New Champ Build Problems...

[Bruce / Mission Amps]

I admit I don't know the exact circuit your are working on but...
It is very rare indeed to see a 10K screen node resistor (assuming it is the screen node resistor) go bad... and end up being 0 ohms... I think you have something else wrong there. The choke isn't going to fix this and it will cause the screen voltage to rise very high. The power tube will then draw even more plate current through the B+ rail, pulling it out of regulation even more..

[Meaux]

How does it work if you install a good 10k resistor?

I don't know, I don't have one... I was just about ready to dump it...

[Meaux]

I admit I don't know the exact circuit your are working on but...
It is very rare indeed to see a 10K screen node resistor (assuming it is the screen node resistor) go bad... and end up being 0 ohms... I think you have something else wrong there. The choke isn't going to fix this and it will cause the screen voltage to rise very high. The power tube will then draw even more plate current through the B+ rail, pulling it out of regulation even more..

Hey Bruce, you most certainly know more about this than I do, and I didn't suspect that part to be bad either... In my frustration I was giving it one last check before giving up, so started checking all the wires for the umpteenth time and stuck my digital meter on the 220K and got a 228K reading... On to the 22K and got a 22K reading... On to the 10K and got a .000 reading... I thought what? So I unsoldered one side, plugged her in and didn't get any humm, and that makes sense... So I thought I'd clip in a plain wire in it's place... I got the exact same Humm I've been trying to fix!!!... I've been pulling my hair out for weeks... I sure hope that Resistor is my problem, if it ain't, I give up...

[jmaf]

So I unsoldered one side, plugged her in and didn't get any humm, and that makes sense... So I thought I'd clip in a plain wire in it's place... I got the exact same Humm I've been trying to fix!!!... I've been pulling my hair out for weeks... I sure hope that Resistor is my problem, if it ain't, I give up...

Unless I'm missing something(it's the first series resistor at the power supply right?), I think you disconnected the HT to the preamp when you unsoldered it, that's probably why you had no hum. When you jumpered it you also lost that filter so hum had to increase.

Also, important note: when you jumpered over the 10K resistor, the voltage to the preamp tubes went much much higher... you need that resistor there even if you add a choke, because it reduces the voltage to the preamp.

[MWJB]

Meaux...STOP replacing parts. Work through this methodically. Otherwise you are going to replace good parts with more good parts & still have hum.

I would start by rerouting the wire feeding pin 5 of your power tube away from the plates wire at pin 3.

You apear to have 4 wires attached to your speaker jack, ther should only be 3 - OT speaker tap, OT common tap, NFB wire from 22K on the board? If you are running a ground wire to a point shared by the rest of the circuit this may increase hum.

You have the preamp filter cap (along with 220K grid load and 470ohm cathode resistor) grounded with the main & screen filter caps. Run the preamp filter ground to the input jack grounded lug.

Some shots of the 12AX7 wiring might be useful?

You have left off the 25uf 25v cap at 12AX7 pin 3.

If 80uf reduces your hum, then use 80uf at the first filter cap, the rectifier tube can take it.

[Meaux]

Unless I'm missing something(it's the first series resistor at the power supply right?), I think you disconnected the HT to the preamp when you unsoldered it, that's probably why you had no hum. When you jumpered it you also lost that filter so hum had to increase.

Also, important note: when you jumpered over the 10K resistor, the voltage to the preamp tubes went much much higher... you need that resistor there even if you add a choke, because it reduces the voltage to the preamp.

Maybe I didn't word it right... When I unsoldered one side of the resistor, I plugged her in and there was no Humm... I thought, 'Of course you don't have any humm, it's almost like pulling the Power tube to make the humm go away"... So, I removed the Resistor completely and found a direct short in the resistor as if it were a piece of wire... I then clipped in a wire in place of the resistor (which isn't in the amp at all) and got the same amount of Humm I've been trying to lose... If that Resistor (which has the same Zero resistance) as a simple peice of wire, what good is it?

I'm no Engineer, just a remove and replace artist, but I see no reason to have a Resistor that has no resistance...

[MikeH]

what does the 10k read out of the circuit? If it reads fine when it's removed from the circuit, then the problem isn't the resistor; the problem is a short circuit in your wiring. If it doesn't, then, yeah, you have a bum resistor. For the record, I've never had a brand new resistor be no good. Usually when they're no good, they look pretty scorched.

[Meaux]

what does the 10k read out of the circuit? If it reads fine when it's removed from the circuit, then the problem isn't the resistor; the problem is a short circuit in your wiring. If it doesn't, then, yeah, you have a bum resistor. For the record, I've never had a brand new resistor be no good. Usually when they're no good, they look pretty scorched.

OK, In Circuit measuring the 22K resistor that is between the 2 8uf Filter Caps with the Digital Meter on Scale 2M, I get .022... On Scale 200K, I get 21.7...

In Circuit measuring the 10K Resistor (in question) w/ Meter on Scale 20M, I get 0.00... On Scale 2M, I get .000... On Scale 200K, I get 00.0... On Scale 0.01... On Scale 2K I get 0.10...

Am I reading this all wrong? Bear with me... I figured if I'm getting a .022 reading from a 22K Resistor on scale 2m, I should be getting a .010 reading from a 10K resistor on the same scale... Am I applying just too much common sense to something I know nothing about?

Isn't it odd that I get the exact same kind and amount of Humm with a piece of wire in it's place?

I reckon as far as Amp Building goes, I'm snakebit...

[MikeH]

Are you sure it's a 10K? You may have been slipped an incorrect resistor with a similar color scheme, or you could have possibly read the code wrong, or it could be just plain no good. But, yes, that is NOT a 10k resistor. It may or may not have been at one point, but it is not anymore. As long as the resistor looks ok (not burned, no cracks, no dark spots) I'd get a replacement (measure it before you put it in) and see how it goes.

It's possible (although I don't know how likely) that another error in your build caused the resistor to fail. Although that usually results in smoke, fire, etc... hahaha.

Having that resistor be 100ohm instead of 10K might as well be a short circuit, which would basically make the second 2 nodes of your power supply filter look like one 16uF instead of 2 8uF caps. In my mind that seems like it would mean one less filtering stage and therefore more hum. Also it would result in higher voltages on your preamp tube. I don't know if that increases noise, but I doubt it makes anything quiter.

[Meaux]

Meaux...STOP replacing parts. Work through this methodically. Otherwise you are going to replace good parts with more good parts & still have hum.

I would start by rerouting the wire feeding pin 5 of your power tube away from the plates wire at pin 3.

OK...

You apear to have 4 wires attached to your speaker jack, ther should only be 3 - OT speaker tap, OT common tap, NFB wire from 22K on the board? If you are running a ground wire to a point shared by the rest of the circuit this may increase hum.

No, that black wire is a Ground going to the bottom bolt of the Transformer... All the other grounds (except the 115vac) are going to the Top Transformer Bolt... I originally had a 1/4 inch Jack for the Speaker till someone suggested I isolate the Speaker jack from the chassis, then I put in the plastic RCA, and added the ground wire...

You have the preamp filter cap (along with 220K grid load and 470ohm cathode resistor) grounded with the main & screen filter caps. Run the preamp filter ground to the input jack grounded lug.

So I remove the ground to the 8uf, and run another ground wire to the Input G?

Some shots of the 12AX7 wiring might be useful?

I'll try...

You have left off the 25uf 25v cap at 12AX7 pin 3.

Didn't come with the Kit... Some say its used, some say it isn't... I ordered one anyway this morning...

[MikeH]

The 25uF cap on pin 3 of the 12ax7 is the cathode bypass cap. Early champs didn't have it. It's not necessary, but adding it will add more grit, although perhaps more noise (fyi)

[MWJB]

@Meaux "No, that black wire is a Ground going to the bottom bolt of the Transformer... All the other grounds (except the 115vac) are going to the Top Transformer Bolt... I originally had a 1/4 inch Jack for the Speaker till someone suggested I isolate the Speaker jack from the chassis, then I put in the plastic RCA, and added the ground wire..." I'd put the 1/4" jack back in, or ground the RCA speaker jack much closer to it's mounting (power tube socket bolt?). All champs have the speaker jack grounded to the chassis.

"So I remove the ground to the 8uf, and run another ground wire to the Input G?" Yes.

it's very, very rare to see a 5F1 without the 25uf cap at 12AX7 pin 3...there may be some that never had it, but by far the vast majority do have it. Adding the cap will increase gain, but not noise if the construction is properly executed. I appreciate the 5F1 schematic does not have it, but then the 5F1 layout has no ground reference for the vol pot...and I wouldn't recommend copying that either! ;-)

[Meaux]

I recieved my Choke this AM, Gator clipped it in, and BINGO! Very slight Hum, and that small amount is to be expected with a tube amp...

The Supplier apologised last evening for sending me the wrong resistor... Now I have to remount the circuit board, inputs and Vol. (I have another plain Volume Pot without the switch comming) and hunt up some shielded wire... Do I ground both ends of the shield, or just one end? I also have the 25uf comming...

Does it matter (in relation to the other 2 transformers) where on the chassis to mount the Choke?

I guess I'll start another thread on that, and if I should use a GZ34, or keep the 5Y3 with the 6L6?

Looks like I'll finish before 2012...

This problem is finally solved!!!

[Bruce / Mission Amps]

So? Is the resistor actually a 10 ohm resistor or 1 ohm or 100 ohm or what?
All you did with the choke is make the two capacitors separate and act like they should, with the added feature of some inductance between them... the DC resistance of the choke is likely very low... like 90 ohms to a couple hundred.
But the choke is not a dead short, like your resistor was measuring.
Which is why some of us suspect your 10K resistor is a 1 ohm, 10 ohm or something like that, but not really a 10K.
Really, I can't ever recall seeing a 10K resistor go shorted internally! ha ha.....
In my opinion, you should now put 2 watt 10K or a 5K1 to 7K5 resistor in series with your choke leads to get your screen and preamp voltage down where they belong.

[Meaux]

So? Is the resistor actually a 10 ohm resistor or 1 ohm or 100 ohm or what?
All you did with the choke is make the two capacitors separate and act like they should, with the added feature of some inductance between them... the DC resistance of the choke is likely very low... like 90 ohms to a couple hundred.
But the choke is not a dead short, like your resistor was measuring.
Which is why some of us suspect your 10K resistor is a 1 ohm, 10 ohm or something like that, but not really a 10K.
Really, I can't ever recall seeing a 10K resistor go shorted internally! ha ha.....

I think it's a 10 ohm... And it's probably a good one, but it dosen't matter now...

In my opinion, you should now put 2 watt 10K or a 5K1 to 7K5 resistor in series with your choke leads to get your screen and preamp voltage down where they belong.

I'll probably check those voltages tomorrow... Hopefully my parts will come in... I'm gonna replace the Champ Style On-Off- (not wired anyway) Volume Pot with a straight Volume Pot... I'll have you know you were the inspritation for the seperate On-Off Switch, and Volume Pot, from one of your posts I read awhile back... Thanks!...