Fender blues Junior.

I did changed all the tubes and the sound is great.
Its all NOS they been tested before installed...
Now how normal is the Hum in those things....
without plugging a guitar theres the hum when playing you dont care much..

Any advice..thanks

there is potential for hum in any/all guitar amps. some is acceptable, some is not. how much is too much?

Hum in a guitar amp is pretty typical... they're really high impedance input, really high gain, and completely unbalanced. In a PP stage, you get some common-mode rejection, but never enough. But as pucho said, how much is too much?

Well the owner have had this amp for around 2 months before retubing and he said before there was no hum.
But i think the amp was defective since he bought it and never sounded 100% as now.

I dont really care he tried in fron of me and it sounds good better distortion than before.

We'll see what happens next.

Thanks.

do a bit of research on guitar forums - the blues junior is a particularly known culprit of hum and noise and there is much conjecture on the best way to ground that 5E3 circuit.  It is possible to get rid of most of it, i am just saying that it is a common complaint from users of that design

Does it humm less with the guitar volume pot turned down than with the jack lead unplugged from the amp?

Maybe check the normaling ground contact on the input jack... Those often goes bad.

Axel

It did not hum before the new tubes were installed?

I reads like one or more of installed tubes might have an issue.  NOS means nothing.

60Hz or 120Hz hum?

also check the tightness of sockets for the pins in the tube sockets

I've known a few NOS tubes to have bad heater/cathode isolation, causing excessive hum. I wonder if there's a shelf life on the oxide coating they use to insulate the filament.

3nity said:

Well the owner have had this amp for around 2 months before retubing and he said before there was no hum.

Click to expand...

There's a clue. If you changed the output tubes (EL84/6BQ5), chances are they will need to be biased correctly.

If the amp is humming, the bias voltage may be too low which means there is too much idle current through the output tubes and they will be running hot.

(This is sometimes accompanied by a red glow from the plates. Have a look in dim light and see if they look red. If they are, the bias should definitely be reset.)

Magneto sound.
The fixed bias was set to -10V .

Then i found this website: http://home.comcast.net/~machrone/bjr/bjbias.htm
They say to change R52 from 22K to 27-30K.
I changed to 30K....with this it should read around -13V but voltage wont change from -10V.

Gus. even if the tubes were tested??
Thanks.

3nity said:

Then i found this website: http://home.comcast.net/~machrone/bjr/bjbias.htm
They say to change R52 from 22K to 27-30K.
I changed to 30K....with this it should read around -13V but voltage wont change from -10V.

Click to expand...

"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 was going to say R31 - make sure you have the right resistor ...

They've given you the right advice, sort of. I would probably just put a 22k trimmer in series with R52 (or R31), that should give you the right amount of range, and make it easier to adjust next time you change tubes.

Just lift one end of the resistor and insert a 10k or 22k trimmer in series. Now you can gradually raise the bias voltage and see if the hum goes away.

(If you raise the bias voltage too much, you will hear crossover distortion when you play the amp, so don't be tempted to add 'one for luck'. Just set it where the hum is acceptable.)

If you do this correctly and the voltage will not increase, there is a good chance that you have a bad tube which is sucking the bias voltage in through a leaky grid. Needless to say, you should discard that tube (or the pair) and start again.



(I have no way of knowing if you have done this kind of work before, so I will just remind you that tube amps contain potentially LETHAL voltages. If you are not confident that you can carry this out safely, leave the job to someone who is. NEVER WORK ON A LIVE TUBE AMP WITHOUT ONE HAND BEHIND YOUR BACK.  )

Very often hum comes in an otherwise quiet amp after changing the output tubes because the tubes are not matched. Quietness of a push-pull tube amp requires good matching of the output tubes (amongst other things).
You may also want to check the screen-grid resistors (100R 1/2W).

Yes, good point! The OP said tested, not matched. If you don't put in a matched pair of outputs, all bets are off.

FWIW

http://home.comcast.net/~machrone/bjr/bjrhum.htm


http://home.comcast.net/~machrone/bluesjunior.htm

and the new website

http://billmaudio.com/wp/

I have not read or indorse the above pages but it may have info you want.

I once sampled some distorted hum from fender blues junior and edited it into a d'n'b bass preset. true story.

The input jack is some plastic crapola too. I had to replace that after about a year.

Great links, pucho.

I loved this bit ...

"You can find a difference between the sound of a cool-biased Blues Junior and the hot factory bias if you look--and listen--hard enough ... as near as I could measure with my setup, the cooler-biased amp is 1dB louder. That's imperceptible."


I mean that's, like - one louder!  ;D

MagnetoSound said:

Great links, pucho.

I loved this bit ...

"You can find a difference between the sound of a cool-biased Blues Junior and the hot factory bias if you look--and listen--hard enough ... as near as I could measure with my setup, the cooler-biased amp is 1dB louder. That's imperceptible."


I mean that's, like - one louder!  ;D

Click to expand...

our amps go to 11  ;D

Wow.
I havent had time to do any further test.
the owner was so impatient to try it out...take the amp today.

Are you sure ? the difference between 1 watt and 2 watts Vs 51 watts and 52 watts ,
Sometimes my amps sound different depending on how much coffee I've had .