How to Bias a Fender Concert amp. (Title Changed)

[wsscott]

So that's my question.  I haven't seen the amp yet, but I've checked out the schematic, attached.  Do you get the plate to cathode voltage and use Rob Robinette's calculator for the DC voltage value, and then adjust the value of the 470 ohm caps on the screens of the 6L6's? 

[shooter]

you start with the -55vdc FIXED bias and plate vdc, run it through your "calculator" to see what the tubes are dissipating
ballpark starting point is ~~~~~~60% dissipation at idle, no signal.


I start by adding 1 ohm Resistors cathode to ground on the PA tubes, makes calculating much easier.


screen current is calculated by the Voltage drop across the screen resistor then ohm it, V/R = I and compare to tubes data sheet.
now you play til your fingers bleed and "decide" if you want to "tonally" change things

[wsscott]

I have one of the basic model Eurotubes bias probes that plugs into my DMM.  So I can get the bias reading with that and the cathode current reading.  So to adjust the bias, and maybe it won't need it, would the two 220K resistors be changed to get the bias to read -55VDC, or would it be done by changing the 8uf/150V(is that what it is?) electrolytic cap?  Maybe I'll be lucky and it will have the pot installed to adjust. 

[wsscott]

Attached are the results from the Robinette bias calculator using the 6L6GC tube and the 456VDC voltages shown on the schematic.  It shows 55 DC milliamps for a cool Cathode Bias.  So is cathode bias value what we're using here?

[Willabe]

So to adjust the bias, and maybe it won't need it, would the two 220K resistors be changed...

No.

Changing them will not change the bias of the tubes. There's no current flowing so there's no voltage drop across those 2 x 220K grid leak R's.

.... get the bias to read -55VDC,

The -55dcv is not the magic -dcv number. Not how it works

Back then, in the golden age of tubes, Fender used RCA mostly. And those RCA 6L6GC tubes were very close to each other in output current. So Fender could/would find a -dcv that would work with all, or nearly all, of those RCA tubes and give them the output current they wanted. 

But any set of 6L6GC's you get now, NOS or new production, all bets are off. They probably won't bias up right with that -dcv.   

or would it be done by changing the 8uf/150V(is that what it is?) electrolytic cap?

No.

The -bias circuit has single solid state rectifier to get the -dcv from the PT secondary acv. So that 8uF/150v is needed as the ripple smoothing cap. It does nothing to change/set the -bias.

You need to read these 3 links on -bias.

https://www.aikenamps.com/index.php/what-is-biasing

https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm

https://www.valvewizard.co.uk/bias.html

[Willabe]

......  So is cathode bias value what we're using here?

No.

That Fender amp is class AB grid bias.

Read those 3 links, until you understand the different types of bias, what -bias is and how to set it.

If you don't get this right you can blow up the power tubes, maybe the OT/PT. The power amp has the highest dcv's and the highest current in the amp, it's the amps heart.

Your just guessing and you can't just guess and run numbers through a bias calculator expecting to get it right. 

[wsscott]

Thanks, this is very helpful. I'll check the sites out.   I've only had the opportunity to be exposed to maybe 8-10 amps or so, and they have either been cathode biased or fixed biased with a pot.  So this is a new circuit for me to learn from.

[Willabe]

And read these 3 from our host Doug;

https://el34world.com/charts/Biascircuits.htm

https://el34world.com/charts/bias_conversions.htm

https://el34world.com/charts/fenderservice5.htm

[Willabe]

.... and they have either been cathode biased or fixed biased with a pot.  So this is a new circuit for me to learn from.

There's fixed bias and there's adjustable fixed bias.

There's 2 resistors in that amps -bias supply. As you'll read and see drawings of, you replace 1 of those R's with a pot and a pot tail R to make it adjustable. You need the pot's tail R, so there's always some -dcv. Otherwise you could burn up the tubes by setting it too low accidentally.

[wsscott]

I haven't had a chance to read all of the articles, but I did read Doug's article on How A Bias Circuit works.

Am I correct that to change the bias on this amp, without installing a pot, I would change the 10K resistor that connects between the - side of the diode and the 56K resistor, as shown on the schematic of the Concert amp posted above?

[Willabe]

Well, what happens if you raise or lower the value of that R?

It will give you a higher  -  voltage or a lower  -  voltage. Lets say -50dcv or -25dcv. (It's easy to get mixed up with higher or lower with this because it's a  - voltage.  )

But, there are limits to how much changing that R can go. Sometimes you have to also change the 56K R.

You start with taking out the power tubes and measuring the -dcv at the tube socket pin. Then after knowing there's a -dcv that looks about high enough to keep the tubes from running away,   you put the power tubes back in while watching the meter for current. AND watching for red plating of the tubes plate. And you'll need the tubes plate dcv at that current draw.

Then put those #'s in a bias calculator and see where your at. Then go from there. 

[Willabe]

Is this amp a real Fender, or a clone?

If it's a clone, don't even mess around, put a pot in to make it adjustable bias.

[wsscott]

I'll be interested to see what the bias reading actually is.  It looks like the goal is -35mA as a good bias for 6L6GC's. Maybe nothing will be needed!!

[wsscott]

I haven't seen it yet, but it's supposedly an original 1964(?) Fender Concert Amp.  Not sure of the year yet, but its old.

[Esquirefreak]

The current (mA) is only half of the equation. The plate/anode voltage is the other half.

If it is a '64 it should be an AA763 with adjustable fixed bias?

/Max

[Willabe]

I'll be interested to see what the bias reading actually is.  It looks like the goal is -35mA as a good bias for 6L6GC's. Maybe nothing will be needed!!

No, that current draw will change with the dc plate voltage.

Generally to get the same dissipation from a set of tubes, the higher the plate dcv the lower the current draw is set, and the lower the plate dcv then the higher the current will be set.

A lot of guys go for ~70% dissipation, some like only 60% better. I go with 60%.

[Willabe]

I haven't seen it yet, but it's supposedly an original 1964(?) Fender Concert Amp. 

No, 6G12-A is a brown face amp, 6 series of Fenders. It's older than 64.

Unless Fender had some left over parts they wanted to use up and built it after the black face amps were released?

Edit; Looks like it could be 1 of the last 6G12-A's, left over parts?

From wiki;

.....  model updates for the Concert starting with the 5G12 (late 1959, early 1960) and continuing with the 6G12 (in late 1960, early 1961) and the 6G12-A by 1962 into 1963.

[Esquirefreak]

I'm aware.

There's also some major differences between 6G12 and 6G12-A.

/Max

[HotBluePlates]

How to Bias a Fender 6G12-A Concert. ... Do you get the plate to cathode voltage and use ...

you start with the -55vdc FIXED bias and plate vdc, run it through your "calculator" to see what the tubes are dissipating
ballpark starting point is ~~~~~~60% dissipation at idle, no signal. ...

My Opinion:

   1.  Do as Shooter said, and connect a voltmeter from Pin 5 of the output tube socket to ground.  See that the amp develops approx. -55v DC at Pin 5.
   2.  Connect a voltmeter from Pin 3 to ground, and measure the plate volts of the amp.
   3.  Use your Eurotubes probe to measure the actual plate current of the output tube.
   4.  Multiply the Voltage reading from Step 2 with the Current reading of Step 3.


It's possible to adjust the voltage sent to Pin 5 to bias the tubes used, but the average tubes shouldn't need much (if any) adjustment.  Changes mostly take the amp from "Happy to Glad."

The -55dcv is not the magic -dcv number. Not how it works

There's a little bit of magic, once you understand how to design a power output section:


Why did Fender bias to -55v as shown in their schematic?

   -  Output tubes distort when the peak drive-signal equals or exceeds bias.  So we can assume "55v peak" is "fully driving this power section."

   -  6L6s have a transconductance of around 5000 to 6000 micromhos, which could also be said as 5-6 milliamps of plate current per volts of G1 voltage.  Or the European way of "5-6mA/volt."  Let's assume an "average Gm" of 5.5mA/volt.

   -  Multiply the peak drive-signal by the tube's transconductance to find the Peak Plate Current pulled by the tube:  55v Peak x 5.5mA/volt = 302.5mA Peak

   -  This is a Class AB amp, with an output transformer primary of about 4kΩ.  The load to one side of the push-pull power section when fully-driven (and the other side cut off) is 4kΩ / 4 = 1kΩ.

   -  We know an Impedance, and we know Peak Plate Current.  Use Ohm's Law to find Peak Plate Voltage Swing:  302.5mA Peak x 1kΩ = 302.5v Peak

   - Peak Power Output = Peak Plate Current x Peak Plate Voltage Swing = 302.5mA Peak x 302.5v Peak = 91.5 watts Peak

   - RMS Power Output = Peak Power Output / 2 = 91.5 watts Peak / 2 = 45.8 watts ----> exactly what we expect from a Concert/Super Reverb-sized amp.


This is the logic behind Mesa's non-adjustable fixed-bias:  if you use tubes that conform to "average specs" then you shouldn't need a different grid-voltage to bias the tubes.

There's a little more involved, especially if the amp runs close to Class B and needs a cooler idle bias to keep the tubes from overheating when driven.  But this covers the main idea.

I haven't seen it yet, but it's supposedly an original 1964(?) Fender Concert Amp.  Not sure of the year yet, but its old.

I haven't seen it yet, but it's supposedly an original 1964(?) Fender Concert Amp. 

No, 6G12-A is a brown face amp, 6 series of Fenders. It's older than 64.

Unless Fender had some left over parts they wanted to use up and built it after the black face amps were released?

There were blackface Concert amps in 1964, which were like a Super Reverb but with no reverb.  See the AA764 Concert schematic.

Since the OP hasn't seen the amp, it's like the model is not correctly identified yet.

[Esquirefreak]

The transitional and mystery models are my absolute favorite Fenders. I think between 1959 and 1963 Fender made some of their biggest changes both in looks and electronically

[wsscott]

Here are some photos of the amp.  Don't know the date.  The serial number is F214526.  Will open it up in the next day or so.

[HotBluePlates]

Here are some photos of the amp.  Don't know the date.  ...

That's an 80's Rivera-era Concert.

The bias supply is fixed (at -50v), and there's a bias-balance pot to adjust to get equal plate current in the output tubes.  You will need your own means to measure idle current.

[shooter]

I start by adding 1 ohm Resistors cathode to ground on the PA tubes, makes calculating much easier.

[wsscott]

My "co-conspirator" who has possession of the amp now, says it's a 1983, based on the PT.  So just need to download the correct schematic.

[wsscott]

Shooter-Any reason not to use my Eurotubes device?

[HotBluePlates]

My "co-conspirator" who has possession of the amp now, says it's a 1983, based on the PT.  So just need to download the correct schematic.

There's a link to it in my post.

[shooter]

simplicity, you can monitor current "naturally" without adding an extra "layer" to the mix.
I've never liked "breaking" a circuit if you could get the same information without breaking into it.


as a salaried employee, the faster I got what I need, the sooner I was 100ft up a vertical face in a wonderful place 

[wsscott]

HotBluePlates-So am I correct that with this circuit you don't really ever adjust the bias which is set at -50v like you might do with a Princeton or Deluxe Reverb, but you just adjust the current on the 6L6's with the 10k pot so that they're reading about the same?  And you could use either a Eurotubes probe or the 1 ohm resistors setup on Pins 8 of the 6L6's to get this reading?

[shooter]

this is a FIXED bias AMP
NOT a SELF biased amp


fixed bias is the VOLTAGE at the control grid (G1)  that VOLTAGE controls the conduction of the tube


the BIG "tell" fixed bias will have a negative VOLTAGE applied to the control grid and NO resistors at the cathode
Self biased amps have a large power resistor at the cathode to ground connection and NO negative VOTAGE at G1

[wsscott]

So Shooter, where are you reading the voltages to balance them with the pot?  Are you reading them at the pot, between the center leg of the pot and each of the other 2 legs of the pot?

Sorry, but I've never seen this type of layout before.

[shooter]

i'm NOT seeing it in the schematic I have;
https://el34world.com/charts/Schematics/files/Fender/Fender_concert_6g12a_schem.pdf


that said, with NO pa tubes installed you should be able to measure the NEVITIVE voltage at the control grid socket.  it should be stable over time.  The pot should be able to adjust any "Imbalance" in the 2 sockets.


So if one grid reads -52 and the other is -48, the pot should be able to adjust so both read -50.

[wsscott]

Sorry Sluckey-Here's the one that is this 1983 amp.  The first schematic was what I posted before we got the amp.

[Willabe]

So am I correct that with this circuit you don't really ever adjust the bias ....

No.

Did you read those links I posted yet? There all, except for Doug's, only 1 page, won't take that long. It's not like reading a whole book. Will be quicker then asking questions and waiting for reply's. You guys need to do some of the research for your selves. Then ask about what you might not have fully understood.

All tubes need to be biased. And not all tubes, even if they are the same type, will pull the same current with the same dcv. For some unknown reason, Fender later decided to go with adjustable balance instead of adjustable bias. And yes you can have both working together. 

which is set at -50v like you might do with a Princeton or Deluxe Reverb,

Your way too hung up on the  -dcv number. That -50dcv happened to work with the 6L6GC's they had at that time. They found a range that the tubes wouldn't red plate and then were able to balance the current, so they didn't need matched tubes. Some set's probably ran cooler than other sets, but they didn't care how 'hot' an amp sounded. The tubes were probably not as close in current output as the earlier made tubes.     

but you just adjust the current on the 6L6's with the 10k pot so that they're reading about the same?

Yes, they probably used a scope, there's no 1 ohm K R's.

And you could use either a Eurotubes probe or the 1 ohm resistors setup on Pins 8 of the 6L6's to get this reading?

Yes.

[Willabe]

So if one grid reads -52 and the other is -48, the pot should be able to adjust so both read -50.

Putting the same -dcv on both grids won't work to balance the 2 tubes. That's why it's adjustable, because their going to need a different -dcv on each grid so they draw the same current.

[Willabe]

Sorry Sluckey-

       Shooter.

[Willabe]

I haven't seen it yet, but it's supposedly an original 1964(?) Fender Concert Amp.  Not sure of the year yet, but its old.

What happened? Did the seller tell you it's a 1964?

An '80s 2 x 12" Rivera Concert is a long ways of from a '64 Concert amp.   

[wsscott]

He just said he had a really old Fender Concert amp.  Didn't know the year.  Didn't mention Rivera.  I didn't know that amp had been modified so much over the years. 

[shooter]

 
I was just trying to keep it simple for now, by NOT having tubes in and "playing" with the pot, measuring VDC at the grid pins, wsscot gets a "sense" of how things should be/react before letting the magic smoke outta the PA tubes

[wsscott]

Shooter-you got that right!

[shooter]

to willabe's point, that pot does balance current not voltage in an operating system.


all that goes out the window once you plug-in and start wailing on the guitar, that's why we typically start out with a cool to cold bias in fixed bias amps, that will "limit" the swing until you get a sense of how that effects "Tone", then if you bias hotter for tone, you need to accept tubes will expire faster, but sound better, there's no free lunch 

[wsscott]

Ok, finally I've got more information about the amp and why the tubes were replaced.  I'll shorten the story.

The owner said that it had been playing well, and then recently turned it on and the amp played but had a LOW VOLUME and really NO POWER.

Also the Output tubes were burning BRIGHT RED.

He talked to his buddies and they suggested he get new tubes.  So he got a new set of ALL the tubes.  They also told him that the new tubes would require a BIAS job.

So that's how the amp gets to where it is today, and why it was brought in to my friend.  It may need biasing, but that's not likely the problem.

So I wanted to let you know what's really going on with this amp, and get your thoughts on whether this repair is going to be worth taking on the challenge, and where you would recommend I start a preliminary exam.

Attached are some photos.

Thanks for your time and patience.

[Willabe]

The owner said that it had been playing well, and then recently turned it on and the amp played but had a LOW VOLUME and really NO POWER.

Also the Output tubes were burning BRIGHT RED.

... got a new set of ALL the tubes.  ... told him that the new tubes would require a BIAS job.

... It may need biasing, but that's not likely the problem.

Yes, it probably IS the problem.

Power tubes red plating on their own with nothing having been done to the amp? Sounds like the -bias went out/failed. Could be bias cap died/shorted to ground.

Most of the e caps look original to me. Only 2 new e caps on the dog house board.

[Willabe]

Take the power tubes out and measure for -dcv on pin 5 of both power tube sockets.

According to the schematic ~  -50dcv.   

[tubeswell]

Also the Output tubes were burning BRIGHT RED.

That's loss of bias voltage. Before switching the amp back on pull all the output tubes out and measure the -VDC in the bias supply. You're wanting a negative VDC that's about 1/10th of the B+ voltage at each of the control grid pins (Pin 5 on a 6L6 socket)


The existing 6L6 tubes will likely be knackered if they've been red-plated already, so throw those ones away

[wsscott]

Willabe-I meant that a bad tube wasn’t the cause, it was something else that caused the tube to red plate and fail, like maybe the bias went crazy because of something else, like bad caps.

[shooter]

that a bad tube wasn’t the cause, it was something else that caused the tube to red plate

were the tubes tested in another system??


Too much current causes a tube to red-plate, current is controlled by your -50vdc bias
spend quality time there, as already mentioned, WITHOUT pa tubes pugged in.

[wsscott]

I'm thinking that this topic has now really changed from the initial question, I should shut this one down.  I can start up a new topic and maybe a moderator can move the posts starting with #40 over to the new topic.

[Willabe]

There seems to be really only 2 things with this thread, 1. It's a different amp, 2. Power tubes are red plating, so the bias went out.

Just start a new thread.
 

[wsscott]

Thanks.  Will do.

[wsscott]

One last question:  Can a failing filter cap cause red plating on the 6L6 power tubes, even if the bias setting for this 1983 Concert is reading correctly?