Need New Blues Junior Schematic 1/03revC

[Tele-Dave]

I need the current new C revision schematic for my new Blues Junior. Only one I can find is for older Green Board and all component numbers are different.

Anyone having a copy to post, especially in PDF format, would be greatly appreciated.


Thnx----------Dave

[Billm]

It's on the Fender site.

http://http://www.fender.com/support..._Schematic.pdf

The online one is marked rev A, but all the part numbers are correct. Rev B added a fuse on the filament line and Rev C corrected a backwards filter cap in the -15V supply to reverb circuit, C34.

All Rev A and B cream board Blues Juniors have this cap backwards and it's blown out. Power to the reverb chip is a little dirty, but it doesn't affect the performance. I replace the cap as a matter of course when rev A and B amps come in for mods.

[Tele-Dave]

[QUOTE=Billm;1708917]
The online one is marked rev A, but all the part numbers are correct. QUOTE]

The one on website is marked rev D and on the area of schem I have that is clear enough to read the voltages on test points are different and parts are numbered differently and testpoint voltages are different on TP17 and 18. What's up with that???

[Billm]

You didn't scroll down far enough. There are two schematics on that PDF. The cream board schemo is on page 3.

[Tele-Dave]

Originally Posted by Billm

You didn't scroll down far enough. There are two schematics on that PDF. The cream board schemo is on page 3.

AAH HAA!!!!!!

[Billm]

Just for kicks, measure the voltage drop across the output transformer primary, red to brown. stay away from the blue wire or it'll oscillate and give you a false reading.

The winding is nominally 100 ohms, so move the decimal point two places for the idle current in milliamps for V5. Multiply it by the plate voltage (probably 330V) for the idle dissipation. So 3 volts is .030 amps, times 330V is 10 watts. That would be kind of high for the rated 12 watt plate dissipation for an EL84.

I set them for 8 or less and the amp generally sounds better and runs a lot cooler.

[Tele-Dave]

[QUOTE=Billm;1709033]Just for kicks, measure the voltage drop across the output transformer primary, red to brown. stay away from the blue wire or it'll oscillate and give you a false reading.

The winding is nominally 100 ohms, so move the decimal point two places for the idle current in milliamps for V5. Multiply it by the plate voltage (probably 330V) for the idle dissipation. So 3 volts is .030 amps, times 330V is 10 watts. That would be kind of high for the rated 12 watt plate dissipation for an EL84.

I set them for 8 or less and the amp generally sounds better and runs a lot cooler.[/QUOTE


Have .031 X 335 = 10.385 idle watts. SCHMOKIN!!!!!!!

I could probably use this thing as a space heater.


Only worked with single ended EL84's. Noticed no cathode bias resistor. Take it it is grid biased. Looks like R35 and R36 is bias resistors??? FP for Flame Proof.

[Billm]

I'm not surprised at that reading. I've seen BJrs and PJs idling at 13 and 14 watts--above the maximum dissipation spec. It's not uncommon for a single-ended EL84 to be cranking 12 or 13 watts--it has to be at 100 percent or you get cutoff distortion. But it's just silly on a push-pull amp.

As you say, it's grid biased. R35 and R36 are the screen resistors. The bias is set by the voltage divider formed by R51 and R52. A quick and dirty fix is to drop a 27K into R52. It won't be too hot anymore, but it might be for the next set of tubes. As I said in the other thread, I'm not trying to hawk kits here, just solve a problem. I've got a nice little trimpot that drops right into the bias circuit so you can match any EL84 pair. And believe me, they're all over the lot.

[Tele-Dave]

Originally Posted by Billm

The bias is set by the voltage divider formed by R51 and R52. A quick and dirty fix is to drop a 27K into R52.

Which resistors would I replace with trimmers to have separate Bias? R31 R32?

Billm, your help here has been extremely appreciated, Thnx.

Not trying to dis you on your mod kits, but I have buckets and buckets of parts and several dozen trimpots of various ratings.

Will probably PM you in a few weeks to get the Clean Boost module and also the A-taper pots because I dont have any pots like those. BJ is brand new so I dont want to void the warranty just yet, but with C/B module installing on component side should be able to install then remove if necessary cleanly enough to not be too noticeable if warranty work is ever needed. Pots I would like to have on hand.

What kind of tube life should I get if I dont re-bias right now because I really don't want to void my warranty just yet unitll i've had this thing long enough to see if there are going to be any other issues from normal use just incase some problem arrises in the near future. Most problems arise usually in the first 100-200 hours of use for new electrical/electronic equipment. Want to burn in for a few months at probably 10-12 hours a week to make sure of no problems.


Thnx---------Dave

[Billm]

R31 and R32 are isolation resistors. I wouldn't make them variable.

If you want individually adjustable bias, you'll have to replace R52 with a pair of trimpots and and cut some traces so you can send different DC voltages to each tube. I used to do that; it's not worth the effort.

Regarding burn-in, I don't think that you'll accomplish much. The most common failure is cracked solder joints on the output tube sockets--caused by heat and expansion/contraction each time you cycle the power. So your burn-in is part of the problem.

The next most common failure is a screen short in one of the EL84s. I think Fender's intent was for the little screen reisistors, R35 and R36, to act as fuses and burn out if there's too much screen current. But they always survive and R47 burns. Is heat a factor in screen failure? Seems likely to me.

The only "infant mortality" failure that I can think of on the Blues Junior is the broken wire inside the reverb tank. Easy to fix, easy to replace. The other failure I see on new-ish Blues Juniors is the 22uF filter caps leaking. A tiny amount of tan foam around the + lead is all it takes. The Illinios Capacitor caps are not great. For some reason, the one with the least load, C28, seems to be the first to go. I replace 'em all with Xicons.

[Tele-Dave]

Originally Posted by Billm

R31 and R32 are isolation resistors. I wouldn't make them variable.

If you want individually adjustable bias, you'll have to replace R52 with a pair of trimpots and and cut some traces so you can send different DC voltages to each tube. I used to do that; it's not worth the effort.

Regarding burn-in, I don't think that you'll accomplish much. The most common failure is cracked solder joints on the output tube sockets--caused by heat and expansion/contraction each time you cycle the power. So your burn-in is part of the problem.

The next most common failure is a screen short in one of the EL84s. I think Fender's intent was for the little screen reisistors, R35 and R36, to act as fuses and burn out if there's too much screen current. But they always survive and R47 burns. Is heat a factor in screen failure? Seems likely to me.

The only "infant mortality" failure that I can think of on the Blues Junior is the broken wire inside the reverb tank. Easy to fix, easy to replace. The other failure I see on new-ish Blues Juniors is the 22uF filter caps leaking. A tiny amount of tan foam around the + lead is all it takes. The Illinios Capacitor caps are not great. For some reason, the one with the least load, C28, seems to be the first to go. I replace 'em all with Xicons.

When I said "Burn-In" what I really ment was to play it a lot for the first few months to test for "Infant Mortality".

As to R35 R36 and R47, at least I see that they are F-P.

As to bias, will go single trimmer.

Filter caps, never liked IC too much anyway. Seen too many leaking in older equip. Have a few Nichicon's around here somewhere. I think I even have some O.S Siemens 22's also burried somewhere. When I'm ready to "Void" my warranty, was going to replace all of them anyway. Will double up on C25.


Thnx Billm, appreciate it-------------Dave

[Tele-Dave]

Question, Since there is no choke on High Volt secondary and dont have to worry about Fly-Back, I should be able to install Standby switch using jumpers J11 and J12, right?

Just a thought.

Also from what I've read on your web and others, I should have no problem running into a 4ohm load?
Thinking on building ext cab like yours!! Looks neater than "all get out"


Thnx-------------Dave

[Billm]

Yes, you can interrupt the high voltage AC line for standby, but I think it's a waste of time. It does next to nothing for tube life, although in your current hot-biased condition, it would cool the tubes a bit. If I were going to void the warranty, the bias would be a much higher priority than standby. Standby is unnecessary if the plate voltage is less than 500V. It's a handy mute switch on amps that have them, but there's no other reason for them.

Yes, the BJr will push 4 ohms without problems.