Introduction
Perhaps the most critical part of any audio tape deck is the tape head assembly. The tape head is the magnetic pickup that transfers the audio signal to the tape and back. A worn or otherwise defective tape head can cause poor audio recording, with lack of high frequencies (usually) or possibly no sound whatsoever out of one or both channels.
This article will address how a tape head can be defective, and how to potentially determine if a tape head is worn, or worn out when auditioning a reel to reel deck prior to purchasing.
**Note that for this article, we assume that the entire tape path has been cleaned. See our ātape path cleaningā article here***:
https://reeltoreeltech.com/reel-to-reel-head-and-tape-path-cleaning/
How a Tape Head works
Without getting into deep magnetic tape theory, a tape head consists of a tiny coil of wire, one for each channel, embedded within a small sealed housing. When an audio signal is applied to the head in record mode, the tape head transfers the audio signal into magnetic pulses, which is then recorded onto the tape. In playback, the reverse happens, the magnetic pulses are picked up by the tape head, and turned into a small alternating current, which is then turned into audio through the use of a high gain amplifier/preamplifier located within the tape deck.
In a standard stereo Ā¼ track tape head using Ā¼ā wide tape, there are two tracks in the forward direction, and two in the reverse direction. Therefore, the width of each track is roughly 1/16 of an inch. The actual width of each audio track is actually slightly less than 1/16 of an inch, as there needs to be a tiny space between tracks that is not recorded onto, so that adjacent tracks arenāt picked up during playback or recording.
Given the narrow spacing between tracks, and the narrow width of each track, the proper alignment between the recording tape and the record/play head is critical. Any deviation from a perfect alignment will result in a less than stellar audio recording or playback. Things get even more critical when it comes to cassette decks, since a cassette deck uses 1/8ā width tape rather than the Ā¼ā of a standard consumer reel to reel.
When new,Ā the tape head(s) have a nice rounded taper, as roughly illustrated below, and the tape moves across the head, making even contact with the tip of the head.
A typical reel to reel deck uses the head casing made of steel. As many thousands of feet is run across the tape head, the tape wears on the steel, eventually causing a flat spot on the front of Ā the head
The picture above shows a Ā½ track (2 track) head, with a Ā very large flat spot on the front center of the head. The head no longer has a rounded tip on it, instead, the tape is dragged over this large flat spot, causing a lack of fidelity.
This head also shows two additional problems:
At the top of the tape path, thereās a distinct ridge at the top of the head caused by countless hours of tape running. When the tape now moves over this head, itās possible for the top edge of the tape to move in and out of this ridge, which will cause high frequency and level variations on the left channel. On this head, the right channel is the bottom track, and with a Ā½ track head, assuming the same groove is worn into the bottom portion of the head, the right channel volume and high frequency fluctuations will also be affected.
The other problem with this head is that the right channel (bottom track) has been worn right down to the iron pole that the head coil winding sits on. Once the tape head has a visible gap, very low or no audio will be heard on that channel.
The above head is well beyond saving, or any potential use. The deck likely came from a radio station where it was heavily used.
Hereās more examples of bad heads:
Sony 4 channel head, showing a wide flat spot on the head, and weāve used black Sharpie to show where thereās a visible gap on channel 2, causing low audio.
The same 4 channel Sony head without Sharpie, showing a wide flat spot, but one that is a bit tapered, with the flat spot being wider at the top of the head than at the bottom. This indicates that the tape path is likely mis-adjusted as well, with a bit more tape tension at the top than the bottom, causing the uneven head wear.
A worn out full track mono tape head. Old tape oxide aside, this head also clearly shows the ridge and large flat spot that the tape movement caused over time, and a slightly wider wear pattern at the top of the head compared to the bottom.
Yet another completely worn out Teac Ā½ track head out of a Tascam 32 deck. Both channels of heads are worn down to the gap, with absolutely no audio on the left (top) channel, and attenuated audio in the right channel (thereās still a little bit of metal left on the right channel, so likely youād hear a bit of audio.
Tape Head Life
Thatās the million dollar question, isnāt it? Heads do start to wear with the first foot of tape run through the deck, and generally speaking, the Ā heads are rated at 2,000 hours of use. After researching online, there seems to be no set hard rule as to those 2000 hours. Is that the point where you start to hear audio fluctuations? Is that the typical point where the head gap opens? We couldnāt find a hard and fast answer to that online.
Thereās exceptions to the 2000 hour rule. The first are the older tape decks from the 1950s and 1960s, specifically the older Sony machines. Those heads are known to use soft metal, and I estimate that those head ratings are closer to 500 useful hours before they wear out. Ditto for many other similar consumer machines from the 1960s and early 1970s.
The other exception are ferrite heads, and the āglassā GX series heads that Akai came out with in the early 1970s, and continued through the last tape deck that they made, in the mid 1980s. Here in Canada, the GX glass head life was advertised as ā24/7 play for 17 Ā½ yearsā, or in the US, it was advertised as 150,000 hours life span.
Indeed, both ferrite heads that Sony and Teac (late models) used amongst other manufacturers, and the Akai glass heads (which were a variation of the ferrite heads), do indeed last a lot longer than a steel head used by Otari and Teac, however these longer lasting heads have their own unique problems that are harder to visually check for.
The above head came out of an Akai GX series of tape deck, likely a GX-280 or similar, circa 1973. To the eye, thereās no flat spot on the head, the curve is still nice and rounded, giving no physical indication of wear. These heads however, can develop microscopic pitting on the head surface, which causes a lack of high frequency response.
From our extensive servicing of all brands of decks, the earlier Akai GX series seem to be far more affected by this pitting than the later models. Weāve seen pitting numerous times on pre-1974 models of Akai, such as the GX-210, GX-220, GX-215 and GX-280. When you listen to the tape recording compared to the source, the tape sounds muffled, with the high frequency dropping off usually around 7-8Khz, whereas it should be flat to 20Khz. This pitting can affect one or both channels, or one channel may have more severe high frequency drop-off than the other. Head replacement is the only cure for a pitted head.
The later generation Akais, post 1975 seem to succumb to the pitting problem significantly less than the older models, however weāve seen both a late model Akai GX-635 and the top of the line GX-747 heads have poor frequency response. Given the number of working late model Akai decks out there however, this problem is rare.
Sony later model decks like the TC-765 and 766 (and other models using ferrite heads) can also have pitting of the heads, although as with the Akai models, this is relatively rare. Your only source for replacement heads is a donor deck, or eBay, as theyāve been discontinued decades ago.
Revox Heads
The Revox heads in the A77 models and later used fairly standard steel heads, however Revox manufactured their own heads, and did a couple of things differently than other manufacturers:
Revox cut a slot into the head, top and bottom of the tape path as shown below. This meant that a ridge could never form as the head wore down, and thus tape would never ride in and out of the ridge caused with conventional heads, extending the head life considerably
This innovation of the slot top and bottom in the heads caught on in the studio world, and other manufacturers followed suit. JRF Magnetics and other tape head vendors will cut a slot into the head of your choice if itās possible to be done.
-Secondly, the arc of the Revox head was far more gentle of an angle, resulting in longer life than other typical steel heads.
According to Revox and several other manufacturers, the width of the flat spot of a head can be measured in mm. Any flat spot wider than 4mm indicates a worn out head, and a flat spot less than 4mm indicates some usable life left. Most tape head suppliers will provide a form of āhead life leftā spec if a set of used heads is sent in for evaluation.
Electrical Faults in heads
Outside of the physical wear and tear on the tape head surface, the tiny wires that form the coil within the head can go open and is generally not repairable. If you donāt get continuity on any of the coil(s) embedded in the head, that head winding is open, and the head needs to be replaced. Depending on the design of the head, it can measure under 10 ohms, or over 300 with a standard ohmmeter. As long as each head winding measures closely toĀ the adjacent windings, the coils within the tape head should be in good shape.
Head Lapping
Most steel heads can be lapped, meaning the virgin metal area causing the ridge that the tape rides in and out of is removed, which brings the entire tape to head contact surface down to the worn area. Once the excess metal is removed, there is no more wear area, and the head contour returns to its original shape, restoring the tape to head contact, and thus the fidelity of the head. (Yes, to be technical, with the removal of the metal on the head, the frequency response is changed slightly, but this article isnāt meant to get that deeply into the theory of things. Thereās lots of articles online that dive deeply into tape heads, magnetism, and other technical details that are beyond the scope of this website.
While we lap our own Ā¼ā tape heads here in our shop, we would require your entire tape deck to realign the heads once they have been lapped.
Because itās easy to completely mess up a head when learning to lap them properly, we choose not to post that information on our website. We highly recommend that you look at the tech referral list on our website, and contact a company like JRF or JM Audio to do the work rather than try to do it yourself.