(10-05-2014, 05:20 PM)beedoola Wrote: Is it important/need to replace the bias resistors? - R49, 50?
Really, again, why not?
I'd make it more critical to replace that diode and the bias capacitors (make the diode it the last part you replace in the bias network so it has the least soldering heat time).
You don't want your bias straying as the amp warms or over time, period. Note that carbon comps (if that's what any of them are) are most likely to drift in value when warm and current is flowing.
If you want to spend the time--my time is more valuable to me than this--measure the values, see how far out of spec they are especially when warm and the amp is hot, but be careful as always. Ok, if they measure within spec, fine, you saved a couple of cents on resistors. I just change them. I'm the "overhaul the amp while it's unbuttoned" guy through and through though. Once and for all, and you are being proactive...especially in a V-series where instability can take out tubes running at or above max spec already.
Some people change the stock resistor values for bias point tweaking. I'm a fan of cold bias here--fatter bass if you ask me. The Fender 60%-70% bias at idle rule will eat tubes here, and with my real-time experiments, warmer biasing adds a kind of midrange but makes the lows mushier. This amp's great features are that it has tons of midrange available, and the lows can as FAT or FATTER than any other amp I have played without mushing out. The cool bias clears up the mids to let the mid knob do what it does, and keeps the lows fat and powerful. So, why bias this one hot? Some loose sleep (like I once did) because some forum doods claimed that it is across the board better to have hottest bias... Don't do it, at least not with a V-series...but hey, for anyone that thinks the amp lacks mids(?) or the bass isn't mushy enough, and wants to replace power tubes as often as possible, hot bias is your ticket here. But maybe they should play a Fender tweed or blackface amp, if so (which are no doubt quite nice amps)... ;D
I prefer a really cool idle bias with this amp, tonally (as low as ~40% is quite good IMO). IT also, serendipitously, prolongs tube life. The stock values are wisely chosen and good IMO as well.