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10-08-2018, 12:29 AM
I have all of the TUT books but am still confused when it comes to matching Primary Impedance on Output Transformers to suit the particular tubes being used. Furthermore, how important is it and what difference does it make?
In regard to the construction projects in TUT 3 & 5 the most common specified output transformer for 50 watt power amps is a Hammond 1650KA. This is being driven from a 4x octal tube complement. The transformer primary impedance rating is 3,400 ohms with taps for 4,8,16 ohm speaker outputs. You can find this transformer on the Hammond website under the general heading: Push Pull "Ultra Linear" HI-FI and it specifies for use with 4 Tubes (6L6GC, 807, 5881, EL34, 6146B, 6550B). So far all good and it seems that this transformer is perfectly matched to a 4x Tube complement.
However, if you look further on the Hammond website and look at a direct Marshall JCM800 replacement output transformer for 50 watt you get the recommended 1750N. This has a primary impedance of 3,200 ohm. But this is driving only 2x tubes! Why is it so low? Shouldn't it be higher at around 5000 to 6000 ohms if driving only 2 tubes? It is interesting to note that the 100watt JCM800 replacement transformer (Model No. 1750U) driving 4 tubes has a primary impedance of 1,700 ohms. Do Marshall deliberately mismatch their primary impedances which gives them the sound they want? How important is it to match impedances and what is the ideal impedance for a octal tube complement?
If anyone can answer my questions and explain this confusion I'd be very grateful.
In regard to the construction projects in TUT 3 & 5 the most common specified output transformer for 50 watt power amps is a Hammond 1650KA. This is being driven from a 4x octal tube complement. The transformer primary impedance rating is 3,400 ohms with taps for 4,8,16 ohm speaker outputs. You can find this transformer on the Hammond website under the general heading: Push Pull "Ultra Linear" HI-FI and it specifies for use with 4 Tubes (6L6GC, 807, 5881, EL34, 6146B, 6550B). So far all good and it seems that this transformer is perfectly matched to a 4x Tube complement.
However, if you look further on the Hammond website and look at a direct Marshall JCM800 replacement output transformer for 50 watt you get the recommended 1750N. This has a primary impedance of 3,200 ohm. But this is driving only 2x tubes! Why is it so low? Shouldn't it be higher at around 5000 to 6000 ohms if driving only 2 tubes? It is interesting to note that the 100watt JCM800 replacement transformer (Model No. 1750U) driving 4 tubes has a primary impedance of 1,700 ohms. Do Marshall deliberately mismatch their primary impedances which gives them the sound they want? How important is it to match impedances and what is the ideal impedance for a octal tube complement?
If anyone can answer my questions and explain this confusion I'd be very grateful.
