Princeton aa1164 new build issues

[ElPedro]

I just completed wiring this build.  All I'm getting is a hum, a teeny bit of guitar, and a melting 56k resistor (the one that ties straight to ground, directly in line with pin 7 of the first ax7).

Overall voltages are about 10% low.

I can hear the vibrato working.

I don't get it!!!! Obviously something is wrong but I cannot find it.

Anyone have any ideas?

[John]

I must be blind, I don't see a 56k anywhere on pin 7, V1.


But if it's "melting", that's where your voltages and signal is going, more likely. Something isn't wired right. Maybe that resistor is hooked up wrong?

[bnwitt]

ElPedro,
You're going to have to give us a little more to work with like photos.  Of course as Doug is famous for stating "If it were wired right it would be working" so I'd go over the amp with a fine tooth comb and look for component value errors or something connected incorrectly.  Which layout are you using?  The only 56k resistors I see in that amp are the ones on the phase inverter tube.  The one on the cathode of that tube does not tie directly to pin 7 of the tube but has a 1meg resistor between it and pin 7.  The other one ties directly to the plate or pin 6 of that tube and has the B+ going through it.

[ElPedro]

Bnwitt - that is the resistor I was talking about. I do have the 1 meg resistor in there, I suppose I misspoke on tying directly to pin 7 of the PI....

I'm using monotone's layout, which is based from fenders aa1164.

I actually grabbed another eyelet board and rebuilt this amp again, and achieved the exact same results.

Perplexed.

[sluckey]

There must be a wiring error if that 56K is melting. Unless you are using a 1/4 watt resistor.

Click on the link in my sig line below...
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[sluckey]

I'm using monotone's layout

Who is monotone? Can you post the layout or a link to it?

[ElPedro]

Sorry it's Mojotone....

http://www.mojotone.com/Amp%20Kit%20Schematics%20and%20Wiring%20Diagrams/Blackface%20Kits/MK-PR-WD.pdf?ck=RT2rRNcCAoLQtZni&vid=RT2rRNcCAobQtbnZ&cktime=131799&gc=clear

[sluckey]

Mojotone's layout is OK. My bet is on a wiring error.

[EL34]

I am not seeing a 56k on pin 7 of the 1st 12AX7

You are probably staling about V4 not V1?

[EL34]

You need to double check your wiring using this method first

http://el34world.com/Forum/index.php?topic=19361.0

[bnwitt]

ElPedro,
Did they give you a schematic and a layout to follow?  If so, can you share them or it?  I thought Mojotone just used the original Fender layout and schematic in their kits.

[bnwitt]

Mojotone's layout is OK. My bet is on a wiring error.

Except for the light blue lettering which made my eyes cross.

[sluckey]

Except for the light blue lettering which made my eyes cross.

How stupid is that! Cyan text is readable on most dark backgrounds. Looks like crap on white though.

[bnwitt]

I actually grabbed another eyelet board and rebuilt this amp again, and achieved the exact same results.

Perplexed.

ElPedro,
That reminds me of the phrase "I've cut it three times and it's still too short"  Obviously there is a repeated error in your assembly process.  Maybe a misinterpretation of an under board wire or something else in the layout

[ElPedro]

I have moved all of the underboard cx to the top to help eliminate confusion.

Here's a pic (I hope) of the board/chassis. I know the wiring isn't dressed real well, but that'll get fixed once I figure out what's going on with this amp....

[bnwitt]

There are some differences in the way the wire IDs like X, C, B are shown on that Mojo layout and the original Fender layout.  I'm looking that over right now.

[p2pAmps]

There are some differences in the way the wire IDs like X, C, B are shown on that Mojo layout and the original Fender layout.  I'm looking that over right now.

I wish we had a "Like" button for posts

[ElPedro]

I'm thinking something in the power supply circuit has to be wrong.  I checked my wiring with the highlighter method, and that all checks out.  And making the same mistake twice isn't something I generally do - so I must be reading something wrong. Or I just screwed it up twice - completely plausible 

[bnwitt]

Well after looking at the Mojo layout I'm suspicious of it.  See the cleaned up black lettering version attached and notice all of the letter id-ed points which do not correspond to the Fender version of the layout.

[bnwitt]

It looks like the filter capacitor wiring is incorrect in the Mojo layout.  If I were you, I'd use the original Fender layout to rewire the amp.

[ElPedro]

Ok, I'll give that a shot tomorrow and post my results.

Thank you!

[p2pAmps]

It looks like the filter capacitor wiring is incorrect in the Mojo layout.  If I were you, I'd use the original Fender layout to rewire the amp.

agreed...  this time make her really neat as you go.  It will make troubleshooting later much easier IMHO.  You might consider dumping all that heavy gauge wire too as it's harder to handle.  Besides correct wiring, wire dress really matters which I have learned the hard way myself.

My 2 cents...

[bnwitt]

Ok, I'll give that a shot tomorrow and post my results.

Thank you!

You're welcome.  Good luck on the rewire.

[bnwitt]

By the way, here is a photo of a Princeton done by a fellow some years back (Texas Amps I think) that shows how neat the wiring can be.

[bnwitt]

and here's an original PR gut photo

[sluckey]

It looks like the filter capacitor wiring is incorrect in the Mojo layout.  If I were you, I'd use the original Fender layout to rewire the amp.

It's different from the original, but it's an improvement IMO. The original PR did not use node C. Instead, it used node D to power too much stuff. Mojo's version uses node C to power the PI like the AB763 amps did.

If he wired the amp IAW Mojo's layout, that 56K would not be melting.

[ElPedro]

I haven't had a chance to go up and retwire yet - will do that tomorrow.  However, I think I may have found an issue based on the two drawings. The mojo drawing has a spot called "x" on the filter cap, and two spots called "x" on the board. I connected all three of those together.  Based on the fender drawing, it looks like only the x's on the board are connected. Does that sound correct?

[bnwitt]

Elpedro,
Yes, that is what I was talking about when I said the filter capacitor was wired wrong.  Forget the Mojo layout.  Start fresh with the original Fender layout I posted.  However, if you want to take Sluckey's advise and follow the Mojo layout You've got something else wrong.  All of the X's should be connected together with jumper wires as should the C's and D's etc.  Those are B+ voltage points at different stages of the power supply output.  Notice how the red wire comes in from pin 8 of the rectifier tube.  That is the highest voltage node.  It goes from the board to the output transformer's red wire and to pin Y on the multi section filter capacitor.  Then the power goes through the 1K 2 watt resistor to drop the voltage a bit where it then goes to pin X of the multi-cap and to pin4 of each power tube as well as all of the points on the board marked B.  The next drop is across the 15k 2W resistor which goes to pin U of the mulitcap and all of the C points on the board.  Then the voltage is dropped again through the 15k 2W resistor on the multicap to pin O which feeds all of the D points on the board.  So each of the three resistors (2 on the board and 1 on the multicap) are used to drop the B+ voltage for each different stage of the amp.

[sluckey]

The "X" on the cap can is simply how JJ labeled the caps. That X has nothing to do with the Xs on the board. Look at the labeling on the side of the cap in this pic. The four sections of the multi cap are labeled Y, X, U, and O. That same labeling is next to the actual cap terminals also. Don't connect that X cap to points X on the board. That X cap connects to point B on the board, just like Mojo's layout shows.

[ElPedro]

Bnwitt - no offense to Slucky, but I'm going to take your advice and go with the Fender layout.

[sluckey]

You really should read my last post.

[bnwitt]

ElPedro,
It may be the quicker path to a working amp to follow Sluckey's advice and double check all of your wiring to the mojo layout rather than rewiring the entire amp to the Fender layout.  Just be aware that the multicap has 4 separate capacitors built into it with a common ground pin.  So each pin that is not the ground pin is tied to a different part of the amp's B+ rail  Just ignore the letters on the capacitor, identify the ground pin, make sure it is wired to ground and then follow the layout to connect the other 4 pins to the proper place on the board. Or realize that the O pin on the cap feeds the D points on the board, the U pin feeds the C points on the board, the X pin feeds the B points on the board and the #4 pins on the power tubes and the Y pin is the capacitor that smooths the B+ to the OT.    Also, the X points on the board are for a wire from the preamp section's last amplifier output stage to go to the phase inverter's grid input pin.  It is not a B+ connection point.  And by the way, if the MOJO layout came with a matching schematic you can use that to understand how the power section works.  Just imagine each filter capacitor section of the multicap is a water balloon on a piston pump fed water line that stores water pressure on every piston stroke of the pump and then when the piston is drawing back the balloons push water into the pipe to reduce the pulsing of the water pressure.  That is what capacitors do with electricity.  They store electrons on the pulse out of the rectifier and pump them into the amp on the lower output to smooth out the ripple.  The resistors are there to reduce the voltage for each stage of the amplifier.  The power tubes get the highest voltages and the preamp tubes get the lower voltages.  One more thing, Sluckey is seldom wrong. 

[EL34]

I checked my wiring with the highlighter method, and that all checks out. 

Good deal, glad you double checked the wiring

I did a color replace on the Mojo layout here
Cyan is a bad color against white for sure

[p2pAmps]

One more thing, Sluckey is seldom wrong.

Funny, I told my wife the same thing about me LOL

[HotBluePlates]

One more thing, Sluckey is seldom wrong.

Funny, I told my wife the same thing about me LOL

And we know that's because you always agree with your wife. 

[ElPedro]

Ok, so I found problem number 1...it was indeed the power feed marked "x" that I connected to the x terminal on the can cap.  Lifted that and the amp sprang to life...

Thanks for the tips on that!

Now I have problem #2.


The vibrato is almost non existent...and the reverb. Whoah! As soon as I plug in the reverb pedal, the radio stations come thru the amp. Also, if I move the reverb knob on the amp at all, it gets super loud.

All of my rca jacks are grounded. I have a 200k resistor running from the center pin of the pedal jack to ground and the pedal and reverb output are connected together at their center posts.

Wiring looks correct.

I am using a classic tone reverb transformer.  The one I purchased prior to this one was bad.

Any thoughts?

[EL34]

Question
Did you not catch that error when you were doing the layout diagram tracing?

[ElPedro]

Yes and no.  I thought x on the board was supposed to go to x on the cap. So I knew it was there the whole time but didn't realize it was incorrect.

[EL34]

X on the cap does not show a wire going to X on the board

X from the cap leads over to B on the board

Whoever designed the layout diagram really should not use an X on the layout diagram
They should use another letter
E would have been a better choice

[ElPedro]

Yep, I realize that now.  I am very new to amp building, and obviously learning along the way.

This particular amp sounds great.  Once I have the reverb issue figured out, it'll be 100%.

After that, I plan on building a couple more of these same amps, and maybe mod them a bit, like add a mid tone control, more gain, less gain.  Don't know. But it's a fun hobby, and also very educational.  A win win. Having the folks on this board to help is an invaluable resource. I hope to some day have enough knowledge to help someone else!

[bnwitt]

ElPedro,
Well I'd say you need to start shortening all of your wires and clean up the lead dress.  Radio or inter-stage noise can definitely be caused by poor lead dress.  Also, as you're doing that, check every single component value and make sure it is correct as well as every single wire run.  Make sure you have an exact copy of the layout in your real world amplifier and everything should work properly.  Many times one can put in an incorrect resistor value due to misinterpreting resistor color code bands.  Since this is a kit you assume all of the values supplied are correct but make sure they are and that all are in the correct place.

[bnwitt]

Whoever designed the layout diagram really should not use an X on the layout diagram
They should use another letter
E would have been a better choice

I would probably use a number or other symbol to distinguish it from the power rail letter system

[ElPedro]

ElPedro,
Well I'd say you need to start shortening all of your wires and clean up the lead dress.  Radio or inter-stage noise can definitely be caused by poor lead dress.  Also, as you're doing that, check every single component value and make sure it is correct as well as every single wire run.  Make sure you have an exact copy of the layout in your real world amplifier and everything should work properly.  Many times one can put in an incorrect resistor value due to misinterpreting resistor color code bands.  Since this is a kit you assume all of the values supplied are correct but make sure they are and that all are in the correct place.

It also doesn't help that the 700WLW broadcast tower is literally across the street!!!

I know my wiring needs improvement, and I'll work on that.  Respectfully, I don't think this reverb issue is due to lead dress, as this interference and loudness is like flipping a switch - super loud...ill check my component values for sure...again ;). This amp wasn't a kit, btw.  I've built a few other amps and have accumulated parts over time, an I sourced the rest of the parts myself that I didn't have.

I did purchase the transformers and chassis. The next one I'll use my own chassis.  I can get old burglar alarm boxes for free, so may as well use em for something....

[Willabe]

ANY and ALL grid wires in an amp act as an antenna and will pick up stray signals from/in the air.

Longer grid wires need to be shielded.

Also the open side of the chassis can greatly benefit from a shielded cover plate. 

[sluckey]

Whoever designed the layout diagram really should not use an X on the layout diagram

Mojo just copied what Fender did on the original layout. Fender has been using X any Y on their layouts to show connections between the preamps and PI since the '50s. It was never a problem until JJ came along and built a multi cap can but used O, U, X, and Y to label the cap sections instead of the old square, triangle, and half circle markings used on the old cap cans. BTW, the original PR layout shows the old style cap can labels.

Still confusing though.

[ElPedro]

I just finished checking the wiring on the reverb circuit. It looks good. Tried a different au7 as well. Same issue.  I know the foot switch and reverb tank are good, as they are installed in my fender twin.  Just double checked them and they work fine with the twin.
The only thing I haven't checked is the 3m ra pot and the reverb transformer.  Don't have extras of those lying around to test with.  Have some on order tho....

[PRR]

> the 700WLW broadcast tower is literally across the street!!!

For those who don't know:

WLW was owned by Powel Crosley, refrigerators and small cars. Started at 50 Watts when that was a lot. 500, 5,000, 50,000 Watts. But Powel figured out that 50,000 Watts is "only the power of a speeding Buick" (around 33 horsepower) and commissioned a 500,000 Watt transmitter. People in Russia learned English (midwestern american) from it. People in Canada and much of the US complained loudly. Neighbors' lights dimmed on modulation peaks. By 1938 the general feeling was that 50,000 Watts should be enough for any US broadcaster, and the 500,000 Watt temporary broadcast permit was not renewed. (The experimental permit lingered into WWII, on the chance we would need a super-station for national defense.) (The transmitter was idled but not scrapped for decades; may still be there.) (The 500KW rig was driven by the 50KW rig which was still operational in 1999, though a newer 50KW was primary.)

WLW is now "only" 50,000 Watts, but 50KW AM stations have become increasingly rare. Coastal 50KWs have to beam their signals away from Europe to avoid protest. And it's a big electric bill. Repairs/replacements must be costly. And AM radio is not the cash-cow it once was. Many stations have chosen to down-size their powers.

WLW apparently still uses the Blaw-Knox tower built for 500KW use, plus two smaller towers to distort the coverage away from Canada. They get occasional coverage in at least 38 states.

I once lived in sight of the WCAU (now WPHT) 50KW rig in Moorestown when it was new. I learned a lot about radio interference in audio, but nothing about radio. ANY thing would pick-up WCAU, only the better superhets would get anything else.

50KW AM in your dooryard is a significant challenge.

[sluckey]

It also doesn't help that the 700WLW broadcast tower is literally across the street!!!

There is a 220K resistor mounted directly on the reverb return jack, right? Put a 100pF cap directly on the reverb return jack also. That may help with the RFI. But you will likely always have RFI problems in that location.

[ElPedro]

Yeah, WLW certainly has an interesting story behind it. Just down the street is Voice of America, which is not defunct...but was impressive in its day.


ON THE BRIGHT SIDE OF THINGS - The Princeton Reverb is finished!  It's dead quiet when it's supposed to be.  The people with the RFI - a grounded rca jack that "looked" grounded, but wasn't. I forgot to remove the plastic insulator so the outer shaft was never making contact with the ground lug.

[drew]

Congrats on getting it going.  If you're going to be building more of them, you might want to consider trying the Hoffman version - http://el34world.com/schematics.htm#Hoffman_Princeton_Reverb_  There are several recent build threads here on the forum about them.