately it doesn't. My BIL is a lawyer for an insurance company. They have little use for personal responsibility. And I love wine, and Bourbon, and beer, but none make it near my workbench.

[swingarm]

 POMS and and they're having a wee glass  BTW your signature quote fits nicely here! Insurance banking etc has ruined this realm.


Now lets hope Bieworm can mix up some of his own spirits and maybe use it medicinally on his PT LOL

[bmccowan]

Yup. Recently, said BIL defended a lakeside lodge owner who was sued for not posting the water depth along their dock. A kid had dove in head first. He lost. To my surprise...

[Bieworm]

I’m from Belgium.. so beer is our national pride! Maybe I can melt some chocolate over the laminatings😎

[Bieworm]

I decided to leave it as is. It’s not bad in a way it is audible while playing and an amp serves best when played, not listened to it at idle😎
It resides in this awesome early 60’s Gretsch cab

[bmccowan]

Wow - that's a perfect fit in that cabinet - love it. Good decision on the PT I think, as you can always go back to it

[passaloutre]

I think I saw this amp on one of my favorite guitar player’s facebook page today

I know it will be rocked as hard as it deserves!

[bmccowan]

well give up that page, man

[Bieworm]

I think I saw this amp on one of my favorite guitar player’s facebook page today

I know it will be rocked as hard as it deserves!

If you mean Walter Broes, he’s a friend of mine who wanted a lower powered 5F6a with bias vary tremolo built in hos old Gretsch cab. He’s insanely happy with the result 😎

[passaloutre]

Yes, it's Walter. He's been an old guitar forum friend of mine from the Gretsch Pages for 20 years now. Though I've never met or seen him in person, I've learned so much from him over the years.

[Bieworm]

Yes, it's Walter. He's been an old guitar forum friend of mine from the Gretsch Pages for 20 years now. Though I've never met or seen him in person, I've learned so much from him over the years.

He’s a library of knowledge and the most sincere and loveable guy on the planet. If I could buy him.. I would, and keep him😎😎😎