Modified Heathkit TA-28: Help me understand this

[Frukost]

OK, so the other day I finished building a modded version of the TA-28 according to http://tagboardeffects.blogspot.com/2012/09/heathkit-ta28.html (The one with the added trimmers).

Everything works as expected, but I noticed today that if I turn down the tone controls completely on my guitar it adds the most of the overtones, really bringing out the velcro sound, and vice versa: Having the tone controls on the guitar way up dampens the overtones. This happens when the pedal settings for both fuzz and tone are fully on.

I don't get it. Did I wire something backwards, or is this expected? And if so, could someone please explain why? I'd like to learn more.

Thanks in advance!

[Mark Hammer]

I'm not familiar with that model.  Does your guitar place the Tone pot on the input lug of the Volume pot, or the wiper of the volume pot?

[iainpunk]

I'm not familiar with that model.  Does your guitar place the Tone pot on the input lug of the Volume pot, or the wiper of the volume pot?

he has a 60's tribute, and the 60s wiring for tone is before volume.

i think it has to do with bias shift through high frequencies, or something similar.
i didn't have that symptom when i build a clone of the TA28

cheers

[pinkjimiphoton]

i love that circuit. but it is supposed to run at a really low power, like 1.5v or something stupid.

the behaviour you're experiencing means its biased wrong. there's different ranges of biasing for it. the t28 should clean up fairly well when ya turn it down,
the velcro with the tone knobs down as i recall is normal.

dangit, not i gotta build me another one....

[iainpunk]

its supposed to run on a AAA battery, but any 1.5-ish volt battery or power supply would work. i used a simple voltage regulator, consisting of a BC547, three resistors and a capacitor, like this:



(i build mine using a BC547 and BC557, instead of the original transistors)

cheers

[pinkjimiphoton]

yeah, those are a lot hotter than the original transistors!
but i bet it still sounds ace

cool trick doing the voltage supply! i forget what we did years ago, let me see if i can find my old layout from days of yore...

[iainpunk]

yeah, those are a lot hotter than the original transistors!
but i bet it still sounds ace

yes, but IIRC this is the first project where i used 'reverse beta' transistor for lower gain.
it sounded quite generic, but my playing was way worse back then, i think it would sound beter if i played it now.
i also left out the gain control, as my guitar has a volume control for that.

the perf board is gone, and the enclosure now houses a different pedal

cheers

[pinkjimiphoton]

i use reverse beta a lot . its funny with ge how often it ends up sounding better "backwards" in a fuzz if the q is leaky enough.

[Frukost]

the behaviour you're experiencing means its biased wrong. there's different ranges of biasing for it. the t28 should clean up fairly well when ya turn it down, the velcro with the tone knobs down as i recall is normal.

Thanks for your reply. Could you elaborate on this? Not sure i follow.

[pinkjimiphoton]

there's a bunch of possible "ranges" that the transistor can work in and still be "biased". some of these ranges are linear, some not. if the transistor's gain is off far enough, it can make the tone controls seem to work "backwards". its weird, and not super common, but i've had it happen to me a few times, too.

if ya look at the photon phuzz thing guitarpcb.com is selling, its really simple. but the bias pot takes you thru all the possible ranges of bias, and they all sound and react differently.

munky put up a thread about it, there's a link to the project in there for the build doc.

but anyways, really high hFE q's in that circuit can make the phenomenon you describe happen at some bias settings.


edit: oooops, my dumb ass forgot the linky

https://www.diystompboxes.com/smfforum/index.php?topic=120976.0

[idy]

One thing, we don't know how much the OP knows...Is he really saying the signal gets more trebly (can hear the overtones) when his tone knob goes down?

One really common thing I encounter is with the tone pot down everything (many pedals) sounds distorted. Great for octavia, not so when you want a cleanish sound. I'm nit sure if my ear/mind just is calling a low pass "distorted" or if... it is messing with impedance or... It really sounds slightly fuzzy to me.

[pinkjimiphoton]

One thing, we don't know how much the OP knows...Is he really saying the signal gets more trebly (can hear the overtones) when his tone knob goes down?

One really common thing I encounter is with the tone pot down everything (many pedals) sounds distorted. Great for octavia, not so when you want a cleanish sound. I'm nit sure if my ear/mind just is calling a low pass "distorted" or if... it is messing with impedance or... It really sounds slightly fuzzy to me.

that's what we call the woman tone back in the day.
ya roll it off, it goes "oooooooh" until ya go off or almost off, and then it snarls. when ya cut the highs
it makes the snarly, vocal midrange more prominent and lively, it gets a bit more amplification in those frequencies.

i believe... maybe i'm wrong... he was talking about it getting more trebly when ya turn the guitar knobs down... like, the opposite of expected.

[anotherjim]

As you roll the tone off, the more the tone pot cap is active making the LC relationship with the pickup inductance stronger- then you have a resonant peak. The boost and width of the peak can be wide & flat beyond notice if the pure resistance (mostly the pickup winding) is high.  The closer it is to just the L&C in circuit then the stronger the peak. Playing with some possible values I've got the peak +10dB before a 300Hz low-pass rolloff.
Now that +10dB or whatever (it can be higher) really is a boost above normal output, so more distortion is likely I would have thought.
But, you can't have a big peak without the width getting narrower.

[Frukost]

OK, I think i understand a bit more now. Thanks for your input!

This leads me to the question what to do to adjust the pedals behavior. For starters, when i adjusted the trimmers I know for certain that the circuit is fed close to 1,5V. I adjusted the volume trimmer to fairly match the distorted signal to the clean bypassed level.

Perhaps i need to replace other components? Not sure how i can adjust the bias.

[anotherjim]

It's really a big ask to match fuzz volume with clean when there is no output volume control from the fuzz. You want a panel knob for that because you can find that what's right playing on your own is different when competing with the rest of the band. Unlike the clean sound, a fuzz has no dynamics so the pick attack doesn't attract attention as a cleaner tone does.
Do you have room to add a volume pot?

[Frukost]

Do you have room to add a volume pot?

One of the trimmers on the board controls volume, and theres no room for more knobs. So it's a set and forget thing. No big deal.

My main question is, what I can do about the inverted relationship between guitar tone and fuzz overtones described first.

[pinkjimiphoton]

if the transistors are socketed, try lower gain ones. or higher. somewhere you'll notice a change in behaviour of the tone controls on the guitar. once ya figure out which direction to go, fall down the rabbit hole a while til ya find something that works.

i believe tayda does sell the 3391's now, too. i may have to revisit this circuit, its been years and i really don't remember much about it.

[anotherjim]

If you don't like how the circuit interacts with the guitar controls, you can add a buffer circuit in the input which will act as a barrier to the interaction without adding any gain. Difficult to work a decent buffer off 1.5v, but you have 9v in there also.
If you have a classic Boss pedal then on bypass it will act as a buffer so you can try it between your guitar and the TA-28 to see what happens.

[pinkjimiphoton]

foggy-ass memories come back slowly sometimes...
yeah, jim nailed it i think.
what is probably causing the biasing to get weird is actually the input from the guitar
making the bias shift. when that thing was designed, a "hot" paf may have been 6k or so, and
many guitars had pickups that were 3-5k.
a modern pickup may put out enough voltage where it literally overpowers the circuit, and makes it act so funny.

you CAN run this at a higher voltage with a bit of circuit adjustment, but i postulate that if ya change the input gain pot from 50k to maybe, say, 500k, it will have a bit more balls and the higher impedance may make the issue abate.

maybe. i'd have to have a play with the circuit again, but sometimes when ya need a buffer, a simple passive resistance in series with the signal will do the trick.

something about changing the input from a voltage to a current by doing that. smarter folks than me could probably explain it way better. you probably know more on this part than i do for sure already

[iainpunk]

foggy-ass memories come back slowly sometimes...
yeah, jim nailed it i think.
what is probably causing the biasing to get weird is actually the input from the guitar
making the bias shift. when that thing was designed, a "hot" paf may have been 6k or so, and
many guitars had pickups that were 3-5k.
a modern pickup may put out enough voltage where it literally overpowers the circuit, and makes it act so funny.

this seems to make like 66% sense to me.
maybe put a germanium clipping pair in front of the fuzz, to prevent overloading the circuit like that.

cheers