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FDP Forum / Fender Amps: 1985 - Current / Does Blues Jr. need bias modification?

Contributing Member

united states

Jan 21st, 2004 04:50 AM   Edit   Profile   Print Topic   Search Topic

I've read in various places that the Blues Jr. runs too hot and risks overcooking the power tubes. The suggestion is to modify the bias, so that the amp runs cooler. Is this really the case? I was thinking about buying a set of NOS 84s but don't want to spend the money if they are just going to cook too soon.

By the way, if I do install another set of EL84s, is it necessary to check the bias?

Contributing Member

Mulvane, Kansas

Trout make good pets!
Jan 21st, 2004 07:37 AM   Edit   Profile   Print Topic   Search Topic

Gina D. from Fender posted once before that no bias adjustment was needed on the Pro Jr. I'm going to assume (a dangerous practice) that since the BJ is similar in design, that the same is true for it.


New Jersey, USA

Hey! What exit?
Jan 21st, 2004 09:07 AM   Edit   Profile   Print Topic   Search Topic

The problem is that Fender biases the EL84s at 39 or 40ma, while the recommended bias for class AB1 operation is around 24ma for the plate voltage you find in Blues Junior. The hot bias lets the power tubes start clipping a little earlier, which some players may like. But it wears out the tubes faster and generates more destructive heat in the amp.

And if you like clean headroom, the hot bias doesn't help at all.

I've been meaning to do some temperature measurements of factory-biased and modified Blues Junior output tubes, and I have a new meter with a temperature probe. I'll make the measurements tonight and post them, along with a revised recommendation for changing one of the bias resistors for cooler fixed bias. Doing the adjustable bias mod is still the way to go for maximum flexibility and control over your tone.

I've also noticed that most of the "matched" tubes I've come across lately are not particularly close--I've seen 7 to 10ma differences in plate current. Individually adjustable bias solves that problem. While unbalanced tubes can add a little interesting grit to the sound, it works against clean headroom.

Bill's Blues Junior Mods

Contributing Member

Mulvane, Kansas

Trout make good pets!
Jan 21st, 2004 09:46 AM   Edit   Profile   Print Topic   Search Topic

I like what THD is doing with their Flexi 50. Aside from the ability to use different types of tubes, they have individual bias pots. Just as you stated, this negates the need to buy matched tubes.

btw - I've purchased two matched quads of Svets 6L6s from The Tube Store. Both sets were matched within 3 mA. I didn't think that was bad. I used the rather crude and laborious method of checking each tube in V7. I then put the 2 closest matched tubes in V8 & V9. I put the highest mA tube in V7 and it's closest match in V10. This was in my Twin Reverb of course. I set my amp at 33mA (V7). I use my ears. I go hot to where it sounds good to me and then back off to as cold as I can get (within reason) and still have a good tone and call it good. 30mA - 33mA on a Twin Reverb is very reasonable setting considering the plate voltage. It definitely isn't running too hot or too cold.

One more interesting note billm. The stock GTs were set to a bone chilling 23mA.

Now back to our originally scheduled programming. Your mods look interesting, but I don't have the skills necessary to do them. Of course, I own a PJ though, so they probably wouldn't synch up completely anyway.

(This message was last edited by Lesterstrat at 10:02 AM, Jan 21st, 2004)


New Jersey, USA

Hey! What exit?
Jan 22nd, 2004 12:21 PM   Edit   Profile   Print Topic   Search Topic

OK, I did some temperature vs. dissipation measurements on a bunch of different Blues Juniors, and moved the same output tubes from amp to amp and measured the plate current on each. The output tubes were Sovtek Groove Tubes.

The first amp was a stock Rev C (green board). The second was a Billm-modified Rev C with cooler fixed bias. The third was a rev B with full Billm mods, including adjustable bias.

The maximum rated plate dissipation for an EL84 is 12W, so for class AB1 operation, it should dissipate approximately 70 percent of that, or 8.4W, at idle.

Amp 1: 11.8W/364F 10.8W/345F
Amp 2: 9.2W/286F 8.3W/283F
Amp 3: 7.6W/250F 6.7W/250F

I also tested Amp 3 with a pair of Electro Harmonix EL84s, which was what I usually have in there, biased a little warm, but nowhere near as hot as the factory bias:

Amp 3: 9.2W/318F 9.2W/320F

All of the measurements were made in the same place on the glass envelope, halfway up, with a bead thermistor and a digital meter.

The difference of anywhere from 50 to 100F is significant in my book, and the increased clean headroom is important to me and a lot of players. So if you want to cool things off but not go all the way to adjustable bias, just move the 33K resistor (R52 cream, R37 green) to R51/R31 and insert a 27K resistor in R52/R37.

For clarity:
The stock bias resistors in a green board BJr are R31 and R37, 22K and 33K respectively. In the cream board, they're designated R52 and R51, also 22K and 33K respectively. I recommend that the 22K resistor be changed to 33K and the 33K resistor changed to 27K, as with Amp 2, above. Going below 8W makes the amp sound a little cold and gritty.

Obviously, there are a lot of BJrs out there cranking away at 11+ watts idle dissipation. They're not bursting into flames or melting their Tolex or anything. But they'll last longer and sound better if they run a bit cooler.

Bill's Blues Junior Mods

(This message was last edited by Billm at 12:41 PM, Jan 22nd, 2004)

Contributing Member

Mulvane, Kansas

Trout make good pets!
Jan 22nd, 2004 12:32 PM   Edit   Profile   Print Topic   Search Topic

Good info Billm. Thanks! 50 to 100 is indeed a significant difference. I wonder how significant it is on tube life.

The typical life of EL84s according to Mike at KCA is around 500 hrs, a little more for NOS. I'd be curious in quantifying the effect on tube life those temp differences would have.

The clean headroom is a different matter. Those differences would most assuredly have an impact on clean headroom.

FDP Forum / Fender Amps: 1985 - Current / Does Blues Jr. need bias modification?

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