Buzz in my 190L

[slippingglimpser]

  Ok, so I've had this amp for a while now, and it's always been etremely reliable, but just yesterday when I was playing (at normal 4-5 volume) the amp started buzzing (and amplifying this buzzing) whenever I'd play an A on any octave.  
I'm thinking it's one of the tubes but am worried it might be something like the reverb spring rattleing on certain frequencies.  The only problem with this theory is that even when I unplugged the reverb in the back, it still amplified the buzz.  
The tubes are only about a year and a half old.  Could they need to be replaced already?  Also, can I replace them myelf?  Last time an amp guy charged me an arm and a leg for labor time just to simply switch new for old.  Thanks to anyone who can offer me advice.

[JoeArthur]

Well, this will probably get me into trouble.  But the primary reason that you should take your amp to a "professional" to have your tubes changed is so they can make some money on the deal.  All of the hocus-pocus around the need to re-bias in one of the 50 thousand superior methods supports this.  If you stick to the same brand of tubes the chances of needing a drastic change in bias is like slim to none.

If you haven't changed tubes yourself before, just do it one at a time so you don't wind up getting the wrong tube in the wrong socket.

Tubes generally fail mechanically before they fail electronically.  Take out each tube, hold it up to your ear and give it a slight shake like some people do with a burnt out light bulb (why they do this I ain't got a clue).  If it rattles replace it.  Oh, do this with the amp off and cool BTW.

With the amp on and running, you can tap (gently) each tube inside the amp with the eraser end of a pencil and if it produces too much noise through the amp (or the same type of buzz) then replace it.

One thing that I have found causing this, is a loose metal shield around the preamp tubes.  If it wasn't placed on too snug to begin with, it can work itself loose over time.  

If you have to change an output tube (Does a 190L use 6550?) replace all of them.  Preamp tubes (the smaller ones with a shield) can be replaced individually.

[slippingglimpser]

Thanks for the info, Joe!  I did the shake test and the tap test and what I found is that both 6550's rattle just a little bit.  Don't know why it happened that they both did this at the same time but I guess I'll replace them both.  Any advice on what's the best kind of tubes to put in?  Right now they're svetlana.  If I change the brand will I have to change the callobration?  Is that hard to do?  Thanks for any help!

[slippingglimpser]

drrrrrrr... re-biased, I mean

[JoeArthur]

I use Tungsol myself.  They just seem to last forever.  The last place I found them was Antique Electronics in Tempe, AZ.  They have a website... http://www.tubesandmore.com.

It wouldn't hurt to check the bias.  You can do it yourself with a VOM.  The positive lead (usually red) should be securely attached to the chassis (I use a test clip - wire with alligator clips on both ends).  

The negative lead needs to be connected to the center tap of the bias adjustment potentiometer.  I don't know if this is true for the 190L, but my Model T has a bias test point on the top of the chassis where I can stick the lead in.  It's a labled, small round red plastic thing with a hole in the middle.

The VOM needs to be set on a DC voltage selection, whichever setting is closest to but greater than 55 volts.  If you get a voltage reading of 55 volts, you are all set and done.  

If not, then you adjust the bias potentiometer until you do (use a slotted screwdriver with a plastic handle) - but be careful with this and watch the meter carefully and do not change the voltage drastically.  Just easy does it... very gentle movements and give the amp some time to "self adjust" to the new setting before deciding to turn it some more.  It doesn't take much movement to change the setting and your amp should be fairly close anyway.  Especially be careful if you have to reduce the voltage and don't slip... too little bias voltage can damage the tubes.

See... no big deal.  However, safety is an issue... these amps have high voltages and if they bite they can kill you.  Make the VOM connections with the amp unplugged and preferably unplugged several hours before so that the caps can discharge.  Turn the amp on and let it warm up... at least 10-15 minutes to stablize.  Never place more than one hand into the amp's innards and keep the other one in your pocket or behind your back to resist the temptation.  Do not touch the chassis and innards at the same time.  Make sure the chassis is supported and do not use your hand to do it!!  This is assuming your amp doesn't have a test point... test points make it much safer.

If you have even the slightest doubt about your ability to be safe with high voltages, then pay a tech to check and adjust - it shouldn't be very much.

[slippingglimpser]

Ok!  Thanks for all your help, Joe!