Silverface Champ Noise When First Turned On

[peetez]

So when I turn on my champ from cold, it has no issues whatsoever

If I turn it off after having it warm up, then turn it on again when it's warm, it lets out a huge 60 cycle hum for maybe two seconds, then it quiets back down and doesn't give me any issues after.

I'm worries there's something wrong with it, any ideas? It's a non vibro champ, likely a aa764

[Latole]

I could be a tube.

Did this amp see a tech ? Amp may need new filter caps .

There is nothing to worry about. If you want to be safe for next 20 years do a complete service:

3 prong power cable: a must for safety and for quiet amp
Tubes
Filter caps
100 k plates resistors
Tubes's cathod 'lytic bypass caps
Check if fuse is the rignt one 1 A slow -blow

[peetez]

the power tube has been swapped not long ago, I also tested with a different rectifier tube to the same result

the amp has been 3 pronged but the filter cap is original, plus the first cap is actually a 40mfd instead of 20 as called for on the schematic, I'm planning on replacing it with 3 axial 20mfd caps. It's just the problem is so strange I've never found any mentioning of this sort of problem anywhere

[Latole]

I did not read you try know good 12XX7 tubes; Issue my be there more than Power tube.

Rectifier you try ; they are good ?

I don't think 40 mfd for first cap is not good. No need to be a 20mfd.

3 axial 20mfd caps ; Do you mean 3 in 1 can capacitors ?   I'll buy JJ's

https://www.tubesandmore.com/products/capacitor-jj-electronics-500v-40202020-f-electrolytic

Other;

 https://www.tubesandmore.com/products/capacitor-ce-mfg-450v-202020-f-electrolytic

https://www.tubesandmore.com/products/capacitor-ce-mfg-525v-202020-f

Schematic

https://el34world.com/charts/Schematics/files/Fender/Fender_champ_aa764_schematic.pdf

[PRR]

> I turn it off ..., then turn it on again

Are you waiting? A lot of things don't like rapid power cycling.

[Latole]

> I turn it off ..., then turn it on again

Are you waiting? A lot of things don't like rapid power cycling.

On-Off switch with burn contacts ?

[peetez]

>

Are you waiting? A lot of things don't like rapid power cycling.

yeah I wait for a minute or two, not just off and then on right away

[peetez]

On-Off switch with burn contacts ?

I've tried to turn the amp off and on with an external power strip to the same result, maybe the switch is having contact issue when voltage is first applied but I don't see how that could lead to a short burst of noise

[peetez]

I did not read you try know good 12XX7 tubes; Issue my be there more than Power tube.

3 axial 20mfd caps ; Do you mean 3 in 1 can capacitors ?   I'll buy JJ's

tbh I never looked into the possibility of a bad preamp tube since I turn the amp on with 0 volume, but hey you know it could be the post volume knob stage

I looked into the cans, but if the 3 in 1 can cap isn't the secured to the chasis the same way, I figured I might as well just remove it and zip tie it in the amp.

Now that I think of it, the only time I've had bad 60hz noise is when this old amp started outputting 100v more than normal out of the power transformer and started frying the filter caps, I'll try playing the amp with a variac, also haven't done that

[Latole]

n
tbh I never looked into the possibility of a bad preamp tube since I turn the amp on with 0 volume, but hey you know it could be the post volume knob stage

n

1- Yes. See schematic.
2- Issue with a tubes amp; first thing to do is to check all tubes

[jjasilli]

Ditto to all of Latole's posts, especially "first thing to do is to check all tubes"

Assuming the hum is truly 60 Hz, this tends to suggests heater hum.  Failure to post a schematic is not helpful.  Merely saying it's a Champ does not specify if it's a scratch build, a Fender build, modified, etc.  The point is that stock Fender Champ heater wiring is not twisted pairs, but just one feed wire, with chassis grounds at each tube socket.  This is more likely to cause 60 cycle heater hum to blend into the signal path, regardless of the vol pot setting.  Does you amp have twisted pair heater wiring?  If not, upgrading the wiring scheme may help.  But always suspect tubes first. 

[Latole]

+1 jjasilli

Layout ;https://cdn.shopify.com/s/files/1/2325/2729/files/Champ-AA764-layout.pdf?5926397210715640943

[peetez]

Hi yall

Changed the ONLY pre amp tube and the problem is gone, thanks for the input, I can not believe that I didn't think of that.

Feeling very stupid at the moment, but hey, learned something today

[Latole]

We always learn and this forum is a very good place.

Enjoy your amp