Heathkit TA 28 Fuzz Booster

Discussion regarding early stompbox technology: 1960-1975 Please keep discussion focused and contribute what info you have...
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pinkjimiphoton
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Post by pinkjimiphoton »

hi guys,
was surfing around evilbay earlier, and spotted this:

https://www.ebay.com/itm/150894762099?s ... 1423.l2648

Image

they were kind enough to provide this:

Image

and a gutshot!

Image

and even a schematic, too:

Image

so i looked up the transistors, to see what would sub, and it came up mpsa18 for q1, and our old friend 2n3906 for q2. i decided to suggest germanium there, but pretty much anything should do i would imagine.

that said, i was bored, so i worked up a vero...

Image

gonna build it at some point just for sh*ts and grins...

just thought i'd share...was mighty nice of them to provide enough info to not need to build the pedal. ;)

if anyone whacks it together, please let me know!

happy fumes!
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Post by roseblood11 »

R8 and C3 could join at line f if you add a cut at f8 and
R3, R5 and +1.5V could join at line c.
You could omit lines a nad b and jumper...

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Post by LucifersTrip »

here's my reply from the other forum just in case...
hi guys,
was surfing around evilbay earlier, and spotted this...
hey jimi!

cool to see you're still looking out for the odd fuzzes.

I built that one a while ago from the same schematic:
http://www.muzique.com/schem/heathkit.gif

To my ears, it is most similar in sound to the Sam Ash Fuzzz Boxx/ Astrotone...but better. It's one of the few 1.5V fuzzes that are silicon. Still my favorite is the Sun Buzz...
so i looked up the transistors, to see what would sub, and it came up mpsa18 for q1, and our old friend 2n3906 for q2. i decided to suggest germanium there, but pretty much anything should do i would imagine.
I actually had some vintage 2N3391's laying around, which had hfe's ~ 350 -400. The MPSA18 is way higher, 800+.
You know how it is with cross references...they're many times ridiculous. Coincidentally, I did use a 2N3906 for Q2 with a gain of around 200 - 250. Note that my vintage ones are in the 100 - 150 hfe range. I have no idea why you're thinking about a germanium there. I doubt it'll bias well.

according to the analogguru schematic, Q2 would be cool with BC309B, which has min hfe 250.
http://analogguru.an.ohost.de/193/schem ... _TA-28.gif

I did try many different Q2's before deciding on the 2N3906.

gonna build it at some point just for sh*ts and grins...

just thought i'd share...was mighty nice of them to provide enough info to not need to build the pedal. ;)

if anyone whacks it together, please let me know!

yah, you know i'm gonna try it with a 9volt.
There are a couple things that are unusual about that pedal...It uses an NPN / PNP combo and it's a 1.5V silicon...so it'd be sacrilege to change to 9V...the 1.5V is part of the fun! just kidding...you should always try everything.
from owner of original:
As for how the example I have sounds, it's a sort of fuzzy overdrive, but there is a volume cut with it engaged. It needs another amplifier stage at the end.
When tweaking, I noticed it was not loud enough and there was a simple fix. Just lower or remove the 10K before the tone control. I actually wound up putting a 10K pot there.

I also used a switch between the original input cap and a .033uF which smoothens out some of the raspiness when the drive (50K) is higher up.

Anyway, here's my final version with voltages & notes.

Image

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Post by pinkjimiphoton »

roseblood11 wrote:R8 and C3 could join at line f if you add a cut at f8 and
R3, R5 and +1.5V could join at line c.
You could omit lines a nad b and jumper...
yah, i could have made it smaller, but at 10x10, it's pretty small already.

i am beginning to not like standy-uppy resistors...too easy to break all too often.

thanks tho! ;) :-D
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Post by kaycee »

This is a really nice fuzz, one of my favourites actually. I build mine as per the schematic and don't seem to have volume issues. I don't use specific transistors in mine, just sort through and tune to ear, the tone control although crude works quite well. Don't be put off by the 1.5volt operation, it has plenty of output. Its a bit like the Jordan Bosstone which also use a NPN/PNP combi.

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Post by pinkjimiphoton »

cool! i may give it a go in a couple days :thumbsup
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Post by drewl »

Was that a kit like other heathkit stuff?

Back in high school I had an electronics class, and I built a fuzz pedal kit as a project.
I think that was a Hallibar (sp?) kit though.
Anyone ever hear of Hallibar?
Maybe I should start another thread for that. I did some searching but have not found anything yet.

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Post by pinkjimiphoton »

yah, it was a kit.
built it the other day, nice fuzz for 1.5v!

made a daughterboard so i could run it at 9v, sounds a little tighter with the 9v...less sag.

here's the stupid pedal tricks video:



couple mods on the other forum, notably using 10k resistors at the ground ends of the tone stack, and swapping the 10k before the tone stack to either a pot or in my case a 3.9k resistor.
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Post by LucifersTrip »

drewl wrote:Was that a kit like other heathkit stuff?
yes...did you read the ad?
https://i605.photobucket.com/albums/tt1 ... ooster.jpg
Back in high school I had an electronics class, and I built a fuzz pedal kit as a project.
I think that was a Hallibar (sp?) kit though.
Anyone ever hear of Hallibur?
Maybe I should start another thread for that. I did some searching but have not found anything yet.
never heard of em...it would be cool if you could dig something up...around what year are you talking about?
you definitely have the spelling right (Hallibur)?

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Post by drewl »

Says kit right there, sorry.
Hmmm, Hallibar I built in high school would be around 1980/81.

I also turned a mini transmitter/reciever kit into a wireless.
Have no idea where those got to.

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Post by pinkjimiphoton »





screw it, i can't get this page to embed the damn video...
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Post by LucifersTrip »

remove the "s" from https


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Post by pinkjimiphoton »

thanks lu!!!
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Post by johnk »

I always wanted one of these so I finally got around to building one. mine is stock with the exception of adding a 1.5V voltage regulator(so it'll run on a standard 9V power supply), a relayed true bypass, led indicator and a clean Jfet output boost with level control. I LOVE this pedal!
TA-28-01.jpg
TA-28-02.jpg

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Post by johnk »

here's quick clip of it with a p bass thru my amp:


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Post by deltafred »

johnk wrote:here's quick clip of it with a p bass thru my amp:

That sounds really good, definitely one to go on the "to build" list.
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Post by ljn »

You know, I read that Mark Farner of Grand Funk Railroad used a Heathkit TA-28 in the early,70s. I've heard some of their early stuff that he used it in and from what I can tell, it seems like a decent sounding fuzz. I may build one later on.

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Post by pinkjimiphoton »

these are really good fuzzes, they kinda remind me of a percolator in some respects tonally.
check diy for lucifer's trips notes on achieving unity gain, and how to drop voltage to get it to work with a regular supply.
fwiw, i make the pnp ge on mine, and it's incredibly rich and tube like... after seeing this thread i dug up my old layout and schematic, and am gonna probably build one of these today just for shits n giggles. :twisted:

lot of fun!! don't get hung up on the transistors too much.. in my experience, most anything seems to work with a little bias adjustment.

it WILL be "cleaner" and less sputtery with a higher supply dropped down... try like a 7802 or something to drop it, or a couple diodes will work too if memory serves.
check my lame ass vid to get a taste of the tone.

happy building friends.
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Post by ljn »

It's definitely on my list of fuzz circuits that I want to build. I just have too many projects going on right now (mainly a couple of tone benders ). I'm thinking that I'd like to build it as the original schematic suggests. I don't mind a bit less output on a fuzz. Of course, I had read this was common problem with the Ampeg Scrambler. I built one of those and it's quite loud. I believe I have a vintage 2N3391 in a non working reel to reel. I'm definitely tempted to put one of these together, but I don't really have all of the required components at this time.

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Post by drewl »

Deja Vu
Got one of these in for repair and stumbled across this thread while researching it!

Wow, the volume is pretty low on these, lowering the 10k does help, raising the two 1k resistors really kicks up the volume!

I'm going to experiment with values to find a sweet spot of more volume while retaining the tone of the pedal.

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