Not sure if i am posting this in the right spot. But anyway, i'm planning on doing the bias mod to my 6505+. There is a lot of information on it on the internet, enough that i'm pretty sure i will be fine to do it. But i have read a few warnings with the screen resistors, I'm not sure what the problem is but it seems like you are supposed to change them if you do the bias mod? Is this true? I don't want to risk hurting the amp... And is there anything else i should be worried about doing this mod besides making sure i drain the capacitors and don't kill myself? I know not to use the bias test points on the amp. I will get a bias probe.
Also if i only end up biasing around 30ma should i even worry about checking plate dissipation? (i think it was plate dissipation, i might be using the wrong term)
When i do the mod (if i do, i haven't decided i will for sure) i will do a before and after comparison. Seems to me that every time i read "i will post some clips of this" i can't find any clips, and the threat was years old. empty promises? hah. As long as i do the mod i will post clips of before and after.
It's off topic, but i have a comparison of my amp at low volume, post 3, post 5, and post 10 volume, all balanced out to the same volume in the computer to see what the difference is without the high volume difference distracting me. Would anybody be interested in hearing this?
I'm not an amp tech but I figured you reposted over here since your identical thread in GGnA got closed. I've not done the mod before so maybe consider this a free bump. Yes, it makes sense that the screen grid resistor needs to be changed. Yes drain the filter caps. Do not touch anything on the inside of the amp until that is done. The FJA mod on that blueguitar.com page looks pretty good. Just follow that. You don't really check the dissipation per say, what you need is the plate voltage. What that guy (glpg80) is saying below is that because of the way the amp is designed you would have to mod it further to get a linear reading from voltage to current as it relates to the bias so what's the point.

That said, I alway bias my amp to the best sound not some predetermined number. I know from experience that biasing to a number is a waste of time. However, if you know the plate voltage and general plate dissipation number you want to target (say 60%) then you can determine a bias current mA to shoot for to get started.

These are the two threads I usually link people to...

This detail from the Peavey Forum that you've probably already seen:

Then read this one too and pay attention to what glpg80 says

I also have a blog in my profile on how to bias an amp with a bias probe and multimeter that has lots of info and safety tips if you are interested.

Now. Honestly, I would just say get someone who knows a little bit more about electronics to help you.
I would like to have an somebody who knows what they are doing help me haha, but i live in a town with 2500 people... the odds of somebody knowing electronics well aren't to bad, but that and being interested in guitar/ guitar amps seems pretty rare haha. If somebody isn't into guitar amps they probably don't need to know much about tube amps.

And i'm not sending it off to have this done, it looks so easy. I know how to solder, i'm at least good enough with electronics to wire guitars without schematics (not becuase i memorized where everything goes, i figure them out as i go because every guitar i've wired has been different). I know that's not impressive but i do have a basic understanding of how electricity works.

Anyway, everything to me makes sense at this point except for whether or not i need to do the screen resistors. Why do you think i would need to do them? I've not seen on any post of how to do the bias mod somebody actually changing the screen resistors. Doesn't seem like most people do it. A few people have just mentioned it may need to be done. It seems like with so much information of the bias mod, if the screen resistors need to be changed it would be mentioned more. But i don't know... Why would the screen grid resistors need to be changed when the amp is biased properly?

Oh and yeah i moved the thread because i figured amp building is probably closer to what i am asking.

Thanks for replying!
That is where my knowledge falls off. Assuming R68 is not a screen grid resistor I'm assuming that this aspect refers to what glpg80 was saying. He was basically saying that with the 6505 is that due to the way it is designed the mA and the mV are not linear (doesn't follow ohms law). Because of this, biasing with a multimeter is pointless.

Like I said though, you should bias with your ear anyway.

Cathbard or Colin may know more.
You don't need to replace the screen resistors.

If people are replacing them when doing the bias mod, it would be for repair or improvement reasons, not by necessity.
If the screen resistor was blown or damaged, you'd need to replace them.
If the stock screen resistors suck or you want more heat dissipation, it would be a convenient time to replace them while you were in there.
If you were planning to use a different type of power tube, you might want to re-spec those resistors.
If you were getting oscillation or wanted to change the amount of sag from the power section, you could do that too (though neither of those are likely scenarios on this amp).

All of those are totally optional, though (except if they were blown). You do not need to replace those resistors for the bias mod. It's just a common thing to do while you've got the chassis out, is all. That's why you'll see it mentioned in passing when people are doing other mods.
I see no need to change the screen resistors but I probably would. Here's what I think they are talking about. With the bias mod done you will be running the tubes hotter and the chance of the screen resistor blowing goes up with that. The 6505 has a tiny screen resistor, I'd pull it out and stick a 750R to 1K resistor anyway, 100 ohm is just too small for a 6L6 imo.

The bias probe will allow you to check the plate voltage as well as current so you'd be mad not to measure it just to be safe. Don't just measure the current and hope for the best, measure both of them.

I'd recommend using a shaftless rotary pot for the bias adjuster. Not a trim pot, a full sized shaftless pot. Then you can mount it so you can adjust the bias without removing the chassis.
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Last edited by Cathbard at Aug 16, 2013,
ok thank you guys! I will leave them as they are then because i don't want to do something i don't have good directions on doing. And i will check the plate voltage. Also my 6505+ comes stock with a pot that is accessible without removing the chassis. It just doesn't do much as it is haha.