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

[shooter]

in the PS??  which TAP?  probably need more info to say yes or no


red-plating is to much current being drawn by the tube

[wsscott]

Assume the A or B nodes of the PS.

[pdf64]

Assume the A or B nodes of the PS.

I can't think how that would play out.
As I see it, either ecap failure mode * would tend to reduce the node voltage, thereby cooling the operating point.

* excessive ESR or excessive DC leakage current.

[wsscott]

The owner brought the amp in to his friend because he said the 6L6 tubes were glowing bright red and his buddies told him to get new tubes and have it biased.  So my buddy and I haven't seen the tubes glow bright red (with either the old tubes or the new tubes installed), and the bias is fine.  So we can't figure out what's going on.  I thought maybe there was some kind of an intermittent failure in one of the components.

[shooter]

maybe there was some kind of an intermittent failure in one of the components.

and that's probably the case
don't just "watch it", abuse it, raise a corner of the amp and let it drop, rock it back n forth, with a wooden stick tap on everything, play it like you stole it with volume at "The cops are on the way"


ALL while monitoring the bias VDC, the PA tube PS tap, if there's gremlins they will show up
otherwise;
re-build the PS including the Bias supply, tension the PA tube sockets, touch up sketchy solder joints, test the board itself for conductivity, look at the laminations and mounting points, the underside wires...

[wsscott]

I've noticed that the lugs in the 6L6 pin sockets are really wobbly, even though the tubes seem to be holding okay in them.  Maybe one of the solder lugs was touching another lug and creating a problem.  We'll let the owner know they should be replaced and see what he wants to do.

Thanks as always for everyone's insight.

[wsscott]

As to those lugs I just mentioned, I noticed that the lug for Pin 7 (heater pin) is really close to Pin 6 where the 470 ohm resistor bridges to Pin 4 (screen grid) and picks up the Node B power of 467 VDC.  What would be the result if those pins were in contact with each other?

[Willabe]

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?

Not enough information to answer this. There's no context.

Is this with the old tubes? What do you consider bias reading correctly? How was the -bias dcv determined? What filter cap? Is it the -bias filter cap your talking about? How do you know the filter cap if failing?

Power tubes red plate because of not enough -bias on the tubes grid. 

[Willabe]

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]

With the tubes out, it read -48.9VDC consistent with the schematic of -50VDC.

We never saw either the old tubes or the new tubes glow bright red.

The ESR readings for all the e-caps are within their ranges, even though they're 40 years old.  Will be replacing them anyway.

[mresistor]

If both power tubes were redplating the problem was not likely a pin on a tube socket, because it would have to be two pins on adjacent sockets at the same time.
More likely a problem with the bias balance pot or the 10uf 100v capacitor connected to the input wiper of the pot.
I don't think those pots are available anymore. 


If replacing those two 10uf 100v ecaps I think I'd go a little higher in uf like 30-47 uf.

[Willabe]

.... to the input wiper of the pot. I don't think those pots are available anymore.

There is no adjustable -bias on this amp.

Yes the 10KL bias pots are still made, CTS, Doug sells them; https://hoffmanamps.com/MyStore/catalog/parts5.htm

[pdf64]

.... to the input wiper of the pot. I don't think those pots are available anymore.

There is no adjustable -bias on this amp.

Yes the 10KL bias pots are still made, CTS, Doug sells them; https://hoffmanamps.com/MyStore/catalog/parts5.htm

Only 3 lugs?
Stock pot has an extra tap.

[Willabe]

Yes the 10KL bias pots are still made, CTS, Doug sells them; https://hoffmanamps.com/MyStore/catalog/parts5.htm

Only 3 lugs?
Stock pot has an extra tap.

I think your thinking of the bias balance pot Fender used later on?

[HotBluePlates]

The owner brought the amp in to his friend because he said the 6L6 tubes were glowing bright red ...

... We never saw either the old tubes or the new tubes glow bright red. ...

On another forum, someone said their tubes were glowing bright red.



They were referring to the end of the cathodes (glowing orange), where the end of the heater was also visible.

[pdf64]

Yes the 10KL bias pots are still made, CTS, Doug sells them; https://hoffmanamps.com/MyStore/catalog/parts5.htm

Only 3 lugs?
Stock pot has an extra tap.

I think your thinking of the bias balance pot Fender used later on?

Oops, correct, sorry I got mixed up 

[mresistor]

.... to the input wiper of the pot. I don't think those pots are available anymore.

There is no adjustable -bias on this amp.

Yes the 10KL bias pots are still made, CTS, Doug sells them; https://hoffmanamps.com/MyStore/catalog/parts5.htm

Not like that pot, those have 4 leads on them.

[Willabe]

.... to the input wiper of the pot. I don't think those pots are available anymore.

There is no adjustable -bias on this amp.

Yes the 10KL bias pots are still made, CTS, Doug sells them; https://hoffmanamps.com/MyStore/catalog/parts5.htm

Not like that pot, those have 4 leads on them.

1st, There is no adjustable bias on the brown face Fenders.

2nd, Fender used 3 lug pots with a tap (4th lug) for the brown face TS treble pot and much later for the bias BALANCE pot.

The 10KL pot that Doug sells has 3 lugs.   

The black face amps had that 10KL pot for the bias pot.

[Willabe]

I'm not sure but I seem to remember that some of the later silver face amps might have used a 3 lug pot with a tap for the adjustable bias.

[mresistor]

This amp has a 4 terminal bias balance pot.  I've encountered them in a few Fender amps. Look at his pictures you can see that it has 4 terminals with a ground wiper on the backside.
They can be substituted in circuit but they cannot be replaced.  The pics below are of the schematic not his photos of the actual amp you'll have to scroll to look that those.


IOW I'm telling the OP to forget trying to find one of these 4 terminal pots   they are not made anymore ..   

[Willabe]

I'm still looking at the title; 6G12-A. 

And wsscott changed the amp to a later Fender Rivera era Concert amp. 

[wsscott]

Yea, the subject matter has certainly changed from the Subject Title!

Speaking of which, the Rivera Fender Concert Amp has had all the filter caps replaced, and the bias balance is -49.7DCV.

The only real issue is trying to resolve a hum.  The 100 ohm Hum Pot (why a pot and not just a couple of 100 ohm resistors connecting to the 6.3 V filament wiring) which connects to the filament wiring at the Power Bulb can be adjusted to reduce the pitch of the hum, but it's still there.  The hum increases in intensity when the Volume pot for either channel 1 or 2 is increased.  If you remove V1 tube, the hum is still there and obviously there is sound coming through the speaker.  But if you remove V2, the hum goes away and there is a much lower level of sound.

Any thoughts on what's the cause?

[Willabe]

... The 100 ohm Hum Pot (why a pot and not just a couple of 100 ohm resistors connecting to the 6.3 V filament wiring) which connects to the filament wiring at the Power Bulb can be adjusted to reduce the pitch of the hum, ....

You answered your own question.

The way a transformer is wound the wind is not perfect. Even without a CT they're not always perfectly quite. And the faux CT resistors might not be perfectly matched. 

If it has a CT the earlier winds take less wire than the winds after the CT. And the CT might not be perfectly centered from start to finish of the heater wind. They try but it's just a guess as far as being perfectly centered. 

Either way, with a faux CT or a real CT the hum-dinger pot can dial in to a much more balanced place. They work if done right. 

[shooter]

maybe post the schematic to whatever amp ya'll decided we're helping out on 
if V2 "kills the hum" the problem is there or between there and V1
drifting part, bad solder, sensitive wires crossing big signal wires, sloppy sockets....

[Willabe]

I think it's this;

https://el34world.com/charts/Schematics/files/Fender/Fender_concert_ii_83.pdf

[Willabe]

I tried to find some chassis gut shots and the only 1's I can find are Reverb web page shots and the forum won't let me post them here.

It's kind of a rats nest inside. I can believe it humms.

They did use a LOT of shielded wire. 

[wsscott]

Yes that is the correct schematic for the amp at issue.  The original one I posted is not the correct one. 

[shooter]

looks like they hired a fresh outta collage engineer and said "have at it" 
so what are the squares pt# 013556 relays?

[wsscott]

Shooter they are VacTec Led/Ldr diodes.  I think the foot switches use them for switching between channels and effects.  Probably a Rivera idea.

[shooter]

hum increases in intensity when the Volume pot for either channel 1 or 2 is increased.

those both happen after the TS's but before V2 best I can tell


does this amp have the brass ground plate?
do the wires going to n from the TS's look like they were originally laid or like someone got in there n rat-nested them up some?

[wsscott]

Yep it has the brass plate.  The wiring rat's nest is the way it was built as far as we know.  It's really not had any work on it before.

Sorry Shooter but what do you mean "going to n from the TS's". I assume TS's is Tone Stacks, but what is "n"?

[shooter]

the wires leading from board TO the TS (tone stack) and back to the board.  Those are what I call "sensitive signals", if they lay near, parallel to "High signal wires" (power supply high volts, PI, or PA signal wires) they will "cross-couple" or cross-talk.


the brass plate has been known to cause oxidation at solder connections, causing sketchy bonding of those connections


with hum present;
does any of the FS actions change the hum?
any of the switches like "bright" et al cause change in hum"
have you jacked in a signal source to the return FX and verify the PA section is hum-free?

[wsscott]

The owner never has had a Foot Switch, so can't test any of that.

The hum changes with changes in Treble or Bass, but that's just a tonal thing.

I haven't put a signal into the Return FX, but can give that a try tomorrow.

Thanks Shooter.  Best.