First Express build, but weird harmonic

[BLT]

I finished my first express type build with few problems and was pleasantly surprised with the results, except for one thing. I was playing a Memphis scale style riff when I got an odd octave lower 3rd or fifth harmonic playing above the ninth fret on the E & G string. Anybody have any experience with this?

[BLT]

Anyway, here's a couple of pics. I used what I had on hand, and bought good but less expensive parts. I managed to keep the price tag right at or just under $500.

I started off very angular on the tone stack in the first pic, but when that turned out to be too brittle and bright, I switched it over to resemble Francesca and Ingrid.

The Edcors courtesy of RJ!

[M Fowler]

Looks good and keep working at it to adjust the circuit to meat your needs.

[Tillydog]

IMHO, the most likely problem is an ultrasonic oscillation. The circuit is notoriously fickle!

My suggestions...

Get the grid wires all tight to the chassis - in particular where you have pin 2 of the PI running near the wire from pin 6 and where the wire to pin 2 of V2 (with the green dots) is up in the air;

The wiring around the bright caps needs to be really carefully run and tucked away;

You might need to tuck the wire from the volume control to V1B down out of the way a bit more, and/or try shielded wire here (as a diagnostic, at least).

Try it with a bottom cover on the chassis.

My £0.02, anyway

[rooster]

Maybe create a different ground path for the Presence control, separate it form the the input/tone stack. Too, the power caps you are using are going to make a difference to your overall tone. On this note, caps that are not formed can cause some ghosting, so maybe burn it in for 8 hours - just turn it on and let it sit. And then there's speakers and the cab. Maybe you have some weird sympathetic vibration there. Or maybe your power tubes or preamp tubes are causing this.

The biggest surprise in my sound room (garage) was a Leslie cab. I sold off the Leslie amp and installed a crossover in the cab, and then use an external ten speed motor controller. I just plug a guitar amp into it when I need it, 8 ohms. But I digress.... the way the stock amp was installed, there is a large L-bracket on the floor of the cab towards the front. It's a hold down affair that you can slide the stock amp chassis into and then bolt the front. OK, it turns out this L-bracket was acting like a tuning fork and would vibrate every time I played a C /C# above middle C. And it was loud! It took me 6 months to figure this out I'm sorry to say......

[KellyBass]

The wiring around the bright caps needs to be really carefully run and tucked away

Absolutely! And, on one of mine, the wire runs around the brite caps as short as possible.

[BLT]

All great suggestions. I have changed a number of things, but it still exists. I will shorten the wiring in the bright switch when I get caps that are a little less bright.

So far I've swapped speakers and tubes, moved the grid wires tighter to the chassis, and tried to deaden some components with pieces of foam, just to see if the oscillation would at least change. I tried shielded wire from volume to grid pin2 on V1b, but that didn't do it.

Pissed off? Not yet, like many have stated before, this is a great learning experience for builds in the future. I really appreciate all the advice.

[fishy]

Have you checked all the grounds to make sure they are all good.
A poor ground can cause this and will be like a bad / high impedance cap.

I always check grounds relative to the chassis as a matter of course before firing an amp up. I'm always suspect of the pot buss.

Good luck.

[passfan]

Dont forget to listen too and properly evaluate each change you make . It's part of the education.

[geetarpicker]

I think the overtone you are hearing is not even a problem with the amp. The guitar is never perfect intonation wise and in many cases the two notes of a double stop may be out of tune with each other to actually cause a slight beating effect as the two notes frequencies go in and out of sync. This beating effect can actually be fast enough that it creates an audible note in itself, and can typically be lower than the notes you are playing. Bending one note of each double stop carefully may help but IMHO it isn't the amp at all. That said it is a high resolution amp and it may bring out stuff you might miss on other amps.

I've experienced some odd low overtones on 4x12s where a low overtone would come in on certain single notes when played distorted. I have a cab where sometimes when you play a low C note on the
A string an overtone a full octave below would sometimes kick in. Pretty cool to hear a note come out below the range of the guitar. It might have also been a very low example of cone cry.

[BLT]

I think you may be correct GP. This amp, when you really analyze what's going on, is very overtone rich. It can add a great deal of color to your tone. Apparently, some of the colors are less pleasing than others, but until I get this into a different environment from where it sits right now, I won't be able to assess what it's capable of producing, sound wise.

I also now realize that this circuit really does deserve the best of components. It doesn't demand it, it just deserves it. It has been a true pleasure to noodle around on, and I look forward to getting it into the cab. My only thing left to do before it gets tucked away, is to make the bright switch a little less bright and to shorten its lead.

Thanks for the advice everybody!

[vibratoking]

I think Glenn is right. I hear these types of overtones on many amps, and not just an Express. There are many possible explanations. Cone cry, bad solder joints, cabinet vibrations/resonance, internal oscillation, intermodulation distortion, etc...

I hear quite a bit of intermod on my Express and Boogie Mark III. Any non-linear circuit generates integer multiples(harmonics) of the input frequency and sum and differences of the input frequencies and it's harmonics. Imagine the input and all the harmonics that are generated. Imagine an internal oscillation. Then imagine the sum and difference of all those frequencies. That's for a single stage. Now imagine cascading that through several stages. That's a lot of frequency content and quite a bit of it is loosely related to the input (out of tune). Sometimes the player can compensate with micro type bends or volume and tone tweaks. Other times, it is not in the players control.

[Paultergeist]

Greetings,

I realize that the issue is not a discernable 60-cycle hum, but the heater wiring seems to violate the *rules* that I have gleaned from reading, photos, etc. Is this amp quiet -- with respect to AC hum? There seems to be a lot of extra heater wiring around those sockets......curious.....?

[BLT]

Actually, very quiet. The only hum I get is, of course, when I use my single coils.

I do get some hiss, but I will try to mitigate that with the usual remedies mentioned here.

[gingertube]

Greetings,

I realize that the issue is not a discernable 60-cycle hum, but the heater wiring seems to violate the *rules* that I have gleaned from reading, photos, etc. Is this amp quiet -- with respect to AC hum? There seems to be a lot of extra heater wiring around those sockets......curious.....?

Can't recall where now but once saw a scholarly work on heater wiring - the relevant part which stuck in my mind was 3 twists per inch minimum but more than 5 twists per inch gave no improvement in noise supression.

So 3 to 5 twists per inch it is - that is 3 to 5 twists per 25.4 mm for those of us who use proper units.

Cheers,
Ian