Triple7
The Decepticon
Thanks for the info, not sure if this is something I can do myself though.
6505 bias mod.
- Thread starter Leuchty
- Start date
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sami
porkshop sammiches
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.
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.
Link above is to mod the 5150. Is this is the same for the 6505+?
rectifryer
Banned
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.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?
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.
And 35mA is still cold hahahaha
Yes, at 50 mA, as they are rated.
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.
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+.
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.
NinjaRaf
Well-Known Member
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.
espmetalhead
Member
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.
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.
TheWinterSnow
Well-Known Member
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.
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.
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.
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).
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.
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.
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.
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).
