Fender Music Master Power Cord wired in reverse

[wsscott]

A friend asked me to look at his mid-70's Music Master amp, and replace the filter caps since there's never been any work done on it.

So the first thing I noticed was that it has a 3 prong plug, but both of the prongs to Neutral and Line are the same width.  That didn't really bother me, but then I noticed that the Neutral/Blue wire was connected to the Black/Line input on the PT, and the Line wire was connected to the White/Neutral wire on the PT.

It looks like it was factory installed, but maybe not.  I haven't listened to it, but he tells me it has a lot of hum.

So any thoughts on this wiring scheme?  I'm going to put in a fuse holder and a 2 Amp fuse because it doesn't have one, so I'll be wiring that anyway, but I feel like I need to reverse this wiring pattern.  Could that be a part of the cause for hum?  Oh, and it has no polarity switch, but it did have a death cap which I have removed.

Thanks.

[shooter]

it's "best practice" to wire properly
will it fix hum, donno
where does the 3rd, ground wire connect?

[wsscott]

The 3rd Green ground wire connects to an independent bolt and lug on the chassis.  It had been on the transformer bolt, but I removed it from there and added the separate mounting point.

[sluckey]

I noticed that the Neutral/Blue wire was connected to the Black/Line input on the PT, and the Line wire was connected to the White/Neutral wire on the PT.

Nothing wrong with that.

[wsscott]

Thanks guys.

[acheld]

NEMA 5-15 plugs mostly have blades the same width, because polarity is enforced by the ground pin.  But there are some plugs that do have the neutral "wide" blade. 

It's true that -- in terms of the amp itself -- the polarity of the plug makes no difference.   However, fusing should always be for the Line In, or both wires -- but not the neutral alone.   Sounds like that is what you're doing.

I can't stand it when I see polarities crossed in amps.   Drives me nuts.   

[wsscott]

Yes, I'm running the Line lead from the plug wire into the bottom of the new fuse holder, and then out the top of the fuse holder into the bottom of the On/Off switch, and out the top of the Switch to the PT.  The Neutral wire from the plug is connecting to the White/Neutral lead coming out of the PT.

[wsscott]

I was going to get started on the caps, but before doing that I wanted to see what the voltages are coming out of the PT and also after the full wave diode rectifier circuit.  All the 3 tubes, ie. 2 output tubes and the 12AX7, have been removed. So the schematic shows 245 VAC on the secondary, and I'm reading 260 VAC--Ok I think; and after the diodes but before the filter caps the schematic shows 295 VDC and I'm reading 362 VDC.  Does that sound right for the secondary, or is it really high because there's no load on the circuit?

The schematic and the actual caps are 300 V, but I'm going to replace them with 500V caps.

Thanks.

[sluckey]

is it really high because there's no load on the circuit?

BINGO! 260 x 1.414 = 367

[wsscott]

Ah yes, the square root of 2!

[wsscott]

Well, I got all the caps replaced, and turned it on with a current limiter and Variac in place, and no smoke.  Only problem is the Output Volume is weak and there's hum, I think 120HZ.  I can turn the amp volume all the way up and it's maybe like a 4-5 watt amp.  It has 2 6V6GT's so it should be a 14 watt amp. Not loud.

If I remove the preamp 12AX7 tube, no sound as expected, but no hum.  I've changed tubes and no difference.

I've got 295VDC coming out of the rectifier diodes, so the PT must be okay.

I'm attaching the schematic.

Any thoughts on where to start.

[wsscott]

Sorry, I should also say that the hum is present even if nothing is plugged into the input jack, and the volume of the hum increases/decreases with turning the Volume pot.

[sluckey]

Plug the power cord directly into the wall.

[tdvt]

Sorry, I should also say that the hum is present even if nothing is plugged into the input jack, and the volume of the hum increases/decreases with turning the Volume pot.

Is the switch on the jack actually grounding out when nothing is plugged in?

[wsscott]

Amp was plugged directly into the wall after the initial startup test to make sure I didn't have any shorts.  So all of these symptoms are direct to wall mains power.

The jack is actually grounding out when nothing is plugged in.

[shooter]

Any thoughts on where to start.

AWAYS start with data;
NO signal
VDC at PA cathode
VDC at PA plate


VDC at cathode n plate of PRE tubes


hum (AC) comes AFTER DC checks

[mresistor]

So this is the actual schematic of the amp you are working on.  Do the listed voltages line up with what you are seeing? 

[wsscott]

mResistor-Yes all of the voltages are in line, maybe 3-4 volts different at most on the high voltages.  The voltages on the plates and cathodes on the 3 tubes are right on, as well as before and after the diodes, and on all 3 of the electrolytic caps. Also the PT primary and secondary are perfect.

So I guess this indicates that PT and also the OT are both working fine.  I guess as to the OT, all I can say is that the voltage on its CT and on the 2 Plates is perfect.

Maybe the OT Secondary is the issue?  I am getting sound, but its no where near where it should be for these 6V6's.

[shooter]

Maybe the OT Secondary is the issue?  I am getting sound, but its no where near where it should be for these 6V6's.

scope n sig-gen time
that "T3" PI is direct coupled (No caps) they tend not to like much current, if the windings are partially shorted that could cause "low output"


start with a 100mV-ac RMS signal into the amp, monitor the 2nd PRE tube plate, adjust the input signal til distorted.  Measure the VAC signal across the speaker.  post some screen shots and measured values.

[sluckey]

Swap tubes should have been the very first thing you did. I don't see any mention of that.

[wsscott]

I swapped the tubes yesterday, it was the first thing I did, no effect, however, see below.

I think I solved it.  Before doing the cap replacement, I did a mod that I saw to increase the brightness of one of the channels by adding a 1M ohm resistor on one of the 2 input jacks.  I hadn't even turned on the amp before.  So I added the resistor to the 2nd jack to the right as you face the amp.  After I changed the caps, and turned the amp on to play, I could only get sound at a low volume on Jack 1, but no sound from Jack 2.  So I let that be and focused on the low volume.  After taking a break today I decided I would change the wiring on that new resistor.  Now I'm getting full volume on Jack 1, but low volume on Jack 2.

So it looks like it's just something in the wiring of the jacks.  Just got to figure out that wiring.  They're both switched jacks, so I've obviously got to figure that out.

Still has some hum, but I don't think its as bad.

[wsscott]

The mod for the jacks adding the 1M ohm resistor is a copy of the Princeton 5F2-A layout.  I've added the resistor so the 2 jacks are wired identically to the Princeton now, but still have the problem with the jack where the 1M is installed having a much lower volume than the other jack.  The other jack has great volume and tone now.

Attached is the Layout and Schematic for the Princeton.

[Esquirefreak]

Isn't the 1M resistor just for grid reference?

/Max

[sluckey]

If you really wired the jacks like the 5F2A then the jack with the 1M would be much louder than the other jack.

Did you remove the .0047µF and 470K?

[wsscott]

I didn't remove those 2 components that you asked about.  I only removed the 100K R6 and the .02uf C4 as shown on the schematic going between the Tone and Volume pots.

As to the loudness, your comment is what I was expecting to happen.

[sluckey]

If you really wired the jacks like the 5F2A then the jack with the 1M would be much louder than the other jack.

[wsscott]

Should the Ground and Switch lugs on either of the 2 jacks be jumpered together?

[tdvt]

Check out this vid

&t=549s

[sluckey]

Look...

     https://el34world.com/charts/CommonHookups.htm

[wsscott]

I've checked the wiring that I did, as shown in the attached photo, compared it with other schematics wired the same way, and reviewed the video and the hookup chart, and mine appears wired correctly.

However, the jack with the 1M ohm resistor has a much lower output volume than the other jack that only has the 68K resistor.  I don't know what I'm missing.

Maybe this photo will lead somewhere.  Thanks for your time.

[shooter]

as viewed;
the right jack, bottom tab appears to have a wire looped around, but not soldered.

[wsscott]

No, Shooter, that's an empty Ground tab hole, and it's just a tiny bit of solder. Nothing is connected to it.  It is grounding just to the chassis with the jack's mounting washers and nut. Above that jack is the Switched lug, and then the Tip lug on top.  At least that's how I identified them.

The specs for the wiring show that the Jack with 1M resistor is at 0db, and the other jack is at -6db.

I just don't have any sense of how that difference "sounds" in real life, ie. the "loudness" difference.  Maybe it's okay.

Still I can't figure out why the jack without the 1M is so much louder than the jack with the 1M.  Doesn't make sense.  I don't think I have the lugs on the jacks mis-identified.

[sluckey]

Still I can't figure out why the jack without the 1M is so much louder than the jack with the 1M.  Doesn't make sense.  I don't think I have the lugs on the jacks mis-identified.

A 1M resistor would be BRN-BLK-BLK-YEL. Looks like your resistor is BRN-BLK-BLK-BRN which is a 1K resistor. Verify with your ohm meter.

[wsscott]

Sluckey-GREAT EYES--You're right.  Don't know how I missed it.  I'll make the switch and see what that does.  Thanks.

[wsscott]

That did the trick!  Now the Brighter channel is the one with the 1M, and the other channel is the Normal and has very good volume level too, just a little bit lower.

Still has some hum, but it may be what's expected for an amp of this age and design.

Thanks again for your patience and great eyes!  I actually have an eye appointment tomorrow! 

[wsscott]

So if I could ask for some education on this whole project:  Why does adding a 1M resistor to one of the input jacks result in a "brighter" and maybe "louder" output sound?  Is it cutting out low frequencies in the signal, or does it affect the signal going to Ground thus leaving more gain?  Or something else?

Hope this isn't too stupid.

[sluckey]

The jack with the 1M is DEFINITELY louder. The voltage is about 6db larger (that's twice as big) than the other jack. Perhaps the larger signal only seems brighter. And it could be that the .0047µF and 470K are a factor. I suggested earlier to remove those and go straight to the tube from the junction of the two 68Ks.

[wsscott]

I also solved the hum problem by moving the Ground for 2 of the filter caps to a different location on the board away from the AC line in.

Very pleased.  Very clean sound.  I would say no hum, except a very minor amount when the single coil pickups are used.