Output Transformer Primary Impedance Affect on Tone

[makinrose]

Hi Guys

This is an old thread but a very important point was left out with respect to other factors that make hearing the different OTs easier: bias method and tube idle dissipation, and tube type.

The OP said he was using a Fender 5E3 to try out different output transformers. The 5E3 is cathode-biased which makes the tubes run hot. This heat can stress the screens of the tube especially if the screen is physically a grid rather than beam-forming plates. Even with the latter, too much heat can cause thermal distortion of the signal going through the tube.

Being able to reduce the idle heat will reduce the smearing of the sound as the amp is driven harder. The easiest way to control the tube idle is with fixed-biasing that has adjustable bias (not a contradiction).

So, having a lower idle heat reduces thermal distortion and will allow the OT swaps to be more audible. Using fixed-bias allows this to be easier. Using more neutral tubes will keep the subtlety of the OT tones from being swamped by the tube character.

Of course, you may want to use cathode-bias and/or a "sonicly rich" tube type in which case assessing the OT differences is more difficult and may result in no audible difference. That could lead you to having an incorrect conclusion regarding the efficacy of the method if you do not consider the context of your test.

Personally, I find the "beam-forming plate" tubes to be the cleanest and most neutral sounding. KTs and other gridded screen structure tubes have a distinct tone of their own.

You make an excellent point.  In the past few years I have found that majority of 5E3 and similar hot cathode biased builds that came in my shop using the usual Rk values are actually dissipating well above 100% with most tube samples.  There is muddiness to the sound.    With the Tweed Deluxe I think this partially because Fender never amended their Rk values as plate voltages rose in their amps.  Fenders first Deluxes ran about 350 VDC on the plates---by the end production the plate voltages were about 415 VDC.  Guys building kits tend to copy this. 

I've increased the value of the Rk in my own builds and they do sound much clearer.  After doing so I heard more of difference when accessing other components.