5e3 New Build Troubleshooting Hum/Oscillation

[choosebronze]

Sorry, I originally added this to my turretboard thread, but I don’t think anyone had a way of knowing it was there. I can't thank you guys enough for helping with those issues. Doug's board soldered easily and powered up working the first time. Doug is the best and I've learned my lesson about other shops. 

I've finally had a chance to play the amp. The tones are great, and holy heck it’s loud! Standard layout but added RobRob’s PrePI MV (wired w/ RG174 grounded at one end) and his 3-way feedback switch.

I have a few problems that feel related to me. Fingers crossed:
 
There's a really bad hum. Hum is changed by the volume knobs and present on both channels. If I hover my hand about 1” in front of the pots, the hum gets a lot worse. If I touch the chassis the hum gets a little bit quieter. If I tap the chassis with my fingertip, I hear that thud through the speaker. It sounds like 120 cycle hum with the volume at 0. As soon as I turn the knob, it’s a different tone. Almost like static on a TV. That tonal change only happens on the Bright Channel, on the Normal channel it just stays a regular old hum. But loud.

Around volume 3 I think I start to hear a VERY faint and slow oscillation, almost like a Phaser at it's lowest settings. Around volume 7 it sounds like there’s a helicopter in my speaker cabinet. This happens on both channels, and only happens with no instrument connected. If an instrument is connected I can crank it to 10, I still have the loud hum I mentioned before, but no helicopter noises.

Heater CT, PA ground, and HT CT grounds all go to a PT bolt. Power cord ground to it's own PT bolt. Everything else goes to the Preamp Ground. I probably have a couple of leads that are a tad longer than they need to be. I’ll try to shorten them. I could see that causing some hum, but I can’t imagine that causing this extreme hum and oscillation. Tried all new tubes. All the wires are bent around to make sure nothing is touching. I've seen lots of issues where the V1 and V2 wires are touching causing hum, but all mine are airborn and not making contact.

I don’t know if I damaged a component following AES’s advice to use the “highest heat possible.” Not sure what the most likely damaged component would be in that case? Grateful for any advice on where to start looking. I’m assuming it’s ground related based on how it sounds and the position of my body changing things but I’m no expert.

[Latole]

" There's a really bad hum. Hum is changed by the volume knobs and present on both channels. If I hover my hand about 1” in front of the pots, the hum gets a lot worse. If I touch the chassis the hum gets a little bit quieter. If I tap the chassis with my fingertip, I hear that thud through the speaker. :

-choosebronze

Something is not grounded, a ground is missing somewhere.
Good pictures of the amp may help.

[pdf64]

Perhaps the shorting contacts on the #1 input jacks aren’t wired up properly?

[jjasilli]

www.geofex.com > Tube Amp Debug Page > Hum

[Latole]

Do you have a meter  ? Can you check how many ohms you read between input jack external nut and chassis ground.
And to chassis metal

Input jack must be a metal one, not plastic.

[choosebronze]

Thanks guys. I always forget to check RG Keen's site, and I shouldn't forget. He's got all the answers. I'll go over his troubleshooting steps tonight.

Yes, open metal jacks. Resistance from the outer nut are:
To chassis metal: 1.2 ohm
To preamp ground bus: 0.9 - 1.5 ohm (it was different every time I measured)
To power amp/CT/HT ground screw: 0.2 ohm

[vampwizzard]

You should be measuring from the grounded terminal (tab connected to the innermost metal pad on the amp side of the input jack. Its really easy to mess this up on a 5E3 4 input build. Do you have pictures of it?

[choosebronze]

Measured from the ground terminal of the jacks, all other ground points in the amp, and the chassis itself are all 0-0.5 ohms. Here are some pictures. Some of the solder joints look gooey, but it's reflections from the chassis and overhear light. In person everything is smooth and shiny. I've matched it to the drawings over and over and it looks right to me. I'm off to explore Geofex for a bit.

[choosebronze]

Okay, there's a lot less hum with V2 pulled. The hum doesn't seem to change with V1 pulled.

For the helicopter chopping, which I gather is "motorboating," RG says it's "almost always a signal that the decoupling capacitors in the B+ lines of the preamp section are going high impedance," but he says he's only discussing amps that previously worked. I haven't found anything about new builds specifically. I could just try different filter caps. I don't have any other 16uf on hand. No reason I can't use 500v 22uf, right?

Filter caps are listed as a possible cause of the motorboating, hiss, and hum, so I'm inclined to think that's where the problem is. It's entirely possible I just overheated one during installation.

[Latole]

You should make sure that the solders on the ground are also strong; those with body pots for example.
Try to pull them towards you with long nose pliers. They must resist.

Amps building problems always come from only 2 things;
Assembly error or bad welding.

Something else that is unrelated but will be a future problem; many of your wires are too long or badly installed. For example the blue wires coming from input jacks should run along the metal chassis and be as short as possible.

Look at my photo

These wires act like an antenna and will bring noise in amp

Same for all the wires.

The wires that need to cross must do so in 90 degrees. And no parallel wires.

[Latole]

Motor boating may come from bad wring dress and /or power supply. Not from filter caps.

Power supply include power supply rail, not only where are the filter caps.

[Latole]

Is this cap polarity is right ?

[DummyLoad]

questions about your layout - see marked-up pic. green wire seems redundant and what's under the heatshrink attached to the green wire?

--pete

https://www.dropbox.com/s/7f8fuo3d03kg0l3/bronze-5e3-wiring-not-following-layout.png?dl=0

[choosebronze]

Thanks Pete. That green wire is the shield ground heatshrinked to RG174 for the MV. Same reason the component you listed is missing. So hidden under that green wire is a run of RG174 that goes to MV lug 1.




The resistor where you put question marks is for his 3-way feedback switch.




Robinettes stuff is used by a lot of people and I consider it to be known-good, but maybe I should undo the mods, go back to the stock Fender circuit, and get that working. Then add the MV and feedback switch back in.

Is this cap polarity is right ?

Thanks, I'll tidy up the lead dress. The cap looks right to me.

[Latole]

Robinettes stuff is used by a lot of people and I consider it to be known-good, but maybe I should undo the mods, go back to the stock Fender circuit, and get that working. Then add the MV and feedback switch back in.

 

I agree.

I'll do mod after stock amp will work right

[choosebronze]

I rewired the entire preamp ground bus and removed the MV. Hum is definitely lower, and the helicopter chopping is gone. I also think the hiss is gone. It's hard to say, because the airy/hiss noise was a bit intermittent.  The hum is still too loud, it can be heard over playing. Tomorrow I'll clean up the lead dress on the tube side of the board and see if that helps.

[choosebronze]

Okay, I think the hum is gone. I still get some hum in the 10-12 range of each volume control. That seems reasonable to me since I'm pushing the amp hard, and it's not loud enough to get in the way of playing.

But, I have this airy hiss. Only when the Normal channel is in use. And only when the Bright volume is  above about level 9. - For example, if I'm playing through the Bright channel everything is perfect. If I play through the Normal channel everything is good. Playing through the Normal channel I started adjusting these infamous controls, and if the unused Bright channel gets around 9 (out of 12), the loud hiss kicks in. I probably don't care, I can't see that as a setting I'd ever use. I just want to make sure it's not a symptom of a bigger problem.

This feels connected to me: The tone control seems a little wonky. It's an audio taper pot. But the taper I hear doesn't sound that way. The perceived low cut I'd expect from a Tone pot seems to increase to about 2 o'clock, then the tone darkens a bit, and then start brightening again around 4 o'clock. Could a faulty Bright cap be causing both problems?

[Latole]

Hiss may come from bad wires dress and / or too long wires.
Mostly wire to tubes grids

[jjasilli]

www.geofex.com > Tube Amp Debug Page > Hiss

Always suspect tubes 1st.

[choosebronze]

I’ve tried new tubes in all positions and the behavior is the same.

[shooter]

Could a faulty Bright cap be causing both problems?

unsolder one leg n test

[Champ_49]

From the pictures the soldering on the back of the pots looks beaded meaning it doesn't look like its soldered properly. I usually never solder to the back of pots for risk of applying too much heat to the pots and potentially damaging it. And it takes alot of heat to solder well to it. I use a buss but its along the turret board. From there all the grounds are connected to it.
But if you plan on soldering to the back you should reflow the solder between the buss and the pot. Both the contacts (back of pot and buss) should be heated enough for the solder to flow onto it. Never try to transfer the solder from the iron to the contacts to make it stick. Heat the contacts first and apply solder onto the contacts when it gets hot enough. Also tin the iron and apply some solder flux to the contacts to help it flow better.

All the other solder joints look good.

Another thing to mention is the green ground wire at the input. The mechanical connection to the turret is good but it doesn't look soldered.

[PRR]

> Bright channel gets around 9 (out of 12), the loud hiss kicks in

"Kicks In"??  Like 9.1 is low-hiss and 9.2 is big hiss? That's oscillation, supersonic. Layout.

[choosebronze]

unsolder one leg n test

The severity is intermittent, which makes troubleshooting hard. A couple times I've thought I solved it but then it turns out it was just taking a break. But with the Bright cap disconnected I think the hiss noise is much quieter, to a usable level. I'll pick up another cap and try it out.

"Kicks In"?? Like 9.1 is low-hiss and 9.2 is big hiss? That's oscillation, supersonic. Layout.

Yea, basically. But I listened to some clips of ultrasonic oscillation. That doesn't sound like what I have. Maybe I should be calling this something else. Here's an audio sample. I have a guitar plugged into the Normal input turned up pretty high, and about 5 seconds in I start turning up the Bright Volume from ~5 to 12.

https://www.dropbox.com/s/xjko47vo4eiquyy/5e3hiss.m4a?dl=0

One more wrinkle to add, the B+ voltages are weird. B+1 reads 347 VDC. B+2 reads identical, 347 VDC. B+3 reads 257 VDC. I've verified that reading a few times and B+1 and B+2 are always identical. I checked the first voltage dropping resistor, it reads 4.67 Kohm (it's a 2W). The solder joints on both sides look good and I re-did the joints on both sides of that first resistor just to be sure, problem persists. I don't know why it wouldn't be, well, dropping voltage.

On the plus side, apart from these small issues the amp sounds amazing. Thank you for all your help so far.

[Latole]

From the pictures the soldering on the back of the pots looks beaded meaning it doesn't look like its soldered properly.
All the other solder joints look good.

 

I agree with reply #9

I often solder on back of pots, it work well if you sand the back before

[sluckey]

Is V1 a 12AY7?

[Champ_49]

From the pictures the soldering on the back of the pots looks beaded meaning it doesn't look like its soldered properly.
All the other solder joints look good.

 

I agree with reply #9

I often solder on back of pots, it work well if you sand the back before

Yes. I forgot to mention. Sanding it helps.

[Champ_49]

unsolder one leg n test

The severity is intermittent, which makes troubleshooting hard. A couple times I've thought I solved it but then it turns out it was just taking a break. But with the Bright cap disconnected I think the hiss noise is much quieter, to a usable level. I'll pick up another cap and try it out.

"Kicks In"?? Like 9.1 is low-hiss and 9.2 is big hiss? That's oscillation, supersonic. Layout.

Yea, basically. But I listened to some clips of ultrasonic oscillation. That doesn't sound like what I have. Maybe I should be calling this something else. Here's an audio sample. I have a guitar plugged into the Normal input turned up pretty high, and about 5 seconds in I start turning up the Bright Volume from ~5 to 12.

https://www.dropbox.com/s/xjko47vo4eiquyy/5e3hiss.m4a?dl=0

One more wrinkle to add, the B+ voltages are weird. B+1 reads 347 VDC. B+2 reads identical, 347 VDC. B+3 reads 257 VDC. I've verified that reading a few times and B+1 and B+2 are always identical. I checked the first voltage dropping resistor, it reads 4.67 Kohm (it's a 2W). The solder joints on both sides look good and I re-did the joints on both sides of that first resistor just to be sure, problem persists. I don't know why it wouldn't be, well, dropping voltage.

On the plus side, apart from these small issues the amp sounds amazing. Thank you for all your help so far.

Before you look at what other issues might be at fault. I would check your solder joints first. You are focusing on other things that might be solved first and foremost from good solid solder contacts. You should brush up on that first. Process of elimination and proper soldering is first and foremost a very important aspect of it.

[Latole]

Solders and wire dress

[Champ_49]

Solders and wire dress

Seond that. Lead dress is VERY important.

[choosebronze]

Is V1 a 12AY7?

Thanks Sluckey, yes 12AY7 in V1. I don’t have a spare on hand but I tried a 12AT7 and the same symptoms persist.


As for soldering on the back of the pots, there are no components grounded to the back of the pots. The bus is only soldered to the pots so it doesn’t fall down. I can’t see how that would affect the signal chain. And they are scuffed and fluxed. The solder is stuck to them firmly.

[choosebronze]

Dumb mistake... I had the B+ wired to the next lug over, at the second filter cap. I went over the back before installation and then once it was in the chassis I couldn't see it anymore and forgot about it. That's an embarrassing one. The voltages are now appropriate (B+1 367v, B+2 326v, B+3 241v).

That airy noise persists and I think is actually worse now. I'll try another 500p cap once I have one.

[Latole]

Solders and wire dress

Seond that. Lead dress is VERY important.

I never heard about choosebronze about what he do to have a better lead dress. Or I'm  wrong ?

[Champ_49]

Solders and wire dress

Seond that. Lead dress is VERY important.

I never heard about choosebronze about what he do to have a better lead dress. Or I'm  wrong ?

I don't think so either. Choose bronze, from the looks of it it seems like you rushed your build to get a "working" amp. You really have to take your time when building these. From the pictures your wires are all curled up, too long, the pots and solder on it dont look like its properly soldered and messy, you are now realizing you wired things in wrong.  You REALLY have to take your time with these builds and do everything very carefully. Check and double check. Start in small segments. Do one part first for day 1. For instance day one do heater wiring, day 2 power tubes, etc.  Do each part and do it WELL the FIRST time so you dont have to or at least minimize the "fixing" after.

From the looks of it it just seems like you wanted this thing to magically make great tone by just wiring it up with messy wires and without checking and double checking. Yeah you get sound but you have hum and oscillations. 

I would either

1) Gut the whole thing and start over OR
2) clean up your wires, make sure the joints where it connects dont expose too much lead, make sure all solder joints are smooth and shiny, make sure the eyelets holes are covered up with solder, i can go on and on.  Heater wires are pushed up close to the chassis and twisted all the way up to the pins. Make sure the wires are not too long and curled twisted up near the inputs, make mechanical connections to ALL your joints before soldering (the input solid wires look like its just poking through the eyelets).

I suggest you research and read up on how each part of the amp should be wired up such as the heater wires which can be a source of hum.  Go to valve wizard. Look at how you can improve grounding etc.

Now you are just doing a shotgun approach to to fix the issues you are having (changing the tubes for example). It's most likely not the tube's fault it's your lead dress and wiring based on your photos you provided.

Any monkey can just look at a diagram and solder a bunch if wires but if you want a good sounding amp that has no isssues you have to really do this slowly and carefully with much thought, consideration and some research.

The 5e3 is a good place to start a simple enough build but its not simple if you rush everything and think it will just magically have great tone with no issues.

[smacbride]

Where is your ground bus grounded?

[BrassElephant]

Try un-soldering the ground buss from the back of the pots. Leave it connected to chassis at only one place. Right now you have multiple circuit ground connections to chassis thru each pot.