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Excessive amp hum? Bad Cat Fat Cat 50 - FOUND IT

sixstring531

Silver Supporting Member
Messages
3,221
I got this one in a trade not too long ago and have noticed it idles a little louder than other amps I've owned. That being said, I've always owned 30 watt AC style amps and this is a 50 watt bassmanesque Bad Cat, so I don't know that I have a good idea of when hum is really excessive. (so I may not have an issue - but thought I would bring it to the group)

The hum goes up and down with the volume knob on the amp, regardless if an instrument is in or not. It really doesn't even bother me that much, though I know my previous Bad Cat (Black Cat 30) was quieter when at the same volume (around noon) and Bad Cats are notoriously quiet.

I'm pretty sure the TAD 6l6s are original (2012 I think) and I have a new set coming. I changed the PI for a good 12ax7LPS and put brand new Tung Sol 12AX7s in the other two slots.

Should I be checking anything else? Speaker cable? Clean the tube sockets? Or does a 50 watt amp just have a little more noise and I'm nuts?

Thanks gents!
Rich
 
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dbun

Member
Messages
844
Could be output tubes.

I know with AC30s (which are cathode biased, so may not apply), they can hum if the output tubes are not balanced properly, which is why getting a matched quad is important.
 

sixstring531

Silver Supporting Member
Messages
3,221
Found the culprit! I bought two brand new Tung Sol 12ax7s before I sold my last amp and threw em in this one. I had the chassis out tonight and decided to clean all sockets and try some sound tests with different tubes. Yup - the new Tung Sols have a massive hum associated to them. Does that mean they're simply hotter than the EH I put back in it?

I also checked the plate voltage (still waiting for my bias adapter to put the new power tubes in) and got 444. Easy enough to remember.
 

dbun

Member
Messages
844
More likely a bad tube, or a tube on the way out.

12ax7s should all have the same gain factor more or less.

Glad you found the problem though! With Badcat amps the problem is rarely, if ever, the amp itself. It's good as you can have confidence that the amp is fine and the problem lies elsewhere.
 

Diablo1

Member
Messages
620
Some tubes are just born hummy, and some turn hummy after some use. It's not by design, but bad quality.
 

KeyserSoze

Member
Messages
39
Damn, I just retubed my JTM-30 a few months ago (TAD 6L6's and Tung-Sol 12AX7's) and have recently noticed a lounder than usual hum ...but I've also been playing my LP Junior alot and just assoiated it with the single P90!
I'll have to swap some preamp tubes around and see what's up!

Thanks for the info! :aok
 

sixstring531

Silver Supporting Member
Messages
3,221
Yeah - I did the same. I played with my Strat and LP and both had the hum. Is the tube "bad" or does it just hum more than others? I will email the vendor and see what they think.

Thanks again!
 

KeyserSoze

Member
Messages
39
Awesome! Good for them ...thought they'll probably just put them back on the shelf and send them to someone else! :eeks
 

Mark Robinson

Platinum Supporting Member
Messages
8,858
Some tubes are just born hummy, and some turn hummy after some use. It's not by design, but bad quality.
This is not actually the case. Some 12AX7 types have humbucking filaments. Some 12AX7 types do not. Sometimes the layout of an amplifier will place a tube in proximity to a transformer field which will induce hum in a tube without humbucking filaments. So this means a tube that is strong and sounds great, can turn out to be a poor choice for some positions in certain amplifiers.

Like everything else in life, I had to learn this the hard way, banging my head trying to have fun rolling tubes in amplifiers.
 

TimmyP

Member
Messages
2,488
JCM900s are rather picky in this manner, especially V1. The hum balance control does not always help, and a tube swap is the solution.
 






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