heathkit ta 28 fuzz booster

[kaycee]

If anyone knows how to light up an LED from a 1.5v battery that would be neat  

there are many ways...as I mentioned earlier in this thread, I use the easiest (they're not super bright but cool enough for an indicator):
http://www.futurlec.com/LED/Red_2mm_LED.shtml

Thanks, I'll have a look around.

[pinkjimiphoton]

mac, that's way over my head...are you suggesting that at 9v you could get an octave up effect?

i DID try it at 9v, but it sounded real farty, and i didn't want to have to change the circuit to get it to bias.

total hack newb here bro!!

[Mark Hammer]

You know, I thought that the TA-28 was the same distortion a high school buddy of mine had made back in the day, to go with his Heathkit amp.  But trading e-mails with him today, I learned that it was NOT, in fact a Heathkit fuzz, but an Allied Radio unit he had bought from Chicago.  At the time, from what I understand, Allied and Knight were effectively the same.

The effectsdatabase shows a Knight fuzz here.  Clearly not the same.  Note the diode pair off in the corner.  can't see the transistor or caps clearly but do those resistor values ring any bells?

[pinkjimiphoton]

ff with a diode clipper?

[Mark Hammer]

Not with a pair of 2M2 resistors in there.

I'll keep looking through my files of 2-transistor fuzzes for something using those values.

[LucifersTrip]

Not with a pair of 2M2 resistors in there.

I'll keep looking through my files of 2-transistor fuzzes for something using those values.

I had this bookmarked

[Mark Hammer]

Wow!  If that isn't it, I don't now what is!

The transistors still remain unidentified, but there aren't all that many candidates in the button style from that era.  I'm guessing 2N3565 or maybe even the EHX favourite - 2N5133.

Thanks.  I owe you one....and I think I'm gonna build one tonight.

[Mark Hammer]

Okay, built.  For some reason, the 2N3565 button-types I had in the bin had hfes under 100, so I thought better of it and simply used a pair of 2N3904s that were in the mid-100s.  I'm kind of kicking myself that I didn't wire up sockets to swap out transistors, but so be it.  The diodes are unspecified type that I believe are germanium (I think from a Radio Shack "variety pack") but have a forward voltage around 350mv or so.  The input has a 470k fixed resistor feeding the input of a 100k pot.  I figured I'd make it a little more sensitive by using a 250k pot with a 330k fixed resistor in series.

An interesting sound.  Not a fuzz, as such, but certainly not a conventional overdrive sound.  Really more of a growl than a fuzz...and that's using a Tele.  Thicker-sounding with humbuckers.  Listening to it again, the sound that immediately came to mind was Stephen Stills' (or maybe it was Neil Young's) distorted tone on the Buffalo Springfield tune Bluebird http://www.youtube.com/watch?v=55q2rRh5UUU&feature=related (Jeez what a great tune; holds up well).   

I thought I might want to stick a cap in parallel with the diodes to tame any potential excessive treble but the 1500pf base-to-collector cap takes care of all that.  An input pot and volume pot are just fine.

Not for everyone, but worth building.

[Electron Tornado]

Put a cap on the first transistor between collector and base and it kind of looks similar to a BMP with the diodes at the end. What does a BMP sound like with hard clipping?


Here's a question - the schematic doesn't show what transistors are being used. Assuming they are both being used simply as amplifiers to get maximum gain, and that most (if not all) of the clipping is meant to come from the diodes, and given the other compnent values, input power, and input signal level, can one make an educated guess at the transistors' specs and use those to come up with likely devices to use?

L ooking at the schematic, I'm guessing both transistors are the same.



I hate typing on an iPad.  

[LucifersTrip]

Wow!  If that isn't it, I don't now what is!

excellent...

The transistors still remain unidentified, but there aren't all that many candidates in the button style from that era. 

Okay, built.  For some reason, the 2N3565 button-types I had in the bin had hfes under 100, so I thought better of it and simply used a pair of 2N3904s that were in the mid-100s. 

Definitely not nearly as many candidates as metal can,  but I must have at least 15-20 different button numbers.  Most that I have are form the mid to late 60's (I have one bag labeled 1964) and are one color, black. The hfe's of mine generally tend to be on the lower side (<100). But, I believe the ones in the pic you posted are a bit newer, which are black & white. When I have both black colored ones and black & white colored ones in the same #, the black & white ones are usually higher gains....and do have a newer look. The ones in my garage sale (#36) actually have gains 300 - 500:
http://www.diystompboxes.com/smfforum/index.php?topic=90624.msg864111#msg864111

An interesting sound.  Not a fuzz, as such, but certainly not a conventional overdrive sound.  Really more of a growl than a fuzz...and that's using a Tele.
Not for everyone, but worth building.

thanx for posting a report...I don't think I ever did breadboard that one, but might try real soon. Again, it would be cool if you could post your voltages as a reference. Did you screw with the biasing?

ps: we might consider a new thread instead of infiltrating TA-28

[LucifersTrip]

can one make an educated guess at the transistors' specs and use those to come up with likely devices to use?

probably someone as good as PRR or RG could, but honestly, with those type of silicon fuzzes, it's pretty much all about the gain...and when you get to a certain point (~150-200), it doesn't make as much difference going much higher...

after building a sh*tload of vintage fuzzes, it's very common that the higher gains will just give you a bit harsher sound. every circuit is different, but many times I get a warmer sound with hfe's under 100, the best sound with vintage fuzzes in the 100-200 range, then an increase in harshness over 200...with, as I wrote before, small changes after that. Ie, there'll be a bigger difference in sound between hfe 50 & 200  and very little difference between  hfe 300 and hfe 500. You can actually see this by taking voltages. You'll realize that there is a sharper curve from 50 - 200, then a much slower curve after that...

Looking at the schematic, I'm guessing both transistors are the same.

yeah, probably...check the pic above

[Electron Tornado]

Thanks, LucifersTrip, that's some interesting info on older fuzzes. I have a feeling that the design formula was to use cheap garden variety transistors and bias to taste.

[mac]

mac, that's way over my head...are you suggesting that at 9v you could get an octave up effect?

Someone posted long ago about transistors biased near vcc. Spice graphs showed how a transistor distorted the signal into something "similar" to 8-up. I use "..." because it was not exactly 8-up, just think of a cousin who is far away.
Besides I guess q1 has not enough gain to exhibit this "8-up".
If you want to hear the difference just breadboard a single transistor circuit sitting at 8.5v or so. Then take the output from HK q1.

i DID try it at 9v, but it sounded real farty, and i didn't want to have to change the circuit to get it to bias.

total hack newb here bro!!

Use this values for 9v:
q1 - 270k, 56k, 27k, 5k6, 33k
q2 C should be near 6v or so.

mac

[Tony Forestiere]

Not with a pair of 2M2 resistors in there.

I'll keep looking through my files of 2-transistor fuzzes for something using those values.

I had this bookmarked

I could be wrong, but I think the input jack wiring is wrong. The tip seems to be going to the supply. ***in his best Sgt. Schultz voice "Ahhhh, but I know NOTHING!"***

[LucifersTrip]

I always ignore the jack wiring since I've done it so many times, but yeah, I don't think that would be the standard way of drawing it.  I remember an old one that also caused confusion
http://www.montagar.com/~patj/mfuzrite.gif

[pickerdude]

Hey jimi, I built one of these way back in the 60's. It sucked. Might have been the way it was put together...

[pinkjimiphoton]

it is what it tis... lol

[Electron Tornado]

Finally got mine done last week. I used a Radio Shack enclosure since it was cheap and available, and seemed to work out well with a previous build.

   

Playing hard with the 0.1uf input cap in the circuit and the drive on full it will not only sag, but will actually cut out at times. Halving that cap might help prevent that, though 0.1uf is stock and can get some nice full fuzzy sounds playing single notes. Switching to the 0.0047uf cap is more crunchy, but less sustain, but playing with the tone control can give some great tones.

I used 100k pots for the tone and volume since I already had them on hand. If I were to build another I might go for 50k for the volume, but 100k works well for the tone control.

I eliminated the 10k resistor on the output of Q2 for more output.

The pedal plays well with others and plays great with a wah.  

Here's my final schematic:

[pinkjimiphoton]

cool, thanks for the build report!!

if you run it on a 1.5v battery, it will indeed sag as you report...

if ya use the little daughterboard so you can run it at 9v tho, it probably won't...mine doesn't, anyways.

it is a nice little fuzz...so glad them peeps on ebay posted it. 

[Electron Tornado]

Playing hard with the 0.1uf input cap in the circuit and the drive on full it will not only sag, but will actually cut out at times. Halving that cap might help prevent that, though 0.1uf is stock and can get some nice full fuzzy sounds playing single notes. Switching to the 0.0047uf cap is more crunchy, but less sustain, but playing with the tone control can give some great tones.

I'm kind of dragging up an old thread, but I'm still stumped about something.

After some fiddling, I opted for a 0.01uf cap for the input cap. With a 0.1uf, as used in the original TA-28, it will cut out when playing hard, especially on low strings. I ran it from a One Spot. Tested an original TA-28 using a fresh AA battery and it did the same thing.

Does anyone have any idea why it will cut out with a 0.1uf cap on the input?