Choke on tube amp

[Connoisseur of Distortion]

Where should i put this thing? will it suffer being too close to any particular things? I'm not sure if it develops a strong enough field to bother any other chassis parts...

[R.G.]

Placing parts in a tube amp is an exercise in running from the noise.

You wan the AC power lines and rectifier/filters as far from the input circuits as you can get them. Once that's done, you want the output transformer as far from the AC power transformer as you can reasonably get it. The choke is less sensitive, so I'd put it in the middle.

And did.

[H.Manback]

Slightly off topic, but I think it's related enough to the topic. I've built a p1 extreme, at least the chassis and electronics part, still have to do the wood work to make it a combo. Now I think I got lucky and I dodged the humming noise, but I can't really believe it to be honest since single ended amps are so sensitive to hum.

My question is, how do you find out you have a hum problem , sounds stupid, but when should you be able to hear it. I've tried turning up the master volume and preamp volume with input grounded (no jack connected), and it's only noise, no hum. My guess is that means it's ok, since you should hear power supply hum regardless of the input. Is that correct?

And another question, does a choke also affect the tone of the amp in any way? I would imagine having a inductor in the power supply which isn't a stable voltage could influence the sound?

[H.Manback]

Oops, pressed quote instead of edit...

[Stevo]

This is off stompboxes too!  But I have a Mesa Subway Rocket that just makes a hum not from the guitar or pot sensitive just a hum when it is on... Not annoying at all just a slight idle hum...Now I have heard these do that, because of the gain of the amp.. Also I have heard that the PCB is the wrong material for the tone control and somehow and causes a little hum in these amps......JUST ASKING sorry not to be about stompboxes

[fikri]

I have built several tubes project, and it always hums mostly because a bad power supply or grounding or maybe a bad layout. Lucky you if you have only hiss !  . But i have a question for this topic : Do we still need a choke ?

[Connoisseur of Distortion]

just finished the champ!

unfortunately, i can barely hear the guitar over the hum 

i guess i messed this up pretty badly...

[geertjacobs]

unfortunately, i can barely hear the guitar over the hum

No worries, hum is mostly caused by silly mistakes. Especially when working from a known schematic and a proven layout, you will be able to solve it in practically all cases.

1. Did you use star grounding?
2. Are all your ground connenctions soldered well?
3. Did you ground the input connection properly? (Did you use shielded wire for the input connection?)
4. No signal wires too close to power supply wires? (probe with a wooden chopstick to move wires around and see whether it affects hum)
5. did you ground the secondary of your PT?

If these are all ok then you start tracing the hum methodically:
- ground the grid of the poweramp tube (with an alligator clip or so) and hear if the hum is already present.
If yes, then the hum is in the poweramp so check power amp/power supply. Else continue:
- ground grid of the last preamp stage and check if the hum is coming from there...
- same for next stage...
Find the guilty stage and check what's causing the problem.

There are tons of threads on hum hunting on http://www.ax84.com/bbs
As always using the search function will give you all you need.

BTW: be sure to do this with a fresh mind. It will go so much better, don't ask how i know 
BTW2: when using a single coil guitar, STEP AWAY FROM THE AMP (5m or so)) before thinking there's an issue, again don't ask how i know 

[fikri]

Uh..! dont forget to ground one of your fillament wiring ! those will give you huge result too !
dont ask me why ! 

[zachary vex]

not put tube amp in mouth!  very dangerous!




okay, i couldn't resist.  8^P

[PaulC]

Actually you can have the output transformer and choke pretty close to the pwr tranny, and not have any home.  It's all about the alignment of the transformers.  You should try and get as many different 90 degree turns in alignment as you can.  You can actually get them right up on each other, and have less hum then if they were 2 feet apart.

A dead give away for output tranny hum induced by the pwr transformer is you'll hear that when the amp is on standby.  To really get things quite you can actually find the best quite spot for the OT before you mount it.  don't wire the tranny into the amp, but hook up some headphones to a speaker output on the tranny.  Then just pwr up the pwr tranny, and move the OT around untill you find the best spot.  Don't forget to rotate it a few different 90 degree planes. 

Also you should not ground one of the leads of the fil supply!  The 12a-7 style preamp tubes have a humbucking setup with the internal heaters.  Pin 9 is the center tap between pins 4 & 5.  what you want is a 3.15 - 0 - 3.15 vac fil winding with the center tap grounded - not one of the outside legs.  If you don't have a center tap (like a champ) then you want to make a fake one by adding 100ohm resistors from each leg to ground.  Doing that while removing the ground connection will go along way to get rid of the hum in a champ or an old AC30 caused by the heaters.

There's a ton more, but do these first if you've not done it.  Later, PaulC

[Connoisseur of Distortion]

thanks for the advice... on this project, i actually star grounded at the base of the filtering capacitors, so i thought i had it just about right. i'll run through that little checklist (thanks for that!)

another concern i have is that the 6V6 is glowing VERY brightly when i turn on the plate voltage. i can't say for sure, but it looks like the top spacer assembly is the cause. should i be concerned?

[Connoisseur of Distortion]

here's a question... should my 5Y3GT be before or after the standby switch? does it need to do something with the DC voltage? just a concern, as it looked like something was releasing some heat when i flipped the power switch...

[geertjacobs]

- The 5Y3GT is the rectifier tube. It's the predecessor of a diode rectifier.
But just as all tubes it has a heater, that should be already on during standby so the tube can heat up slowly without needing to "work" immediately.
- the 6V6 glowing very brightly? Double check the resistor values in your power amp, so you are sure that the 6V6 is biased correctly.

I second PaulC's note on the 100 Ohm filament reference resistors. Very important. You can also refer the center point of these two resistors to a dc voltage in your amp instead of to ground. Info on this can be found at the end of the P1 theory document.

The P1 is an that is very close to the Champ. I reallly really recommend reading the P1 theory document at least three times. It will help you understand your (Ch)amp and will help you to locate where the problems may be.

edit: spelling

[PaulC]

You can also refer the center point of these two resistors to a dc voltage in your amp instead of to ground. Info on this can be found at the end of the P1 theory document.

Yep - that's a great trick for knocking down heater induced hum.  It can help alot with single ended designs because you don't have the humbucking nature of a push/pull output stage in regardes to the heater hum.  An easy trick is to tie the reference point to the cathode of a cathode biased output tube.

The standby switch comes after the rect tube in most cases.  But there are better places to put it.  The switches are not a fan of High DC voltages.  You can put the stby switch on the center tap of the high B+ winding lifting it from ground.  Or you can use a double pole switch and switch out both ends of the primary winding to the rect. 

One of the best things to do for hum is to have the center tap of the pwr tranny going to the main filter cap, and then both of those would go to the start ground.  Do not have any other ground points in the amp going to this filter cap/CT connection.

Then what I'd do is start working backwards to find the hum.  Have just the pwr tube in and see if it hums.  If so try and fix it.  Then add the the other tubes in a backwards order trying to knock out any hum as each on goes in. 

Star grounding is cool, but not the best.  what works best for me is a modified method that has everything fed off of a filter cap going to that cap, and then the cap goes to the main ground point.  This includes input jacks, and reverb jacks - they need to be isolated from the chassis so they can tie to the cathode circuit they feed.  Then that connection goes to the filter cap, and then to ground.  doing this will get rid of pretty much all of it.

But a single ended output design by it's nature has alot of hum with the standard pwr supply.  Look at maybe adding a choke and another cap before the OT tap.  You can see this in things like the old AC15 and Sunn amps.  Also the dynaco ST70. 

If your pwr tube is going cherry check the biasing. If you could post numers that would be great!

Take care, Paul

[Connoisseur of Distortion]

ok, i got the 6v6 under control. phew. it was pretty creepy for a while. 

my 6.3 volt hookups came with a center tap, so i used it. i hooked the CT to ground, and ran one leg of the transformer to 2 on 6V6 and 9 on 12AX7, and the other to 7 on 6V6 and 4/5 on 12AX7.

the schematic suggested placing one leg to ground, and running the other to the filament. should i try this instead of the CT method?

EDIT: thinking transformer, said transistor