Bias/balance question for Blackface Super Reverb

[Newguy]

Hello

I have BF SR Fender.

I think/assume I only have a power tube balance pot. I read on forum to balance this, take tubes out and measure pin 5 to ground on both PT sockets, and it should be around -55 VDC.

Questions

1. You only take power tubes out correct, the preamp tubes do or do not have to be taken out, I assume leave the preamp tubes in?

2. You guys ever use https://ssl.eurotubes.com/cart/index...ategory_id=109

This is eurotubes bias probe ($25) and cheap digital meter $10? I only have a few amps to do at home. I know it is not the best, but looking for a cheap way to set bios.

Thanks for any input

[Amp Kat]

Depends on which model you have. One is -52 and one is -48 however that is really irrelevant when it comes to biasing because the wall outlet voltage and probably tube wattages have changed since then so the bias should be set to a good % of dissipation of somewhere from 60 to 70 % where you won't burn the tubes up and not go into crossover distortion at higher volumes. The bias probe at Euro should work fine as all you really need is a way to monitor the current of the tube and the rest is simple math of plate voltage X tube current = wattage.

[pontiacpete]

leave the preamp tubes in

If you have a DMM you check the bias with that. Aiken has a good discription of how to do that:

Aiken Amplification

[Newguy]

Amp Kat Amp Model Number

Sticker says AB76 (smudge on last digit).

Guy I bought it from back in the 80's gave me schematic and it says AB763.

It has been a long time since I have had chassis out. I seem to remember it only had one pot (which probably means bias balace right?).

pontiacpete: I did not see directions on that site, can you link it to me and I can see if directions are any better then this one:

Bias FAQ

For just setting the Bias balance pot from the link above,

Turn your amp on, but leave it on STANDBY. Set your DMM to the highest DCV scale, ground the black probe to the chassis, and take a reading from pin FIVE of any power tube socket. You should see a negative voltage in the -35 to -50 volt range if the amp has EL34s, or in the -45 to -60 volt range if the amp uses 5881s, 6L6s, or KT66s. KT88s, 6550s, KT90s, and KT100s can have bias ranges that go as high as -100 volts. Amps which use 6V6s will usually have bias supplies which produce voltages that are similar to EL34 amps...but not always. Note that you should *not* have any power tubes installed in your amp yet.

Nest paragraph:
Next, adjust the bias control until you have MAX NEGATIVE voltage on pin FIVE

Question

1. The way the article reads I am not sure if tubes should be in or out for max negative voltage on pin 5, I assume out?

2. What does this balance adjustment do, if any thing tone or performance wise?

The amp itself sound fantasti!!! However vibro part only clicks, and reverb is noisey from 1-2, and 8-10. No matter, as I use it around 5 any way, and it is great. I am not worried about the vibro for now.

HOWEVER, if left sitting with out playing, and amp turned on, I get some little crackles and pops (from reading maybe arcing??). Would it be possibe for just the bias balance cause this? I tried another set of 6l6 tubes w/o bios change, and same thing.

The amp is 45 years old, so I figure it plays well for that age. Even with it's problems, I have never plugged into any amp that did sound that good. I would love to find a good tube amp guy, but there are none in my area. I would have to drive 4 hours one way in order to find a good tech. I may some day for a full tune up, but I can not make that trip (twice) any time soon.

Thanks for any input/help

Again, for now, just one step at a time, I am just looking for fail safe directions on how to set bias balance pot.

[Newguy]

Follow ups please, or other members advice. I would like to get this done today and on the way to Radio Shack to get digital meter as all I have is old analog meter.

Review:

1. Right way (or any way) to set bias "balance"
2. Power tubes out for both pin 5 Power tube for setting balance, and should I try to get max neg voltage for pin 5 to ground (for just balance), and do power tubes always stay out for trying to max out balance? It would seem a balance adjust would only balance out even, and I would not be able to increase voltage with this adjust?
3. All vol and tone controls to zero for this right??
4. Does this bias "balance" effect tone in any way, and does it do any thing else IE Tone/noise/any thing??
5. If not voltage at pin 5 with standby, do I flick the other switch in back, as mention on forums that some amps to not supply voltage on just standby to Power Tube pin 5?

Thanks

[pdf64]

Aiken has a great new bias meter, likely superior to anything else out there.
http://www.aikenamps.com/Wombat.htm

Your amp, if it is AB763 / real blackface has an 'overall bias' level pot, not bias balance.

[Newguy]

I am pretty sure it is real, as I had it to one Fender Gold Rated tech in Orlando Florida..Randy, amp never sounded so good. Also another Fender silver rated tech, neither said it was any thing but a blackface? Not sure what your meant?

Any way, that is out way out of my price range. I need to do it the cheapest way possible.

What do you mean overall balance??? I assume it means it raises or lowers bias to both power tubes, but they are not matched?? Explain as I have not a clue what you mean.

Thanks

[Jazz P Bass]

Your amp, if it is AB763 / real blackface has an 'overall bias' level pot, not bias balance.

Here is the BFSR AB763 schematic.
It has an adjustable bias supply.

[Jazz P Bass]

What do you mean overall balance??? I assume it means it raises or lowers bias to both power tubes, but they are not matched?? Explain as I have not a clue what you mean.

Certain Fender amps had a hum balance pot.
Yours does not.
You have an adjustable bias voltage circuit.
The schematic shows -52Vdc at the output tube grid pins (pin #5)
In a nutshell: Push-pull output tube Bias:
In a push-pull configuration, Class B, there is a notch on the output waveform when one tube stops pushing & the other tube starts pulling.
By going less negative with the grid bias, you enter Class A.
The mix is termed Class AB.
When the notch (crossover distortion) disappears (Class AB), you no longer need to go less negative with the bias voltage.
So how do you set it?
Without test equipment (a scope, an ammeter, soldering tools) you set it by ear (crossover distortion sounds bad) or you set it to Fenders -52Vdc & play the amp.

[Newguy]

Certain Fender amps had a hum balance pot.
Yours does not.
You have an adjustable bias voltage circuit.
The schematic shows -52Vdc at the output tube grid pins (pin #5)
In a nutshell: Push-pull output tube Bias:
In a push-pull configuration, Class B, there is a notch on the output waveform when one tube stops pushing & the other tube starts pulling.
By going less negative with the grid bias, you enter Class A.
The mix is termed Class AB.
When the notch (crossover distortion) disappears (Class AB), you no longer need to go less negative with the bias voltage.
So how do you set it?
Without test equipment (a scope, an ammeter, soldering tools) you set it by ear (crossover distortion sounds bad) or you set it to Fenders -52Vdc & play the amp.

Jazz..got it..clear as bell. Wish I could read a schematic, looked over it a few times before I saw the -52v. Not sure you know that is pin 5?

Hope you are some one knowledgeable is still on site for next question

[Newguy]

I watched video tonight and guy setting bias and plate voltage on blackface deluxe (point to point) so it should be the same.

He measured bias by using meter (on milliamp setting) and red lead to center tap (main power to fiber board or what ever that is), and black meter lead to pin 3 power tube.
He measured plate voltage by using meter (on volts setting) red lead to pin 3 power tube, and black lead to chassis grond.

By messing with both he pretty much got into spec (he was using different rectifier tubes to show how that effected readings).

Plate voltage on deluxe around 390v
Bias reading on both tubes close to 24 each.

Question

1. Any one do it that way with just meter?
2. What plate voltage am I looking for on the 6L6
3. What bias reading should I see.
I was not sure where to look for specs but this may do:
Fender Super Reverb (1965)

Am I looking at plate voltage of 460-470 and bias reading from pin 3 at -48??

Even though very dangerous, looks like an easy way to do it, and the way I will go unless some one tells me this is not the way to go for some reason. I do realize I have to try and be in spec and also listen to the sound.

Thanks for the help guys

[Jazz P Bass]

6L6 datasheet.
Pinout is a socket side view.

[Newguy]

I take that -48 back.

What reading should I get for bias with red @center tap and black at pin 3 power tubes?

Is there a spec on this, or should they just be real close, and plate voltage less the 500v?

[g1]

The current you measure from pin 3 to center tap is your idle current. In your example it was 24mA for the deluxe. The current times the plate voltage gives your power dissipation for each tube. So, for the deluxe, .024 X 390 = 9.36 watts.
You have 6L6 tubes which can handle up to 30W. So if you want 60 to 70% of 30W you need 18 to 21 watts idling dissipation.
As you adjust the bias the plate voltage will change so you keep checking it and adjusting your calculations. For example, at 450 plate volts, 40mA per tube would be 18W (.040 X 450).
Be aware that when your meter is in current (mA) function it can be extremely dangerous. Don't touch anything but the points you are measuring and don't forget to change back to volt mode to check the plate voltage.

[Newguy]

G-One

Thanks for that post. I get it (sinking in).

Wow, it looks so simple on video, but I can see it would be so easy to make a mistake, get hut, blow up the amp or meter, or both.

Thanks to you folks here, I can see why it is not easy to explain how to do this. Even watching the guy on video, (who made it look easy), thinking about all that can go wrong, makes me a little nervous.

Maybe setting it to -52vdc and maybe a small tweak by ear may be a smart thing to do. I know I will prob go for it though lol.