Ampeg B18 loud HUMMMMMMMMMMMM

[nateflanigan]

Hi I'm working on an Ampeg B18 that has developed a very loud hum.  Here's the facts...


- It has been recapped relatively recently (within the past 10 years)
- All voltages look normal
- I've tried swapping out the power tubes and the preamp tubes, no change.
- OT tests good with resistance tests
- Volume and tone controls effect the sound.  With both volumes down lots of hum, as one volume is brought up it starts to buzz, at around 10 o'clock the buzz and hum seem to hit a null and cancel each other out a bit, beyond that buzz increases.  Both channels behave this way, one channel has to be all the down in order for the "null" effect to occur. 
- Hum is just as bad with preamp tubes pulled.
- Hum is just as bad with PI pulled.
- I've replaced the bias diode.
- The pitch of the hum reads 120 hz on a tuner app, and placing my meter across the output jack it bounces between 60 and 120, so I dunno.


The only tube I haven't been able to verify is the 7199 PI.  They're going for about $50 these days, I'm thinking maybe I should rewire the socket to accept 6U8A tubes...
https://tubedepot.com/t/tubes/preamp-tubes/7199
http://ampgarage.com/forum/viewtopic.php?t=16804&sid=1ddda3e432a056f1904adf4787f47a80


In the mean time any thoughts on narrowing down where the hum is coming from would be wonderful.
Thanks!

[sluckey]

Filter caps are at the top of the suspect list.

[Jack_Hester]

The Bias cap was a serious source of hum on a couple Ampeg 'B' models that I have.  Your schematic shows a 100uf just downstream of the diode.  Swap it out for something greater than 100v rating.  I used caps rated for 160v.  This voltage goes pretty high, before the tubes start conducting.  Something over 100v, but not greater than 150v on the ones I measured.

Jack

[nateflanigan]

The caps are pretty recent.  The Bias cap is a 1000uf 150v sprague, one of the black ones with green writing.  The cap can is from ce manufacturing, https://www.cedist.com/products/C-EC40X3-525.  And the 33uf 600v cap is a gigantic Angela instruments branded cap.


Would you still suspect the filter caps?  It does seem that the hum is everywhere in the amp.  I'm suspicious of the power transformer, how would I check that?

[sluckey]

The Bias cap is a 1000uf 150v sprague

Surely you mean 100µF? If it's really a 1000µF you need to replace it with the correct value.

Would you still suspect the filter caps?  It does seem that the hum is everywhere in the amp.

Anytime you have hum such as you describe, filter caps are suspect. Even if you just changed them yesterday. Other easy to check suspects are rectifier tube or grossly mismatched output tubes.

I'm suspicious of the power transformer, how would I check that?

Why are you suspicious?

[nateflanigan]

Hey Sluckey, you're right.


I disconnected the cap can, it's hardly reading any capacitance at all on any of the terminals.  Almost every time I get stumped on an amp it's because of an assumption I'm making.


Bias cap is 100uf, I need new glasses.


I think I'm going to order this, cap kit from fliptops, it has an increased voltage rating of 600v. 
http://www.fliptops.net/catalog/p-100023/cap-kit-for-ampeg-b15n-b18n-sb12-v.2-x-seriesamps


If a standard re capping doesn't get rid of the hum I'll come knocking again.  Thanks as always.

[nateflanigan]

Finally got the re-cap kit from fliptops (long story), all is well, hum is gone.  Sometimes it's easy.  Thanks again Sluckey.