Hi Leon; they aren't octals ... noval 9 pins. The Thunder 1 10" and 12" models and the Thunder 1 RVT are all powered with twin 6GW8s / ECL86s. It and the more common 6BM8/ECL82 are two tubes in the same bottle; a triode, exactly like half of a 12A_7, and a 'sharp cut-off' pentode power section -
6GW8 tube data sheet. Because of the manufacturing economics in the two-tubes-in-one-bottle format, both the 6GW8 and the 6BM8 were particularly popular in European table radios. As a result, Mullard, Siemens, Telefunken, Tungsram .. all the big Euro mfrs made the tube and, in my opinion, the best-sounding versions are used European versions.
Guild chose the sister 6BM8 tube in the stand-alone reverb amp in the RVT models while using the 6GW8 for power. I think whoever designed the T1 / RVTs was after a dry, tight tone and thought the 6GW8 better suited. In the late 50s/early 60s, the 6BM8 was used in Gibson's GA1RT,
this beautiful GA8T, and Guild's RC30 remote reverb amp (functionally comparable to the Gibson GA1RT). Guild's T1 RVT reverb amp is a near part-for-part dupe of the GA1RT circuit. Univox later borrowed or licensed the GA8T / 6BM8 circuit in its U45B Jimmy 'Stairway' Page amp and 6BM8s appear in the Hilgen Meteor, another dupe of the GA8T circuit. Despite the similarities, the 6GW8 and 6BM8 are not drop-in substitutes. Can be done but must disconnect / relocate / reconnect 4 of 8 pins on each tube. John