Little help with Heathkit TA-28 Clone

[omarvolta]

I've just built a TA-28 using this schematic http://www.euthymia.org/DIY/HeathkitT28Fuzz.jpg
The Fuzz is working with a 2N3391 and a 2N3906 substitution for Q2.
What I need help with is getting more volume out of it for it's not enough
when turned all the way up. Also, when rolling down the tone control all
the way down there's a huge loss of volume. I've never played an original
so I don't know if the tone control should do that. Just out of curiosity,
has anybody built a TA-28 in fully working condition?? Thanks.

[Dragonfly]

theres definitely a volume drop with them...a friend of mine has one...great tone, but loses volume. you could tack on a boost circuit like the LPB at the end of the effect to make up the lost gain from the tone stack....

You might also try messing the value of R6 and see if that helps without totally screwing up the tone. not sure if that will do the trick or not, but worth  at try...

[nordine]

its that last tone shaping stage the one that screws things,

make that C3 (10uF) cap your OUTPUT, and you'll have plenty of volume... or else, put there a proper tone snippet (Big muff or similar)

[omarvolta]

Thank you guys. Used a pot for R6 and it didn't  help with the volume at all. C3 as the Output seemed to do the trick, but the fuzz got really muddy. So I'll try the LPB circuit and/or  putting another Tone snippet(which I don't know how to do but will try). Thank you again.

[zachomega]

I have a TA28 and the output is not even close to unity gain. 

I had this discussion with somebody else, but never tried it...but try reducing the value of R8 to increase the gain. 

-Zach

[omarvolta]

Reducing R8 gave it a more volume but it lost definition and got too bassy. I also tried the Big Muff Tone Stack and it also helped with the volume but I'm not getting enough treble to match that of the original circuit. How could I get more treble out of the BM Tone Stack??

[nordine]

With this:

http://www.duncanamps.com/tsc/

tweak the values and you'll see the changes in freq response, so you know when you have enough treble

[omarvolta]

Ok now, I had used the BM Tone Stack that I found some where in here which had different values than the one in the Tone Stack Calculator. I'm now getting the treble I wanted but when I turn the tone knob to the treble side it looses volume. When on the Bass side the volume goes backs up. I'll see what I can do about that. Thank you!

[omarvolta]

I found this on the forum,  "One thing I did was to use a 9V supply and a Mosfet variation of a series pass circuit to provide the main circuit with 1.5V. Basically all this entails is connecting a 100K trim pot across the rails and attaching the wiper to the gate of a Mosfet, just move the wiper up from ground till 1.5V is measured at the source. You also need to decouple the source (100u cap). The advantage of the 9V supply is that you can incorporate a LED indicator, and I find a 9V battery less hastle to incorporate into a box." and since the TA-28 uses 1.5v, could this then be used to power the TA-28 and a Linear Power Booster from a single 9v Battery??  If so, what Mosfet would I need to use??(I dont quite get all of this but I'll try it anyways). That way I could keep the TA-28's original circuit and tone and get the boost from the lpb.

[Joe Viau]

Omarvolta, did you have any luck?

[Mark Hammer]

If muddiness is the problem then I think you might look to the Univox Square Wave for some ideas. ( http://www.schematicx.com/schematic/univox-sq-150-square-wave-effect-schematic/ )  There, the output is fed to a small value capacitor, and a pot in parallel.  The cap always lets highs through, and the pot lets varying amounts of the entire signal through.  As the pot  resistance is decreased, the output gets made up more and more of the entire signal, bass and all.

So, consider flipping the order of C3 and R8, and make R8 smaller, say 4k7.  From R8, run the signal to a small-ish value cap, like .01uf and then to a 50k log volume pot.  In parallel with that .01uf cap, stick another 50k-100k pot.  That pot will determine the amount of bass content (mud) in your final output.

[omarvolta]

 Well I gave up on this a long time ago but this past weekend I put together a Mosfet Booster and remembered the TA-28. Before it it added distortion but no volume. After it with the booster on full is barely unity gain and very noisy but I don't care!! In sounds great. I'll try marks advice later today and bring back any news.

[omarvolta]

OK, this is what I did just now... followed Mark's directions and the thing has plenty of volume all by itself and even more gain than before so it'd be good to add a gain control. Now if the pot in parallel with the .01 cap goes as a variable resistor, 50k didn't do much and didn't have a 100k so i put a 250k and that does the trick OK. However, I think it sounds different now. It lost the little fartsyness that It had before or maybe It's the volume gain that makes the break up sound different. I'll make the original circuit once again and compare between the two.

[Mark Hammer]

Even when the provenance of a schematic can seem above reproach, sometimes even the "original" can have errors in it.  Keep in mind that you are working with a schematic only.  Anyone buying the kit from Heathkit back in 196x might have received an assembly guide and circuit board that conflicted with the schematic, and when the person looked at the 3 information sources they had, they went with the "majority" (2 out of 3) opinion.  In your case, all you have is the schematic.  And if a bonehead change to the output improves matters, then perhaps there IS an error in the schem. 

In this case, I'm starting to think the values shown in the last bit of the output portion are incorrect.  For example, a network of .022uf and 1k (C4/R12) gives a highpass filter function at 7.2khz  Now, what earthly good is that to anyone playing guitar?  If, however, R10 and R12 were 10k instead of 1k (i.e., if a "missing zero" was NOT accidentally dropped), then the C4/R12 network would pass content above 723hz (bass-less but still plenty of zip), and the R9/R10 network would not attenuate the full signal nearly as much.

Maybe there is your answer.

[omarvolta]

and there it was!!!! Thanks a lot Mark, replacing those 1k(R10 and R12) with 10k brings the original circuit to unity gain. I've omitted the original volume because it's not really needed and it's working like a charm. It's noise maybe because it's not boxed up yet but I'm loving it as it is. Thanks again Mark Hammer!! Finally Mark Farner in a box.

[Joe Viau]

Thanks for the replies! There's a lot of variety here for a circuit with such a small parts count.

[soggybag]

A friend of mine just gave me his original Heathkit to fix. This was an original, it had the two 1K resistors R9 and R10. The complaint was, loss of gain. Replaced these with two 10K and bam, more than unity gain and the thing sounds pretty good. Thanks Mark great thinking. After trying this myself, I'm sure that Mark is correct in thinking that R9 and 10 should have been 10K in the first place.