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#1 2013-02-24 12:34:23

Dan56
Member
From: RI
Registered: 2012-09-02
Posts: 37

B Sharp Music

Want to give a recommendation to Dave Martinka at B Sharp Music, 265 Broadway, Providence.  He did the recap and filters, tune up you could say on my V4.

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#2 2013-02-24 15:12:41

Dan56
Member
From: RI
Registered: 2012-09-02
Posts: 37

Re: B Sharp Music

I asked Dave about biasing and why mine was set as it was.  He gave me the following answer.  It was more than I ever expected.  I thought people would appreciate this as a reference to his work:

"I like to use all of those methods together. I generally start biasing an amp at 50-60% of the maximum dissipation rating, using a "biasprobe" which inserts a resistor between the cathode of an output tube and ground. I measure all of the voltages, do the math to calculate the idle dissipation, then play the amp a little bit to see how it sounds. Then I hook up a sine wave generator, and monitor the output waveform on my O-scope. I look for crossover notch distortion, and if I see any I will usually move the bias to run the tubes a little hotter, until the notch disappears. Then I check the current draw again (signal gen. off) just to see where it's at. At this point, if everything is within acceptable tolerances, I will hook up a dummy load and run a sine wave into the amp for 30-60 minutes for a stress-test. At the end of this, I let the amp settle for about five minutes, then check the bias again. The bias has usually shifted a tiny bit since the tubes have burned in a little bit, so I bring it back to where it was previously, then play some chords through it and make sure it sounds good.

I have discovered that I can get very repeatable results this way, and that certain amp models almost always end up at about the same idle point. With a V4, I find that if I start out at a cathode current of 27mA, I'm usually right where I need to be at about 15 watts/ tube. As you bias hotter, you'll get more compression, but you can get closer to the safe dissipation limits, and you will wear out your tubes faster, so I usually end up going for around 55-65% idle dissipation, which I have found to be a nice compromise between tone and tube life.

One thing about the V4, is that the EQ has a huge dynamic range - if you crank up the bass, you WILL lower the headroom of the amp. Same goes for the mids if you set them for 300Hz. Ease up on the bass, watch the midrange, and generally don't use a lot of EQ boosting, and you'll find you get more headroom out of those amps. If you really want more clean headroom out of a V4, the best thing you can do is switch to 6550 output tubes. I have done this many times, and it gives you a huge clean tone, like 2/3 of an SVT, but at that point it starts sounding more like a bass amp... smile"

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