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Post subject: Pro Junior fans?
Posted: Thu Dec 08, 2011 1:28 pm
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Location: Camrose, Alberta, Canada
I love my Pro Junior. What a sweet tone it has. I have a Tweed version from a few years back.

I just got tubes replace a while back, and it's been great. However, since the last little while, it has started to hum abnormally loud. The tubes look alright. I'm no amp tech though. It sounds like normal, I think, it just way more hum than there used to be.

I also noticed when touching one of the power tubes, there is a little bit crackling happening. Perhaps a bad socket?

Any ideas?

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Post subject: Re: Pro Junior fans?
Posted: Thu Dec 08, 2011 5:08 pm
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Hi dolphinstreet,

Do you have the hum with nothing plugged into the input of the amp?

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Post subject: Re: Pro Junior fans?
Posted: Thu Dec 08, 2011 5:34 pm
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shimmilou wrote:
Hi dolphinstreet,

Do you have the hum with nothing plugged into the input of the amp?



Yes, I do.


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Post subject: Re: Pro Junior fans?
Posted: Fri Dec 09, 2011 7:28 am
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It very well could be your output tubes, or some other problem. Easiest thing to try would be known good tubes if available. If you don't have tubes on hand, try this. With amp off, remove V1 preamp tube, power amp back on and check hum. If it still hums, power amp off, replace V1 and remove V2 PI tube, power amp back on and check hum. If it still hums, then it is possibly the output tubes causing the hum. If removing one of the above tubes eliminates hum, possibly that tube is bad. When tubes were replaced before, was the bias checked?

If not a tube issue, the amp has a five year warranty, any time left? :idea:

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Post subject: Re: Pro Junior fans?
Posted: Fri Dec 09, 2011 9:49 am
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No, I didn't have it biased... I should've had that done.

I don't know how to do it myself, how do I find out how to do the biasing?

No warranty left - the amp is way old.


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Post subject: Re: Pro Junior fans?
Posted: Fri Dec 09, 2011 10:12 am
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That's OK, I just asked because some tubes would run hotter than others in this amp, and there is no adjustment pot, so it is normal for most people to just replace the tubes and go. Some tubes last longer than others in the PJr.

Try replacing the output tubes if you have a known good spare set, but first try removing the two smaller tubes, one at a time as described in my previous post, before proceeding.

Do you have a meter and/or bias probe, soldering iron and schematics?

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Post subject: Re: Pro Junior fans?
Posted: Fri Dec 09, 2011 10:22 am
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I have a digital meter, soldering iron. No schematics but I imagine that can be found online?

What is process like to adjust the bias?

I will try your suggestion with removing the preamp tubes this weekend.


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Post subject: Re: Pro Junior fans?
Posted: Fri Dec 09, 2011 12:09 pm
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The bias refers to the amount of current flowing through the output tubes at idle. The Plate voltage and current determine the watts dissipated by the tubes at idle, and should be checked whenever output tubes are changed. The preamp tubes need no adjustment when changing them.

Adjusting the bias in the PJr involves either replacing a resistor in the bias circuit with a different value, which might have to be done again with each output tube change, or replacing the resistor with an adjustable pot and resistor. Many people just change the tubes without worrying about bias in this amp, some have problems, most don't, except for maybe shorter tube life.

The easy way to check the bias is with a bias probe. If you only have a meter, with amp off and caps discharged, check the resistance of each half of the primary of the output transformer, note the readings. Then power up the amp, let tubes warm up and check the DC Plate voltage at each tube referenced to ground (chassis), note the readings (should be close to 316 VDC on each tube Plate). Then measure the DC voltage drop across each half of the transformer in the same places as the resistance was checked earlier, note the readings (maybe 2 or 3 VDC). Then divide the voltage measured by the resistance measured to get the Plate current for each tube (I = E/ R), it will be in mA, like 26 mA to 30 mA or something close to that range. Take the Plate current times the Plate voltage to get the watts dissipated by the tubes at idle. (P = I x E) When finished you will have the idle wattage dissipated by each tube. If your idle is within an acceptable range, no adjustment needed.

The EL84 being a 12 watt tube, generally would run in the range of 50% to 70% of it's max rating. Some feel that the very hot bias in the PJr and BJr is intentional and adds to the sound. I don't agree with better sound from an unreasonably high bias, and the hotter the bias, the shorter the tube life. I installed an adjustment pot on my BJr and it sounds better than ever at a much more reasonable bias setting. The stock tubes showed definite signs of excess heat before the change.

Schematic:
http://ampwares.com/schematics/pro_jr.pdf

The resistance of the OT primary halves are read from the red wire to blue wire for V3, and the red wire to brown wire for V4. The voltage will be read at these same two places.

Plate volts for the PJr are pin 7 of each output tube, easier to check at the connection for the blue wire for V3 and brown wire for V4, referenced to ground (chassis). Checking voltages at the tube socket pins solder connections can be dangerous, one slip and you can short a couple of pins together, and fireworks ensue, we don't want that. Be careful!

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