# 6505 bias mod.



## Leuchty (Oct 13, 2008)

Can anyone give me info on these bias mods?

I have a stock 6505+ and heard that one can get even better tones with a bias mod.

What is it exactly?


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## 7stringless (Oct 13, 2008)

CYBERSYN said:


> Can anyone give me info on these bias mods?
> 
> I have a stock 6505+ and heard that one can get even better tones with a bias mod.
> 
> What is it exactly?


The tech will install a pot which will allow you to increase the plate voltage and get more drive from the tubes. At least that's my understanding. I think some amp companies bias they're amps cold which makes for longer tube life ( decrerases the watts)and isn't an issue for 90% percent of the people that own them. 

Here's an interesting post from Steve Fryette (VHT Amplification) I copied from the VHTusers forum. It shows just how cold some amps are biased from the factory.......





F
Many amp companies do not rate their amps based on actual power performance. Instead they just let the consumer assume that 2 tubes = 50W, 4 = 100W, etc. If pressed some would have to admit their power ratings are based on distortion figures as high as 20% or more, which would translate to ratings that are 20 - 40% greater than RMS at onset of clipping. Hey, you want distortion anyway, right? 

Some examples:

D60
Published - 60 Watts
Actual - 62 Watts RMS

D120
Published - 120 Watts
Actual - 118 Watts RMS

50/CL 
Published - 50 Watts
Actual - 50 Watts RMS

2/90/2
Published - 90 Watts/Channel
Actual - 105 Watts RMS/Channel

2/50/2
Published - 50 Watts/Channel
Actual - 62 Watts RMS/Channel

5150/6505
Published - 120 Watts
Actual - 72 Watts RMS

Dual Rec
Published - 100 Watts
Actual - 72 Watts RMS

Triple Rec
Published - 150 Watts
Actual - 108 Watts RMS

2:90
Published - 90 Watts/Channel
Actual - 65 Watts RMS/Channel

TSL100
Published - 100 Watts
Actual - 78 Watts RMS

P50 - 50 Watts RMS in a sealed enclosure. Closer to 35 open back.
V30 - 75 Watts in a sealed enclosure. Closer to 50 watts open back.
G25M 50 Watts RMS in a sealed enclosure. Closer to 35 open back.

All amplifier power figures above are based on identical test procedures: 
AC Output RMS at onset of clipping (assumes amp under test will actually produce a symetrical wave form at clipping. If not, we use whichever half-cycle clips first).
8 ohm/200W non-inductive test load.
Regulated 120V AC source.
Bias set to manufacturers spec and stock tubes.
Signal input directly to power amp stage for accuracy.
All power amp controls set for maximum clean power (typically Presence - 0, Depth -0, SS Rect Mode, Class AB, etc).


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## Leuchty (Oct 13, 2008)

whoa!!! the awful truth. 

So does it drive all the tubes or just the power tubes?

What tone benefits would I be getting?


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## sepherus (Oct 13, 2008)

IT decreases crossover distortion in the poweramp section. It onoly effects the power tubes. It makes the amp sound a hell of a lot better too. If you take the back plate off the 6505+ and look inside above the ground switch near the power transformer. There is a bias adjustment knob there already. Bias it as hot as it will go, which is WELL within the range of the 6l6s that it has in it. It only goes to like 20-25 mA they should be more around 35mA.


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## Leuchty (Oct 13, 2008)

Awesome!!!

how do I do that?

Can you run me through it? or point me in the right direction.

thanks heaps.


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## sami (Oct 13, 2008)

DEFINITELY worth getting. Too bad you're not in the US. Jerry from FJAMods does it and he's got other mods that make the 5150 series sound better.

Here's a pic of mine with the bias mod:


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## DrakkarTyrannis (Oct 13, 2008)

So then biasing the amp correctly helps increase the wattage? Would it be possible to get a 6505 closer to the 100 watt mark with proper bias?


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## 7stringless (Oct 13, 2008)

DrakkarTyrannis said:


> So then biasing the amp correctly helps increase the wattage? Would it be possible to get a 6505 closer to the 100 watt mark with proper bias?


Biasing the amp HOT may not be "correct", considering it will chew your tubes up. But it gives some people the sound they're after.......

J


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## Leuchty (Oct 14, 2008)

sepherus said:


> IT decreases crossover distortion in the poweramp section. It onoly effects the power tubes. It makes the amp sound a hell of a lot better too. If you take the back plate off the 6505+ and look inside above the ground switch near the power transformer. There is a bias adjustment knob there already. Bias it as hot as it will go, which is WELL within the range of the 6l6s that it has in it. It only goes to like 20-25 mA they should be more around 35mA.



Are you saying the 6505's have the "mod" already?


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## Ishan (Oct 14, 2008)

No, he's saying 6505s have a bias pot but it won't bring it up enough so you can set it to max anyway. I remember there's a resistor you have to change in those to make the bias pot more useful, but I don't remember the details of it


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## Leuchty (Oct 14, 2008)

Oh OK. sorry misunderstanding. 

I'm not the best at electronics but I'm also not an idiot with electronics. I will open it up later and see what I'm dealing with.

can I do this myself? anything else I should know?

much appreciated.


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## Drache713 (Oct 14, 2008)

If you have a 6505+ (like I do) then it comes stock with adjustable bias. However as others have said it's in the cold range, so you need to mod it to be able to get the bias up where it should be (35-40 mA idle current). Refer to the link below and read the 5150 II/6505+ section about the resistor you need to change, I did it and it works perfect.

euro-k


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## Jan (Oct 14, 2008)

I am at the stage of wandering whether it's really worth increasing the bias in my 6505... I mean, is the difference in sound THAT great? Isn't that just another myth? A local guy who is into amp construction advised me against it. He said that the difference in sound would be negligible, but the tube life woud be considerably shorter...


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## Drache713 (Oct 14, 2008)

Jan said:


> I am at the stage of wandering whether it's really worth increasing the bias in my 6505... I mean, is the difference in sound THAT great? Isn't that just another myth? A local guy who is into amp construction advised me against it. He said that the difference in sound would be negligible, but the tube life woud be considerably shorter...


It's not going to turn your amp into a new amp, and if tube life is an issue for you then you can set it wherever you like. If it sounds good to your ears as it is right now then I wouldn't worry about it. That being said I do perfer mine biased hotter to get the amp out of crossover distortion, makes it cleaner louder and warmer, and I HIGHLY doubt it decreases tube life THAT much.


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## sepherus (Oct 14, 2008)

I loaned my amp to a friend who has the original 5150 (single EQ not the II) which is pretty much universally regarded as the better sounding amp, and he wanted to steal mine. His was being loaned to a friend on tour.

It really makes the rhythm channel better. the cleans can aactually be called clean. It also adds more clarity to the lead channel, and more responsiveness in the power section.


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## Leuchty (Oct 14, 2008)

Drache713 said:


> If you have a 6505+ (like I do) then it comes stock with adjustable bias. However as others have said it's in the cold range, so you need to mod it to be able to get the bias up where it should be (35-40 mA idle current). Refer to the link below and read the 5150 II/6505+ section about the resistor you need to change, I did it and it works perfect.
> 
> euro-k




Thanks man, interesting read. I will take that info to my amp tech and see what he can do. I took off the grill and cranked up the bias. I'm assuming turning it clockwise was correct. Not a major difference but I can hear that something has changed for the better.


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## 7 Dying Trees (Oct 14, 2008)

sepherus said:


> I loaned my amp to a friend who has the original 5150 (single EQ not the II) which is pretty much universally regarded as the better sounding amp, and he wanted to steal mine. His was being loaned to a friend on tour.
> 
> It really makes the rhythm channel better. the cleans can aactually be called clean. It also adds more clarity to the lead channel, and more responsiveness in the power section.


Comnpletely agreed. It also takes away the sound of bees flying through the tubes. Really makes it a bit louder, punchier and clearer. Sounds awesome, well worth doing on a 6505/5150


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## Cypher (Jun 11, 2009)

Getting my 6505+ back tuesday. Bit of bad luck with em lately, my 2nd head in 2 months as the first one had problems too. As I'm endorsed they sent me a new one right away. 

Went in to record some stuff in the studio and my rhytm channel was all gone (month new amp). We needed the amp back quick to do some re-amping so my bud took it too his local Amp dr. where he get's his Amps modded. 

Heard today he fixed the problem and my amp is working perfectly again, and as a extra service he modded the bias. Curious about the result, the amp already sounded super, but gonna testdrive it tuesday again.


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## 7 Dying Trees (Jun 11, 2009)

sami said:


> DEFINITELY worth getting. Too bad you're not in the US. Jerry from FJAMods does it and he's got other mods that make the 5150 series sound better.
> 
> Here's a pic of mine with the bias mod:


Yep, it is this simple, however, using a pot like this isn't good. SHould be using a screwdriver adjustable mini pot to be honest. Priinciple is the same, execution is way cleaner and neater.



Jan said:


> I am at the stage of wandering whether it's really worth increasing the bias in my 6505... I mean, is the difference in sound THAT great? Isn't that just another myth? A local guy who is into amp construction advised me against it. He said that the difference in sound would be negligible, but the tube life woud be considerably shorter...


Tube life is shorter, but not that muchh shorter. It is, however, worth it. Turns it from a beast into a beast with powers of cthuulu. Makes it a hell of a lot louder, removes fizz, adds grind.



sepherus said:


> I loaned my amp to a friend who has the original 5150 (single EQ not the II) which is pretty much universally regarded as the better sounding amp, and he wanted to steal mine. His was being loaned to a friend on tour.
> 
> It really makes the rhythm channel better. the cleans can aactually be called clean. It also adds more clarity to the lead channel, and more responsiveness in the power section.


+1 to everything here


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## 7 Dying Trees (Jun 11, 2009)

Drache713 said:


> If you have a 6505+ (like I do) then it comes stock with adjustable bias. However as others have said it's in the cold range, so you need to mod it to be able to get the bias up where it should be (35-40 mA idle current). Refer to the link below and read the 5150 II/6505+ section about the resistor you need to change, I did it and it works perfect.
> 
> euro-k


THis is SO happening to the 6505+ i have this weekend


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## Triple7 (Jul 6, 2009)

Thanks for the info, not sure if this is something I can do myself though.


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## sami (Jul 7, 2009)

If you don't think so, definitely find someone who's comfortable with this kind of work. And like it was previously suggested, get a proper pot. My old amp in that pic, it was done by someone who's their own tech.


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## Rotatous (Jul 8, 2009)

I turned the knob in the back all the way up, and I definitely noticed a difference. I might have mine modded after I get a new cab.


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## Tapew0rm (Mar 1, 2010)

7 Dying Trees said:


> THis is SO happening to the 6505+ i have this weekend



Are there better instructions for doing this? I've got a 6505+ and I'd like to get this done.


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## Necky379 (Mar 1, 2010)

http://www.blueguitar.org/new/schem/peavey/modnotes/fja_5150_bias_mod.pdf


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## Tapew0rm (Mar 1, 2010)

Necky379 said:


> http://www.blueguitar.org/new/schem/peavey/modnotes/fja_5150_bias_mod.pdf



Link above is to mod the 5150. Is this is the same for the 6505+?


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## Necky379 (Mar 1, 2010)

on the 5150 all you have to do is change the resistor, there's a pot installed on it already it just doesnt get hot enough without the resistor swap.


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## Tapew0rm (Mar 1, 2010)

Necky379 said:


> on the 5150 all you have to do is change the resistor, there's a pot installed on it already it just doesnt get hot enough without the resistor swap.



Cool man, THANKS!


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## rectifryer (Jun 8, 2014)

CYBERSYN said:


> Can anyone give me info on these bias mods?
> 
> I have a stock 6505+ and heard that one can get even better tones with a bias mod.
> 
> What is it exactly?


The Bias for the 6505+ is simply replacing a single resistor with a lower value to shift the existing range of the pot into "hotter" bias territory. 



7stringless said:


> The tech will install a pot which will allow you to increase the plate voltage and get more drive from the tubes.


Voltage will not be increased. At all. I am not even sure what this guy meant by "drive". He probably realizes this now, it's an old thread. 


sepherus said:


> IT decreases crossover distortion in the poweramp section. It onoly effects the power tubes. It makes the amp sound a hell of a lot better too. If you take the back plate off the 6505+ and look inside above the ground switch near the power transformer. There is a bias adjustment knob there already. Bias it as hot as it will go, which is WELL within the range of the 6l6s that it has in it. It only goes to like 20-25 mA they should be more around 35mA.


And 35mA is still cold hahahaha



DrakkarTyrannis said:


> So then biasing the amp correctly helps increase the wattage? Would it be possible to get a 6505 closer to the 100 watt mark with proper bias?


Yes, at 50 mA, as they are rated. 


7stringless said:


> Biasing the amp HOT may not be "correct", considering it will chew your tubes up. But it gives some people the sound they're after.......
> 
> J



Once again, we are still talking well below what the tubes are rated for. Unless you need your tubes to last for 5+ years, IT WILL NOT BE an issue. 



Jan said:


> I am at the stage of wandering whether it's really worth increasing the bias in my 6505... I mean, is the difference in sound THAT great? Isn't that just another myth? A local guy who is into amp construction advised me against it. He said that the difference in sound would be negligible, but the tube life woud be considerably shorter...



It's worth having the options of multiple tones from your 6505 or 6505+. 



Necky379 said:


> on the 5150 all you have to do is change the resistor, there's a pot installed on it already it just doesnt get hot enough without the resistor swap.


You mean the 5150 II or the 6505+. 


Sorry for bumping this thread, but it comes up when people google how to bias mod a 6505, and it is a top result. I figured I would highlight the correct points of the thread.


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## NinjaRaf (Jun 9, 2014)

I bias modded a 5150 a few years ago. I dont think its worth doing the mod. The differences are very subtle unless you are running the post gain VERY high, IMO.


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## espmetalhead (Jun 26, 2014)

I did it, and there is a huge difference in tone - the amp actually has a clean channel now. However I found it sounded best in the 32-34 ma range versus the 38-40 ma that most guys say are ideal, out of the box it was at 8ma which was so cold there was crossover distortion. Gutting all of the stock tubes made a huge difference too, SED Winged "Cs" or Ruby's in the power section were the best, the pre amp section has a mix that Doug's tubes said would be sweet, and it is 

Here's a pic of the mod, it's the little blue thing directly under the 6 black cylinders (capacitors). All I had to do was clip a resistor out, and solder this guy in. 

I will say, that the volume needs to be at a 2 or louder to notice the difference.


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## TheWinterSnow (Jun 26, 2014)

The value of the pot should be changed to as a larger value will allow for larger range, the resistor simply limits the maximum bias when the resistance of the pot is set to 0 ohms.

In the 6505+ amp though the pot is indeed 10K which is the ideal value, you just want to reduce the 12K resistor down to 4.7K, then you have a little more to work with.



rectifryer said:


> Voltage will not be increased. At all. I am not even sure what this guy meant by "drive". He probably realizes this now, it's an old thread.



The control grid voltage is increased by adjusting the bias, that is what the bias control does. This controls how on the tube is when no signal is present. This is done so that tubes on opposing sides of the transformer are on at the same time during the transition where crossover distortion is present. There is a delay for tubes to turn back on after they go into cutoff and that is the result of crossover distortion, for some time there is no signal and creates a gap, the harmonics that are created from that is what we hear as crossover distortion. By biasing both tubes on when no guitar signal is present just enough, you eliminate that crossover distortion.

The catch side to that is you have all tubes pulling current and the hotter you bias the amp, the more tube sag you get, the tone gets squishier and more compressed sounding. Biasing any higher than the required need to get rid of crossover distortion therefore becomes a personal taste thing and willingness to replace tubes sooner.



rectifryer said:


> And 35mA is still cold hahahaha



For a 500v Anode voltage 35mA puts the quiescent power consumption puts the amp closer to Class A territory than it does Class B. Remember, we are only biasing out of Class B towards Class A just enough to remove crossover distortion, Class A amps are horribly inefficient.

35mA would be a bias of about 58% of maximum power, in Class AB amps you should never bias higher than 70% which would be about 42mA. The common range for bias modded 5150/6505 amps is anywhere from 32mA to 42mA and that is all based on preference.



rectifryer said:


> Yes, at 50 mA, as they are rated.



Bias does not control output power, you are always getting full power unless you start biasing in Class C territory, so far into it that the maximum output voltage of the phase inverter can't provide the 60Vp-p signal required for the power tubes to reach full power. Virtually impossible to do with tube amps and it would sound really really bad, crossover distortion so bad it would sound like the amp was cutting out (gating).


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