Many of the Custom Series used 7189A's as the power tubes. These are similar to 6BQ5/EL84s, but the max plate voltage rating is 440V instead of the 6BQ5's 300V (or 415V in ultra-linear applications). Magnatone engineers took advantage of this, and in most Magnatone applications, plate voltage is well over the 6BQ5 limit. The 7189A was introduced in late 1959 and was designed specifically for high end audio use. The datasheet details use in ultra-linear applications (which Magnatone used).
It should be noted that besides the 7189A, there is a 7189. The 7189A was a design improvement over the 7189. The most significant design improvement of the 7189A was General Electric's adoption of the use of a new composite material made possible through a new process called Explosive Forming. This allowed for improved tube stability at higher operating voltages, and this the 7189A had higher Grid No.2 voltage rating (also, the 7189A made the the internal connection between Pin No.1 and No.6 that the 6BQ5 and 7189 did not have). See wikipedia's article on Explosive Forming for more on this topic.
The 7189 without the "A" suffix works in Magnatone amps, so using 7189's when 7189A's aren't available is acceptable (see basing below).
6BQ5's are still in production and are very cheap. It is common to find 6BQ5's in a Magnatone where the 7189A was originally specified, however most Magnatone's have plate voltages that exceed the 6BQ5 specification. Mixed results are reported. Running a tube at higher voltages than specified can result, not only in tube failure, but internal shorts that can over-draw current. The time between this happening and the fuse blowing is the danger zone wherein transformers fail. Proceed at your own risk with 6BQ5s!
RadioMuseum indicates that the 6BQ5B is identical to the 7189 (not 7189A).
There is also a "high voltage EL84" that was made by Reflector in ex-USSR states that is a military tube. These "EL84M" tubes (AKA the 6P14P-EV) have a higher plate voltage that can be used where 7189A's are specified (see basing below).
Basing: 9CV vs. 9LE
All tubes have a basing specification, or pin-out code. This code can beused to see what tubes can be interchanged, at least as far as the pins are concerned. For the 7189A, the code is 9LE, for the 7189/6BQ5/EL85/EL84M, it is 9CV. The unique 9LE base is similar to the 9CV, but on some Magnatones, the difference might prevent interchangability.
The difference is that the 9LE (7189A) internally connects both pins 1 and 2 to the control grid, and the 9CV (7189) only connects pin 2 to the control grid. Similiarly, the 9LE internally connects both pins 6 and 9 to the screen grid, and the 9CV only connects pin 9 to the screen grid.
On early amps, Magnatone used pin 2 for the control grid, and pin 6 for the screen grid. This will prevent a 9CV tube (7189) from working in the amp. On later amps, Magnatone continued to use pin 2 for the control grid, however the jumpered pin 9 to pin 6 on the socket, so both 9Le and 9CV basing code tubes would work.
Magnatone made 5000+ of the amps between 1963 and 1968, across two different plants and several engineers and wiring technicians. Its possible that only pin 1 was used for the control grid on some amps, and its possible that some later amps didn't have the 6/9 pin jumper. You (or your amp tech) will need to look at your sockets. Please contact me if you find variations so I can improve this page.
Using 7189/EL84M in place of the 7189A:
A 9LE socket can easily be wired to accept a 9CV tube. There isn't any re-wiring to do, only installing jumper wires on unused socket pin leads. If pin 9 is not connected, install a jumper between pin 9 and pin 6. If pin 2 is not connected, install a jumper between pin 1 and pin 2.
Below left is an M10A with a jumper wire between 6 and 9, to the right is an early M14 where the 9 is not jumpered. On the M10A, the cloth covered wires from the ultra-linear transformer are faded, orange is to the left, and yellow is the right. The same wires on the M14 (right), are connected to pin 6. In both pictures, you can see pin 1 is unused. (right click on pictures to view larger images).
Thanks to Keith Lynch for submitting corrections to errors I had previously made about the 7189A nomenclature.
Also see Duncan Amps - 7189A