New Champ Build Problems...

[jmaf]

On the OP's first post he said it was modified to run "6L6's" so I though this champ had 2 tubes. Now it's been clarified it's a 5f1.

Edit: I should've guessed from the 470 ohm cathode resistor.

[Enzo]

First, it is single ended, there is no push pull hum cancellation.


Fellas, please, can we post schematics instead of layouts please?

The plate supply of the powr tube comes directly off the first filter. All the ripple on that node will come right through the powr tube stage.

I have a service bulletin from Fender from long ago telling us not to try to "fix" humming Champs. It was normal and not a defect.

If you were to move the red output transformer wire over to the second cap, I bet the hum would go down. Not a recommended solution, just a demonstration of the concept.

Those cap values, 16uf and 8uf, may be authentic for historical reasons, but those small values were a matter of being cheap back when that circuit was new. Increasinjg them to more modern values like all of them 22uf shoul dimprove hum.

You can't pull the 5Y3 and hear anything, since that removes the power supply.

[Meaux]

OK, I've checked and rechecked everything as far as wiring goes till I'm blue in the face... Which components should I be looking to replace/check first? Or, should I just find the nearest trash can?

I want to stick in a choke while I have the PITA apart...

[MikeH]

I certainly wouldn't send it to the trash can!

Perhaps it would be easier to assess the issue if you could post a recording of the hum? Since the hum is still present with the preamp tube pulled you should be checking things in the power section - output tube (and all associated wiring and components), output transformer, and filter caps. Enzo had a good point about upping the filter caps. I have a champ-ish clone that I used 2 20uF's and 1 8uF (simply because that was what I had on hand) and it is quieter than the stock 8/8/8 or 16/8/8 configuration.

Although it would help to hear the hum. Champs are already on the noisy side, and upping the output (meaning going from 6V6 to a 6L6, etc) is only going to up the noise. High wattage amps are almost always push-pull and hum cancellation is one of the main reasons, I think.

[Tom Phillips]

OK, I've checked and rechecked everything as far as wiring goes till I'm blue in the face... Which components should I be looking to replace/check first? ....

I say back up and apply more thinking & research instead of jumping into parts replacement.
We cannot tell how bad (or normal) your hum problem is by the words that have been posted. I suggest that you listen to other champs and determine if they "have a hum problem" too. If so then you will need to improve the basic design your particular build with better power supply filtering or possibly transformer orientation. If other Champs don't exhibit excessive hum (to your ears) then you can suspect a flaw in your build and you need to look at it until you are even bluer in the face. I suggest that you do the comparison with a 6V6 installed as the power tube. Then you will be comparing apples to apples.

My personal opinion is that Enzo's post #17 nails the situation. Your 6L6 "Mod" draws more current from the wimpy stock power supply, increases the ripple at the power tube plate and makes the hum even worse than a stock champ.

Regards,
Tom

[Meaux]

I certainly wouldn't send it to the trash can!

Perhaps it would be easier to assess the issue if you could post a recording of the hum? Since the hum is still present with the preamp tube pulled you should be checking things in the power section - output tube (and all associated wiring and components), output transformer, and filter caps. Enzo had a good point about upping the filter caps. I have a champ-ish clone that I used 2 20uF's and 1 8uF (simply because that was what I had on hand) and it is quieter than the stock 8/8/8 or 16/8/8 configuration.

Although it would help to hear the hum. Champs are already on the noisy side, and upping the output (meaning going from 6V6 to a 6L6, etc) is only going to up the noise. High wattage amps are almost always push-pull and hum cancellation is one of the main reasons, I think.

I'm not able to record Hum... But, my 60's Pro Reverb on 10, is a mere whisper compared to the Champ...

I thought the fact that the Filter Caps dump themselves to 1v, would have been a better clue... I'll look thru all the old Caps I pulled from the Pro Reverb and see if I can come up with something... I still wanna install a choke though...

[Jag]

My second thought was transformer orientation.

My first thought was, can you detail your "Star grounding on Power Transformer Bolt" method. Do you mean you have everything grounded there? I would ground the first two filters at the bolt, and star ground everything else near the input.

Do you have any pics of the innards?

[Meaux]

I'm not good with pictures either...
Board before chassis...


Power Transformer side


More Trans...


Back of circuit board...


6L6 wiring... Edit... Funny, the picture shows the Resistor between the 8uf caps to look like it fell off a truck... Dosen't look that bad in real life... Could that be my problem?


Star grounding to trans. bolt...

[Tom Phillips]

I thought the fact that the Filter Caps dump themselves to 1v, would have been a better clue...

That is normal behavior. Immediately after the amp is turned off the tube heaters remain hot and the amp circuitry continues to draw current from the caps. In this case the relatively small caps are discharged rather quickly.
Tom

[Meaux]

Thanks!

[Meaux]

OK, I found a good 20mfd 450v Cap and clipped in on top of the 16mfd 475v and there was a little difference... Then I tried an 80mfd and the Hum was cut to half, but still there... There was a difference so I must be getting closer???

Now I reckon I'll look for Resistors to swap out on those Filter Caps???

[MikeH]

Woah there! 80uF is too high, I think. Rectifier tubes can only keep up with so much filtering capacitance. I'll let some one with more knowledge chime in regarding what a 5Y3 can do in this instance, but I've heard you don't want to go much over 20uF in the first node on a champ.

You say that the OT was replaced with a higher wattage OT to run a 6L6, but what about the power transformer? Is it still the same stock 5F1 tranny?

[Meaux]

Woah there! 80uF is too high, I think. Rectifier tubes can only keep up with so much filtering capacitance. I'll let some one with more knowledge chime in regarding what a 5Y3 can do in this instance, but I've heard you don't want to go much over 20uF in the first node on a champ.

Yeah I know 80mfd dosen't go there, I was just making a Test... I'm at the end of my rope with a pile of Electronics that has turned to crap...

You say that the OT was replaced with a higher wattage OT to run a 6L6, but what about the power transformer? Is it still the same stock 5F1 tranny?

Specs on Power Transformer... http://www.magneticcomponents.net/40...ev_B_Stock.pdf
"Upgrade and/or general replacement power transformer for all of the above mentioned amp models. Plate winding 550 or 630V CT @ mADC. Voltage compensated for Tweed Series amps as well as later models. Heavier current capacity (100mA vs. 70mA) provides cooler operation and better regulation. Provided with thermal cutout for added safety. The center tap on the heaters is an additional feature. Paper layer wound like vintage era originals!"

Specs on Output Trans. Output Trans. http://www.classictone.net/40-18031.pdf
"This is a larger, multi-purpose version of the original single ended, 5W output transformer used in Champ**, Tweed Princeton**, Vibro-Champ**, Harvard** and Bronco** style amplifiers. These can be used as an upgrade for these amplifiers or in higher wattage, single ended amplifier projects that requires up to 15W. They can be used with 6V6, EL84, EL34 and 6L6 output tubes. These can also be used with twin 6V6 or EL84 tubes wired in parallel. They will provide increased headroom and clarity when used with single 6V6 and EL34 stages. Also, these feature 5K and 8K Ohm primary impedances and 4/8/16 Ohm secondary impedances. These are paper layer wound output transformers like the vintage originals to provide that ultimate tone! If used as an upgrade in original Champ**, etc. style amplifier chassis, they may require chassis hole modification since they are slightly larger than the originals...See specifications for details."

Layout... http://site.triodestore.com/5F140-18027120V.pdf

[Meaux]

I think I have found my problem... The 10K-2W Resistor between the 16uf and the 8uf filter caps is bad... .000 Resistance... I disconnected it and gator clipped in a plain wire in it's place and I have the same Humm...

Dang, I knew I should have installed a Choke before finishing... I'm gonna order a Choke and Report back...

[Tom Phillips]

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