#1
I decided to ask from GB&C as the guys in GGA's 6505+ official thread were really quiet. Trying to figure out everything about the mod for about a week now. Got pretty much nearly everything in my head already. Exept the following...

Looking at the components I'd need. I need 2 resistors (other one for discharging the caps) and a pot. Or should I just discharge the caps without a resistor, creating a spart?

Resistor:
5-7K
0.5W<
What ohmage?
Also my local store sells different types of resistors eg. metaloxide resistors, carbon coating resistors, carbon mass resistors etc. (bad translations). So What type?

Pot:
10K
0.5W<
My local store sells maaaany many types of pots, but also one thats called "CTS 10K bias pot". Should I just get that?

Also how do you prefer to check the bias? Is it worth getting a bias tester which is stuck between the pin holes and power tubes? How much would one cost? Any kind of link to one would be greatly appreciated.

EDIT: oh yeah I've been using these links:
http://www.gearo.com/Files/5150biasmod.pdf
http://www.blueguitar.org/new/schem/peavey/modnotes/fja_5150_bias_mod.pdf
Last edited by fc89konkari at Oct 31, 2011,
#2
Quote by fc89konkari

Looking at the components I'd need. I need 2 resistors (other one for discharging the caps) and a pot. Or should I just discharge the caps without a resistor, creating a spark?

No

Quote by fc89konkari

Resistor:
5-7K
0.5W<
What ohmage?

5-7K

Quote by fc89konkari

Also my local store sells different types of resistors eg. metaloxide resistors, carbon coating resistors, carbon mass resistors etc. (bad translations). So What type?

Well metal film are best I believe; they are low noise and have the most accurate tolerances. But people say they all sound different and IMO it doesn't really matter.

Quote by fc89konkari

Pot:
10K
0.5W<
My local store sells maaaany many types of pots, but also one thats called "CTS 10K bias pot". Should I just get that?

Sure, as long as it's within spec and the form factor is what you want then get it.

Quote by fc89konkari

Also how do you prefer to check the bias? Is it worth getting a bias tester which is stuck between the pin holes and power tubes? How much would one cost? Any kind of link to one would be greatly appreciated.

It depends, some amps need them and some have reliable test points. IDK about yours.
www.amp-head.com/product_info.php?products_id=30{8}23

Quote by emad
jthm_guitarist
Warned for trolling!


Quote by metal4eva_22
Didn't you say that you had a stuffed fox that you would occasionally fuck?

Quote by Axelfox
It's not a fox,it's a wolf.
#3
Quote by jthm_guitarist
5-7K

Oooh so K = Kohm I'm quite a noob at this (oh really)

Quote by jthm_guitarist
Well metal film are best I believe; they are low noise and have the most accurate tolerances. But people say they all sound different and IMO it doesn't really matter.

My local store sells this metalfilmresistor 0.6W 1%. What does this 1% mean, nothing to worry about?

Quote by jthm_guitarist
It depends, some amps need them and some have reliable test points. IDK about yours.
www.amp-head.com/product_info.php?products_id=30{8}23

My amp is a 6505+. Don't know about it..

That link of yours:
This probe measures the cathode current on pin 8 of the tube.

Couldn't I just measure the current of the pin 8 in another way with a regular voltage meter? And would the - or + go onto the tube (I assume I'd have to open the amp up and poke with the meter from the inside), and where would the other end go?

And finally, what kind of a resistor should I use to drain the caps? When I stick it inbetween wire, do I have to shrink wrap the resistor or can it just be uncovered?

Thanks!
#4
Quote by fc89konkari
My local store sells this metalfilmresistor 0.6W 1%. What does this 1% mean, nothing to worry about?

That means it's within 1% tolerance of the printed value, which is great.

Quote by fc89konkari
Couldn't I just measure the current of the pin 8 in another way with a regular voltage meter? And would the - or + go onto the tube (I assume I'd have to open the amp up and poke with the meter from the inside), and where would the other end go?

Well, the tube needs to be plugged in... that device is the easiest way to measure the current.
There might be test points somewhere in your amp that you can measure with a multimeter.

Quote by fc89konkari
And finally, what kind of a resistor should I use to drain the caps? When I stick it inbetween wire, do I have to shrink wrap the resistor or can it just be uncovered?

I would use a resistor rated for like 10W, they are surrounded in a cement block.

Quote by emad
jthm_guitarist
Warned for trolling!


Quote by metal4eva_22
Didn't you say that you had a stuffed fox that you would occasionally fuck?

Quote by Axelfox
It's not a fox,it's a wolf.
#5
Allright, thanks! All I gotta figure out now is whether my test point of the 6505+ is usable.
And yeah, my local stores guy is awesome. He can probably tell me what kind of a resistor I'd need to drain the caps exactly.

Just now I started to think about the specs of my soldering gear. I haven't checked what kind of my stuff is, but:
What wattage should my soldering iron be, so I wont burn the shit inside or anything?
And would some solder stick easier or be better in some way than another? I've had some problems getting my solder to stick..

Thanks!
#6
Update. I opened up my amp and took some pics. Hard to figure out some stuff here...
Ahh I can't post the images here (I suck). Here they are anyway, sorry about the quality:
http://imageshack.us/g/845/kuva0070.jpg/

Now I'm following this to do the bias mod (and some others):
http://www.gearo.com/Files/5150biasmod.pdf

From those pictures you can see that we got the 15K resistor that we gotta replace, but in my amp I just got some fat red block! Wtf is this about, how am I gonna do the mod?

Another thing. I got pictures showing the pins of the power tubes (a little). I took them because they are circuited to the other stuff by the board, not any wires. I thought that was weird and it makes figuring things out a lot harder. I just wasn't able to figure out my next question:

How can I manually check the bias (no probe, just Volt+Ampere+Ohm meter, measurements and a lot of calculations)?

Thanks!

EDIT: just to mention. The red block thats where the resistor should be has a black and a red wire going to the bias point (pot and test point).
Last edited by fc89konkari at Nov 1, 2011,
#7
UPDATE: looked inside the amp again. The red block has the black and red wires going to the bias stage. My bias stage also has a resistor. Looking from the color codes, it should be 12K. Can I just do the mod by taking that out, replacing it with the pot+6,8K resistor in that place where the 12K is now?

And the only other open question is the bias. How to check it without biasmaster or any kind of probe like that, just a volt/ampere/resistance meter.
#9
30-50W iron is fine, you can spend 50 bucks and get an XYTronics and have a digital temperature setting, spend another 20 and you can get one with a built in fume extractor if you plan on soldering a lot. If just doing this mod, a 30W rat shack iron will suffice. 60/40 solder is standard is works just fine.

To check the bias, take a volt meter with the + side at the cathode, and the - side at the grid of the tube.

Are you able to get crisp photos? If that 12k is what is setting the bias currently, then yes, replace it with a new bias setup.
#10
Quote by blandguitar
To check the bias, take a volt meter with the + side at the cathode, and the - side at the grid of the tube.

I'm really a noob at this. Is the cathode like pin 8 and the grid pin 5 or something? And when I'm inside the amp (I gotta do this from inside, right?) which way am I looking from (so I don't mix up the pins)?
And this method is quite accurate, right?

I managed to get some better pics (realised that my webcam can do pretty OK)

http://img585.imageshack.us/slideshow/webplayer.php?id=20111106195929970.jpg

There you can hopefully see the bias point and the place where the mod is usually done. What I'm wondering now is where will I stick the new pot+resistor combo if i remove that R68 12K Resistor? At the location of the 12K resistor or on the blue cap where its usually done?
#11
Quote by fc89konkari
Can I just do the mod by taking that out, replacing it with the pot+6,8K resistor in that place where the 12K is now?


Unless I'm missing something I cant see what the new pot is for

I modded mine to bring the bias into a useable range by simply replacing the resistor on the bias pcb.
#12
Yeah, I wondered about it myself. I just thought that "everyone with more experience and knowledge are doing it that way, so I might aswell".

What value resistor would I replace it with then? 6.8K? but the pot was supposed to me 10K so if I only used a resistor would it be around 16.8K then?
#13
If I remember correctly I used 6.8k but I cant say for certain. The idea is to lower the resistance which will let you bias your amp hotter. I have read about people using anything from 5k to 10k.

Now I think about it the pot would be used if you were doing this mod to a 5150/6505 not the 5150II/6505+ models, that could be where the confusion lies.
#14
My method will tell you the voltage from grid to cathode, due to variations in the tubes themselves, most people use a current method rather than voltage as each tube will pull a different current for a given grid-cathode voltage. Measuring the voltage across the cathode resistor tells you the current through ohms law. I=V/R=E/R depending on how you learned ohms law you'll either use V or E, they both stand for the voltage in this equation.

The resistor 6k8 combo is where it'd go, you could even go down to 4k7 or 5k1 (5k1 would be a safer choice) which would give you variability from 5.1 thousand to 15 thousand ohms. The 5k1 sets a minimum resistance value so that the output stage can self-bias. The 10k pots allows you to play with the value to find one that you prefer, ideally, you'd replace this pot with a resistor of the closest value to whatever value you liked best on the pot. So if you toyed with the pot and found that you liked the sound best when it was at 7400 ohms, you might replace it with a 7k5 resistor, or, if ordering from a place like Mouser (check the resource thread for other places) you could find a value even closer. The reason is that like all components, potentiometers will drift, but it will most likely drift in value more than an equivalent resistor will.

You can remove the legs of the 12k and solder into the holes where the component legs were, or you can solder onto the legs of that cap. Electrically, they're about the same, personal choice. Soldering onto the cap will be less of a headache for you though. Be sure that you are following safety practices, while the voltages of the bias circuit may or may not be of concern, they should be treated with the same fear as the HV. And just as you could damage yourself and your amp from the bias, you could make a mistake and handle the higher voltage wires and be even worse off. The voltages in any amp are not laughable.
#15
Quote by blandguitar
My method will tell you the voltage from grid to cathode, due to variations in the tubes themselves, most people use a current method rather than voltage as each tube will pull a different current for a given grid-cathode voltage. Measuring the voltage across the cathode resistor tells you the current through ohms law. I=V/R=E/R depending on how you learned ohms law you'll either use V or E, they both stand for the voltage in this equation.

I still didnt't really understand. I mean, I do understand what your saying here. Its just that I literally don't know where to stick my voltmeter "grid and cathode" yes yes, but if you could supply a picture to indicate what they are (where, what the look like etc. etc.) so that I would know what to measure, it would be awesome!

Quote by blandguitar
The resistor 6k8 combo is where it'd go, you could even go down to 4k7 or 5k1 (5k1 would be a safer choice) which would give you variability from 5.1 thousand to 15 thousand ohms. The 5k1 sets a minimum resistance value so that the output stage can self-bias. The 10k pots allows you to play with the value to find one that you prefer, ideally, you'd replace this pot with a resistor of the closest value to whatever value you liked best on the pot. So if you toyed with the pot and found that you liked the sound best when it was at 7400 ohms, you might replace it with a 7k5 resistor, or, if ordering from a place like Mouser (check the resource thread for other places) you could find a value even closer. The reason is that like all components, potentiometers will drift, but it will most likely drift in value more than an equivalent resistor will.

Just wondering cause my original resistor is different (12K) so can I use the regular 6.8K (or 5K), I guess I can (and will) but after I get the bias checked, I'll check back here to check if the numbers look allright.

Quote by blandguitar
You can remove the legs of the 12k and solder into the holes where the component legs were, or you can solder onto the legs of that cap. Electrically, they're about the same, personal choice. Soldering onto the cap will be less of a headache for you though. Be sure that you are following safety practices, while the voltages of the bias circuit may or may not be of concern, they should be treated with the same fear as the HV. And just as you could damage yourself and your amp from the bias, you could make a mistake and handle the higher voltage wires and be even worse off. The voltages in any amp are not laughable.

If I'm correct, I can still wire the resistor to the caps legs, even though mine is located very differently?
#16
The grid and cathode will be connected to the pins of the tube. Pin 5 is the grid, pin 8 is the cathode; however, measuring the voltage across the cathode resistor(s) is truly a better method as it gives current and voltage. http://media.photobucket.com/image/octal%20vacuum%20tube%20pinout/vkung/2011_06_12_20_34_420001.jpg http://www.learn-about-electronics.com/images/Pin-Identification-all-tubes-are-viewed-from-the-bottom.gif You're interested in the octal layouts.

The idea is to allow variation but if you wanted, you can also return it to stock value. 6k8 and a 5k1 in series is pretty close to 12k.

It should be the same thing, but a different layout and component values. You can always examine the traces on the board to see if the resistors and capacitor are connected.
#17
Quote by blandguitar
The grid and cathode will be connected to the pins of the tube. Pin 5 is the grid, pin 8 is the cathode; however, measuring the voltage across the cathode resistor(s) is truly a better method as it gives current and voltage.

...could you clarify this a bit. I can't just stick the ends of the voltmeter to pin5 and pin8 themselves? Or can I? Or is it so that I can stick to the pin5 but not to pin 8, but somewhere where that leads to?

And I can do this with any tube (of the 4), right?
#18
You can; however, each tube will draw a different current based upon the grid-cathode voltage, so the power dissipated through each tube is really not able to be determined without the current. With vacuum tube amplifiers, they are typically loaded with an output transformer, that said, the plate rests at the high voltage. P=IV, the current through the tube idling, and the voltage across it while idling is the quiescent power dissipation. To return to the concept of measuring the voltage, voltage is a measurement taken with a certain reference, for instance, there is no such thing as a voltage by itself, it must be in reference to another. Therefore, to measure the grid voltage, you must also measure another voltage (in this case the cathode) to arrive at the measurement you desire.

Yes, any and or all of the 4 tubes may interest you. If they are matched (as the should be) you need only to measure one of the tubes.
#19
I apologize for my stupidity. I do understand what you just said, all the concepts. But.. Whats the other measurement that I have to do? I know I have to measure current after I've measured the voltage, and calculate what the power (Wattage) is, but how do I measure it? Like where do I stick my shit?
#20
You only need the voltage from plate to cathode and the current through the tube (the same as the current through the cathode resistor. The voltage across the tube, which is the same as the high voltage minus the cathode-ground voltage. Using the voltage across, and the current through, you have the idle power dissipation. Connect the voltmeter across the cathode resistor, I=E/R, that's your idling current. The voltage across the resistor is the cathode-ground voltage, you can measure the high voltage, or if it's known, just use the schematic approximation. This voltage, minus the cathode-ground voltage is the voltage across the tube. You only need to use a voltmeter and the cathode resistor.
#21
Quote by blandguitar
You only need the voltage from plate to cathode and the current through the tube (the same as the current through the cathode resistor. The voltage across the tube, which is the same as the high voltage minus the cathode-ground voltage. Using the voltage across, and the current through, you have the idle power dissipation. Connect the voltmeter across the cathode resistor, I=E/R, that's your idling current. The voltage across the resistor is the cathode-ground voltage, you can measure the high voltage, or if it's known, just use the schematic approximation. This voltage, minus the cathode-ground voltage is the voltage across the tube. You only need to use a voltmeter and the cathode resistor.

Your bringing up a lot of new stuff for me here. First of all, where is my cathode resistor supposed to be located?

Lets break this down. I want to know the power dissipation of my tube. P=E*I (I've actually learned this P=U*I so that U always stands for voltage). So if I'm correct, U (or E, which represents voltage) is my so called plate voltage. This is measured by comparing/measuring the voltage of the plate and cathode. Which end (+ or -) goes to which?

I=E/R (I know this as I=U/R). To measure the current, I have to measure the resistance and voltage of a resistor wired in series with the tube. Then I simply do the math. Both are measured by sticking - and + on both sides (right?), but which goes on which side?

Then we go back to P=E*I and do the math. I'd want my 6l6GCs biased for 70% of the max power handling (or whatever its called). So I'd bias it 21W (max is 30W). From the actual bias happening I don't really know much. Meaning, I'm not sure but does the bias pot control the current, right? So when I'm turning the bias pot, what do I do to check the numbers again, assuming I don't have to do all of the things again.


This is how I think things are. If some stuff is incorrect, please let me know. I got confused as you started talking about cathode-ground voltages and high voltages. What do they have to do with the equasion?

Thank you very, very much.
#22
The cathode resistor is the resistor from the cathode of the power tube to ground. Find where the cathode of the power tube is, find the resistor that connects it to ground. Doesn't matter, you'll just get a negative value if using a digital meter. If using analog; however, you need the + to be at the plate, and the - to be at the cathode.

The resistance can be determined by the colored bands on the resistor. If you measure the voltage across it, you can use Ohm's Law to find the current. Same rule applies, doesn't matter if using digital, if analog, you want the + to be at the more positive voltage, which in this case is the cathode rather than ground.

It essentially controls the current. If you lower the resistance, more current flows, but more current means a larger voltage across the same resistance. This counter-action is why the cathode resistor bias method is called self-bias. Sounds like you've got it about right. If it's a class AB Push pull amp, you can go upwards or 80% at idle without too much concern.

It's is very important to understand that the voltages within your amp are highly dangerous. You should spend an hour looking up tube amp safety practices before you go poking inside of your amp.
#23
I cant find that god damn cathode resistor... look at the pics. It's hiding somewhere!

Tips on finding it?