Need help with tape head circuitry - UK Vintage Radio Repair and Restoration Discussion Forum

sourbiscuits · 26th Dec 2023, 6:41 pm

Hello everyone. I want to try and build my own stereo three head tape box - recreating an analog tape delay unit myself. I know that I will need one erase head, one recording head and one playback head to begin with. Obviously, a motor, but I don't want to worry about that just yet. I want to focus on the basic circuits that I will need for the three heads alone. I'm assuming I will need a preamp, and I need to generate bias signals, but since I'm very uneducated in this field, I thought I'd start off with a post here and take it from there. Any information will be appreciated!

paulsherwin · 26th Dec 2023, 7:14 pm

Start with a tape recorder circuit. You may be able to adapt the electronics scavenged from a scrap 3 head tape recorder.

G.Castle · 26th Dec 2023, 7:18 pm

Look up 'WEM Copycat" later transistor versions will give you good ideas.

sourbiscuits · 26th Dec 2023, 11:31 pm

Thank you Paul. I do have an old mono tape player/recorder/radio in one. I was wondering if I could use some of it's circuitry? I think the radio has it's own pcb apart from the tape segment. But the heads are mono and it's a two head setup,so I would think I'd have to build a dedicated pcb from scratch for the unit I have in mind. I actually got a response from another platform on this topic containing a very intriguing piece of literature. I am attaching it to this thread as it may be very handy to others.

http://www.tubebooks.org/books/burstein_tape.pdf

It's called "Elements of Tape Recorder Circuits" by Herman Burstein and Henry C. Pollak. It gives a thorough breakdown of separate components. I will go through that and probably ask questions based on the relevant information I find there.

The copycat is a great delay module, I know it, but never had the chance to test or own one. I want to potentially end up with something similar, but add some missing features.

ButchJames · 27th Dec 2023, 12:30 am

On the electronics alone, this is going to be complex.

Very basically ....

For record: You'll need to pre-amp and regulate and shape the input audio to the head - called pre-emphasis. This is to force a near constant audio signal current (hence constant mag flux) from the head to the tape with increasing frequency. Then of course you'll need a bias oscillator - a higher voltage RF signal upon which the audio 'sits'. This is used to push the audio signal into the head's linear flux-density vs current input region.

playback: You'll need a very sensitive/high-gain voltage pre-amp with a playback de-emphasis curve, with a corner-frequency that'll make the output sound correct, give or take.

As an engineer, even I would have to pull my socks up on this one, although I could do it.

Why re-invent the wheel?, true experts have already done this.

However, I do understand the excitement of making your own tape recorder from scratch. ;o)

G.Castle · 27th Dec 2023, 10:00 am

Did this as a project at college. Cassette versions are mechanically and quality limited.

A ¼" R2R chassis is a better basis, you can add outrigger heads or mechanical loop delay more easily: deciding on a tape transport mech. should be the first project decision of course.

Not easy.

If you go for VSD of the capstan drive for instance you then have to try to track your bias and emphasis if you want any pretence of quality.

This is a project that will be fun, but it's not a beginner level one for sure. You will get your analog audio engineer laurels if you succeed with it.

Good luck!

jamesperrett · 27th Dec 2023, 8:22 pm

My first attempt at tape electronics was the playback part of a Mullard design published in the 1972 Henrys Radio catalogue. I used the tape mechanism from a Brenell MkV for the heads and mechanics. Considering that many of the components were extracted from an old TV, I was amazed that it worked as well as it did!

If I was going to start on a project like yours, I would be looking at the service manuals for some of the classic old machines - and maybe the manuals for the various Roland Space Echo models. Some of the designs, like those on the Soundcraft multitrack machines, are relatively simple. The exact design will very much depend on what heads you can get hold of - especially on the record side where the inductance of the head is important.

26th Dec 2023, 6:41 pm	#1
sourbiscuits Pentode Join Date: Sep 2022 Location: Amsterdam, Netherlands. Posts: 124	Need help with tape head circuitry Hello everyone. I want to try and build my own stereo three head tape box - recreating an analog tape delay unit myself. I know that I will need one erase head, one recording head and one playback head to begin with. Obviously, a motor, but I don't want to worry about that just yet. I want to focus on the basic circuits that I will need for the three heads alone. I'm assuming I will need a preamp, and I need to generate bias signals, but since I'm very uneducated in this field, I thought I'd start off with a post here and take it from there. Any information will be appreciated!

26th Dec 2023, 7:14 pm	#2
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,992	Re: Need help with tape head circuitry Start with a tape recorder circuit. You may be able to adapt the electronics scavenged from a scrap 3 head tape recorder.

26th Dec 2023, 7:18 pm	#3
G.Castle Heptode Join Date: Dec 2020 Location: Swaffham, Norfolk, UK. Posts: 587	Re: Need help with tape head circuitry Look up 'WEM Copycat" later transistor versions will give you good ideas.

26th Dec 2023, 11:31 pm	#4
sourbiscuits Pentode Join Date: Sep 2022 Location: Amsterdam, Netherlands. Posts: 124	Re: Need help with tape head circuitry Thank you Paul. I do have an old mono tape player/recorder/radio in one. I was wondering if I could use some of it's circuitry? I think the radio has it's own pcb apart from the tape segment. But the heads are mono and it's a two head setup,so I would think I'd have to build a dedicated pcb from scratch for the unit I have in mind. I actually got a response from another platform on this topic containing a very intriguing piece of literature. I am attaching it to this thread as it may be very handy to others. http://www.tubebooks.org/books/burstein_tape.pdf It's called "Elements of Tape Recorder Circuits" by Herman Burstein and Henry C. Pollak. It gives a thorough breakdown of separate components. I will go through that and probably ask questions based on the relevant information I find there. The copycat is a great delay module, I know it, but never had the chance to test or own one. I want to potentially end up with something similar, but add some missing features. Last edited by sourbiscuits; 26th Dec 2023 at 11:36 pm.

27th Dec 2023, 12:30 am	#5
ButchJames Tetrode Join Date: Dec 2021 Location: Newcastle Under Lyme, Staffordshire, UK. Posts: 84	Re: Need help with tape head circuitry On the electronics alone, this is going to be complex. Very basically .... For record: You'll need to pre-amp and regulate and shape the input audio to the head - called pre-emphasis. This is to force a near constant audio signal current (hence constant mag flux) from the head to the tape with increasing frequency. Then of course you'll need a bias oscillator - a higher voltage RF signal upon which the audio 'sits'. This is used to push the audio signal into the head's linear flux-density vs current input region. playback: You'll need a very sensitive/high-gain voltage pre-amp with a playback de-emphasis curve, with a corner-frequency that'll make the output sound correct, give or take. As an engineer, even I would have to pull my socks up on this one, although I could do it. Why re-invent the wheel?, true experts have already done this. However, I do understand the excitement of making your own tape recorder from scratch. ;o) __________________ Interested in tape recording since about 1971. http://cassettedeckman.blogspot.com

27th Dec 2023, 10:00 am	#6
G.Castle Heptode Join Date: Dec 2020 Location: Swaffham, Norfolk, UK. Posts: 587	Re: Need help with tape head circuitry Did this as a project at college. Cassette versions are mechanically and quality limited. A ¼" R2R chassis is a better basis, you can add outrigger heads or mechanical loop delay more easily: deciding on a tape transport mech. should be the first project decision of course. Not easy. If you go for VSD of the capstan drive for instance you then have to try to track your bias and emphasis if you want any pretence of quality. This is a project that will be fun, but it's not a beginner level one for sure. You will get your analog audio engineer laurels if you succeed with it. Good luck!

27th Dec 2023, 8:22 pm	#7
jamesperrett Octode Join Date: May 2007 Location: Liss, Hampshire, UK. Posts: 1,876	Re: Need help with tape head circuitry My first attempt at tape electronics was the playback part of a Mullard design published in the 1972 Henrys Radio catalogue. I used the tape mechanism from a Brenell MkV for the heads and mechanics. Considering that many of the components were extracted from an old TV, I was amazed that it worked as well as it did! If I was going to start on a project like yours, I would be looking at the service manuals for some of the classic old machines - and maybe the manuals for the various Roland Space Echo models. Some of the designs, like those on the Soundcraft multitrack machines, are relatively simple. The exact design will very much depend on what heads you can get hold of - especially on the record side where the inductance of the head is important.