Hi, I recently purchased a used Audio Research VSi55. I have been enjoying it immensely. After reading up on tubes and tube BIAS, I started to wonder if I should replace my tubes or not. My VSi55 has Svetlana 6550C tubes in it, from Audio Research. I'm guessing that they are at least a year old.
I got out the ARC manual and decided to try to check the BIAS on the tubes. I am a novice with the voltmeter, so I need some help. I've turned the voltmeter to what I think is the correct setting for volts DC. (The V with what looks like an = sign next to it, although the bottom line in the = sign is broken into 3 dots)? I've selected the "2" setting, as opposed to 200m, 20, 200, etc. Is this the correct setting for checking BIAS? According to the ARC manual, the VSi55 normally ships with 6550EH tubes, which should have a BIAS reading of .065 Volts DC.
So, here are my questions: 1) Am I using the correct setting on the multi-meter? 2) Should the 6550C's be close to the 6550EH's in terms of proper BIAS?
When I used the settings above, I'm getting a reading of right around -.150 on each set of test ports (one for each of the four tubes). All are within .005 of this reading.
This seems WAY off from .065, and I'm negative! Help!
Also, any recommendations on retubing? Should I use KT88s or stick with the 6550Cs?
I would say your meter is doubling the voltage and your reading should actually be about .075. And, I would suggest you spend about $20 on a digital meter which shoud read out the actual voltages. Take that meter down to the shop where you bought the ARC amp and have them show you exactly how to check and set bias. Though your tubes might not be spot on according to the manual, they would appear to all be in the ball park and evenly matched.
Bias is something a tube amplifier owner can often use to their advantage by upping or lowering the bias voltage. If you begin searching for new tubes, any good tube dealer can give you advice regarding which tubes might work best in your amplifier. Many of the tube dealers have some descriptions of the basic sound of a certain tube and you can begin your search with these sites in mind. Remember the sound of a tube is dependent upon the circuit it is placed in. The same tube in two different amplifiers might sound radically different.
The sonic difference between the various 6500's and KT88's is a matter of the tube itself. In general, the KT88 offer slightly a smoother sound than a 6550, but that is a very broad statement that could be disproven in any given situation.
Assuming you are using an incorrect setting on your VOM, however, I would say you are nowhere near ready to re-tube your amplifier out of necessity.