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?

[g-one]

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.

[pdf64]

'or you set it to Fenders -52Vdc & play the amp'
With respect, that shouldn't be a method of setting bias recommended for use today. Because the bias voltage needs to be set in relation to the B+, which is dependant to the line voltage, amongst other variables.
That -52v is with RCA tubes and at 117v line.
As you don't have access to Fender's tube supply of the 60s, and your line voltage is probably not 117v (generally I understand that it's higher), then there's little benefit from setting the bias to -52v.
A 65 BF SR is a pretty valuable amp, a lot of that value is in the transformers which may be damaged if the bias gets screwed up.
Are you sure that this is an amp that should be scrimped on?
Plus there's your personal safety and that of other members of your household.

[Jazz P Bass]

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.
Please be aware there are a couple of ways to set the bias.
On this post, the method is tho disconnect the wire that goes to the output tube pin #3.
Each tube, one at a time.
You set your meter to read DC current.
The meter gets placed betweejn the disconnected wire & pin #3.
In this way you are measuring the tubes idle current.
Another method is to measure the resistance of each side of the output transformer.
Then with the amplifier on, meter set to read Volts dc, you measure the voltage drop across each side of the transformer.
Using Ohms Law, I=V/R, you get the tubes idle current in milliamps.
The third method is to place a low value, high tolerance resistor in between the 6L6's cathode & ground.
The voltage reading at the resistor can be converted to current. I=V/R.
Hope this helps.
(not easy?? this is why god made techs.)

[Newguy]

OK Jazz, you have me confused again.

Methods:

1. method 1 Take both power tubes out. Take/adjust reading of power tubes pin 5 to gnd to approx -55vdc. This would be a ball park and not an actual bias adjust.

2. method 2 (power tubes in)

a. (Bias) Using meter set to milliamp Red lead to center tap and black lead to power tubes (one at a time), to pin 3 to check bios.
b. (Plate volts) Using meter set to volts, Red lead to pin 3 and black lead to chassis ground.

Adjust as needed using formula stated here IE 40 milliamp (a) and 450 plate volts (b). This example would be for 18 watts.

**I do not see/understand where a disconnected wire is mentioned?

Please post as soon as you can. I did not get meter yesterday, but did take chassis out of amp along with tubes and sprayed contact cleaner in tube sockets and adjustment pots. It is snowing here in East Tennessee and was on way out as one radio shack has deoxit and figured I would get the deoxit and meter and get busy.

Newguy.
Please be aware there are a couple of ways to set the bias.
On this post, the method is tho disconnect the wire that goes to the output tube pin #3.
Each tube, one at a time.
You set your meter to read DC current.
The meter gets placed betweejn the disconnected wire & pin #3.
In this way you are measuring the tubes idle current.
Another method is to measure the resistance of each side of the output transformer.
Then with the amplifier on, meter set to read Volts dc, you measure the voltage drop across each side of the transformer.
Using Ohms Law, I=V/R, you get the tubes idle current in milliamps.
The third method is to place a low value, high tolerance resistor in between the 6L6's cathode & ground.
The voltage reading at the resistor can be converted to current. I=V/R.
Hope this helps.
(not easy?? this is why god made techs.)

[pdf64]

'sprayed contact cleaner in tube sockets and adjustment pots'
Oh dear, did you use the same spray for both?
That's a bad thing.
Pots need a cleaner with lube in it.
Tube sockets don't, high voltages across an oily residue can cause big problems.
Try to clean any lube off the sockets with a high purity solvent cleaner, and retension the contacts.
http://www.fenderforum.com/forum.htm...-01-2006:22:14

[Jazz P Bass]

[QUOTE=Newguy;191851]OK Jazz, you have me confused again.

Methods:

1. method 1 Take both power tubes out. Take/adjust reading of power tubes pin 5 to gnd to approx -55vdc. This would be a ball park and not an actual bias adjust.

2. method 2 (power tubes in)

a. (Bias) Using meter set to milliamp Red lead to center tap and black lead to power tubes (one at a time), to pin 3 to check bias.
b. (QUOTE]

O/K
On your #1, leave the tubes in.
On #2 STOP.
When your meter is set to read CURRENT, it gets placed IN the circuit.(ie: break the connection, insert meter) NOT across it.
If you do as you said, you will ruin your meter & who knows what else.

[Newguy]

'sprayed contact cleaner in tube sockets and adjustment pots'
Oh dear, did you use the same spray for both?
That's a bad thing.
Pots need a cleaner with lube in it.
Tube sockets don't, high voltages across an oily residue can cause big problems.
Try to clean any lube off the sockets with a high purity solvent cleaner, and retension the contacts.
FDP - Forum

OK, I am concerned. Lets get this one right. The cleaner I sprayed on both the pots and sockets was Radio Shack (64-4315), Contact/Control cleaner and lubricant. It says cleans and lubricates contacts and controls, for electro-mechanical devices,especially drum-type TV tuners.

Your link item 5 says"

5. Get some DeOxit or Radio Shack tuner cleaner. (In a pinch, you can use contact cleaner but DeOxit is best.) Put the little tube in the nozzle so that you can squirt the solution into the individual connector holes.

It does appear to leave a residue, but according to the RS description, and the above item 5, I would "ASSUME" this will be OK??? BIG QUESTION MARK HERE, PLEASE CHIME IN GUYS.

[g-one]

JazzP: The OP is describing shunt method of bias where you use the shortcut of shunting out the OT as a short-cut rather than placing the meter in series with the plate lead.
The methods are described on Aikens site.

[Newguy]

Yea, I think it is called the shunt method. I will have to watch video again. I have watched it like 4 times now.

Here is link to a page that describes it (scroll down for shunt method):
Bias FAQ

Questions of concern:

1. Am I an any trouble with the RS contact/control cleaner I used. If so, will deoxit get me out of the trouble?

2. On the -52vdc on pin 5, are you SURE to leave tubes IN?? I have so many links I can not find the one I am looking for yet, but I am pretty sure it said to take power tubes out, and turn down all volume/tone control pots. Then measure pin 5 of power tube to chassis ground and adjust to -52vdc.

Item 2: They did not mention any thing about speaker being connected, I assumed if no power tubes, no speaker needed, but Jazz us saying to leave power tubes in.

I am waiting to hear from you guys before I make any move at all lol

[MWJB]

They should read around early 30's ...you want an exact number? Try 32mA. Your B+ voltage will be what it is, if the amp sounds great, then it is right...it could be as much as 525vdc. Plate current should really be within +/-5mA from one tube to the next.

If you are using the shunt method, use meter leads with clips/wire grabbers, make all connections with the amp unplugged from the wall AC, after taking a reading on one tube, power down, unplug & move meter lead to next pinm 3 & repeat. If you short pin 3 to ground you will kill the tube, if you short pin 3 to pin 2 you will kill the tube, start a fire & burn out any virtual centre tap resistors making the amp hum until you replace said resistors. So triple check connections & ensure no chance of a short...you don't need things to be touching to cause a short, these are the highest dc voltages in the amp and an arc can jump quite happily oves a small gap. Remember to set your meter back to volts before trying to read any voltages, or you will kill your meter/meter fuse.

In short, I strongly suggest that you don't use the shunt method, buy some bias probes then you can read & adjust plate current without even pulling out the chassis.

[Newguy]

MWJB

Yea, I was going to get the mini-alligator clips with the rubber around them, turn every thing off before moving any probes, double and triple check connections and settings on meter before turning amp back on.

You know any thing about the -52vdc on pin 5 and if I should or should not pull the tubes for this measurement? I would like to measure it as is before I make any adjustments.

BTW, in the video I watched (shunt method I assume), the guy did not power down, only put amp in standby (has brass ones).

Going to get some deoxit, only one radio shack in the area has it in stock, and it is 20-25 minutes away.

Added information, amp has been modified with no rectifier tube (solid state rectifier, not a plug in, but mod on top of socket). Power tubes are Groove Tubes (GT-6L6B), hardness rated at #4. I know they are not the best, but all I have right now. The best I can tell from google searches, is the groove tubes are Sovtek 5881WXT

[g-one]

The control cleaner with lubricant should be ok. Deoxit seems to be preferred by most but is more expensive.
The bias voltage measured at pin 5 is with power tubes in. Whenever power tubes are in, speaker (or load) must be connected.
What is stated on the Duncan site regarding your meter quality for shunt biasing is important. If your meter does not have a very low resistance in current mode you will get a reading that is falsely low and you run the risk of biasing your tubes too hot and decreasing their lifespan.

[MWJB]

"You know any thing about the -52vdc on pin 5 and if I should or should not pull the tubes for this measurement? I would like to measure it as is before I make any adjustments." Forget "-52vdc" it was only relevant when the amp was manufactured, & only with the brand of tubes that they were using then, set the bias pot so that you get maximum negative voltage at pin 5, then fine tune by current (mA).


"BTW, in the video I watched (shunt method I assume), the guy did not power down, only put amp in standby (has brass ones)." It's very easy on a BF amp to brush your arm against the hot contacts of the power switch, plus you still have full dc at the standby.

G-one wrote: "What is stated on the Duncan site regarding your meter quality for shunt biasing is important. If your meter does not have a very low resistance in current mode you will get a reading that is falsely low and you run the risk of biasing your tubes too hot and decreasing their lifespan." This is nonsense in this particular case...maybe more relevant with a 100-200W amp with a very, very low DCR at the OT primary, but not to a super reverb.

[g-one]

MWJB, why is this nonsense? Maybe not critical in your opinion but I think being off by 10% or more could decrease the lifespan of the tubes. Especially if you thinki you have biased for 70% but it's really 80%. I'm talking about a 10 ohm shunt as opposed to a 1 ohm shunt across what, 100 ohm primary DCR?

[MWJB]

Even using the cheapest meters I have usually found them to be accurate enough. There's no real reason to bias to 70% specifically, it is not usually necessary. It's not a competition to see how close to the edge you can get. If your amp is biased to 70% (however determined) and still sounds poor unless you set for more current, then setting for more current is what you will do, some amps have to be biased to 85% or thereabouts to sound half decent (I'm not suggesting anyone do that without specific knowledge of the tubes & amp in question). Power tubes are relatively cheap, so one is more likely to live with shorter part life than have a $1000 amp they don't like the sound of. How hard the tubes are worked also depends on how hard the amp is driven in terms of signal, so one pair of tubes running at 60% can conceivably wear out quicker than another biased to 85% that are not worked so hard.

9% margin for error, in reality, is not unworkable...you might have that, or thereabouts when using 1ohm cathode resistors to read bias, plus the additional factor of screen current. Some "6L6GC" will crap out at well under 70% of 25W, the only real way to know what the max dissipation is that your tubes will take is to destruction test them...whichever way you look at it there are variables.

The actual number in mA is less critical than a repeatable process to get to where you know works, backed up by ear testing.

I used to know a guy who drove everywhere at 10% over the signposted speed limit because he said that you were allowed +/-10% before you could be prosecuted. Not having had his speedometer calibrated he didn't actually know whether his speedometer was right or 10% over to begin with, plus tyre pressures can affect reading etc...in short, it's not very wise to constantly run at what you perceive to be the limit.

[Amp Kat]

Newguy I was hoping you weren't going to use the shunt method not because I didn't have faith but because it is so easy to slip and fry an OT and it's dangerous and you can forget to put you're meter in Amperage mode and fry it and circuitry. If you plan on doing biasing it really pays to have a bias rite or cheap tube current monitoring system. As many times as I've done the shunt method having a bias probe is the single best investment I probably ever made in my life and mine is pretty decent but it's just the way to go IMO. This one reads plate volatge,current and wattage.

Compu-Bias™ Computerized Tube / Valve Bias Meter

[Newguy]

Amp kat, I hear ya about messing up and frying every thing. After doing my first bias, I see that bias probe give all three measurements that one needs (would not have understood this until after doing one).

Any way did it/done.

Starting measurement 16.59ma and 479 plate. like 9.2 watts. Believe it or not, it still sounded good lol.

Ending measurement 40ma and 454 plate voltage. That is like 18.1 watts correct? According to specs of 6L6 that should be pretty close to being in spec.

The Dexoit stuff works great! Toothpich method of cleaning sockets worked well also, got lots of black stuff off that way (saw it on tooth pick).

I think amps sound a little better (it already sounded good). The dexoit helped out a lot with reverb static/feedback . The reverb sounds fantastic, although if turning up to above 8.5 it starts to howl (I tried a few different tubes in reverb preamp but no good. Still with reverb turned up to 6 it sounds better then the reverb on the BF Deluxe RI., and Vibroverb RI, and that is saying something.

Pots are quiet as a mouse. Vibrato still does not work, even though I can see light on top of chassis blinking when I kick in vibrato. Bummer, but I do not want to drive 5 hours each way..twice to just have that worked on, and probably pay out the nose too.

Any one have a link on troubleshooting the vibrato on a BF SR?

Thanks for all your help guys, no way I would have tried with out all your help.

[pdf64]

Re tremolo, probably you need to replace the opto-isolator, see
http://www.tubesandmore.com/cemirror...n-moreinfo.png
Re the cleaner lube, the key thing to ensure there's no residue between the terminals / tube pins, as it can arc over.
If there is, give the top of the socket a wipe over with some pure solvent, ipa (propanol-2) type cleaner.
Re reverb howl, does it howl with the reverb tank disconnected (remove the cable from the back of the amp)?
Re the bias check/adjust - well done!

[Newguy]

PDF64

That link to ping image goes and says "more information" (that is all it displays, strange) ? I tried on both Chrome and Firefox. I would like to see it too, if it shows solder connections. I have and do look at ping images btw on cdfreaks.

Where does one get this opto-isolator, as that would be a super easy fix . Is it as easy as replacing or is there adjustment also? It does blink, and it does change the speed of the blinking when the front knob for speed is moved.

ipa (propanol-2) type cleaner What is that exactly, and how can you wipe it with all those wires. I was thinking maybe a small tooth brush or something?

Yea, the Bias was not that hard, something one just has to do once. I was almost at end of adjustment pot with the 18 watts, so I would have to change resister on pot to go much more then 19 watts (which was tops..I backed off a little to 18). What would be the advantage of going hotter, as I think I am on the edge of specs on plate voltage, and raising bias would reduce plate voltage some more (if I remember correctly). At my age, remembering what you did gets harder and harder the older I get.

[Newguy]

PDF

Parts is Parts sells that part, but they are not in stock right now and on order from Fender.

Any one, please link me to a place that sells them if any one knows.

Thanks

[pdf64]

Doh - try
More Info for item R-VOP1020
Just replace it, should be super easy. If the light is flashing there's not much else that can be bad.

Propanol-2, try asking at a drug store for a small bottle, or
http://www.tubesandmore.com/scripts/...em=S-CDCC-V510
is probably pure propanol-2 in a pressurised dispenser.
Surgical clean is not necessary, the area around the plate pin 3 is where it really matters.

Can you consider fitting the 1 ohm cathode resistors to the 6L6GC sockets? It's a superior method of checking bias, and I'm concerned that you've turned the pot as low as it will go nearly. Pete.