# Can someone shed light on the bias adjustment in the Fender Blues Junior

#### Buhl

Can someone please help me understand the way a Blues Junior should be biased? There's a ton of different info on the web, some of it outright wrong info, such as stating that the amp is cathode biased etc.

From the service manual (http://schems.com/bmampscom/fender/Blues Junior Service Manual.pdf) I can read that:

"The output bias is derived from the -15Vdc supply. The voltage divider at R31 (R52 in later Rev. but still 22K) & R37 (R51 in later Rev. but still 33K)
determines the bias voltage (-10.7 Vdc). C14 filters the ripple to 1mV or less."

- or see: http://blueguitar.net/new/schem/fender/blues_jr.gif

Now, looking at the instruction video of the late Bill Machrone (YouTube) he states that one should "remove the output tubes or pull one of the red wires in order to lift B+"

He then places the ground terminal on B+ and measures voltage on the brown (transformer ?) wire, and sets a voltage. From the table on his page (Billm Audio >> Setting the Bias) I can see this should be 2,6volts with the stock tube and transformer.

Now, should the tubes not be in the circuit in order to set the bias? I mean, the bias should reflect the individual nature of the output tubes, and in this case also the transformer?

I also see that he has removed both reistors in the voltage divider and replaced them with a multiturn, but I assume that replacing only one of them - eg. R52 with a 50K variable would give the same option to set the bias the way described here?

I would be very thankful if someone could explain this?

Best regards
Hans

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#### Zung

The best way to set the bias is to use a bias probe; you can make one yourself, or, better still, break the cathode to ground connections and solder 1 ohms resistors in stead (1mV reading = 1mA of bias). You can then set the bias according to the datasheet, give or take 20% to get the tone you want.

#### JonSnell Electronic

I usually remove R52 (22k) and replace it with an 18k and a 10k preset in series.
More than ample to cool the output valves down a bit. I aim for 11.5v on the grids.
If you remove the output valves, the HT will be wrong as will the 15v rail, because the secondaries of the mains transformer will be high in turn making the -ve bias incorrect with EL84s fitted.
Fender do not recommend any adjustment as the DC bias is directly proportional to the HT and secondary voltages. I prefer, as do many of my customers, to run them less hot.
If you want to measure the anode current, place a 10R 1W resistor in between two crocodile clips and place it in series with the anode wire. Use Ohms law to calculate the current.

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#### Buhl

Hi Jon
Thanks for your reply! Am I right that you exchange the 22K with two resistors in series with a combined value of 28K? And this could be done with a 50K trimpot - adjusting while measuring the grid voltage?

Do you find variance between different tubes? is it recommended to recheck the grid voltage every time you change tubes?

best regards
Hans

#### JMFahey

Can someone please help me understand the way a Blues Junior should be biased? There's a ton of different info on the web, some of it outright wrong info,
you bet

From the service manual ..... I can read that:
..........

Now, looking at the instruction video of the late Bill Machrone
Bill WHO?
Will you trust a FACTORY Service Manual, written by ENGINEERS whon studied in a brick and stone UNIVERSITY or the "You Tube University"?
I am amazed you can even MENTION both in the same phrase.

Now, should the tubes not be in the circuit in order to set the bias? I mean, the bias should reflect the individual nature of the output tubes, and in this case also the transformer?
The fact that tubes are all different from each other, that´s a fact of life, even if same brand, is the reason for biasing needing adjustment when replaced.

Biasing without them is clearly a disfunctional way.

FACTORIES can get away without individual biasing and save valuable Tech/Assembly time by setting a fixed negative bias voltage *because* they are BIG buyers and can get pre-sorted/matched tubes in batches or dedicate personnel for that task ,aluxury an individual buyer does not have.

I also see that he has removed both reistors in the voltage divider and replaced them with a multiturn, but I assume that replacing only one of them - eg. R52 with a 50K variable would give the same option to set the bias the way described here?

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#### nigelwright7557

I would un-solder centre tap pin on transformer to B+ and put a multi meter in series with it to measure current.

#### Quard

BillM has been a trusted source of practical, useful info for Blues Jr. mods for decades,
especially for amateur musicians (not necessarily engineering-inclined, well-equipped, or knowledgeable about electronics). My guess is that most of his posts were geared toward those people, providing down-and-dirty methods that might get them closer to proper bias more safely than they would on their own.
billmaudio.com (now run by his son) continues to be a respected, and much-appreciated, source of info on Blues Jr. amp mods, specifically.
it's a guitar amp, not a hi-fi, and these instructions from a guitar-amp tech should be taken accordingly. be safe.

#### Zung

..
If you want to measure the anode current, place a 10R 1W resistor in between two crocodile clips and place it in series with the anode wire. Use Ohms law to calculate the current.

I would un-solder centre tap pin on transformer to B+ and put a multi meter in series with it to measure current.

Gentlemen!

It's OK to play russian roulette with your own life by messing with the hot end of the tubes (anode), but it's not OK to talk other people into it.

It's a lot safer to tackle the cold end by inserting a 1 ohms resistor between each cathode and ground, and measure the voltage across it; and you can leave the resistors there, they won't affect anything. And yes, formally the cathode current is the sum of the anode current and the screen grid current, but it's close enough for R&R.

#### PRR

Paid Member
...
Bill WHO?
Will you trust ...or the "You Tube University....

Bill was a very well known and well respected *computer magazine* writer.

On the side he was one of the first to study the Blues Jr. This was LONG before "YouTube".

I respect his Blues Jr writings; but there are things I would do differently.

And it seems either Bill mis-spoke/wrote or Buhl mis-heard/read this bit about setting up a bias check. Measuring across the OT is dangerous. When the BJ was new, Bill did this to avoid warranty trouble. Now-days I would just cut the PCB and put the 1 Ohm resistors in which Fender should have done in the first place.

#### Gnobuddy

Measuring across the OT is dangerous.
It also tends to make the amplifier oscillate, as I found out with a Blues Junior I owned briefly. The amplifier didn't take kindly to having, essentially, a metre-long wire radio antenna connected to one of the EL84 anodes, radiating an electric field for the amp's input to pick up.

At the time I was new to "toobs", and didn't realize quite how high the voltage at one end of the OT primary could get.

-Gnobuddy

#### nigelwright7557

Gentlemen!

It's OK to play russian roulette with your own life by messing with the hot end of the tubes (anode), but it's not OK to talk other people into it.

If they are that inexperienced they shouldn't be messing with valve amps anyway.
Always turn off amp before working on it and discharge B+ through a resistor.
I always add a resistor and LED to my valve amp designs.
Don't touch until the LED goes out.

#### Enzo

Note the schematic calls for -10.7v for bias. It doesn't specify tube current. Like many makers They know that there is a wide range of acceptable idle current. They determined that with the bias set at -10.7v, then most EL84s that get installed will be within that range.

^^^^^^^ That.

#### PRR

Paid Member
> range of acceptable idle current.

On a similar amp in hi-fi service, there was a slight preference of idle current. Best sound was with about 17 Watts dissipation in the 12W plate. Turned down to a safe heat, there was remarkably little change of sound, even quite "cold".

I like thermal biasing. Use your IR thermometer. Both of a pair should be similar. Ideally you have the same tube type in a chassis which can be properly instrumented, so you can get a temperature for "good" heat.

But back through most of the golden age we did NOT obsess about bias.

#### Zung

...
On a similar amp in hi-fi service, there was a slight preference of idle current...

That's an understatement:
• Setting the idle current is the final goal: too low and you have cross-over distortion, too high and you have red plates
• Setting the bias voltage (Vg) is a mean to achieve the goal. With the wide variation in the current production tubes characteristics, setting the current is safer.

#### Enzo

too low and you have cross-over distortion, too high and you have red plates

True, but in between those extremes is a wide valley of pleasant living. We might find say 20ma is too cold, and 50ma is too hot, but in between settings of 30ma, 35ma, 40ma will all be fine.

#### Gnobuddy

True, but in between those extremes is a wide valley of pleasant living.
With tube guitar amps, my ears agree with Enzo. Bias seems far less critical than you'd think from all those Internet forum posts.

A long time ago, I used to build solid-state power amplifiers for myself, and with those, bias really was quite critical. The "sweet spot" was really narrow - and if you plotted the summed transconductance of both transistors (in the push-pull pair) versus bias current, as Douglas Self did, you could see that this was the case. Bias a little too cold, and you have too little transconductance through the crossover. Bias a little too hot, and you have too much transconductance through the crossover region.

By comparison to transistors, tubes change transconductance quite gradually with change in current. As a result, the sweet spot seems to be much wider.

-Gnobuddy

#### PRR

Paid Member
> too low and you have cross-over distortion, too high and you have red plates

Right, but... 6BQ5/EL84 at 325V, you ALWAYS have crossover. Until you turn it hotter than about 16 Watts dissipation. But from 6W to 12W there is not a lot of change of low-level crossover. And when SLAMMED, it's crossover no matter how you bias it.

If we really gave a bent pick about crossover, we'd run these tubes down closer to 250V where the idle current can be a large fraction of the peak current without melting.

#### Enzo

But these are not hifis, so we accept some crossover and other offenses.

So just my opinion, but if you really hear a difference, and 68.23% dissipation is a real thing for you, then more power to you. But for the most part I think 10-turn pots and such for bias, as if it is some super-critical adjustment, is just overkill. Your mains voltage is not stable anyway, and so all your internal voltages will not be stable either.

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