Name:    Steve Kennedy
Email:   steve@pacair.com  
Subject: tubes,biasing  HELP
Thread:  27
Time:    Mon, 17 Dec 2001 06:55:30 UTC

Your amp originally came Sylvania 6CA7 output tubes. These haven't been made for about 20 years and are hard to find (and expensive if you DO find them).


Many people (myself included) prefer the original "big-bottle" 6CA7 tubes but it is a big investment if you want to find a matched pair or quad! That is why most people use the EL34 output tubes.


The EL34 isn't a bad tube all, actually more of a "hi-fi" tube. They are a bit smoother sounding than the 6CA7 type, but the 6CA7 seems to impart more clarity to the signal. This is just my opinion but I have heard other people express similar sentiments.


Also, the 6CA7 was designed to handle the high voltages used in the Music Man amps (700Vdc at the high power setting). Some EL34 tubes will not last as long in this environment. I have had reasonable luck with both Electro Harmonics EL34EH and the Sovtek EL34.


Some of the early Chinese EL34 tubes would internally arc over at these voltages.


To prevent under wear and tear, I run all my Music Man amps at low power unless I absolutely have to run them at high power. The tubes will last forever under low power conditions as they are subjected to 1/2 the plate voltage (closer to 400 Vdc).


What you should do is to ask around (here and at a few tube suppliers) and make a decision on which type of tubes you want to try. By a matched pair (or quad for a HD-130 model)and have a local amp tech check the bias. IF the bias is at about 15mA and is equal for all output tubes then you are all set to go.


See if you like this new set of tubes. If you stick with the same type of matched tubes there should be little need to have to check the bias everytime except to verify that everything everything else is OK.


I know I check the bias everytime I change tubes, but it isn't often and since I do it myself it is just an investment in time. All you need is to know a little ohm's law and some safety measures and own a VOM to make the measurement.


It is safer to have a professional do this if you are not trained in electronics or if you have two left thumbs! That 700Vdc plate voltage CAN kill you if you do not know what you are doing!


Steve