Some interesting discoveries were made during the course of our research, development and playing a whole bunch of the original Marshall JTM-45 amps to help us achieve our benchmark ToneClone transformer series for this mighty amp.
Many thanks also to our player and collector friends for loaning us their amps and offering their insight for the cause. Many hours of tasty licks included. Incidentally, we do actually own one of the first coffin logo offset chassis amplifiers built by Mr. Marshall and crew.
There are more JTM-45 amps built using EL34 tubes than with the legendary KT66 tubes and there is nothing wrong with that. But a change of tube type also requires swapping out the output transformer to get the correct operating impedance and tone.
All of the original (limited run) handmade amps had 6L6, 5881 or KT66 tubes connected to a Radiospares “De Luxe” output transformers. The “De Luxe” model being the premium offering followed down by the “Heavy Duty” and “Hygrade” models. Primary impedance was selected at 6.6K.
The Bluesbreaker’s legendary tone was made by using KT66 tubes and Radiospares (RS) premium transformers. The transformers did most of the work here with their unique high primary inductance and low leakage inductance combined with padded (surplus) primary impedance as compared to Fenders of the time.
Note: With the exception of the transformers the JTM was a copy of the Bassman. An interesting discovery came to our attention here. It seems that most of the amps made during that era had the output leads connected in reverse phase (backwards). Was it by intent or a happy accident? We and a number of pro players seemed to prefer the reverse phase connection after many A/B tests. We had the impression that the speaker cabs sounded better, more articulate.
When the JTM-45 went into full production, the KT66 tubes were switched for EL34s and the Radiospares transformers were replaced with Drake transformers (judging by build quality and style). All to save BIG on costs since at the time Marshall was being hammered by a distribution deal that shot up retail prices for his amps right through the roof (a deal Jim Marshall later regretted). The EL34s were decent sounding tubes, but the new transformers (not so Bluesbreaker-ish) bore no resemblance to the original RS designs. These simplified transformers had a primary impedance of 3.4K with a much more simple (lower labor cost) design.
The bonus here, though, was in the easier to hook-up speaker connections for the player on the road, which also made it more difficult to connect speakers to the amp in reverse.
Another interesting piece of historical data is that when the EL-34 started earning the reputation as an affordable, reliable and great-sounding tube, the people at Radiospares added it to their compatible list of tubes for the RS “De-Luxe” output without changing the 6.6K impedance. And the EL34 still sounds great through the RS output transformer!!
Source: https://mercurymagnetics.com/pages/_misc/FAQ.htm#EARLY_MARSHALLS
