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Post subject: Champ Hum Mitigation
Posted: Sun Jul 16, 2017 11:33 am
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As much as I love my newly recapped & restyled '72 Champ project, I am going to have to address a problem with 60hz hum. The hum is even present with nothing plugged into the input jacks, and with the volume at zero.

I'm fairly certain the shorting contacts on the input jacks are working fine and have a 1m resistor wired per schematic. I'll double check that AND make sure are grounds are good before I do anything else.

If all that checks out, how effective is the artificial center tap mod for the heaters in combating 60hz hum? Is it a "no-brainer"? Twisted pair runs to heaters that are as perpendicular to signal lines as possible can't hurt...

I've also seen talk of folks adding a dc bias to the heaters by creating a voltage divider with a 220k resistor to B+ in series with a 22k resistor to ground. One side of the heater circuit then ties to the center of this divider. I think the key here was to add a dc bias voltage of more then 35v but less then 40v.

I'd appreciate your thoughts/comments/experience with this issue!


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Post subject: Re: Champ Hum Mitigation
Posted: Sun Jul 16, 2017 3:54 pm
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Ground location issue?

UN


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Post subject: Re: Champ Hum Mitigation
Posted: Sun Jul 16, 2017 4:47 pm
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7chord, the BF and SF era Champ (AFAIK) do NOT have a center tap on their heater lines. Rather, one line is soldered to the chassis and the other goes to the sockets. I don't think adding 100-ohm resistors to the lines will work in this arrangement.

You'll need to lift the line going to the chassis. Then, extend this line to each socket --- to get the typical twisted pair. Also, you'll need to remove all independent, chassis-grounded heater lines from each socket). Then, you can add the 100-ohm resistor, from the two pilot lamp attachment points --- to ground. To get your CT.

I have done this on several Champ and VC amps. And it does lower the 60Hz hum noise.

On your amp --- check the solder point on each of the heater lines. Esp, ones to ground. You may have a cold or broken solder, that is induces this hum.

Good luck! :mrgreen:


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Post subject: Re: Champ Hum Mitigation
Posted: Sun Jul 16, 2017 7:48 pm
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Yep, that's correct. I'm well aware of what's involved - was curious as to the efficacy of the cure! I'm going to go ahead and do the mod, and as I said, will check everything else as well. The hum is starting to annoy me...


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Post subject: Re: Champ Hum Mitigation
Posted: Mon Jul 17, 2017 11:42 am
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Quote:
newly recapped & restyled '72 Champ project
Got a pic? Assuming it has a three prong plug and not two.


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Post subject: Re: Champ Hum Mitigation
Posted: Mon Jul 17, 2017 8:03 pm
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sfceric64 wrote:
Quote:
newly recapped & restyled '72 Champ project
Got a pic? Assuming it has a three prong plug and not two.


See viewtopic.php?f=12&t=107250&start=23

Yes, it has a 3 prong plug - It's a '72 SF champ with a serious makeover... :D


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Post subject: Re: Champ Hum Mitigation
Posted: Tue Jul 18, 2017 1:12 am
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A single end amp always have hum.

Ampware say '72 champ use AA764 circuit wich do not have heater center tap to ground
Adding two 100 ohms resitors will quiet the amp.

Older amp wich use chassis ( ground ) to feed one heater side must need a new wiring to quiet the amp.

Wich circuit did you use ?

http://ampwares.com/line/champ/


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Post subject: Re: Champ Hum Mitigation
Posted: Tue Jul 18, 2017 6:39 am
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7chord wrote:
sfceric64 wrote:
Quote:
newly recapped & restyled '72 Champ project
Got a pic? Assuming it has a three prong plug and not two.


See viewtopic.php?f=12&t=107250&start=23

Yes, it has a 3 prong plug - It's a '72 SF champ with a serious makeover... :D
Didn't realize we were talking about that same amp. Still haven't seen any innard pics, any mods to the circuit?
Have you tried a lantern battery to run the lamp/heater line to ensure that's the issue?


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Post subject: Re: Champ Hum Mitigation
Posted: Tue Jul 18, 2017 6:57 am
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stratele52 wrote:
Ampware say '72 champ use AA764 circuit wich do not have heater center tap to ground
Adding two 100 ohms resitors will quiet the amp.


Stratele, are you sure about this? The schemo and layout for the AA764 in a SF era Champ has one leg of the heater line grounded. Ground is an open triangle. Check the sockets, too.

:)
http://ampwares.com/schematics/champ_aa764.pdf

Here's photo of a silverface era Vibro Champ:

Image


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Post subject: Re: Champ Hum Mitigation
Posted: Tue Jul 18, 2017 7:47 am
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BMW2002Ti wrote:
stratele52 wrote:
Ampware say '72 champ use AA764 circuit wich do not have heater center tap to ground
Adding two 100 ohms resitors will quiet the amp.


Stratele, are you sure about this? The schemo and layout for the AA764 in a SF era Champ has one leg of the heater line grounded. Ground is an open triangle. Check the sockets, too.

:)
http://ampwares.com/schematics/champ_aa764.pdf



You are right , I did not look schematic carefully.
To remove the hum, we should remove the heater side from the ground,( use a wire ) and put two 100 ohms resistors to make a artificial ground.

I do this mod on a Champ clone with success


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Post subject: Re: Champ Hum Mitigation
Posted: Tue Jul 18, 2017 6:13 pm
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I'll report back as soon as I get a chance to do the mod.

The other thing I asked about: Has anyone tried to add the DC bias to the heaters?? How did that work out?


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Post subject: Re: Champ Hum Mitigation
Posted: Tue Jul 18, 2017 11:16 pm
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I've only done DC heater conversion on push-pull amps. I don't think the added cost and extra space needed is justified in most single-ended units.

Hi-fi guys that DIY or buy muy $$$ single-ended amps (like Kondo Kagura amp by Audio Note) --- convert to muy $$$ off-chassis DC heater (with regulation) --- as the performance is deemed worthy. Esp in systems with very efficient and $$$ speakers. Being used in near dead quiet conditions.

Image


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Post subject: Re: Champ Hum Mitigation
Posted: Wed Jul 19, 2017 2:27 am
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I try DC heater on amp clone and it is not more quiet than a well built amp with AC heater.


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Post subject: Re: Champ Hum Mitigation
Posted: Wed Jul 19, 2017 6:50 am
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I'm not talking about converting the heaters to DC.

I've seen talk of folks adding a DC bias to the heaters by creating a voltage divider with a 220k resistor to B+ in series with a 22k resistor to ground. One side of the heater circuit then ties to the center of this divider. I think the key here was to add a DC bias voltage to the ac heater circuit of more then 35v but less then 40v.


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Post subject: Re: Champ Hum Mitigation
Posted: Wed Jul 19, 2017 7:15 am
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7chord,

I think you need to lift the grounded heater line and have two twisted pairs of heater leads, to each tube. You can use the cathode voltage off the 6V6GT to supply the needed DC. Valve Wizard discuss this, in his website.

:)

http://www.valvewizard.co.uk/heater.html


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