Unwanted EQ resonance

[hesamadman]

With this build I've been working on for the passed year I am having some slight tuning troubles. Maybe anyway. Maybe it's in my head. Maybe it's my guitar. But at about 110-131hz I get a weird emphasis. It's almost like it plays that note and it plays a very low volume version one octave below. It's only the A to C notes. Now my guitar needs a set up. It has very slight buzzes ALL over the neck. These buzzes could contribute to the multiple frequencies being amplified. However it's not AS noticeable in my 50 watt (which has the exact same preamp only in an el34 50 watt power amp)

When I click my coil taps to play the bridge as a single coil it's obviously not as much of a problem as the single coil doesn't produce the full low end. I guess I'm just asking if there's any suggestions as to what I might double check in my circuit or could something like this be in my head since it's such a small range in frequency that's affected.

[HotBluePlates]

... any suggestions as to what I might double check in my circuit ...

You didn't mention what amp circuit is being used...

Sometimes an underfiltered power supply (like the power supply topology of a vintage AC-30) leads to subharmonics, but then so does extreme distortion.

[hesamadman]

I'll have to post a schematic. I've heard a lot of people refer to the ac30 as under filtered. Has anyone modified the circuit to incorporate better filtering. Perhaps changing the 8uf to 10 or more or upping the 16? Or even a bigger choke. I think I have a vintage 5 Henry choke. I believe I have a 20 on the shelf. Maybe I'll swap that and see what it changes.

[HotBluePlates]

Has anyone modified the circuit to incorporate better filtering. Perhaps changing the 8uf to 10 or more or upping the 16?

Print out a schematic of the old AC-30 (or even many Matchless amps), then print out a schematic of one of the AB763 Fender amps. Use a highlighter to trace from PT through the rectifier to each filter cap.

You'll see the Fender power supply goes in one continuous line from PT to filter cap for the 1st preamp stage. That means that preamp filter benefits from the cumulative filtering of all the preceding filter stages.

When you look at the AC-30, the power supply branches out like spokes on a wheel. It does this because it starts with lower supply voltage from the PT to suit the EL84's, then has to maintain that voltage by avoiding the voltage drops across decoupling resistors in a long series chain, as in the Fender supply (which likely started with ~50% more voltage). So early preamp stages have less-filtered supply voltage compared to a Fender amp.

If you calculated the existing filtering for each topology and the effect of simply adding capacitance, you'd see the Vox system will never get you as-filtered as the Fender system; however, you can't just switch to the Fender system in the Vox circuit because of the lower starting supply voltage for the output tubes.

Additionally, we don't even know if the power supply is an issue, whether your guitar is doing something strange, or if it's the speaker or tone control settings...

[hesamadman]

Well that information is useful none the less. But I'll mess with it Monday morning and report back.

[hesamadman]

HBP,

My build has a solid state rectifier. After messing with this off and on since my last post I finally decided to try a different rectifier circuit. I'm using a full wave with 2 1n4007 on each leg. Like a Marshall 2204. I have recently decided to try to add a third diode to each leg. My result was very surprising to me. Not only did it dramatically decrease my issue, it changed the overall gain of the amp (to my liking). As this is an amp I have played with on my breadboard for some time before finally housing in a chassis, my goal was to use a quad el84 power section with a solid state rectifier. In the preamp side I was trying to utilize a 3 12ax7 (including PI) design for a high gain tone. So it's similar to the ac30 but an additional gain stage basically. Component and value wise it's completely different.

I was hoping to hear your thoughts regarding my recent modification and the results yielded.

[HotBluePlates]

... full wave with 2 1n4007 on each leg. ... I have recently decided to try to add a third diode to each leg. ... Not only did it dramatically decrease my issue, it changed the overall gain of the amp (to my liking). ...

I was hoping to hear your thoughts regarding my recent modification and the results yielded.

I can't think of any reason the added diodes should make the changes you describe, assuming the original circuit was functioning properly. Therefore, I wonder if there was a loose connection on the breadboard somewhere that was fixed when you added the new parts.

If the original diodes were working the added diodes would provide no extra filtering improvement; the diodes only turn the a.c. into pulsating d.c. (and drop 0.7v per diode). And as long as the input a.c. is below 350v RMS, even a single 1N4007 is sufficient to rectify the a.c. A 1N4007 has a 1kV peak inverse voltage rating, so 500v peak on the filter cap while the diode is reversed biased and 500v peak of input a.c. is the rated limit for that diode; 500v peak corresponds to ~350v RMS.

Likewise, a diode has no direct way to alter gain, especially since supply voltage wouldn't change from adding a solid-state diode to an existing array of solid-state diodes.

If the circuit was in an amp chassis and not on a breadboard, perhaps you had an overheated/shorted diode or perhaps a cold solder joint. If only half of the full-wave rectifier was working properly, you might get slightly lower output d.c., much more ripple/hum, etc.

[hesamadman]

I can't think of any reason the added diodes should make the changes you describe

Thats where I was with it all. It made no sense to me. However, there could have been a bad solder joint or a bad diode. I need a scope. I still havent purchased one as I really dont know a whole lot about them.

[HotBluePlates]

Scopes are very handy, but you can probably do 95% of the troubleshooting of a guitar amp with a multimeter and some thinking.

In this case, you might have measured the ripple at the 1st filter cap (connected to the rectifier). Assuming you knew what was typical for your circuit (5v a.c., maybe more, maybe less) you might have identified the effective half-wave rectifier by higher-than-normal ripple. Come to think of it, you'd have to know the same typical operation of the circuit, as the scope will simply provide the same information but in visual/graphical form.

[hesamadman]

Sometimes an underfiltered power supply (like the power supply topology of a vintage AC-30) leads to subharmonics, but then so does extreme distortion.

I'm just researching into this a bit more. When you say that the under filtered supply like in an AC 30 causes sub-harmonics, is this just something that is present in a vox ac30 or did they do something to eliminate it?

[HotBluePlates]

... When you say that the under filtered supply like in an AC 30 causes sub-harmonics, is this just something that is present in a vox ac30 or did they do something to eliminate it?

It's just something present in the AC-30: http://el34world.com/Forum/index.php?topic=19105.msg196311#msg196311

Likely, the subharmonics in such a system is a result of the note you're playing beating against power supply ripple.

But like I said, heavy distortion creates the same effect. Play through a fuzz or some other extremely thick distortion. Play a "pedal steel bend" of a fretted D at 15th fret of the B-string, and a bent B made from a whole-step bend on the 14th fret of the G-string. Now listen to the sound from the speaker under that thick distortion when you release the bent note back down to A. You'll hear a low note (way below what you're playing) bend upward as you release the A bent up to B. That's a subharmonic created by the distortion.

If you play 2 notes under distortion (non-linear amplification), the output of the distorted stage includes the 2 notes plus sum- and difference-frequencies of the 2 notes. The heavier the distortion, the more prominent those sum and difference frequencies. This is part of the reason power chords sound fine but jazz chords can sound awful with heavy distortion; the newly-generated frequencies may be musically-unrelated to the notes you were actually playing.

[hesamadman]

This might just be me being a perfectionist for all I know. I'd say for the most part, the unwanted resonance is occurs during a heavier gain setting. I've listened to some demos online and though I don't hear exactly the same thing in the demos in my amp, I do hear some similarities. I've done multiple things to try and remedy it or reduce it even. I've replaced my 5h choke with a 30h. I swapped my 30 watt OT with a vintage (reissue) ac30 OT. I rebuilt the board using Orange Drops (not that I suspected this to remedy it). This build was well planned and ought out. PLacement of everything is very convenient. Very quiet amp. So my only question is this. Could changing my filter caps with a higher quality cap affect anything at all? Right now I am using F&T (I think). They were just a couple dollars each. Do you suggest I use a better quality capacitor? Also, how can I go about measuring ripple?


Here is a finished photo. The first ive posted in a while.
If anyone is curious about my grounding, I used an under board bus for the most part. Power amp is grounded alone send to PT lug. Same with preamp. Isolated an ran to PT lug. I like the under board look better. Ive done it using a bus bar as well. I daisy chain my tone stack grounds then tie them to the input jack. From there it goes right above my first gain stage bypass cap ground. This is the same point in which I have a ground home run to the PT lug. 


Also my B+ is connected under board on the bottom of the board. Notice the 3 watt resistors on the preamp side.


My shielded wire is Lava Mini ELC cable. Sheathing is rather large but the shield is great and easy to work with. The conductor is a good gauge and easy to work with..

[HotBluePlates]

... I'd say for the most part, the unwanted resonance is occurs during a heavier gain setting. I've listened to some demos online and ... I do hear some similarities. ...

Maybe it's unavoidable for this amp design and that certain distortion level.

... Could changing my filter caps with a higher quality cap affect anything at all? Right now I am using F&T (I think). ... Do you suggest I use a better quality capacitor? ...

I think F&T makes excellent quality capacitors; I would not expect replacing the filter caps to improve the situation. In fact, I would be entirely unsurprised if you replaced the electrolytic filter caps with ploypropylene filter caps and still had the subharmonic. It's just something that comes up frequently with Vox amps.

Also, how can I go about measuring ripple?

Measure a.c. volts at each filter cap + node. The numbers may not be meaningful unless you compare them to something. Pick another amp you have which has a series arrangement of power supply nodes, rather than the Vox' parallel arrangement.

[hesamadman]

Is that something that people actually prefer in vox amps? Maybe it's bugging me so much because I'm not the biggest fan of it. I mean the amp sounds great. VERY QUIET, which I'm very proud of as I spent a lot of time laying out the design. Full chords sound great. Light gain blues sounds great. All around sounds great. But dang it if I don't stop messing with these when they don't need it. 

I even built a very similar model where I kept wanting to get more gain for higher gain lead tone. Messed with it for days. Finally (I know this is an acquired taste) I added a couple red LED's in anti-parallel in series with a resistor preceding the LED's in the circuit (after first CC to ground). I was happy with the results. It gave it just a slight boost (not in volume obviously). Eventually I had to tell myself to put the amp back in its enclosure and quit messing with it. Maybe I should do the same with this current amp. 

[HotBluePlates]

I think people learn not to run the amp in an ugly way by playing certain sounds or limiting themselves to certain amounts of gain (or getting those things from pedals). Kind of like a player would learn playing jazz chords through a fuzzbox just sounds like garbage, where power chords sound great. Or than power chords played clean are pretty under-whelming... I noticed the sub-harmonic thing trying out a thick distortion a long time ago, and realized the oblique bends I do with good results from other types of distortion pedals simply would not work with a fuzz.