Seeking a different tone.

[Chuck H]

I figured the 680p would determine the frequency, and the pot would determine the amount of cut. Not so?

Unfortunately it's not. The circuit impedance changes with resistance and that shifts the target frequency lower when the mids are turned up and higher when the mids are turned down. If it was only a hundred Hz I might not even mention it, but it's actually a pretty significant shift of hundreds of Hz. You might start with a circuit affecting 1000Hz and find adjustment shifts it closer to 500Hz. That's a big swath of the mid band, so, not exactly a surgical frequency effect. For the T filter the bypass cap and the shunt cap value need to be changed to shift the target frequency with the same resulting cut. All else being equal. Or the shunt resistance could be changed and EITHER the bypass cap OR the shunt cap could be changed )but not both) for a similar result. I'm working on a circuit that uses a dual ganged pot to allow adjustment that reduces the frequency shift. But don't hold your breath.

I remembered that I have a 6-band graphic eq pedal. I'll hook that up to one of my tube amps and compare side by side with the Acoustic to see what I can learn.

I do this sometimes when dialing a design in. It can be a very useful tool. But it's important to remember that changing the frequencies AT A GIVEN STAGE can make more specific differences to performance. If an amp has, say, three gain stages in the preamp then boosting or cutting a frequency at the input to produce a net 3dB difference @ frequency in the final EQ would be equal to the same change at any stage in the preamp when running clean. But if the preamp is capable of overdrive it becomes more important just WHERE you make changes to frequency, Especially if you're trying to find a balance between good clean tones and good clipped tones,

[nevetslab]

Some time ago, while working on my Traynor YBA-1A Bass Amp, never being happy with it's tone circuits, I spent some time revising the circuits to come up with something more to my liking. Granted, I'm a bass player, but, when you look at the graphs & curves I came up with, maybe this will give you some ideas. The link below should take you there. I haven't yet pulled the unit apart again to retrofit this upgrade, but it's on my list of things to do while still here on the planet.

Traynor Bass Master YBA-1A Tone Stack - Music Electronics Forum (music-electronics-forum.com)

[Boy Howdy]

Nevetslab, YBA-1s are bringing pretty good money these days. Maybe you should sell it and get something more suited to bass. Thanks for the schematic. I'll have a look at it tonight.

I hooked up the graphic eq and spent a half hour or so trying to dial it in to match the Acoustic. Very surprising results. I used my Sunn amp set to my usual settings for clean stuff: treble 5, mids 4, bass 1. That amp has a lot of things different from stock, and would have more than the usual amount of midrange as a result. Even so I had to boost the mids on the graphic eq by a fairly substantial amount at 250, 500, and 1K hz. And had to cut both the bass and treble frequencies. I'm not prepared to say that I'm as close as I can get just yet though. One thing, I wasn't able to get the high treble harmonics of the Acoustic at all. That's the place where a lot of harsh transistor-ness lives of course but it's where some pretty cool things happen as well. I wish I had a button on my tube amps I could push to get those extra high frequencies. I still haven't figured out where all that nice snappiness (from the Acoustic) comes from. I sure wouldn't expect it to come from what I dialed in on the graphic.

Here's a really good sample of the amp's tone. Ross Traut used a Strat (neck pickup) with EMG pickups. EMGs tend to be a bit dull in the treble so he's running the treble on his Acoustic higher than what I would consider ideal. Running the treble too high, the amp gets really transistory - harsh and brittle.

Backtalk - YouTube

[g1]

Some of that high end & harmonics may not make it through at guitar amp type output transformer.

[Helmholtz]

Some of that high end & harmonics may not make it through at guitar amp type output transformer.

I don't think so.

The frequency response of 8 different OTs I measured doesn't show a HF drop below 40kHz. And that is even without NFB.

The speaker is much more influential.

[Boy Howdy]

AHAAAA!!! I have been trying to get to this question without starting a new thread since this one started. To wit: The reason I don't love the Garnet that I worked so hard on is that it has no sparkle. (Forgive me. Apparently, I'm feeling a little gay today.) And the more you turn it up the duller sounding it gets. I suppose it could be the tubes (I kinda doubt it). I'm thinking it's the OT. I figure that since the market in Canada for a low powered tube vocal amp even in the early Seventies had to be pretty low, and not an application that would require audiophile level components anyway, that it's a low-quality tranny. Hence, no highs. But I really know very little about the subject. I've been thinking that one day I would get around to replacing it, but I certainly don't want to if it won't fix the problem.

We have one opinion on deck. Can we get some others?

[Boy Howdy]

After nearly an hour and a half over the course of three days, here's the final result of trying to duplicate the Acoustic's sound with my 7-band EQ through my modified Sunn Sceptre. I probably should mention that while my Sunn has a very normal, as you would expect, sort of tone, it does have a lot of bottom - which the Acoustic has substantially less of.

[g1]

After nearly an hour and a half over the course of three days, here's the final result of trying to duplicate the Acoustic's sound with my 7-band EQ through my modified Sunn Sceptre. I probably should mention that while my Sunn has a very normal, as you would expect, sort of tone, it does have a lot of bottom - which the Acoustic has substantially less of.

You've probably tried lots of speakers, but have you tried running (other amps) through the Acoustic speakers in the Acoustic cab?

[Boy Howdy]

There are no speakers. It's just a head, G1.

I'm running everything through a 4x12 cab w/ Celestion K-85s. (which are a bit darker than a normal Celestion, and with more bottom end)

29291368.jpg (1600×1059) (windows.net)