heathkit ta 28 fuzz booster

[pinkjimiphoton]

 

lol... i likes it...

[Electron Tornado]

I have an excel-based bias calculator for this if you want to experiment with vcc.

I'm interested in taking a look at that.

your orig voltages would still be cool if you have the time...  

OK, here are the voltages from the pedal itself:

Supply voltage was a fresh battery and read 1.5 V. All voltages are measured with respect to ground.


Q1   C = 1.0v
      B = 0.60v
      E = 0.31v

Q2   C = 1.15v
      B = 1.0v
      E = 1.5v


On the schematic, the Q1 base voltage is listed as 0.87v.
     
More from the Heathkit manual: from the input,  "...the signal is coupled through capacitor C1 to the base of amplifier transistor Q1. The amplified signal from the collector of Q1 is direct-coupled to the base of overdriven amplifier Q2. This amplifier is so named because it operates in such a manner that a small signal will either saturate it or cut it off. In either state, harmonic distortion is added to the signal that is passing through the stage."

From that, it sounds like much of the tone is determined by the signal level of the output from Q1 and the clipping characteristics of Q2.

Now someone just needs to post a table comparing the specs on the suggested subs for Q2.

[LucifersTrip]

ET...excellent. thanx alot for posting em!

[mac]

I'm interested in taking a look at that.

In my gallery, follow this link,

http://www.aronnelson.com/gallery/main.php/v/mac/apps/9v+Heathkit+TA-28.xls.html

Depending on your system, you have to use period or comma as decimal separator.

mac

[pinkjimiphoton]

thanks mac.

gonna give this thing a go....soon as i finish typing this. found the 3391's at tayda, so i ordered 30 of 'em...will swap 'em in when they arrive.

will letcha know how i make out, guys

[pinkjimiphoton]

wellp, i built it, the vero is good...

the mpsa18 works great in q1...i tried a couple germaniums in q2, but the best sounding transistor i have there is a 3906.

tried it at 9v...ummm...needs the bias played with.

i didn't find it to be quiet at all, but the tone control works almost more like a volume control...kinda like the "attack" control on a tonebender.

so the 10k resistor before the tone stack is good.

at 1.5 volts, it actually sounds really good!! very fuzzy, a lot of sag, useful into a clean or dirty amp. nice sustain, tho at times if ya really lean on it, it begins to sound like many really classic
cheap-ass fuzzes like the kay. i mean that in a cool way!!

right now i got the AA soldered to the power supply...heh heh...i'll get a AA holder tomorrow. gonna try it with a 3v lithium coin cell, too...it was almost useable at 9v, so 3 v it may be ok with....and a cr2032 oughta last about ...oh....the rest of my life span.

very nice sounding little fuzz...the fuzz control doesn't go all the way to 0, and the tone control is pretty weird.. but not bad!!

[LucifersTrip]

i didn't find it to be quiet at all, but the tone control works almost more like a volume control...kinda like the "attack" control on a tonebender.

...then something's gotta be a bit off. the tone has a nice sweep from bass to treble with little volume change..

so the 10k resistor before the tone stack is good.

yes, it just gets you to around unity...try with lower and see if you like. it didn't change the tone or character for me.

very nice sounding little fuzz...the fuzz control doesn't go all the way to 0

note the 3.3K  

...it does get more broken up as you increase that control, as I wrote earlier, if you want to smooth out that break up when the level is maxxed, decrease the input cap.

and the tone control is pretty weird.. but not bad!!

double-check that...and see if your voltages are close. it's not weird.

edit:

[Solidhex]

If you're looking for more output from the circuit don't forget about the two 1K resistors to ground on either of the tone control. Increase them a bit and you'll get more output.

[LucifersTrip]

you beat me to it!...as you posted, I was just looking thru my notes and I found the thread. It seems people believed the two 1K's in the tone control could actually have been an error...and should have been 10K. That was the first thing I changed when I heard that vol drop.

I'll fix my final schematic...

edit:
funny, totally forgot about that old thread from a year and a half ago...hey, Electron Tornado was in on it, too
http://www.diystompboxes.com/smfforum/index.php?topic=90489.msg769429#msg769429

Final Configuration:

Q1: 2N3391, hfe = 305
Q2: 2N3906  hfe = 265
Replace 2 x 1K's with 10K's
Replace 10K with 10K pot
Switch to choose between .1uFd and .033uFd input cap

[Electron Tornado]

you beat me to it!...as you posted, I was just looking thru my notes and I found the thread. It seems people believed the two 1K's in the tone control could actually have been an error...and should have been 10K. That was the first thing I changed when I heard that vol drop.

Those resistors have been a point of discussion before, but the 1k value in the Heathkit schematic agrees with both the parts list and the assembly instructions in the manual. So they might not be a mistake as such, but using 10k might just be a great mod.

Here are some other specs according to Heathkit. These are all with the fuzz control at max.

Input impedance - 28k Ohms
Sensitivity - 30mV rms for a balanced output
Voltage output - 45mV into 10k load at 100Hz output

[pinkjimiphoton]

the clip sounds pretty much just like the one i built...thanks bro.

i will check the tone part out, and get some voltage readings while i'm at it...

gimme a little bit!

i MAY have confused the tone and volume pots...was late 

let me see what i got...everyone is up now, so i can make a little noise!

AHA!!! found a mistake... i made the 1k resistor off the 22n cap 10k...that may explain why the tone stack is acting weird.

i'll probably just go with the pair of 10k's...as one side of the tone control seems much louder than the other.

thanks guys..will update the vero when i can...

[DougH]

There's been some speculation that this may be related to the onboard fuzz built into the Messenger guitar used on the early Grand Funk albums.

[pinkjimiphoton]

hi guys,
swapped out that one resistor,

now the tone control seems to work perfectly, nice sweep from treble to bass.

here's the voltages with a 1.5v alkaline

mpsa18
q1
e  .98
b  .83
c  .30

2n3906
q2
c  1.55
b   .98
e 1.10

this thing sounds really good... pretty high gain!! may have to add a snubber, cuz it's picking up some RFI, but it's not boxed yet.
will try and record a clip shortly

[Mark Hammer]

you beat me to it!...as you posted, I was just looking thru my notes and I found the thread. It seems people believed the two 1K's in the tone control could actually have been an error...and should have been 10K. That was the first thing I changed when I heard that vol drop.

Those resistors have been a point of discussion before, but the 1k value in the Heathkit schematic agrees with both the parts list and the assembly instructions in the manual. So they might not be a mistake as such, but using 10k might just be a great mod.

Here are some other specs according to Heathkit. These are all with the fuzz control at max.

Input impedance - 28k Ohms
Sensitivity - 30mV rms for a balanced output
Voltage output - 45mV into 10k load at 100Hz output

The .022uf cap and 1k resistor form a highpass filter with an 8khz corner frequency, where using a 10k resistor gets you an 800hz corner frequency.  Which would you prefer when you turn the tone pot all the way to the treble side?

Those wishing to "muff it up" might consider placing a cap in parallel with the 10k resiastor on the non-treble side of the tone control.  For example, a .01uf cap gets you a rolloff starting around 2.3khz, which will yield a rounder tone.  Given that shaving off treble tends to reduce apparent loudness, you might want to up the value of the 10k to ground to 12k or even 15k for less attenuation.

By the way, I'm curious.  What does the contemporary pedalboard user power this with?  Do you rely on an AA, or use an external supply and drop it, via suitable zeners?

[Electron Tornado]

There's been some speculation that this may be related to the onboard fuzz built into the Messenger guitar used on the early Grand Funk albums.

That was discussed some time ago, and confirmed that it wasn't the Heathkit fuzz. The Messenger fuzz is completely different.

[Electron Tornado]

By the way, I'm curious.  What does the contemporary pedalboard user power this with?  Do you rely on an AA, or use an external supply and drop it, via suitable zeners?

I've seen at least one person use a AA battery. When I can get back to working on this circuit I plan on using 9v with diodes, both so I can use external power, and because I still think it needs a booster stage on the end.

Mark, what's your opinion of adding a buffer to the front end to increase input impedance?

[pinkjimiphoton]

i'm just using an AA battery on mine.

i don't know enough about diodes to run it at 9v, would i use a 1.5v zener or something?

i AM gonna probably add a small pf cap to nuke the rfi i'm getting....

that said, here's the corrected, verified vero layout:

[pinkjimiphoton]

ok, boxing it now...

gotta definitely do a little mod... it's just below unity when pegged.

can't run an led, either!! 1.5 v just barely turns the led on.

[digi2t]

ok, boxing it now...

gotta definitely do a little mod... it's just below unity when pegged.

can't run an led, either!! 1.5 v just barely turns the led on.

I would do a Zener mod, as Mark suggested. Actually, two standard 1N400X diodes in series (roughly 0.7V each) is a 1.4V zener. Close enough. Just set it up like the Vocalizer. Then you can either use a 9v battery, or power supply, and have your LED as well.

Cake, fork, mouth, eat.

[Electron Tornado]

I agree with doing the diode thing. Look at the Fuzz E-One at Small Bear and see how Steve did the power supply:  https://www.smallbearelec.com/Projects/FuzzE-One/FuzzE-One.htm