Low Frequency Dump using successive CR filtering

[HaroldBrooks]

My latest obsession (as many here probably know) is a Zenith Hi-Fi amp I am steadily converting to a guitar amp. Things have worked out well, with one relatively minor exception. At very high input gain the amp carried along very low frequencies from the sub-harmonics generated through distortion. Tonight I exorcised the offending low sub-harmonic frequencies, by setting up Two high pass filters Just after the Bass and Treble controls. Worked great ! This amp has cost me very little, except for the time I've spent setting things right, and for the experimentation and mods (a labor of love !)

I tried to calculate the frequencies of the high pass filters using an online calculator (quicker than using the formula and a calculator), but I couldn't seem to hit upon what I needed, so I just kept alligator clipping in different resistors after the capacitors, the same ones I used to block the DC that was going to the tone pots in the original setup. The Bass control is now a whole lot more useful, and even at horrific levels of gain the amp doesn't fall to pieces like it used to. I decided on what frequencies to be dumped by setting up my parametric EQ at full gain with a compressor in front as an input to the amp, to create a lot of wide band noise across the spectrum, and then I used the EQ to filter out all the highs and mids, just leaving the REAL low rumble, with a central peak around 75hz. Now I could focus on what I needed to do in the amp to remove those lowest frequencies that muddy things up in high gain input, particularly with power chords being played.

I've done this type of LF filtering (2nd order high pass) on other amps, but this time it helped immensely. Not sure if the negative feedback in this amp changes the results, but I have a suspicion it might. I've also altered the "Presence" filter and in addition filtered the Global NFB using a .005uf cap just off the OT lead, the one headed to the volume pot. The amp can take big boost now and it just swallows it whole.

The amp is now a screamer ! Now it is Kinda like a cross between a small Marshall combo (when I hook it to Greenbacks) and an old Tweed Fender, if that makes any sense. It still sounds great played clean or just in a crunch mode, as the real low frequencies are never audible under "normal" gain for a guitar anyway. I ran the amp using two 10" Greenbacks in an open back cabinet I have, and it sounds awesome, and also sounds great using a single 12" Jensen C12R.

So here's (roughly) What I've changed or added so far, in a list (not in any order) :

1. Used slightly unbalanced power tubes. Picked the best sounding PI tube from my lot. Tried several 12A*7 series tubes, but kept the original 6J5 preamp tube.
2. Raised Power tube screen voltage my lowering Filter cap node resistor (this also boosted PI tube and Preamp tubes plate voltage a bit)
3. Added Screen resistors to the 6V6 tubes
4. Biased the power tubes a bit colder from a 220ohm shared resistor to 300 ohms. This was necessary because raising screen voltage increased plate current.
5. Lowered size of coupling caps from .1uf to .047uf
6. Changed Filter cap on Presence from .047 to .15uf as this brought in more mids to the presence control.
7. Added protection in the form of diodes to the rectifier, and Varistors from the power tube plates to the center tap to absorb potential spikes or shorts.
8. Replaced all suspect caps and out of spec (over 20%) and resistors
9. Added 5.6k grid stopper caps to V1 and the gain section of the PI tube
10. Added new power cord, and disconnected the Electrostatic tweeter (High Voltage !) circuit up top, so I could stop getting a shock !
11. Left the convoluted RCA jack switching input alone, as it's working quite well with a simple adapter to 1/4" guitar style jack. The amp is very quite when cranked, with only a tiny bit of hum.

Please feel free to add anything or comment on any of the above, as I still have a lot to learn from you guys !

Once again, Thanks for any help or comments !

[trobbins]

Apart from possible motorboating due to poor filter capacitors, that low frequency oscillation/peaking/motorboating is typical of feedback amps whereby the feedback rolls off but leaves a small range of low frequency where there is effectively a gain hump. That gain hump moves around in frequency and gain with output transformer signal level (as it is related to the OT primary inductance, which is related to signal voltage excitation).

A common technique used in hi-fi amps is to roll-off the forward path gain - which I think is what you are doing with your added high-pass filters - along with lowering coupling cap values. An additional technique is to modify the coupling caps to form a step network - see App B in http://www.tubebooks.org/books/gec_approach.pdf

PS. if you want to try the step / shelf network then there is a good article on design by Roddam in:
https://americanradiohistory.com/Arc...ld-1951-03.pdf

[Chuck H]

Or... Live with it.?. If you're hearing this only when the amp is clipping hard then it could be a phenomenon called "beating". It's not uncommon at all, though not all amps do it and I've never seen a firm consensus on what it is exactly (though trobbins may have it). You can clearly hear this happening in some recorded solos by Michael Schenker, Angus Young and others. But the most clear example I can think of would be the lead guitar work in All Right Now by Free. You can't miss it. Maybe take a listen and let us know if that's what you have. If it is then I may sticky this thread as it progresses because it's one of those weird things that some otherwise good amps do and I'm sure some inquiring minds would love to get the skinny on it.

[Helmholtz]

The official meaning of beating is the low frequency amplitude modulation produced by mixing two close frequency signals. (Acoustic beating is also used to tune an instrument by ear.)
https://en.wikipedia.org/wiki/Beat_(acoustics)

Low frequency beating (or more likely intermodulation distortion) in a distorting power amp is most likely caused by a mixing of ripple harmonics with a guitar signal frequency, causing a low frequency "ghost note".
One of the reasons is that the ripple content in the power amp's output signal strongly increases during clipping (when the signal hits the supply rail).

[Chuck H]

Yep. Ripple plus note equals sum and difference artifacts. That's what I often hear regarding the phenomenon. Strange then that it's a common characteristic of Marshall amps which are amply filtered. And I've heard it a few times personally when playing relatively new amps that would have fresh filters.

Not that typical filtering is a perfect safeguard against ripple entering the signal though I suppose.

And I do know what actual beating is. I didn't name whatever this actually is. I just know I've read that it's commonly called beating so that's what I call it. I know french fries aren't french also.

[Helmholtz]

And I do know what actual beating is.

No doubt. My intention is to give some general info also for the less experienced readers.

As indicated, I think the phenomenon is more likely caused by intermodulation (aka crossmodulation) distortion. Beating is a linear loudness modulation effect with the difference frequency only. Intermodulation is a non-linear effect which produces real additional difference and sum frequencies.

The influence of ripple filtering is a good point. Generally good filtering should reduce the effect. OTOH any PA asymmetry and an OT having a strong bass response (high primary inductance at high signal levels) will emphasize the phenomenon. The same holds for 4x12 speaker cabinets having a transfer peak around 100Hz to 120Hz.

There may be other effects involved.

[g1]

Strange then that it's a common characteristic of Marshall amps which are amply filtered.

Maybe more evident due to them being much more commonly driven to power amp distortion?

[Helmholtz]

BTW, another effect that produces low frequency artefacts is blocking distortion.

[HaroldBrooks]

Yep. Ripple plus note equals sum and difference artifacts. That's what I often hear regarding the phenomenon. Strange then that it's a common characteristic of Marshall amps which are amply filtered. And I've heard it a few times personally when playing relatively new amps that would have fresh filters.

Not that typical filtering is a perfect safeguard against ripple entering the signal though I suppose.

And I do know what actual beating is. I didn't name whatever this actually is. I just know I've read that it's commonly called beating so that's what I call it. I know french fries aren't french also.

Your point is a good one Chuck, as I realize some amps that could be described as "Harsh" in the high gain range are just what the doctor ordered. That's why I have stopped trying to tune my 6973 power tube amps to clear all the intermodulation distortion, as I did in the beginning. A little of this type of corruption to the sound is a good thing !

This amp by the way is not the worst "offender" from my lot. The goal here was not to change the amp character too much, but just to get rid of some of the low boominess when I hook it up to a 4 x 12" cabinet. All in all it sounds great when played through twin 10" or a one 12" cabinet.

I should actually start buying and installing switches or pots instead of fixed resistors, so I can reverse these types of mods as needed.