Biasing a Fender Twin

[SSLtech]

Hey all,

Thanks for the help with the choke on the Fender Twin; I replaced the choke and it's all happy again. Now I'm looking at biasing.

This is a Silverface master volume, which has this schematic.

I hunted around on the web and they suggest adding a 1Ω/1Watt resistor in line with pin 8 of each of the output tubes, then measuring the voltage in milllivolts, which gives a 1:1 indication of the current in milliamps.

I've done this, and I get about 12mA per tube. -A few places seem to suggest that the expected value should be higher than that, but a quick google doesn't really tell me what to aim for...

So I popped the hood on a black-face reissue (non-master volume) and did the same 1Ω resistor per output tube mod, only to discover that two of the output tubes weren't working! (the 470Ω resistors were open circuit) but that the remaining working tubes (one per side) were passing about 37mA each.

So... Two questions:

Can anyone tell me what current I should aim for? (I'm not looking for "pushing" the sound at all... I just want a safe, reliable bias setting) -Also, is there anything 'bad' about the bias having been so low?

Also, is it acceptable to LEAVE these 1Ω resistors in series with pin 8 on both of these amps?

Cheers,

Keith

 

[abbey road d enfer]

Hey all,

Thanks for the help with the choke on the Fender Twin; I replaced the choke and it's all happy again. Now I'm looking at biasing.

This is a Silverface master volume, which has this schematic.

I hunted around on the web and they suggest adding a 1Ω/1Watt resistor in line with pin 8 of each of the output tubes, then measuring the voltage in milllivolts, which gives a 1:1 indication of the current in milliamps.

I've done this, and I get about 12mA per tube. -A few places seem to suggest that the expected value should be higher than that, but a quick google doesn't really tell me what to aim for...

So I popped the hood on a black-face reissue (non-master volume) and did the same 1Ω resistor per output tube mod, only to discover that two of the output tubes weren't working! (the 470Ω resistors were open circuit) but that the remaining working tubes (one per side) were passing about 37mA each.

So... Two questions:

Can anyone tell me what current I should aim for? (I'm not looking for "pushing" the sound at all... I just want a safe, reliable bias setting) -Also, is there anything 'bad' about the bias having been so low?

Also, is it acceptable to LEAVE these 1Ω resistors in series with pin 8 on both of these amps?

Cheers,

Keith

10/12 mAmp per tube is definitely on the low side; you may notice some crossover distortion and reduced gain too (although NFB compensates it somehow).
Real bias adjustment should be done by a combination of measuring/visualizing THD (distorsion meter/scope) AND ear. Personally, I like it clean and loud, so I would bias each tube at 100 mAmp if I could, but the plate dissipation, with ca. 470v on the plates, allows for only about 60mA (6L6GC) or even 40mA (6L6). I think 35-40mA is a good average, representative of the expected Fender "clean" behaviour.
You can leave these 1R resistors in place. Some manufacturers install 10R cathode resistors and leave them in.

 

[pucho812]

here is a handy reference chart for bias and 6l6 tubes which in your twin with a master volume control should be 4

 

[Biasrocks]

Here's a good guide for biasing tube amps.

http://www.duncanamps.com/technical/lvbias.html

And another at the Tone Lizard

http://www.tone-lizard.com/Biasing.htm

Mark

 

[MikoKensington]

http://www.webervst.com/tubes1/calcbias.htm

 

[hobiesound]

get a bias king it'll make your life so much easier............

 

[peterc]

http://www.geofex.com/tubeampfaq/taffram.htm

 

[Johan]

measure the anode-cathode voltage, find out the tubes max disapation from the datasheet (~25W for 6L6 I think ) and aim for 60-70% at idle..Ohms law gives you the rest.
say  25Watt x 0.65 / 470volt = 0.0345A per tube.  your 37mA sounds pretty good to me..
J

 

[MagnetoSound]

Real bias adjustment should be done by a combination of measuring/visualizing THD (distorsion meter/scope) AND ear.

... I think 35-40mA is a good average, representative of the expected Fender "clean" behaviour.
You can leave these 1R resistors in place. Some manufacturers install 10R cathode resistors and leave them in.

Totally agree, do it on a scope and by ear. Just past the point of no crossover distortion generally works for me, the higher the current the greater sustain and 'roundness' you will hear. Too much and your tubes will run away. The hum will warn you.

Those figures are totally in the ballpark 90% of the time. The resistors make a workable fuse if a tube goes short.

 

[SSLtech]

Romping, thanks!

-This twin wouldn't bias much past 30mA anyhow, so I set it in that area and left it alone.

The tubes are now running MUCH hotter. -I'm going to live with it for a while and see what I think.

Thanks again for all the help.

Keith

 

[Emperor-TK]

Here's one I like:

http://www.aikenamps.com/Biasing.html

Aiken seems to dislike the crossover distortion method and gives reasons why.  I second the bias meter recommendation.  I got one from Weber.

https://taweber.powweb.com/biasrite/br_page.htm

 

[SSLtech]

AH..... I like the Weber/Bias King idea... I have dozens of octal plugs & sockets... I should just make plug-in versions with 1Ω resistors in line, then to a switcher box, and use a fluke to read mA... -Genius!

Keith

 

[Stagefright13]

ACK I have a fender twin reverb here and just slapped in a bunch of new tubes into it... (Actually a couple years ago) Inputs and 4 JJ outs. I know enough to know it should be biased properly but it sounds great now. And nothing is glowing brighter than a 60 watt lightbulb...

Maybe I should look into this... Not many people play that amp but...

Thanks,
John

 

[toobdood]

I've got an octal plug-socket with a 1 ohm 1% resistor.  At 1 ohm: Measured Voltage = Current in mA.  You want to bias a Twin to about 45 to 50 mA per tube.  I measure each tube in a set.  Tolerate any tube-to-tube variance of 10% from the lowest current draw to the highest and set the maximum bias for the tube that draws the most current.  I've been doing amps for 20 years and this is THE way to get the most from tubes.

  Formula is:  Tube Wattage X 0.7 /  Plate Voltage = Bias Current.

  Eg: JJ6L6GC  30 x 0.7 = 21  21 / 435 = 0.48 = 48mA

  For BIG amps with OVER 500vdc to the plates I observe a 50% bias setting.  For amps UNDER 500vdc feel free to set up to 70%. 

 

[toobdood]

  Yes there's more!  What you posted as a schematic is one of the more common Silverface amp circuits that I have Blackfaced MANY times.  One CANNOT get the Bias adjusted on these amps without modding the BALANCE control into a BIAS control.  Take a look at the AB763 Twin schematic as reference side by side with yer Silverface and you will begin to see the differences.  Once you have those worked out you are on the path to a Blackface Amplifier.

 

[SSLtech]

Yes, the bias control is indeed a bias level and NOT a bias balance, so someone else has been there before me.

But the tubes were running distinctly cool (as in physically not extremely hot) at 12mA, and since I set it to 30mA they're running distinctly HOT by comparison.

Since the bias asjustment range on this amp seems to top out at about 35mA, I left it at thirty... it just 'feels' wrong for me to 'max out' a bias setting!

Do you have any documentation/schematic  for the bias 'amount' mod (replacing the bias 'balance') -since I wonder if whoever did this mod before me just used a wrong value component or whatever?

Cheers!

Keith

 

[Biasrocks]

Do you have any documentation/schematic  for the bias 'amount' mod (replacing the bias 'balance') -since I wonder if whoever did this mod before me just used a wrong value component or whatever?

Cheers!

Keith

It's a fairly simple mod iirc, you remove the 15K resistor that goes between ground and the 10KLin wiper change that to 27K attach to one side of the pot and to ground. The other side of the pot goes to the Bias PS which should be changed to a 100uf/100v cap instead of the 50/50v in the schematic, the wiper goes to the junction of the two 220K resistors on the main board. You should have around -52v for bias going into the 220K's as a starting point. You can change the 27K value if you need more or less range.

Here's a overview of the SF to BF mods, but the bias mod description isn't correct so take it with a grain of salt.

http://acruhl.freeshell.org/mga/main/sf_bf_mod.html

Here's the blackface schemo

http://www.ampwares.com/schematics/twin_reverb_ab763.pdf

And the silverface

http://www.ampwares.com/schematics/twin_reverb_aa769.pdf

I would recommend converting the entire amp to BF specs, it results in a much nicer sounding amplifier just as Leo intended.

I've got a DVD describing all the changes with some good vintage amplifier maintenance tips as well.

PM me your mailing address and I'll send you a copy.

Mark

 

[clintrubber]

I had a look at my SF Twin Reverb with one of those octal in-between measuring sockets and found that one of the four ran at zero current  :

The side of that 6L6GC tube also seemed a bit suspicious (like having a big second getter spot).

I don't know for how long this might already ge going on. Amp sounded OK...

I realize the Output Transformer could have been running for quite a while with the current unbalance.
It's no mic-TX, but could non-cancelling DC currents develop adverse magnetization in Output TXs ?

I'm also curious to the possible malfunction mechanism of this 6L6GC (resulting in 0 mA); what actually gets damaged ?

Thanks!

 

[PRR]

> could non-cancelling DC currents develop adverse magnetization in Output TXs ?

No, for two reasons.

1) The OT is stacked with considerable equivalent gap, and won't be stressed by tube idle current.

2) Balance the amp, play a BIG bass note and let it fade to silence. There, you de-gaussed your iron.

> I'm also curious to the possible malfunction mechanism of this 6L6GC (resulting in 0 mA); what actually gets damaged ?

My tractor won't run. What actually got damaged?

Could be anything!! Spark, fuel, squeeze, bearings, even the nut that holds the wheel.

Lack of heater glow is bad. If the cathode gets hot, zero current suggests a busted weld inside. But a G2-G3 (beam rod) short could force current so very low that you think it is zero.

A 66HP tractor is worth tearing into; a 6L6 usually isn't (nor can you repair it in the average workshop).

 

[clintrubber]

Thanks Paul for the response, much appreciated.

> could non-cancelling DC currents develop adverse magnetization in Output TXs ?

No, for two reasons.

1) The OT is stacked with considerable equivalent gap, and won't be stressed by tube idle current.

2) Balance the amp, play a BIG bass note and let it fade to silence. There, you de-gaussed your iron.

W.r.t. 1), good to hear this. Didn't realize a push-pull-OT has a significant equiv gap as well.

W.r.t. 2), shame on me, I once worked on degaussing-coils for CRTs, it's the same principle

> I'm also curious to the possible malfunction mechanism of this 6L6GC (resulting in 0 mA); what actually gets damaged ?

Could be anything!! Spark, fuel, squeeze, bearings, even the nut that holds the wheel.

Lack of heater glow is bad. If the cathode gets hot, zero current suggests a busted weld inside. But a G2-G3 (beam rod) short could force current so very low that you think it is zero.

Thanks for the ideas. Heater works, and cathode current doesn't trigger the zero after the dot of 0.0mA (I didn't switch ranges so far; something was obviously plain wrong).
Was just curious to what actually could have gone wrong, but you're right, hard to pinpoint, could be many things, so for now throw the curiousity & the tube away & just replace & check the bias...

(... as far as is possible in this not yet modified TR... presently it's still in the infamous AA270 bias-balance form)

Best regards