Biasing a 6505+

[kylendm]

Hey guys I'm confused by how to bias a 6505+?

I have a DMM and it's got the bias terminals but what setting on my multimeter do I have to turn it on to bias? Also what is correct? Check thing bias with power on or both power and standby?

I set it to 200ma and only turned power on and it said 18 but when I turned it onto standby as well it went to 16 and the power tube were glowing immensely. I unplugged the multimeter turned it off immediately. Anyone want to give me a hand? I have a bias probe as well but I don't want to take the chassis out.

 

[glpg80]

the bias taps on the back of the amplifier are for negative bias voltage, not current. also, if it is a stock 6505+ you will need to change the resistor in series with the bias trim pot in order to get the amplifier out of crossover distortion - stock with the potentiometer all the way up, it still is not out of crossover.

send me a PM for more details if you would like

-matt

 

[RockStarNick]

the bias taps on the back of the amplifier are for negative bias voltage, not current. also, if it is a stock 6505+ you will need to change the resistor in series with the bias trim pot in order to get the amplifier out of crossover distortion - stock with the potentiometer all the way up, it still is not out of crossover.

send me a PM for more details if you would like

-matt

For my own knowledge, what is crossover distortion?

 

[glpg80]

AC is alternating current, a swing from a negative voltage to 0 to positive voltage back to 0 which counts as a full cycle. crossover is where, instead of continuing from 0 to going positive (or negative) the alternating wave stays at 0 for a period of micro seconds and then continues. think of it as a speaker moving in and out and the speaker stopping in the middle and continuing again. it sounds like ass because the sin wave is not a perfect sin wave, but distorted due to cutoff at 0.

 

[Christian Noir]

Hey guys I'm confused by how to bias a 6505+?

I have a DMM and it's got the bias terminals but what setting on my multimeter do I have to turn it on to bias? Also what is correct? Check thing bias with power on or both power and standby?

I set it to 200ma and only turned power on and it said 18 but when I turned it onto standby as well it went to 16 and the power tube were glowing immensely. I unplugged the multimeter turned it off immediately. Anyone want to give me a hand? I have a bias probe as well but I don't want to take the chassis out.

I had the same problem on my 5150II trying to set to the milliamp setting. Two of the tubes started glowing like crazy and I thought they were gonna blow!

I have a SPERRY DM-350A that I got at Home Depot and it looks like this:

I was reading around on other forums an watching YouTube videos and saw that some people would set it to millivolts and not milliamps, saying that the reading should match. By doing the following, turning the bias post on a stock 5150II gives me a reading of 33.9 (fully anticlockwise) -47.9 (fully clockwise)

I plugged the black lead into the little COM hole, and the red one into the V (volt) hole.
Plugged it into the test points in the back of the amp and then turned the dial to the left to 200 (not 200m).

You can see by the picture, but if your multimeter is different, it's 200 (not 200m) in the section that has a V, a straight dash, and three little dots beneath it.

To be honest I still can't decide whether I like it better all the way one way or the other, and then the results with two different cabs are different (Marshall 1960 X pattern V30's and GT12-75's) and Orange (identical configuration).

But anyhow, you should be able to turn the pot fully either way on a stock 5150II or 6505+ and not damage the amp.

If you do any mods or your amp is already modded, I'd be a little more careful though. The wider range may or may not put you in dangerous territory.

 

[Loudness250]

You bias with your amp ON. In power mode, the mode you would have it in if you were going to play the guitar through it. But turn down all the knobs and boosts off. Just put the black end "ground" into the com hole, the positive goes into the other one, set the meter to read millivolts, it should work fine, I don't see any reason your tubes would glow brighter, that seems really weird. Your tubes should not be in any more danger just because you are biasing, you don't have to turn off the amp or trip out on it.

 

[glpg80]

You bias with your amp ON. In power mode, the mode you would have it in if you were going to play the guitar through it. But turn down all the knobs and boosts off. Just put the black end "ground" into the com hole, the positive goes into the other one, set the meter to read millivolts, it should work fine, I don't see any reason your tubes would glow brighter, that seems really weird. Your tubes should not be in any more danger just because you are biasing, you don't have to turn off the amp or trip out on it.

this is wrong.

the bias points are not bias points technically. they are used to read the voltage (not millivolts) of the half-wave rectified transformer tap as it is, there is no 1ohm resistor in series to dump the taps across to read the equivalent millivolts to milliamps due to ohms law with a 1 ohm resistor.

-55 is around what most end up being out of crossover. really hard to tell though depending on the PT % tolerances that differ with each model, component tollerances, and also the tubes you are using. to get that far you will need to modify it. stock the bias taps were better off not even being installed. they do no good.

the reason he almost blew the hell out of his tubes was probably due in fact he set the voltmeter to read current instead of voltage, and shorted the taps coming right off of the transformer.

ive replaced my fare share of diodes in other people's 5150's due to this which causes the bias to run away.

 

[Loudness250]

My bad, I assumed when he said the amp had an external bias port that Peavey went all the way with it but this is some half assed external bias port plus you have to replace the bias resistor to get a wider sweep. apparently like he said , this amp only gives the negative voltage with this bias port. Might as well just buy a bias probe. My main point was to say that you bias with your amp on not on standby and that biasing an amp should not make your tubes glow more. I was just giving a generalization without being familiar with this amps lame external bias capability.

 

[thunda]

Here's a thought: The 6505+ (just like it's predecessor, the 5150) amp has a pretty much powerful *clean* power amp section. The tone comes primarily from the preamp section. If you aren't playing the thing wide fucking open, you *aren't* getting power tube distortion. Therefore, biasing the power tubes hot will really only decrease the useful life of the power tubes. Just saying, from an old fart's perspective...

 

[Loudness250]

I could see where that mistake of putting the meter on millivolts would be made a lot considering this amp is probably one of the only amps with external biasing that works using negative volt reading. But whatever amp you have, always do your homework on your own amp before attempting to bias it. Peavey didnt put much thought into getting your bias exact when they equipped the amp with this "system", because tubes vary even if replacing with the same tube type and sometimes the recommended negative voltage is just not going to cut it, it could be way off.

 

[Christian Noir]

Here's a thought: The 6505+ (just like it's predecessor, the 5150) amp has a pretty much powerful *clean* power amp section. The tone comes primarily from the preamp section. If you aren't playing the thing wide fucking open, you *aren't* getting power tube distortion. Therefore, biasing the power tubes hot will really only decrease the useful life of the power tubes. Just saying, from an old fart's perspective...

I hear your logic, but it doesn't quite hold true with (at least) these peavey amps.

Changing the bias changes the tone at any volume on the 5150 / 6505.

It is a well known and common mod to do to add bias pots to the 5150/6505, and to modify the biasing system on the 5150II/6505+ to get thicker (or diffrerent) jam. Pretty much most owners that mod stuff regularly have done this. They gotta be the by far the most common mods.