I used to adjust bias and let amp sit for half hour and then re check bias. I would actually play the guitar with probes hooked up and pick the sweet spot but my memory tells me that when i went on the hot side, it would sound too gainy or sizzly. I don't remember too cold though but i think it was like you said......dull.
I know that the probe I have says that you can play while it is hooked up....I haven’t actually done that yet. I worry that I’ll blow something up. Lol When the bias is hot/cold it’s really just small variances in the tone. It still gets tons a gain and basically sounds good but there are slight grainy overtones or a slight lifelessness. Probably things only the player that owns he amp would ever notice. Then there’s the volume flucuation things. That’s the tell tale sign somethings up.
Every since i blew a tube with a loud popping sound......i am ready to change my tubes instantly if i hear volume fluctuations or crackling sounds!! Scared the crap out of me and tube looked like it was about to burst into flames! They were brand new Ruby's that were obviously defective...
I definitely agree that the bias affects the amp greatly. Maybe more so than other amps.
Glad you got your tone back GTR.
I've never really experienced a bias drift upwards. Mine tends to drift downward - but I'm guessing that has more to do the power tubes being used - because that is the big variable. I DID however play my amp with a high bias on purpose one time when I was biasing my amp. I was looking for the sweet spot and not really trying to 'force' my opinion based on the numbers - more just what I thought was best. After my amp got up over that 35-36 mA range it was not a happy place to be.
I blew my first bias probe and Scott told me to never play the amp with bias probes hooked up so that is what I'm going to do even thought it no as convenient. I know some probes out there claim you can play through but I don't know anything about their construction that allows them to do that.
You know what....now that I’m thinking about it, I may have set the original bias on the wrong set of tubes! Normally id use the right hand side inner tube of the quad but I had the amp out of the head shell and upside down....I believe I used the opposite inner tube. I’m not sure how much that might affect the bias numbers. When I rechecked the bias it was high....then when I reset it I used the proper tube slot.
How many probes were you using? So if i am understanding correctly, you checked one tube and then when you flipped the amp around...you checked one tube at the opposite end thinking it was the same tube?
Not sure I'm fully following but Scott told me to bias the inner pair together and the outer pair together so that is how I described it in the tutorial I made. You wouldn't think it would make a big difference but I do know that power tubes in a Class A/B amp work in pairs. I only have a single socket probe. I also only have 2 (inner) power tubes to deal with because of running at half power.
The key is to just keep track of what tube is rated what - and then match up the closet pairs.
After I did some more research I was a bit surprised to find that some manufacturers (including some Marshall amps) pair them like this: 1 2 1 2 instead of: 2 1 1 2 - like Splawn and many others
Last Edit: Feb 5, 2018 17:31:26 GMT -5 by splawndude
Yeah i match up the pairs that are as close to each other as possible and use four probes checking all are in same ballpark before letting amp sit for 30 mins on and then come back and check all 4 again.
I ordered a matched set from tube depot. They were labeled: 2- labeled “22” 1- labeled “23” 1- labeled. “24” But since there is only 1 bias adjustment screw I figured there’s not really a way to bias them as individual pairs. Soon used the right hand side inner tube as my bias tube. You’re saying that I should check all of them individually and them pair them up as closely as I can and then go back and use the right side inner tube as my final bias number?
Ok... What I originally did was this 24-22-22-23 BUT...that was when I biased on the wrong tube. Then for some reason I swapped 24 with the 2nd 22 and rebiased. That is how it is now. I’ll swap them back and rebias again.
Yeah they are closely matched but i would still pair them up like Splawndude said. I use four probes but that is not necessary as long as you paired them up.
I bought four probes because i just want to know what every tube is doing before i close it all back up. Ever since blowing brand new tube i am a little paranoid. Like i said it's not necessary unless you are paranoid lile me lol
Ok, I’m currently rebiasing the amp. I matched the pairs as described above. I emailed with Scott a little while ago and he recommended biasing at about 30mA on the full power side. That will yield about 40mA on the Drop B side. So that’s what I’m doing. Every video I see of Scott playing he has KILLER tone.... It’s been holding fairy steady between 29.9-30.3 for about 30 minutes. I’m happy with that.