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Rola 33C Mk II Amplifier Unit

Mk II Amp front

Recently I acquired this unit and for it's age it is in quite good condition. Basically it is a replay amplifer for Mk II Series tape machines. As there was no manufacturer's label on it the original use of this particular was unclear. The circuit connections and the unit's condition when I received it may hold the key to this question, but more about that later.

I was supplied doumentation with this amplifier but as it was for a Rola 66 Mk II and of no use. This is the first unit of this type I have had and during all the years of collecting and I've never turned up any documemtation. After some investigation it was obvious that the circuit of this amplifier is almost identical to the the replay chain used in the Byer/Rola 77 Mk II.

The important differences are:
1. Higher high tension supply is used, +320V dc instead of +260V dc.
2. Two E80F special quality pentodes are used instead of EF86 pentodes.

Unlike other Byer/Rola equipment there is no screen printing on the chassis near the valve sockets to indicate the valve types to be used. I'm assuming that the use of E80F is correct as they came with the unit whereas there were no 12AX7 and 12AU7 valves plugged in. It is likely that someone would pull the 12AX7 and 12AU7 to use elsewhere but a special quality valve like the E80F which is a very close equivalent to an EF86 wasn't removed because it wasn't an EF86.

Rear of the amplifier chassis

Mk II Amp rear

Underside of the amplifier chassis

Mk II Amp underside

The image of the underside of the chassis has been overlaid with component names for easy cross-referencing with the circuit schematic which I've traced form the unit and cross checked with the replay circuit in the Byer 77 Mk II manual. Until some orginal documentation for this replay amplifier is found my schematic is all that is available.

Now returning to my thoughts about the about the original function of this particular unit. From the literature of the time there were two distinct functions of a replay only amplifier. The first function mentioned in the Byer and Rola 66 Mk II operation manuals, a "Simultaneous Replay Monitor Amplifier", available as an accessory which was used for replay monitoring while the 66 Mk II is in RECORD mode. Initially I thought that this is what my unit was used for as it came with a three core mains lead, and not the 8 way cable for connection to the tape transport. Upon further investigation I've come to the conclusion that later on it may have been used as a simultaneous replay monitor amplifier but I think originally it was used as part of a replay only tape machine in a Rola Auto Station. The reason I believe this comes from some detective work, the power transformer has the necessary 18V winding for the transport control circuits and the terminal strip connections for the 18V windings were not clean of solder, rather they had obviously had some wiring disconnected at some stage. My guess is that originally there was a cable to connect with a 66 Mk II transport so I've included that cable in the schematic. Also, the picture of the amplifier chassis of the of the Rola 33C looks identical to this unit.