fender pro junior mod failure

[bluestom50]

I made the mistake of putting a fromel upgrade kit in a brand new fender pro junior 3 amp. I have no sound at all-- red plating el84 in v3-- i have tried but can find no reason why. Any suggestions on where to start looking will be appreciated. Thanks, tom

[Mick Bailey]

Compare the bias voltage with V4. Red plating is caused by too much current and the first thing to do is fix this. You should have about -10v on pin 2 on both tubes. R20 or R21 could be open. Are C8 and C9 original?

[52 Bill]

I made the mistake of putting a fromel upgrade kit in a brand new fender pro junior 3 amp.

I know that we've had others come here with problems like yours, but in order for any of us to help you, you might want to post a link to the kit that you are asking about, or detail what you did.

If your outputs are redplating, then you have somehow messed up the bias voltage. This could be a problem with the bias supply, or a problem with positive voltage leaking from the driver/phase inverter, etc.

[drewl]

These amps run the output tubes very hot, so it's possible a tube died.

I usually add a bias adjustment circuit to these to run the output tubes cooler.
pull the output tubes, check them one at a time to see if the tube is bad or the socket.

As stated you may have caused the problem, so double check your work and socket voltages.

[Enzo]

No offense, but look for a wiring mistake. When a working amp has a new problem after you worked on it, it is almost always something you did. Guys at my experience level are certainly not immune to making wiring mistakes. One suggestion I like is this: when you have stared at it a thousand times, turn the thing upside down and go over the wiring. It won't fade into the familiar that way.

[Jazz P Bass]

My guess is that one of the ribbon cable connections broke.

BluesJuniorIII_schematic_Rev-D.pdf

What a shame that you did not like the stock amp.

I just had one come through with a Cannabis Rex speaker in it.

Sounded pretty good to me.

[Drewline]

Sorry Jazz P but here is the Pro Jr III schematic:

Also if you want to make the bias adjustable you can replace R29 with a multi turn 50k pot.

[Enzo]

With respect, I don't think it is that easy. To make it adjustable, you need to create a voltage divider, and I don't see that there. And I always object to putting a pot to ground that can be turned to zero bias. Tripling that 15k resistor would yield a range of adjustment much of which was not useful. When replacing a resistor with a pot, it is usually advisable to replace it with a resistor and pot pair. In other words, replace a 50k resistor with a 20k pot and 30k resistor, or some similar formula. If we assume a simple R29 replacement would work, a more reasonable value in my mind would be maybe a 10k pot with 10k series resistor or even a 5k pot with 10k-15k resistor. That way the entire range of the pot can be useful, and the guard resistor prevents zeroing out the bias voltage. I personally find multiturn pots overly "precise" for a setting that is not so critical. It is never necessary t set the bias to anything like -45.683v or some such, -45 is close enough. As your mains voltage moves around, so then does your B+, which directly affects tube current. SO that ultra precise setting today is "wrong" tomorrow.

[Drewline]

Enzo,

You're correct. I have used what I believe are 5 turn pots with the middle wiper leg connected to the lower leg so there is no direct connection to ground. I,ve been doing this for the last couple of years with out any problems. I suppose I could fit a resistor in there also. I'll try it that way next time!

[Enzo]

If you tie the wiper to one end, it is now a variable resistor, but if one end is grounded, then the resistance can still be dialed to zero. And that would zero out the voltage, killing the bias. So I always add what I call a guard resistor in series to set the lower resistance limit. Now if you wire one in like that and never turn it to zero, of course there is no problem, but you cannot count on everyone else being so careful. If it is possible to misadjust the control, then someone will do it - Murphy's Law.

The added benefit is range. If you just had a 50k pot, you might find the settings are only in the useful range from say 15k to 30k. Outside of that range, the resulting voltages would be way too high or low. That means the useful setting would be only maybe a third of the pot rotation - all the useful settings are crammed into one little area. So my guard resistor could then be 15k, and I use a 20k pot. Now almost all the pot rotation is useful, and is much easier to set. Like an amp with a volume control that is not loud enough on 1, but by 2 is way too loud, and we have to try to set it just so between 1 and 2. Far better if the volume control was useful over the whole range. Same deal.

[bluestom50]

I feel that I have a bad output transformer Would this cause red plating on v3 el84 when powered on? It sure would account for no sound of any kind. I want to thank all of you who have helped me. If possible could you let me know about my question. Thanks again, Tom

[g1]

See post #2. Do you get the -10V at pin2 of both power tubes?
The output transformer is not one of the first suspects.

[Mick Bailey]

I feel that I have a bad output transformer Would this cause red plating on v3 el84 when powered on? It sure would account for no sound of any kind. I want to thank all of you who have helped me. If possible could you let me know about my question. Thanks again, Tom

Red plating is caused by excessive current draw through the tube and is highly unlikely to be anything to do with the output transformer. Static current draw is dictated by the bias and if this voltage is missing or too low then you'll get red plating. The B+ connects to the plates of the output tubes via each half of the OT primary winding. This is only a few tens of ohms (DC resistance) and even if shorted out on one half wouldn't cause the tube to red plate if the bias is set correctly. You have to know that the bias is right before moving on

[52 Bill]

Just to update you guys, we've already solved the redplating issue in another thread and we seem to think now that the output transformer is okay. But there is still no signal passing through the amp.