Blues Junior power tube Bias

[dadroadie]

Hi to All ,
I recently got my first tube amp in 35 years. It’s a used 2015 Fender Blues Junior in the tweed covering with the Jensen speaker. Right now everything is working fine but I would like to pick up a spare set of EL84s

I’m hoping you might be able to shed some light on the Groove Tubes used in my Blues Junior.
There seems to be a lot of folks that like to bash this amp. I would figure if you made the same amp and it’s been a good seller for going on 20 years it must not be that bad. I understand that it does not have a way to adjust the bias and there are mods to do this. I would rather keep the amp stock if possible.

As a note the current Groove Tubes in the amp have a very clean red number area with no burning of the paint showing so I figure they are running ok.

Right now the amp has the original Groove Tubes that are marked as 4. According to the Fender website the number 4 is on the low end of the medium scale. MEDIUM 4-7 Normal performance, great dynamic range/attack and best all-around rating type for all styles of playing.
For reference I have the amp set for a very clean sound without a lot of grit. The master volume is usually set at around 6 and the regular volume between 2 using a Gibson SG Jr with a single coil or a Fender Strat.

If I understand correctly if I stay with the (4)s as a matched set and the S or R for manufacture I would not need to have the bias reset.
If an adjustable bias is needed to be installed does someone know of a way to install one without removing the PCB?. I believe I have the tools and facilities to do this. I have heard that the board tends to be a nightmare as far as ribbon cables and weak solder tabs

I know it’s a lot of questions but you guys have always come through for me!

Thanks in advance!
dad roadie

[mozz]

http://billmaudio.com/wp/

You have to read everything, all the comments to.

[dadroadie]

Thank for the reply!
I have used the site (very good information by the way) on a couple of occasions and never really found the answer.I probably missed it. I'm an old guy!

The question:
The amp seems to be handling the current tubes ok.( the current Groove Tubes in the amp have a very clean red number area (#4) with no burning of the paint showing. Also After being on for a while I see no evidence of the plates glowing. (red plating?) If I stay with the (4)s as a matched set and match the current S or R for manufacture I would I need to have the bias reset?

Are there members out there that have this amp and have not had the bias mod done? That just match the EL84 tubes and have not had any ill effects ? Even Fender can't seem to make up there mind. In my owners manual I got with it mentions as long as you match the tubes your ok. But when I talked to Fender today they said a service center needs to do this. My 2 local shops want $100.00 labor fee and the tubes at $50 each time the tubes need to be replaced.

I'm retired andI really can't afford that. Should I sell the amp and go back to my old solid state Peavey? Please some one give me some encouraging feedback on the Blues Junior.
It really does sound nice.
Thanks again,
dad roadie

[Randall]

If it sounds good, and the tubes look OK, why do you think they need to be replaced? You may be overthinking this, but if you must have fresh tubes, then yes, replacing with GT rating number 4 will make bias a non-issue in theory at least. All that is is a way to match tubes that draw the same or similar amounts of current. Since there is no bias adjustment on a Junior, this is how you plug and play.

Don't listen to Fender telling you to bring it to a shop to do this. That is just their legal team talking. That amp doesn't NEED a bias adjustment pot, it runs fine without one. some folks just want one anyway, so they can use whatever tubes they wish, and dial them in. And I'll admit having one isn't a bad thing, it's just not really needed.

[alexradium]

What works fine,doesn't need to be fixed,if one day you here a big loss of power,its time to change power tubes.
Internet is full of wannabe experts of nothing.

[vintagekiki]

About biasing tube guitar amplifiers
https://music-electronics-forum.com/showthread.php?t=8291

Thread is from 2008 year. There is a possibility that some links are "dead"
Some "dead" links can be found with http://web.archive.org/


http://web.archive.org/web/20110811053121/http://aga.rru.com/TechTips/disp.cgi?file=LV/BiasAdj.tt

http://web.archive.org/web/20070206020153/http://aga.rru.com/TechTips/disp.cgi?file=TonyNovacheck/DslTslBias.tt

http://web.archive.org/web/20130402154913/http://tone-lizard.com/Biasing.htm

http://web.archive.org/web/20130212213344/http://www.aikenamps.com/BackBiasing.html

http://web.archive.org/web/20120208115718/http://www.mesaboogie.com/US/Smith/biasadjust.html

Bias Calculator
http://www.tedweber.com/webervst/tubes1/calcbias.htm

https://www.easycalculation.com/engineering/electrical/bias-calculator.php

http://www.rogb.dk/tools/bias.html

https://www.google.com/search?q=Bias+Calculator

[pdf64]

Other reputable tube vendors may be able to sell you tubes that bias up the same as whatever GT number you request - just ask them!

[Chuck H]

$150.00 to plug in a pair of el84's and check the bias, maybe change one resistor? But probably not even that!?! That's robbery. Taking the amp apart and putting it back together is included. The job should take about 40 min. if you're not rushing. So let's shoot high on the shop hr. rate and say $80. Add a pair of tubes that DON'T have to be Groove Tubes (at $50). So, a pair of JJ's at $35 (I've been working from a stash, when did el84's get so expensive?). It should cost you about $115. Because it's a common and well practiced maintenance by most shops I would actually expect a charge of about a hundred bucks.

Anyway...

Cathode bias to the rescue! Mod the amp for cathode bias and plug in whatever el84's you can get your hands on. The actual tone would change very little and cathode bias effectively self biases over a pretty wide range. I know this is beyond the scope of the subject as presented, but I wanted to include it in the thread anyway for informative purposes.

And the Blues Junior is a VERY good design (schematically, I can't say about the physical aspect). The model has a bit of a following not just because they're "affordable" but because they perform. Haters make more noise than content players. There are a lot of players that are very happy with their Blues Junior.

[pdf64]

...Cathode bias to the rescue! Mod the amp for cathode bias and plug in whatever el84's you can get your hands on...

Great idea

[dadroadie]

Thanks to all that have replied!
To answer the question ask I just want to order a back up set of the same type and number just in case they are not available if I need them in the future.

I'm really glad that some people like this amp!
I was getting worried that if I didn't have the bias adjusted. (the $150. was the price quoted by both centers) that I would need to sell the amp. I don't have those kinds of funds to play with ever time it needs new power tubes.

The amp sounds really nice at the moment and the #4 Groove Tubes power tubes are not showing any paint burn off. I also have not seen any red glow on the plates after being on for a while.
I play the amp clean with the master at 6 and the volume at 1 to 2. It gets used about 2-3 hours a week.

Thanks again for all the positive remarks on the amp as it is. There is a lot of negative remarks out there!

I understand that the adjustable bias mods will allow you to be able to use a wider range of tubes. I got this amp used (2015 model) from a friend in trade for an acoustic guitar of mine. He said he has never had any issues with it. I would not have otherwise have been able to afford buying something like this. It's nice to have that tube sound again.
Would still like to hear any other experiences with owners of this amp.
Again thanks to all in this GREAT Forum!
dad roadie