Biasing a 6505, ran into issues

[rectifryer]

I bias modded a pv 6505 head I have laying around. Essentially, I simply put a 10k pot + 4.7 k resistor in place of the 15k resistor that was already there.

Now that I am attempting to bias the 6505, which has PCB mounted power tubes, I am following the guidelines setforth here: http://www.carvinmuseum.com/pdf/Ka-Boom_v1a.pdf

The process is basically this:

Measure the plate voltage
Measure the dripping resistor voltage
Measure the current across the standby switch

The pin 3 plate voltages is 458v
The Dropping resistor voltage is 5.8v (it's 400ohm)
The standby current is 14.8mA which seems to be off by a factor of 10??? This is where I am running into an issue. This doesn't make sense. Can anyone enlighten me on what I am doing wrong?

 

[walter]

Do you have all the tubes installed when reading this? It looks like you only have one tube. I didn't read that whole link, so I'm not sure about the details, which dropping resister are you looking at? There is another method for biasing that was not mentioned in that document. You know the amp works and is in the ballpark, crank the bias up, crank the amp up and hit some power chords, watch the tubes for red plating, adjust the bias as needed to be below red plating.

 

[rectifryer]

All 4 power tubes are installed.  All of their heater filaments glow as well.  The tubes also test good.

 

[PRR]

> which dropping resister are you looking at?

I am *guessing* they have you measure the Screen current (400 Ohms is a reasonable common Screen resistor). Or actually probably Screen+Preamp. Then measure total current (across STBY switch, very dangerous!!). Subtract little current from total current, you have power tubes' plate current.

Myself, If at *ALL* possible, I would insert 1 Ohm resistors at both (or four) cathodes. (Yes, PCB construction makes this a pain.) Now you can directly measure EACH power tube's cathode current (without going near High Voltage!!). If you assume that plate current is equal to cathode current, you won't be far out, and in the 'safe' direction.

Does the amp play?

Are the power tubes red-plating?

 

[82Hz]

This is one the useful method i use for biasing guitar amps  ;D

You should measure first the resistor of half primary on the OT. Guitar amp OT are never vell balanced so you will found different values for the two half of prim OT. Follow the three wire from the output transformer.

When you know the resistance you should turn on the amp (connected to a dummy load), measure the anode voltage to ground then you can calculate the optimal bias current with this formula:

Ibias = 0.7*Wmax / Vanode [A], you should divide for 1000 for [mA]

where Wmax is the max plate dissipation of the tube installed. you can consider 25W for EL34, 28W for 6L6, 38W for KT88/6550.

Then is easy to calculate the voltage drop again half primary. Just remember to double the bias current because you have two tubes for each side of push/pull.

Vdrop = Ibias * 2 * R1/2 prim

I don't consider the grid current, but is a very little error.
If you want to be more accurate, calculate the grid current with Vdrop on the R grid and R grid.

Usually i use different meters for read Vanode and Vdrop because Vanode can vary during the bias adjustment. So the optimal current should be re-calculated.

Hope this can help 

 

[rectifryer]

> which dropping resister are you looking at?

I am *guessing* they have you measure the Screen current (400 Ohms is a reasonable common Screen resistor). Or actually probably Screen+Preamp. Then measure total current (across STBY switch, very dangerous!!). Subtract little current from total current, you have power tubes' plate current.

Myself, If at *ALL* possible, I would insert 1 Ohm resistors at both (or four) cathodes. (Yes, PCB construction makes this a pain.) Now you can directly measure EACH power tube's cathode current (without going near High Voltage!!). If you assume that plate current is equal to cathode current, you won't be far out, and in the 'safe' direction.

Does the amp play?

Are the power tubes red-plating?

I tried the one ohm method first ofcourse but I have to completely remove the pcb to do so, and that seems more dangerous than just checking stuff with a multimeter.  I cannot find a 6505 schematic to show where the cathode is going to ground either.  I am very very cautious when working on the amp.  Because of this, I have resorted to just checking the plate voltage and standby switch current instead of cathode current.  To top things off, this amp has a connector that the standby switch plugs into that is very easy to probe without killing yourself. 

The amp does play with no issues, and it doesn't red plate at all even in my highest current bias setting of only 4.7k ohms so I suppose this is all a moot point.  I am actually using KT88s in it, which actually helps the cleans A LOT. 

This is one the useful method i use for biasing guitar amps  ;D

You should measure first the resistor of half primary on the OT. Guitar amp OT are never vell balanced so you will found different values for the two half of prim OT. Follow the three wire from the output transformer.

When you know the resistance you should turn on the amp (connected to a dummy load), measure the anode voltage to ground then you can calculate the optimal bias current with this formula:

Ibias = 0.7*Wmax / Vanode [A], you should divide for 1000 for [mA]

where Wmax is the max plate dissipation of the tube installed. you can consider 25W for EL34, 28W for 6L6, 38W for KT88/6550.

Then is easy to calculate the voltage drop again half primary. Just remember to double the bias current because you have two tubes for each side of push/pull.

Vdrop = Ibias * 2 * R1/2 prim

I don't consider the grid current, but is a very little error.
If you want to be more accurate, calculate the grid current with Vdrop on the R grid and R grid.

Usually i use different meters for read Vanode and Vdrop because Vanode can vary during the bias adjustment. So the optimal current should be re-calculated.

Hope this can help 

I will give that a go if I can't figure out what I am doing wrong. 

 

[Patrick from Davis]

Peavey is notorious for biasing guitar amps very cold.  They did this so you could plug any tube (of the proper make) in and it wouldn't fry.  I have a Webber Bias Rite, which makes most of this quite easy, but there are other ways of doing this.  The plate voltage is required, and then the current going to ground through the bias circuit is used.  Look up the max power dissipation for a KT88 and multiply it by 0.7 to get what you should be using as your max (30% over is to keep things from burning up)  Good ole P=IV is next.  So 0.7*Pmax=Plate V*Imax, rearrange to 0.7*Pmax/Plate V=Imax.  I made up a spreadsheet with a bunch of tube types, but I can't seem to locate it, it is not very hard to redo.  The trick is of course the current.  In a bias rite, it is the voltage across a 1 ohm resistor.  mV = mA when V = IR and R = 1.  Looking at the 5150 II (The 6505 II, they changed the name to avoid paying EVH) schemo it looks like they have a bias section right in the middle of it.  You should be able to measure the voltage drop across your pot and resistor to get the current, well, you need the total resistance as well.  Then you can get it to 55-70% of the power tubes Max dissipation.  My experience is that at some point (usually far below 70%) the power section of the amp opens up and things become much clearer, more high def.  If some one is plucking a guitar, you'll hear it like a pillow being removed from the speakers.  You can't measure anything when the plucking is happening, but once you stop, you can measure and confirm that everything is within range.
Patrick

 

[rectifryer]

The plate voltage is required, and then the current going to ground through the bias circuit is used. 

Measuring across the standby switch should give me the current through the bias circuit correct?  For some reason that only measures ~14-15 mA. 

If I measure the voltage across the bias resistor and the work out the current from ohms law(E/R) then I end up with 3.6 mA lol. 

 

[Patrick from Davis]

The voltage on pin three of a KT88 should be what you measure for the plate voltage.  Thinking a bit more about it, that amp is designed for 6L6's so you might have to adjust the resistor size to get enough oomph through those tubes.  The voltage on pin three will vary from tube to tube.
Patrick

 

[rectifryer]

The voltage on pin three of a KT88 should be what you measure for the plate voltage.  Thinking a bit more about it, that amp is designed for 6L6's so you might have to adjust the resistor size to get enough oomph through those tubes.  The voltage on pin three will vary from tube to tube.
Patrick

I put in a pot so I could adjust it, but my measurements are 1/10th of what they should be.  My plate voltage is 458 volts. 

 

[Patrick from Davis]

Okay, so 458V and 42W max we get, 92 mA is 100% and 64 mA is 70% of Max Power.  A 6L6 would have 30W max so it looks like 66 mA is 100% and 45mA is 70%.  what is the voltage across the resistor and pot for biasing when the amp is on (not in standby) but has no cord from an instrument?  Be careful when it is on those voltages (458V!!!) are deadly.
Patrick

 

[rectifryer]

Okay, so 458V and 42W max we get, 92 mA is 100% and 64 mA is 70% of Max Power.  A 6L6 would have 30W max so it looks like 66 mA is 100% and 45mA is 70%.  what is the voltage across the resistor and pot for biasing when the amp is on (not in standby) but has no cord from an instrument?  Be careful when it is on those voltages (458V!!!) are deadly.
Patrick

55v

 

[Patrick from Davis]

Oops, I need what the total resistance is as well (on standby or off will do fine.)  Then use V=IR and find the current.  Divide by four to get the current for each tube.  If it works out the same, then something else will need to be adjusted.
Patrick

 

[CJ]

Push-Pull amps draw much less current than the tubes are rated for with no signal, (fixed bias)

this is called idle current because the tubes are idle,

this is for power efficiency,

when you crank up the guitar, the tubes will start conducting their rated current,

just turn the bias pot til the amp sounds nice, if the tubes do not "red plate" then bolt it up and let the neighbors have  it,  ;D

humans like to complicate simple jobs, so skip the 41 page article with the 11 pages of disclaimers, 

use to have the bias pot on the twin reverb in a place where i could use it, just like a volume knob, this allows you to bias the amp for the room you are playing,
live room-mushy bias, dead room- cold bias,

or replace the fixed bias system with a beefy cathode resistor for warmer tone and automatic bias for the rest of your life, if you play crunch metal cannibal corpse, you may not like this setup, if you like 20th Century Soundtrack http://stevemichaelsvaultovinyl.com/listen/
then you will be in heaven,

note that with cathode bias, your idle current will be a lot higher, so maybe a fan on the chassis for the tubes and pwr trans,

this is why the Vox AC 30 use to catch fire, Class A = Hot

 

[nashkato]

I(dropping R) = 5.8V(Voltage across Dropping R) / 400Ω (value dropping resistor ) = 14,5 mA
you´re sure you´re measuring across the Stby Switch ?
looking at the schematic (hate those kind , with connections everywhere , you have to look for way too long ) the Stdby-Switch only switches the Screen Voltage , that´s why you only measure Screen current .
This method only works if the Stdby-Switch handles all the Highvoltage Part of the power tubes .
Get a Bias probe , or find a way to measure anode current (through outputtransformer) dangerous too , though

 

[82Hz]

May this can help: http://www.prowessamplifiers.com/schematics/images/6505112.pdf

Carlo

 

[rectifryer]

Thanks everyone.  That completely explains my issue.

I have a bias probe on the way but I am going to try 82hz's method for now as it makes sense. 

 

[rectifryer]

This is one the useful method i use for biasing guitar amps  ;D

You should measure first the resistor of half primary on the OT. Guitar amp OT are never vell balanced so you will found different values for the two half of prim OT. Follow the three wire from the output transformer.

When you know the resistance you should turn on the amp (connected to a dummy load), measure the anode voltage to ground then you can calculate the optimal bias current with this formula:

Ibias = 0.7*Wmax / Vanode [A], you should divide for 1000 for [mA]

where Wmax is the max plate dissipation of the tube installed. you can consider 25W for EL34, 28W for 6L6, 38W for KT88/6550.

Then is easy to calculate the voltage drop again half primary. Just remember to double the bias current because you have two tubes for each side of push/pull.

Vdrop = Ibias * 2 * R1/2 prim

I don't consider the grid current, but is a very little error.
If you want to be more accurate, calculate the grid current with Vdrop on the R grid and R grid.

Usually i use different meters for read Vanode and Vdrop because Vanode can vary during the bias adjustment. So the optimal current should be re-calculated.

Hope this can help 

I followed this and it worked well.  I lol'd when one side of the OT was 29 ohms and the other was 37.5!  Such as life.  To get a bias of 40 mA I would have to bias one side to 2.3v and the other to 3v, but by the time I biased the higher side to 3v, the other was only at 1.5v voltage drop.  Oh well.  I swapped tubes and still had a similiar issue. 

The amp has improved very much.  The tone is much more in front than before with all of the crossover distortion. 

 

[82Hz]

This is one the useful method i use for biasing guitar amps  ;D

You should measure first the resistor of half primary on the OT. Guitar amp OT are never vell balanced so you will found different values for the two half of prim OT. Follow the three wire from the output transformer.

When you know the resistance you should turn on the amp (connected to a dummy load), measure the anode voltage to ground then you can calculate the optimal bias current with this formula:

Ibias = 0.7*Wmax / Vanode [A], you should divide for 1000 for [mA]

where Wmax is the max plate dissipation of the tube installed. you can consider 25W for EL34, 28W for 6L6, 38W for KT88/6550.

Then is easy to calculate the voltage drop again half primary. Just remember to double the bias current because you have two tubes for each side of push/pull.

Vdrop = Ibias * 2 * R1/2 prim

I don't consider the grid current, but is a very little error.
If you want to be more accurate, calculate the grid current with Vdrop on the R grid and R grid.

Usually i use different meters for read Vanode and Vdrop because Vanode can vary during the bias adjustment. So the optimal current should be re-calculated.

Hope this can help 

I followed this and it worked well.  I lol'd when one side of the OT was 29 ohms and the other was 37.5!  Such as life.  To get a bias of 40 mA I would have to bias one side to 2.3v and the other to 3v, but by the time I biased the higher side to 3v, the other was only at 1.5v voltage drop.  Oh well.  I swapped tubes and still had a similiar issue. 

The amp has improved very much.  The tone is much more in front than before with all of the crossover distortion.

Great!

OT in guitar amp are very simple so, don't try to find a good balanced primary  8)

Anyway, bias is important but not too much. If you are in the nice range with a well balanced power tubes you are ok. Try to set bias correct at 0.1mA is really not necessary.

Enjoy the beast! I have mod lots of 5150 with two 6L6 and two KT88. Also try to recuce a bit the gain in the preamp. You will not lose distortion but will have a great increase in definition!

CARLO

 

[Patrick from Davis]

I have found that there is no way to get things right where you want them numerically.  So I just adjust mine to where they sound good, and are not too hot.  I have heard several arguments for having balanced tubes, or not having balanced tubes.  I think people spend too much time trying to prove their opinion of what sounds right is correct.  Again, I find I get more playing time out of just setting it (within reason) by where the bias sounds good and then just having fun playing.  When you get the bias probe, check all of the power tubes, the differences in plate voltages will make you chuckle.
Patrick

Addition:  I keep forgetting to mention this.  If you like the Peavey 5150/6505 sound, they made an amp called the Ultra Plus (Overusing superlatives?) that has three channels and a reverb tank on it.  It was a precursor to the 5150 line of amps and has a lot of the same general sound.  However, the channel switching and clean channel sound make it a very good amp.  On eBay, they commonly go for $400-450, which isn't bad for a 120W head.