Peavey 6505+ tube biasing

[InnerDignity]

Hello, I'm terribly sorry. I was attemting to search for a similar thread but my phone isnt letting me. Anyway I want to bias my power tubes in my 6505+ but I'm having alittle trouble understanding exactly what I'm suppose to do. I get the basic idea but some things I'm still unsure of. Like which setting do you use on your multimeter? And once you have your multimeter set up, what level do you set your tubes at? I also realize that's a relative question so to be more specific, at what level do I adjust the tubes at to get a hardcore, djenty, gainy sound? And also if I adjust the level, does it affect ALL the tubes or just one? And if it's the ladder how do I set ALL of them?

Any help is extremely appreciated. I tried doing research but it seems like every forum dosen't have the same questions as me and every youtube video seems like it's about an amp that I've never even heard of. Please help me lol. Please and thank you, and God Bless!

 

[glpg80]

For starters, biasing isn't going to be something you hear extreme differences with until you're well into band setting volume levels.

The purpose of biasing is to get the amplifier out of crossover distortion. Look at it as much more about proper operation of a class A/B push pull poweramp.

The reason you bias class AB amplifier is because a sine wave has a positive slope and a negative slope. The zero crossing is the point where you want both halves to meet evenly. If there is a pause or lag in the joining of both signals, the speakers will try to stop mid throw in their movement as well. You lose a lot of definition, top end clarity, and bottom end clarity when the bias is not set properly. You do not want to set it too hot, otherwise the power tubes are always conducting even when a signal is not present. This can allow DC to leak into the primaries of the output transformer and cause primary core saturation which causes severe damage (as well as bias runaway/redplating of the power tubes). This is due to the cores of OT's not being air-gap cores.


It does affect the feel of an amplifier. However on that note, you cannot bias a stock 6505+ properly without modification. 5150 II's do not have enough movement to bias all of the crossover distortion out of the poweramp signal.

Therefore if you want to do it right take it to a technician the first time and explain you need a bias modification to allow for proper bias settings.

Afterwards you need to invest in a comp-u-bias meter which is the safest manner of biasing. Do not use the eurotubes adapters for fitting inside the octal sockets, the traces are not stout enough, overheat, and short causing severe damage.

Note you cannot play the amplifier while the bias probes are plugged into the amplifier, the traces inside the octal sockets are not designed for the abuse.

Have fun

 

[InnerDignity]

The only amp technicians in my area are a long drive away, and those technicians have no idea how this amp is suppose to sound. I recently just got it back from a professional because the original problem was a pre amp. It works now and the technician even said it sounded beautiful, and it actually sounds horrible, and I paid $100 for that horrible sound. I really want to learn to do this myself because of my situation and simply because I have the desire to. I don't need to know how everything works, just a step by step process on what I need to do to get this amp sounding normal again. All the questions I need answered are in my original post.

 

[glpg80]

I've done my best to answer your questions and gave you advice on what hardware to use. What you're asking is very generic, so I'm going to break down this reply into two parts.

Part A: Your Bias desires.
The bias modification does not involve tonal tuning characteristics because it is (mostly) an operational modification. You cannot use a (cheap) multimeter to measure cathode current in these amplifiers, and the red/black bias voltage plugins on the back are useless for setting idle cathode current as well. As far as your tube setting questions, they are based on 1950's datasheets that can be found on the internet for class AB applications taking into consideration build specification changes for today's tube qualities. There is a lot to learn and no single step by step writeout is the same for every amplifier or tube choice - you're either going to need to learn the ins and outs of biasing completely, or risk damaging your amplifier or even worse severely hurting or killing yourself. This is why most people prefer to let technicians bias their amplifiers.

Part B: Tonal Desires
For the tonal problems, look over your tubes and consider purchasing different types of 12AX7 preamp tubes. They can be swapped when the amplifier is off. Use the search feature to find what everyone else recommends. Consider looking at what is in V1 and V2 of the amplifier and changing those first.

Summary:
You own a tube amplifier. It is going to take trial and error with not just tubes, but speakers, guitars, pickups, guitar woods, effects, cabinets, and a plethora of variables to make it sound the way you want it to. That involves money and time.

 

[vick1000]

Has the amp been modified for adjustable bias? What were the problems you had that made you take it to the techs? What's wrong with the tone now?

 

[InnerDignity]

The amp has not been modded in any way. The original problem was the amp would turn on but just create static and pops and no guitar sound. The amp tech told me it was a faulty pre amp, which he was right I saw it myself. On the bill it said he replaced the pre amp and re biased the output. And as for the tone now it just sounds like the worst distortion you could think of and messing with the gain just adds more dirtieness to it, and not in a good way. I ordered more power tubes just in case. I figuered if even that wasn't the problem it wouldn't be a bad idea to buy tubes. Whether I need them now or not I know I'll eventually need them.