Help with Fender Blues Junior Valve Amp Hum

[NeilLivs]

Hi Folks,

I'm looking for help with an issue with my Fender Blues Junior, I hope that the knowledgeable people in this forum can add some words of wisdom and also this will help other FBJ owners and perhaps also help amp builders alike.

The amp sounds lovely, however it has a mains hum which I'm struggling to resolve. The hum isn't loud but i don't think it should be there (please correct me if i'm wrong) i expect a bit of hiss from a valve amp but not hum.

I have a knowledge of electronics (i did a degree in EE many years ago now) however I'm not a technician so don't know the tricks of the trade.

Things I've tried are: Replacing filter caps, replacing el84s for brand new set of JJs, resoldering the tube board.

The hum is not affected by any of the controls, however it gets slightly better when the master volume is at full compared to minimum, but it doesn't go away.

The hum goes away when i remove the PI tube. I have swapped the tubes around to eliminate an issue with the PI tube to no avail.

The hum is there even with or without a guitar connected and with the input shorted to ground.

I've tested all of the DC voltages as per the service manual and they all seem close to spec.

Any thoughts would be greatly appreciated and thanks in advance.

[mresistor]

Take a known good preferably new 12AX7 and replace each preamp tube separately with it, one at a time.

[Latole]

" The hum isn't loud but i don't think it should be there (please correct me if i'm wrong) i expect a bit of hiss from a valve amp but not hum. "
-NeilLivs

The difficult is to know how a BJ should hum.
I have many tubes amps I put in good working order and no one hum volume at 0
I don't have a BJ. My amps are all vintage.

First ; Is there any mod inside the amp ?

Issu look between volume pot circuit and Phase Inverter ;  V1a and V2b
You never ear a sound or hum with no PI tube in.

Check bias filter cap for ripple; too much ripple = noise
IMO these not old amp never need new filters caps
Check diodes rectifier
Check all power supply rail ( printed circuit ) from DC to all 12AX7 tubes. And around filter caps.
Are ground are good ? In the amp and power cord to wall outlet ?

 

With new EL84 , check bias adn adhust

Swapped tubes may not always work. You should put few know good tubes

Issue may come from the reverb circuit but you wrote ; " The hum is not affected by any of the controls"


One of many BJ schematics. Wich year your BJ ?

https://el34world.com/charts/Schematics/files/Fender/Fender_blues_jr.pdf

[Latole]

Take a known good preferably new 12AX7 and replace each preamp tube separately with it, one at a time.

100 % right
Issue in tubes amp ; first put know good ( and quiet ) 12XX7 tubes

New EL84's need a bais check and adjust if need

[NeilLivs]

Thanks for help so far.

The Blues Junior is fixed bias so it cannot be adjusted (without component swap at least).

I swapped the PI for a brand new 12AX7 to no avail.

The amp is dated 2002, the schematic is on the last 2 pages of the following PDF:

https://schems.com/bmampscom/fender/Blues%20Junior%20Service%20Manual.pdf

If i remove v1 and v2, the hum is still present. Not sure if this helps pinpoint.

I don't believe the amp is modded.

Where are the bias filter caps?

I have replaced C15, C17, C18 and C10.

Many thanks

[sluckey]

The bias caps are C32 and C33. Not likely to be a problem since the hum dissappears when you pull the PI tube.

[acheld]

I have several BJs -- they do not hum as a rule.

R24 and R25 are prone to overheat, probably not the cause of your hum, but they are underspec'd.

The master of the Blues Junior was Bill Machrone (of PC Mag fame for those of a certain age), who operated BillM Audio until his death a few years ago.  He had a nice website for troubleshooting and for BJ mods, which was easily accessible until about a year ago.

Some of his material is still available via --  https://web.archive.org/web/20190730130448/http://billmaudio.com/wp/

Not sure if that will help.

[NeilLivs]

Thanks, the archive of biillmaudio is very useful and informative.

I haven't found the answer yet but i'm still reading and keeping my fingers crossed for some inspiration.

[Mike_J]

I have found that an unbalanced heater supply center tap can be a source of hum. If you have owned the amp long enough that you knew a time when there was no hum then that isn't it. You can try an artificial center tap with 1% 100 ohm resistors in place of the green/yellow center tap ground. If the heater supply center tap isn't quite centered it will contribute hum in my experience. Not a common source of hum but if you have tried everything else you may look at the heater supply.

[acheld]

The BJ PT does not have a center tap. 

There is an artificial center tap for the heater circuit, with a 47R resistor for each leg, labelled R54 and R55 (on the 2001 schematic).  These are physically located on the main board just above the heater ribbon cable connection to the tube board.

Doug sells a tube board replacement that is robust, though that story is for another day.   

[Mike_J]

The BJ PT does not have a center tap. 

There is an artificial center tap for the heater circuit, with a 47R resistor for each leg, labelled R54 and R55 (on the 2001 schematic).  These are physically located on the main board just above the heater ribbon cable connection to the tube board.

Doug sells a tube board replacement that is robust, though that story is for another day.

Better yet. Just measure each resistor to see they are still measuring the same.

[jamesedmunda]

You could try bending the ribbon cables at right angles.  Double check the reverb does the hum go away when the reverb is turned down.
Maybe chopstick some wires around while its on. Don't forget to check your house plug.

[NeilLivs]

Hi All

So i measured R54 and R55, they are both approx 47ohms.

Interestingly if i turn the amp on, let it warm up, and once its warm carefully lift F2, i.e. turn off the filament supply, the hum stops immediately. So i guess it is connected to the heater supply.  The process continues.

[Latole]

" turn off the filament supply, the hum stops immediately "
-NeilLivs


No fillament / heater supply ; tubes are off .

You can't hear any kind of sound or noise

[NeilLivs]

FYI - When i turn off the filament supply the tubes do continue to work for a good 20 seconds as they are still sufficiently hot to radiate electrons. I checked this by feeding a signal to the amp; when I turn off power to the filaments the amp works while there is sufficient heat in the tubes, it continues to amplify the signal for a good 20 seconds, however the hum stops immediately.

[Latole]

Thanks for help so far.

The Blues Junior is fixed bias so it cannot be adjusted (without component swap at least).
 

You mean there is not pot, nothing new, most fixed bias amps are like that.
Fix bias is a amp with negative bias to power tube 's grid ; with bias pot or not
It is a job for amp tech. You have to change resistor value

You must adjust bias with new tubes.

If you don't know how to adjust bias,  you will have hard time to fix you hum issues. Bias adjust is so easy.

[Latole]

Where are the bias filter caps?

 

Where is the bias negative circuit :
C28,C30 C14

Did you check diodes rectifiers ?

[mresistor]

Heres a thread concerning BJ adjustable bias  https://el34world.com/Forum/index.php?topic=19872.0

[pdf64]

...The hum goes away when i remove the PI tube. I have swapped the tubes around to eliminate an issue with the PI tube to no avail...

Ok so that proves it’s got nothing to do with power tube bias or HT ripple on the HT or bias supplies.

Is it a hum or a buzz?

[Latole]

...The hum goes away when i remove the PI tube. I have swapped the tubes around to eliminate an issue with the PI tube to no avail...

Ok so that proves it’s got nothing to do with power tube bias or HT ripple on the HT or bias supplies.

I won't say that.

About bias ; I know it is not the issue of the hum.

I just mean when you do some work on a amp, do the work properly if you don't want some issue in the futur.
New power tubes need bias check.

[NeilLivs]

...The hum goes away when i remove the PI tube. I have swapped the tubes around to eliminate an issue with the PI tube to no avail...

Ok so that proves it’s got nothing to do with power tube bias or HT ripple on the HT or bias supplies.

Is it a hum or a buzz?

It sounds like a buzz it's not a dull hum if that makes sense?

[pdf64]

...It sounds like a buzz it's not a dull hum if that makes sense?

Great. What's the resistance between any part of the heater circuit and the amp's chassis metalwork? Should be about 23 ohms...

[NeilLivs]

...It sounds like a buzz it's not a dull hum if that makes sense?

Great. What's the resistance between any part of the heater circuit and the amp's chassis metalwork? Should be about 23 ohms...

Thanks for coming back, the resistance between either of the heater supply rails and the chassis is 24ohms, measured on the daughter board and also from R54/R55.

[pdf64]

...the resistance between either of the heater supply rails and the chassis is 24ohms...

This is weird.
Maybe all the 12AX7 you tried in V3 were bad, eg leaky insulation between cathode and heater?

[Williamblake]

Deleted nonsense//

Glad you found the culprit.

[NeilLivs]

Summary so far:

All 4 filter caps replaced, tube daughter board resoldered, noise happens if I remove all 12ax7s apart from the PI. If i remove the PI then it stops. If i temporary turn off the filament supply while the tubes are hot then the noise goes away suggesting issue with the filament supply and not the el84s or the HT supply. R54 and R55 in the filament supply circuit are fine. I have tried 4x 12ax7s in the PI position to no avail. I have also replaced the el84s with new ones. The ground the chassis has been checked and is fine.

So... the next thing i plan to try is building a little rectifier / smoothing cap circuit to feed DC to the filaments. If this works then i'll accept defeat in finding root cause but be happy that it's fixed. if that doesn't help then i'll wait for inspiration lol.

[Latole]

I did not read if you check diodes rectifier.

Heater transformer output winding with good working artificial center tap is not more noisy than DC supply.
I do one many year ago

[pdf64]

DC heaters for all tubes is not a trivial thing.
The nature of capacitively smoothed rectification is that the Iac become maybe twice Idc, and Vdc will be 1.4 x Vac., so needs dropping.
It's more manageable if done to for just 1 or 2 preamp tubes, but it's always done for the input stages.
Far better to find the actual issue rather than generate a whole job trying to work around it.

Maybe the socket, pcb or wiring is bad and allowing the heater signal to contaminate the signal path.
I suggest to remove the link between the balancing resistors and 0V circuit common, then check the resistance between V3 cathodes and 0V.
Try to examine the socket etc under bright light and visual magnification, if anything looks in any way unusal then investigate, eg a bit of pcb that's turned a bit brown.

[shooter]

Maybe the socket, pcb or wiring is bad and allowing the heater signal to contaminate the signal path.

+1
the best fix is something along the lines of what Doug did.  After "fixing" 2-3 I quit taking any in, you wind up somewhere around -12cents an hour for repairs

[mresistor]

the best fix is something along the lines of what Doug did. 

While looking around the net for information to try to help with this problem I ran across this tech in the UK that rewires BJs completely and adds 1 x 12AT7 (maybe 12AX7)  reverb driver tube on the top of the chassis and then utilizes the other half of V2 for the reverb recovery.  He uses a new tube socket board that looks like Dougs. This to me is that ultimate solution to the BJ hum/buzz problem. I asked him a few questions and he voluntarily told me that due to UK guv restrictions he isn't shipping to USA.  Take a look.  https://www.ratvalveamps.com/

[acheld]

Not sure if this will fix the heater supply issue, but the tube board in this amp is a weak point in its design. 

Doug's tube board is very robust, and conceptually simple.

However, it does require removing the main board from the chassis in order to remove all the ribbon cables, and rewire the connections.  I've done several, and it's not a particularly fun job -- not difficult, just tedious.  That said, sounds like the OP is easily capable of this repair.

Based on data provided, that's probably what I would do next, though admittedly without knowing that the maneuver will work. 

[NeilLivs]

Thanks for everyone's input.

I've made progress today. When i removed the main board to replace the filter caps I untangled the transformer leads, when I put it back together I twisted the winding pairs together but I did not twist them as complete bundle if that makes sense. This afternoon I twisted them as much as possible and then used cable ties to squeeze them together. I still have a very slight hum but it is about the same volume as the general hiss of the amp so not loud at all.

The inspiration to try this came from https://www.ratvalveamps.com/fender-blues-junior (item 4 in the list of things he does when servicing an amp).

Thanks for your help everyone. The amp isn't perfectly quiet but i think I'm going to quit while I'm ahead.

I may well replace the tube board as a future project so thanks for that suggestion.