Old circuit as an FX pedal?

Hi all,

I have a couple of old junk tape recorders that I've had sat collecting dust for years. I've been thinking about having a go at making effects pedals lately (I play bass and synthesizers as opposed to guitar) and saw a video of someone using the same recorder as a guitar fuzz/overdrive - and was surprised to hear it wasn't half bad.

It has gotten me thinking about pulling them apart and using the circuit for something similar. I managed to track down a schematic which I've attached, difference with mine being that Q3 and Q4 are 2SB77 PNPs as opposed to 2SB75 like the first two (at least, I assume - I've not examined the cicuit in detail yet to make sure it's not the other way around.) It seems that some of these germanium transistors make decent fuzz box choices from what I've read.

I know little about them, but I'm curious whether it's worth keeping the circuit much the same as it is and just sticking a jack in place of the tape head, or whether it might be better to tear it down and create a new circuit with the components. Given that there is a push pull arrangement on the output meant to drive a speaker/headphones and the transformer etc - is this worth keeping? Or is there a better way?

Cheers!

https://www.imageupload.co.uk/image/DsuY

I'd say start with the easy thing and put jacks in and try it out as is. If you liked what you heard on the video you watched then you might as well go that route first and see what you have.

I can tell you that the original Fuzz Faces used PNP germanium transistors, which you have here. There's not much to that circuit - only 2 transistors if I recall correctly, but it's a bit of a crap shoot to pull old germaniums from gear and expect to throw together a great sounding Fuzz Face clone, because the circuit is dependent on the hfe values of each transistor being within a specific range, and of the leakage of them to be small. Germaniums are not as stable and predictable as silicon transistors and two samples of the same number may have vastly different hfe and leakage values. Most of the germanium transistors that were commonly used in old Fuzz Faces/Rangemasters, etc that are still available have been picked through by boutique pedal makers/DIYers and rejected for not having usable values in those circuits.

So try it as is first and then if you don't like it, you can pull the transistors and hope you hit a winner. A google search will give you the ideal ranges of hfe values for each transistor in a given circuit you might want to clone. It's a very popular DIY project, so there's how-to's all over the place.

BT

> jack in place of the tape head

Huh??

Tape play EQ is wonky, not what you want.

It _IS_ ready to rock. Put in RECORD. Go into the MIC jack. Come out of the EP jack. A 10 Ohm (part-Watt) resistor on the output may be wise. Adjust Volume as desired.

What you have there is essentially a "pignose" or "deacy" amplifier.



I would definitely keep the output transformer and speaker. Just disconnect the motor minimize electrical noise. But the input impedance isn't going to be high enough. If you add a coupling cap and a silicon PNP voltage follower (note the supply is negative and not positive) it will make a HUGE difference.

This amp could sound v. good. If you upgrade to a 5" bookshelf speaker it could put out some bass too.

Note that if you heat the lead on a germanium transistor for more than ~5 seconds it will totally destroy it.

> input impedance isn't going to be high enough

10K will let much bass/mid through, muffle the treble.

This is often "good" for a guitar distortion. I'd start no-change.

PRR said:

> input impedance isn't going to be high enough

10K will let much bass/mid through, muffle the treble.

This is often "good" for a guitar distortion. I'd start no-change.

Click to expand...

The emitter of the input transistor is fully bypassed so the input Z is going to be pretty low.

But I agree that I would try it without changes. Then I would try high Z to hear which is better.

With the motor disconnected it would have plenty of power for an op amp.

PRR said:

> input impedance isn't going to be high enough

10K will let much bass/mid through, muffle the treble.

This is often "good" for a guitar distortion. I'd start no-change.

Click to expand...

Ya!  the high end is not good.

hitchhiker said:

Ya!  the high end is not good.

Click to expand...

Change the R2 resistor for 1Mega, see if you like it better

Thankyou all for your replies and suggestions - I've been a bit busy this week and not had time to experiment yet, but I'm taking it all onboard.

The play/rec head had to come off to free the circuit from the case, but can be reattached if need be - I had always figured (from reading about people who made walkman distortions etc years back) that the head was the place to stick the input due to the heads being high z. Not sure that's actually the case, though. The wonky eq might not be a bad thing considering the purpose is to change the sound in a radical way, but it's a very good point. It might be a good idea to have both options - wonky rec eq and wonky playback eq

Just to reiterate, I'm not a guitarist but I play bass guitar and synthesizers. So high z in would probably be desirable for bass pickups, but synth can be lower. Going off the eurorack modular 'standard' I believe things shoot for 100k in and 10k out. My bigger concern with just sticking a signal through this was whether I would get a full range frequency response adequate for synth, and whether it would be able to handle the lower bass frequencies. And subsequently, would that 8ohm speaker/headphone out be useless at driving another high z pedal, or an amp, or a line input? I imagine a DI following it would be wise, unless I add something in after the transformer.

In terms of power (not sure how clear it is from the schem) the amp circuit uses a 9v battery, and the motor uses a separate supply of 2x 1.5v batteries. I'm only interested in that 9v, so a PP3 or a DC in should suffice.

When I get time I intend to do as suggested - power it, and stick some things through it to see how it sounds. From there if I can make it work, I can experiment with adding other interesting elements to the circuit (tonestack perhaps, some other pots)

Thanks again and of course, all suggestions are welcome I have two (identical) so all is not lost if something goes awry

So last night, I had a few minutes to quickly throw together a test. The original 9v battery leads had been snipped, so I replaced that and put 6.35 jacks in place of the mic and earphone, leaving the play/red head and speaker disconnected.

I first ran my bass guitar through it, straight into my bass amp (2x10 + tweeter) and was really impressed - it has a great overdriven, saturated sound that can be subtle, but still sounds great even when pushed a bit harder. The only problem was that there was a noticeable reduction in low end which felt a little bit thinner than I'd like. It still sounded great, but I would like to keep some of the thump.

Then I tried running my synth, a DSI Prophet 6, through it and again into the amp. This was a bit more tricky, and was harder to dial in a sound that worked.  Tweaking the synth output level helped, but it was hard to nail it just right. Too little volume from the synth rolled the low end off, a tad too much volume on the amp output shook the walls and the volume control in the circuit was a little sketchy. The line level obviously needs a bit of padding to work correctly - I have a reamp box that I will do some more testing with.

I'm also going to try running it into a DI box and a good mic pre, to see how it sounds recorded directly.

I've whittled the circuit down a bit more, playing with the gain staging can get some good sounds from different sources. It still feels light on bass, which I expected - but is there any way I can counteract this, obvious from the schematic? Would making R2 bigger as suggested earlier improve this? I can't see much high pass going on, but I'm curious about whether C4 going into the VR volume control is acting as a high pass when rolled off. To keep the sound relatively 'clean' the pot is staying fairly low in it's travel, and I'm curious whether that could be responsible for thinning it a bit. Of course I expect the teeny tiny transformers are also to blame, but I'd like to be able to get some of that depth back, or at the very least to be able to dial it in. I'm also curious whereabouts I could stick another pot in to let me fine tune things a bit.

I also noticed, the small cap across the T1 primary doesn't appear to be present on mine.

Another option might be to split the signal and mix the distorted, bass-light signal back into the original. Any ideas?

Cheers!