Fender BJr repair

Amp Mad Scientist said:
I don't know why fender keeps doing that...it seems pretty stupid.
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Hi @67plexi , @Amp Mad Scientist

That mod you refer to in the above is then to deal with the fact that the 2x EL84 Valves 'COOK' themselves to death without it, is that right?

This is a whole new world to me, an I've been searching on the topic over the last couple of days.

So to illustrate with an example if an EL34 amp is biased with a plate voltage of 350V and an idle current of 50mA (0.050 Amps), then the power dissipated by each tube would be 350 x 0.050 = 17.5W.
Since an El34's max dissipation is quoted as 25W, and 17.5W is 70% of that, it would be all good.

But I read that the Stock UK Fender Blues Juniors are running an Idle Current of 50ma on each Valve, where it should be around 24~28ma. Anode Dissipation is 17 Watts and it should be 12 watts max. set at around 70% for Class A/B - so 9 Watts would be normal.

Is that Bias Mod Kit you referred to providing the required fix for this?

Thank you
 
STRefugee said:
Got those. all good?

View attachment 105066
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STRefugee said:
Hi @67plexi , @Amp Mad Scientist

That mod you refer to in the above is then to deal with the fact that the 2x EL84 Valves 'COOK' themselves to death without it, is that right?

This is a whole new world to me, an I've been searching on the topic over the last couple of days.

So to illustrate with an example if an EL34 amp is biased with a plate voltage of 350V and an idle current of 50mA (0.050 Amps), then the power dissipated by each tube would be 350 x 0.050 = 17.5W.
Since an El34's max dissipation is quoted as 25W, and 17.5W is 70% of that, it would be all good.

But I read that the Stock UK Fender Blues Juniors are running an Idle Current of 50ma on each Valve, where it should be around 24~28ma. Anode Dissipation is 17 Watts and it should be 12 watts max. set at around 70% for Class A/B - so 9 Watts would be normal.

Is that Bias Mod Kit you referred to providing the required fix for this?

Thank you
Click to expand...
You don't need to search the topic, we can answer pretty much any question right here.
I worked on Fender amps for 40+ years and I have run a Fender service center.

The tubes in the BJ are EL84 not EL34.
Yes they cook themselves to death.

But you don't need any kit to fix it unless you want to do it that way.
You can change 1 resistor in the circuit; adjust the value of this resistor until the plate dissipation is correct.
I always do this by changing / adjusting the 1 resistor.
You can do it either way you want, maybe you want to make it adjustable.

You need to know which version of BJ you have, and you need to know which schematic matches your amp.
Then you can go from there and identify which resistor needs to be adjusted.

Post the schematic that matches your amp, or post a link to it.

It's also helpful to have a resistor kit with different values (it's very handy) for adjusting and tweaking amp circuits.
It's a lot easier than ordering individual resistors one at a time.

Have patience here and work your way through it.....maybe you won't solve everything in 1 day.

I don't know why the hell Fender keeps doing this. It's caused an awful lot of problems.
 
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STRefugee said:
@Amp Mad Scientist

Many thanks again! Fantastic to know there's such a wealth of knowledge and support here!
Brilliant!
I just wish you lived next door.
Ok I'll check the old BJr specs as you advise.
Thanks a lot?
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Can you post some pictures of the EL 84 tubes you are using?
We want to see if the silver inside the glass has turned black, or if there are any dark spots burn marks inside the glass.
You may need to buy a new pair of tubes. The old tubes might have been fried.

Well
if you lived next door you could just bring your amp and I would fix it for you.
But I suppose you are in England? I'm guessing?
 
@
Yes, I am in England.IMG_20250211_183815~2.jpgThey look healthy. Probably because I use it mainly as a bedroom practice amp. Vol rarely over 3.
 
STRefugee said:
@
Yes, I am in England.View attachment 105078They look healthy. Probably because I use it mainly as a bedroom practice amp. Vol rarely over 3.
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Pull out the tubes and inspect for dark spots / burn marks inside the glass.
Look at the silver inside the glass: does the silver have any dark spots or major discoloration? Burn spots in the silver?
The tubes might be OK.
But, because the orange paint is fried off the tube labeling, it tells me that the tube has been over heated.

You can tell a lot about a tube just by looking at the silver inside the glass.
 
Hi @Amp Mad Scientist ,
Great stuff!
Now out for the day. Will check the tubes as you suggest when back home.
So 22k or 42k or... that is where I haven't got a clue, and wouldn't know what works from what doesn't unless the consequences take the form of some dramatic outcome .
So please educate me.
What is a resistor kit, what does it look like? How do you use it etc..?
And which metal foil resistor should I order to replace the toasted R47?
Been looking for a low noise bulk metal foil resistor and it is all very confusing, wouldn't know which to pick.
Thank you
Cheers!
 
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STRefugee said:
Hi @Amp Mad Scientist ,
Great stuff!
Now out for the day. Will check the tubes as you suggest when back home.
So 22k or 42k or... that is where I haven't got a clue, and wouldn't know what works from what doesn't unless the consequences take the form of some dramatic outcome .
So please educate me.
What is a resistor kit, what does it look like? How do you use it etc..?
And which metal foil resistor should I order to replace the toasted R47?
Been looking for a low noise bulk metal foil resistor and it is all very confusing, wouldn't know which to pick.
Thank you
Cheers!
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You don't have a local electronics store? Radio Shack?

R52
the stock resistor is 22K, which makes the amp run too hot.
I suggested changing that to 42K, 47K, or what ever value gives you the correct 70% dissipation.

A resistor kit is just a big box of different values, so you can pick the value you need.
It saves you from ordering resistors 1 at a time.

The burned resistor in your amp:
Replace R47 with 2.2K 2W wire wound. Do not use a resistor larger than 2 watt.
This resistor is a fuse.
 
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Amp Mad Scientist said:
You don't have a local electronics store? Radio Shack?

R52
the stock resistor is 22K, which makes the amp run too hot.
I suggested changing that to 42K, 47K, or what ever value gives you the correct 70% dissipation.

A resistor kit is just a big box of different values, so you can pick the value you need.
It saves you from ordering resistors 1 at a time.

The burned resistor in your amp:
Replace R47 with 2.2K 2W wire wound. Do not use a resistor larger than 2 watt.
This resistor is a fuse.
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Thank you for clarifying. Sorry for being such a thick head☣️ I think I get it now.
 
STRefugee said:
Searching online for a 2.2 Ohm 2 Watt Wire Wound Resistor right now. But so far seems one has to get a pack of 50 or so minimum.
For instance:

Yageo 2.2 Ω Wirewound Wirewound Resistor 2 W 5 % FKN2WSJT-52-2R2 | RS

uk.rs-online.com uk.rs-online.com
Would this be compatible?

Thanks
Click to expand...
2.2 K ohm (2200 ohm) or, in England : 2K2 ohm.
Not 2.2 ohm.
If you can't get 2K2 ohm 2 watt wire wound, you can use metal oxide, ceramic, metal film instead.
I suggested wire wound because it has less noise.

You are going to have a hard time finding single resistors from the RS website.
I'm going to find you a better supplier.
you probably need to find someone who specializes in amplifier parts.

Bulk metal foil resistors: very expensive and may need to custom order.
One resistor will cost at least $10 - $20.
These are the highest quality lowest noise resistors used by audiophiles and probably difficult to buy.

However you can buy them from : Michael Percy Audio
 
STRefugee said:
Searching online for a 2.2 Ohm 2 Watt Wire Wound Resistor right now. But so far seems one has to get a pack of 50 or so minimum.
For instance:

Yageo 2.2 Ω Wirewound Wirewound Resistor 2 W 5 % FKN2WSJT-52-2R2 | RS

uk.rs-online.com uk.rs-online.com
Would this be compatible?

Thanks
Click to expand...
Don't order yet Mouser UK

1X Mouser Part No
594-2222-118-36101
100uF 25V 4,000 hr 2.54 2.54

1X Mouser Part No
594-MAL211838109E3 2.32 2.32
470uF 50V 4,000 hr

2X Mouser Part No
594-2222-118-36221
220uF 25V 4,000hr 2.88 5.76

2X Mouser Part No
594-5083NW2K200JA100 .308 .616
2.2K 2W Metal Film (1 spare)

1 X Mouser Part No
5878-80-3-5 5.05 5.05
5ft 2mm Rosin Flux solder wick
 
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STRefugee said:
@Amp Mad Scientist
Wow! You're Awesome!!
Brilliant!
Ok, I won't order till you tell me we're ready to go.
Thanks so much!
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I couldn't get the 500V caps in UK.
So you are back to the stock 450V caps.
I couldn't get wire wound or metal oxide resistors so you are (so far) metal film.

He has to buy a pack of 10 to get 1 metal oxide resistor.
So I said screw that.
 
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