Heathkit TA-28 Fuzz revisited / Build Report

[LucifersTrip]

I wanted to revisit this because there was very little discussion here, I couldn't find a sound file from an orginal and the sound reported by builds is different than what owners described.



Firstly, here's the one useful previous discussion here:
http://www.diystompboxes.com/smfforum/index.php?topic=64837.0

The one thing that was agreed on is that it's way below unity....and that corresponds with a reviewer.
http://www.harmonycentral.com/products/100357
"The only thing that knocks it from a 10 is that the signal gets lowered a little when the effect is switched on."

I get a much bigger drop than "a little", and others did too, making it pretty unusable at that volume.
The solution was to raise the two 1K's to 10K. I did that and it did bring it up to a usable volume, keeping the tone the same.

The tone I'm getting is a good buzzy fuzz with average sustain for single notes, but a more blatty sound with chords.
Others on the forums have reported the same, one calling it "fartsy". Though, if you read reviews (some or all owners of originals & not clones?), they say the opposite:
http://www.harmonycentral.com/products/100357

"I thought this was going to be a "Fuzz Tone" or "Fuzz Face" type distortion with fizzy fuzz of the '60's.  Nope.  It is more like a Tubescreamer or overdrive pedal.  As mentioned, it is a smooth, round, type of overdrive that has great harmonic content.  Very rich. "

"The Tone control has a nice wide range of Bass to Treble and is real nice to "Hone the Tone". The Distortion is really very smooth and VERY musical.  It is unlike most 1.5 volt fuzz boxes of this era, because it's not a fuzz sound, it's a smooth distortion that is NICE!!!"

--
I've only had the time so far to swap out each component, one at a time. I built it to spec with the exception of Q2.

Transistors: My 3391 has hfe ~ 300. I went thru about 20 different transistors for Q2 and nothing changed the tone. There was
nothing more than a change of output volume and/or a change in sustain. I settled for a 2N3906 with hfe also ~ 300.

Caps: No swaps did any more than the normal... an increase or decrease in  treble/bass

Resistors: Probably since this is 1.5v powered, there's not much room for adjustment. I swapped out each R and it seems that they're pretty much set to the sweet spots. If you change most of the resistances a little in either direction, you get gating and then a cut. The designer was right on.

Though, there were two resistor subs that did make a reasonable change. You can smooth out some of the chordal blattyness and actually get a smoother fuzz by lowering the 82K down to ~ 50K. Unfortunately, when you do that the high strings gate. Secondly, I raised the 6.8K to ~ 20k. This also smoothens some blattyness and gives it a fuller, richer tone, adding a little bass while removing some of the harsh highs that the high E can get.  Unfortunately, with that changed, the tone control is lost...and it almost turns into a volume control. I'm not good enough to know how to recalculate the other parts of the tone control to make it work with the change.

Anyway, if anyone has a sound file from an original, please share the link.  It doesn't sound bad with the semi-blatty chords but I'd like to know I'm starting with something similar to the original before I start tweaking.

The correct voltages are listed on the original schematic:


My readings are almost dead-on to the factory schematic!  

Q1 ECB (.38, .89, 1.02)
Q2 ECB (1.63, 1.02, 1.30)

I would like to make one important note. Unlike some other 1.5v fuzzes I've built which aren't effected by the battery power, this one is! My Sunn Buzz sounds cool on a 1.25v, but here's the Heathkit with just a slightly dying battery:

Q1 ECB (.25, .76, .80)
Q2 ECB (1.4, .80, .82)


If anyone has voltages of one that they think sounds good, please post em.

thanx

[Electron Tornado]

I have an original TA-28. The photocopied schematic you posted comes right out of the construction manual. The pedal does give me a volume drop and the tone is more like an overdrive or distortion than a fuzz. It took some time to research substitutes for Q2. Here's a list of subs I posted on another forum:

D29A4
2N290
2N2907
2N4403
NTE159
BC327
BC638
BC640

Here's a link to the discussion. There's a link to a sound file there as well:  http://www.buildyourownclone.com/board/viewtopic.php?f=9&t=29628

You can solve the volume problem by simply adding a one transistor boost stage at the end of the circuit. Or, you can add a tone bypass switch.

Let us know what you finally use for Q2.

[LucifersTrip]

I have an original TA-28. The photocopied schematic you posted comes right out of the construction manual. The pedal does give me a volume drop and the tone is more like an overdrive or distortion than a fuzz. It took some time to research substitutes for Q2. Here's a list of subs I posted on another forum:

D29A4
2N290
2N2907
2N4403
NTE159
BC327
BC638
BC640

Here's a link to the discussion. There's a link to a sound file there as well:  http://www.buildyourownclone.com/board/viewtopic.php?f=9&t=29628

You can solve the volume problem by simply adding a one transistor boost stage at the end of the circuit. Let us know what you finally use for Q2.

Thanx much for the sound clip. I ignored that when I skimmed that discussion the 1st time because I thought it was from a clone (did I read incorrectly?).
Mine sounds much more buzzy and fuzzy and screams more with single notes, but like I said, there's some blattyness with chords which isn't evident in that sound file.

Thanx also for the Q subs. I did  go thru 20 or more including a couple from your list. Nothing made any tonal change. The one that sounded best was the 3906. I'll try some others. What's the hfe range of the subs you posted?

I have no problem with volume after the 2 x 10K sub, so I won't worry about any extra stages.

Edit: voltage readings moved to original post.

[Electron Tornado]

Here's a couple of things that I've always wondered about this circuit - why a PNP transistor was used for Q2, and why R7 is connected from the collector of Q2 to the emitter of Q1.   Not sure what's going on there.

[Electron Tornado]

Hmm...actually I think the sound file may have been from a clone.

I don't have and hfe data handy. Look up the datasheets for those transistors.

[LucifersTrip]

Here's a couple of things that I've always wondered about this circuit - why a PNP transistor was used for Q2, and why R7 is connected from the collector of Q2 to the emitter of Q1.  icon_question Not sure what's going on there.

It's just another way of designing a distortion circuit & it works. That's a question for someone who has more circuit designing background.  Btw, here's another odd one for you.

Hmm...actually I think the sound file may have been from a clone.

I don't have and hfe data handy. Look up the datasheets for those transistors.

Yes, the unfortunate thing is that datasheets just give you a minimum or a range. Though, I guess I can get an idea, especially since I already tried the 2907 & 4403. My vintage 2907's range from 70 - 120

[mac]

The 8k2 resistor does much of the sound. And at the same time helps a lot to keep bias stable under variations on hfe.
I tried it at 9v also tweaking bias resistors in particular the 8k2, cool sounds, more alive if you allow me the expression.

I've made a bias calculator for this in Excel, email me if you want it.

mac

[R.G.]

This thing is an effects archeology artifact. It's from the days when having a distortion at all set you apart from the other guitarists. Little things like relative volume were secondary considerations.

If I were building this, I'd case it up in some modern wrappings (* see "Wrap, Wrap, they call it a Wrapper at geofex) to make it more polite to modern day setups.

I would
- run it from 9Vdc by making a 1.5Vdc source to run it from
- buffer the input to see if that helped or hurt the sound; chances are it will sometimes be driven from another pedal, which is the equivalent of unintentional buffering anyway
- add a gain stage to the output to let me change it to whatever output level I wanted without mucking around inside it and possibly changing whatever tone it was I valued that make me build this curiousity in the first place; a single dual opamp would do both the buffer and the gain stage

Here's a couple of things that I've always wondered about this circuit - why a PNP transistor was used for Q2, and why R7 is connected from the collector of Q2 to the emitter of Q1.  Not sure what's going on there.

It's a DC feedback pair connection. It was used as an "opamp" back in the days when opamps could not be bought, period, to get reliable gain set by resistors and caps. Q1 = gain to the base of Q2, Q2 = gain to the output. 8.2K and 2.7K force the DC level of the output to be related to the input bias divider and set the output DC level. The 100uF capacitor shunts all AC feedback to ground, otherwise the AC gain would be the same as the DC gain.

Notice that if you replace the left-hand 1K in the tone control with a cap, you get what's become known as the big muff tone control. As it is here, it's a choice between plain vanilla (tone wiper full left) and bass cut (tone wiper full right).

[Steben]

I'ld guess this thing has low impedance due to the cap to ground at first emitter?
That would give it roughly a filter with cap and resistor to ground of (300hfe * 15 ohm internal) 4k ohm.
That gives low roll-of just under 400hz. midhigh boost. Explains a lot of overdrive character.
Change input cap to 1µF and you'll have fuzz splat.

my FAT germs remind of that treble boost effect.

Simple anything simple discrete solid state can sound decent with some pre-bass-cut .

[LucifersTrip]

Change input cap to 1µF and you'll have fuzz splat.

Increasing the input actually decreases the buzz and makes it sound more like a normal distortion / overdrive...

In addition, the excellent tone control give you a nice sweep from bass to treble.

[LucifersTrip]

This thing is an effects archeology artifact. It's from the days when having a distortion at all set you apart from the other guitarists. Little things like relative volume were secondary considerations.

I would
- run it from 9Vdc by making a 1.5Vdc source to run it from
- buffer the input to see if that helped or hurt the sound; chances are it will sometimes be driven from another pedal, which is the equivalent of unintentional buffering anyway
- add a gain stage to the output to let me change it to whatever output level I wanted without mucking around inside it and possibly changing whatever tone it was I valued that make me build this curiousity in the first place; a single dual opamp would do both the buffer and the gain stage

Thanx much for the suggestions, but for me the fun in  "effects archeology" is not to update them with additional parts  and stages and make it friendly to modern guitar players. The fun for me is keeping that part count low and trying to get the sound that made it cool back then with little change to the original circuitry.

In this case, the output volume drop was solved with subbing the 10K's for the 1K's....which didn't change the tone.

I am not a purest, though. I have modded almost every single vintage fuzz I've made with switchable caps, transistors, diodes, etc...but I can usually switch back to the original. The exception is when I found the original totally lackluster, then I'll tweak to my liking.

This one sounds cool as is. My biggest problem here is that I'm not sure what the original sounded like and the reviews (from people who seemed to own originals) contradict the sound I'm getting....and my voltage readings match the original schematic almost exactly. I made only one sub & that's for the (probably) house numbered X29A829.

It would be cool to hear an original and find out the hfe of the X29A829.

thanx again

[kaycee]

Hi,

I built the one in the thread over at BYOC, its also up over on FSB. Mine is as the schematic at the top of the page, didn't sub the tone section resistors - nothing remarkable about the transistors I used. as evidenced from the hurried soundclip (the missus came home early) I've got a decent volume boost on mine, I've got a second on the breadboard (playing with emitter follwer buffers) and thats the same. I did replace the 10K preceeding the tone section with a 10K trim which makes some difference, I have it set about halfway - but doesn't seriously drop the volume. I haven't heard an original either, but I like the sound of this a lot - on the overdrive as opposed to distortion side of fuzz. I did put a 1M pulldown on the input after the pot and before the cap - perhaps thats making a difference to the impedance?

[Derringer]

man ... maybe 12 - 14 years ago I went to a random garage sale and bought one of these fuzzes, a Foxx Clean Machine and a Mutron Phasor 2 for $25.

The foxx was the only thing that really worked but it sounded like crap. So all three effects pretty much just stayed in a box in my bedroom. I didn't know crap about fixing the mutron or the heathkit back then. I eventually threw away the heathkit one day while cleaning stuff up, luckily the mutron looked too cool to throw away and the foxx worked.

The foxx was eventually sold on ebay and I did fix and am using to this day the mutron.

But I do wish I'd held onto that heathkit. It was a quirky looking pedal with a 1/4" cord of about 10' permanently attached.

[PRR]

> Q2 ECB (1.63, 1.02, 1.30)

Should this read "EBC"?

> I'd guess this thing has low impedance due to the cap to ground at first emitter?

Not really. Low, but not cuz of Q1.

Q2 Vbe is about 0.5V-0.6V, its base current about (1.3V/11K)/200 or less than 1uA. Q1 must flow this plus the 0.5V across 27K, say 20uA total. The emitter impedance at 20uA is about 20*28 or 600 ohms. The base impedance is hFE higher, 120K.

We also have 82K, 100K, and 53K all in parallel. These come to 24K; with Q1's 120K, to 20K.

You could make R1 R3 R4 all higher. Double or triple would not lead to sloppy bias.

I suspect they MADE it low-Z input to load the pickup (no buffers those days) and trim the higher partials off the signal so they didn't make too much "tizzzz".

To increase output, ponder making R8 10K lower, 3K or even lower. This starts to change the intrinsic asymmetry, so may not be "authentic".

I suspect it would handle chords better (not great) if C1 0.1uFd were much smaller, even 0.01uFd. This with the treble-loading would bandpass a narrow slice of guitar spectrum, so the wide spread of partials on guitar chords did not intermodulate all over the audio band.

I would love it for what it is: VERY thrifty on a single cell, and a nostalgic benchmark in pedal history.

I can't account for the different impressions, your clones versus the HarmCentral comments. Altho, anybody who hated it probably lost it years ago, and would not be commenting today. Maybe its lovers have unique styles and tastes.

[LucifersTrip]

> Q2 ECB (1.63, 1.02, 1.30)

Should this read "EBC"?

Yes, thanx for catching that. I quickly transcribed the ECB from the 2N3391 and missed the error.

It should be:
Q1 EBC (.38, .89, 1.02)
Q2 EBC (1.63, 1.02, 1.30)

...and for the weaker battery:
Q1 EBC (.25, .76, .80)
Q2 EBC (1.4, .80, .82)

To increase output, ponder making R8 10K lower, 3K or even lower. This starts to change the intrinsic asymmetry, so may not be "authentic".

I tried that already, but thanx for making me re-visit it. I never did try dropping it all the way to 0K. As I lowered the resistance,
I got  a slow increase in volume along with some extra highs.The last few percent down to 0K gives a sharper treble & volume increase. Some of the blattyness disappears at 0k. It basically gets nastier as the resistance is lowered but the overall tone doesn't change too much at all.  I will highly consider a 10K pot there, especially since there is no volume control.  

I suspect it would handle chords better (not great) if C1 0.1uFd were much smaller, even 0.01uFd. This with the treble-loading would bandpass a narrow slice of guitar spectrum, so the wide spread of partials on guitar chords did not intermodulate all over the audio band.

Once again I have to thank you for making me revisit that. I believe I only tried increasing the uFd, especially since that was a common suggestion in other discussions. I guess it seemed logical that softening it with lows [raising uFd] would do the trick, but lowering to .01uFd completely smoothened out the chords. It does remove fuzz and of course makes the tone more trebly, but it is a starting point.
I'll slowly raise it from .01uFd and see where the smooth disappears.

I would love it for what it is: VERY thrifty on a single cell, and a nostalgic benchmark in pedal history.

that it is...

------------
edit: after testing

.1uFd sounds like a standard distortion/overdrive. The blattyness starts to reappear at .022uFd. I'm going with a little higher than that to keep the buzz.

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

[LucifersTrip]

[Electron Tornado]

Got this on the breadboard the other night. Q1 is 2n3391 and Q2 is 2n2907, which I found mentioned on a ham radio forum as a substitute for the X29A829. Sounded great when used with a 1.5V battery, but try to run it off a 9v battery through a voltage divider and it sounded a bit blatty and gated.

With the 1.5v battery here are the voltages I got:

Q1: E 0.25v, B 0.65v, C 1.0v
Q2: E 1.5v, B 1.0v, C 1.15v


With the 9v battery (8.4v actually) and a voltage divider:

Q1: E 0.1v, B 0.36v, C 0.45v
Q2: E 0.9v, B 0.45v, C 0.15v


Not sure what's going on, so I measured the voltage at the divider with and without being connected to the circuit. When not connected, it read 1.5v, when connected it read 1.1v.

I then tried the same thing with an original example of the pedal, and got the same voltages.


Is there something fundamental about voltage dividers that I'm missing?

[LucifersTrip]

Is there something fundamental about voltage dividers that I'm missing?

Will the current remain the same?   

[Electron Tornado]

Will the current remain the same?   

There's a Wiki article on voltage dividers that gives a short explanation why a voltage divider is good for providing a reference voltage, but not for providing a source voltage. It seems current is part of the issue.

Since I want to run this from a 9v supply, the question becomes how to drop that to a 1.5v supply voltage that can be used by the circuit. 

[R.G.]

Not sure what's going on, so I measured the voltage at the divider with and without being connected to the circuit. When not connected, it read 1.5v, when connected it read 1.1v.

I then tried the same thing with an original example of the pedal, and got the same voltages.


Is there something fundamental about voltage dividers that I'm missing?

Pretty much it's what I explained in the article in choosing resistors for a Vbias string at geofex.

A resistive divider composed of, say, R1 and R2 and a voltage Vin, produces an output voltage of Vout = Vin* (R2/(R1+R2)) as you'd expect. However, that voltage appears to be in series with the parallel combination of R1 and R2 for all DC loads. It's only an accurate voltage divider to the extent that currents pulled out of the divider point are insignificant compared to the current running through R1 and R2. So voltage dividers are miserable ways to get a lower voltage for anything except an almost no-current reference. They have to waste about 9 times or more power than they supply to be even partly accurate. Otherwise they sag.

A lot.