Effect Pedal building

[angelodp]

Piggy back T's?? As we know the recipe for great Fuzz is the inclusion of Transistors that have the classic 80/120 - 90/130 hfe in that realm. RG has a great idea on piggybacking T's to get the proper hfe on Si T's so you can get that low hfe going. I just breadboarded the setup and wowza does it ever work well. I have a piggybacked 107A' pairs at 100hfe and another upstream at 140hfe. The pair has a small trimmer to get dialed in and the usual trimmer for bias upstream.

My question, would love to hear any other experiments with this piggyback method? I think this might be the answer to impossible Ge T's, just dial it in with Si.

[pjd3]

That sounds cool, piggy backing transistors to achieve optimal gain. When building my two fuzzes, I purchased sets/pairs from SmallBear electronics who creates sets with optimal gains. (70-90/110-130 I believe where the ranges).

That does make for a very good sounding fuzz pedal.

Good luck,
Phil D

[angelodp]

https://www.diystompboxes.com/smfforum/ ... ic=17513.0

There is a very old (just discovered!!) thread on this forum that RG got going. The results are quite impressive. I have been getting Ge T's from Germany at a fair expense. This method is working for me on breadboard remarkably well. Give a shot on breadboard. 107A works great.

best A

[angelodp]

Here is the idea.

[cdemike]

Got bit by the tape echo bug and am getting some inspiration from the various delay units people have made from old tape decks and Walkmans (Walkmen?). Some are admittedly much better executed than others, but for example, I think these sound really great:






That second one is what pushed me over the edge and decide to start planning to build one: a used Walkman and a spare playback head would cost less than $30, and that build's fidelity is good enough to make for a pretty versatile delay as opposed to a super-low fi novelty delay.

Tape recording, playback, and bias are new ground for me, so this project is likely going to be a learning experience if it gets off the drawing board. Solid state isn't exactly comfortable for me either, but at least it's easy to find information about cloning and repairing Echoplexes. Here's what I have so far. It's basically an early-spec EP-3 but with a modernized, transformer-less bias oscillator along with substitutions for out-of-production transistors. I also deleted the sound-on-sound function.

Edited: found a few mistakes in the schematic. Corrected version attached.

Walkman echoplex.png

[cdemike]

Have been learning a lot about AC bias oscillators in the past few days -- pretty incredible this was common, affordable consumer technology (but hey, so is radio). Technical guides accompanying tape head catalogues have also been great resources. Looking into differences between AC bias with and without transformers, I learned that it's possible to de-couple the LC oscillator network if the coupling cap is sufficiently large so as to swamp the oscillating frequency which should minimize the inductive load presented by the erase head. If I'm understanding it right, that should allow the signal chain "downstream" of the oscillator to present primarily a resistive load to the oscillator, allowing it to provide predictable operation while also providing strong signal to the erase and record heads. Would this also be the case with a transformer-less oscillator?

So right now it seems like I could go one of two ways: with or without a bias transformer. It seems like power supply design may be a little more straight-forward with a transformer-less oscillator since the current requirement could be lower, but I'm attracted to the idea that the transformer might provide a degree of predictability in oscillator design since by providing a degree of isolation between the oscillator and its load. I'm still pretty green with both tape and solid state, so I'm hoping to get some feedback/critique on these schematics.

Here's the version without a bias transformer (oscillator courtesy of JL Hood):

Walkman echoplex rev 1 (1mh osc circuit).png

And here's the version with a bias transformer:

Walkman echoplex rev 1 (T-60-T2 osc circuit).png

These are the resources I've been drawing on lately:
- The JL Hood transformer-less oscillator:

John Linsley Hood oscillator (claimed 16-20VAC RMS output at 50khz).PNG

- Nortronics catalogue page describing the T70-T2 as used in EP-3's (very hard to come by and expensive):

Nortronics-T70-T2.pdf

- Nortronics guide to using the T60-T2 (very useful technical explanations):

ta-t60-e.pdf