Reels are wound on plastic or aluminium flange which diameter ranges from 3.1/4 to 10.1/2” for different playing time. Table below shows recording time referred to different speeds for standard and long, or extended, play tapes (one side only). You can use the table to covert the units of measure between the metric system and the Anglo-Saxon system and vice versa too.


Double (and triple too) play tapes with reduced thickness were available at the same reel diameter. They were more fragile and deformable than standard and long playing tapes and were not used in professional field.

From the left: 3.1/4" (3M) - 5"(BASF) - 7"(SCOTCH) - 10.1/2" (REVOX) CINE - 10.1/2" (AMPEX) NAB

There are three different types of reel supports and locking systems:

• the standard domestic CINE from 3.1/4" up to 10.1/2" reels;
• NAB one for 10.1/2" reels for domestic and professional use. Reel to reel recorders always have the reel support plates with a three splines shaft with rotating upper part to lock both Cine reel or Nab adapter;
• In AEG DIN support, available for studio and professional use only, tape is wound without a flange on a self support metallic core (the so called “bobby”) and locked on an open plate ("pancake") not compatible with Cine or Nab locking system (but easily interchangeable with them changing the upper part of the recorder reel supports in few minutes).

Professional studio recorders can use larger (14") pancake plates for very high speed (30 ips) recording or longer recording time at standard speed. (see photo below).

On the left: a 10.1/2" pancake plate fitting standard CINE locking system for domestic and semi professional recorders (as Revox, Teac, etc) On the right: a 14" professional pancake plate with AEG DIN locking system for studio professional recorder (as Studer, Telefunken, Ampex, etc)

• use large reels or professional pancakes in domestic recorder can seriously damage recoreder mechanical parts; 
• mix different kind of reel (plastic and metallic) and/or different diameter (5" and 10.1/2" i.e.) can damage both tape and mechanical reel parts except in some professional recorder which can manage different kind of reel at same time (i.e. some Otari models, Nagra T Audio, etc.)
  

In few words there are two kinds of tape: obsolete acetate tape and new polyester ones. They can simply recognize acetate tapes from polyester ones watching the reel in backlight: the acetate tape will look transparent. Older polyester tapes had shiny reverse side and matt magnetic coated surface; newer ones (starting from 1970 about till today) had black matt reverse side and magnetic coated shiny surface. In every kind of tapes the magnetic coated side has to face the heads.

Knowing the right kind of tape you are working on is very important in restoration to avoid damages due to wrong procedure. Different kind of tapes have different physical and chemistries characteristics and have to be treated in different ways.

Photo: courtesy by Eddie Ciletti www.tangible-technology.com

Over the years different kinds of metal oxide were tested and used to improve tape quality reducing the signal noise ratio, print trough, distortion and improve dynamic. Each tape was slightly different from the other and every manufacturer suggested the different BIAS and level settings better fit their tape.

Basically speaking the so called BIAS, in magnetic recording field, is an high frequency signal (usually from 100,000 to 150,000 hz about or higher) recorded on the tape at the same time of musical signal. This principle, discovered in 1940 by accident, dramatically improves distortion and high frequencies response giving a very high quality recording.
The Bias frequency is even the same but its level has to be adjusted according to different kind of tapes and speed (see table below) for the best MOL (Maximum Output Level) with less THD (Total Harmonic Distortion).
This is why the reel to reel recorder has to be aligned according to the kind of tape used for the best recording result.
Table shows BIAS recommended setting reffered to Studer rec and play heads. It has to be used as guide only not as absolute reference for all the open reel recorders.