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Want more high frequency extension

I like my current set up, however, I find the high frequency extension to be lacking. Any thoughts on how I can achieve more extension? I was thinking one of two areas, preamp change or tube rolling in my amps.

I currently have an Audible Illusions L-1 with Bugle Boy 6922 tubes. I was thinking of maybe trying a passive pre like a Placette or trying a SS pre. thoughts?

Alternately, I was thinking of rolling out the stock tubes in the Six Pac's. Right now they are Rubey Tubes El34's for the output tubes, Electro Harmonix EL84EH for the current source, and Westinghouse 12BZ7 for the gain/phase inverter. The amps will also take KT-66, KT-88, 6550C, KT90 and KT-99 in the output stage and 12AY7 and 12AX7 for gain/phase. Any tube replacement that will provide more extension?

Thank you in advance
mattybumpkin

Showing 4 responses by eldartford

eldartford
4,262 posts
 
It's hard to believe that the tubes would limit HF response...after all tubes are (or were) used for RF amplification. In a tube power amp the output transformer is the component that limits HF response, and a large amount of feedback around the transformer (often provided by dedicated windings connected to secondary grids of the output tubes) is necessary to provide flat response over the generally-accepted audio range. If your amp is a "low feedback" design the trade-off is limited frequency response.

Also, why not try a supertweeter? It could be your speakers. Or your ageing ears.
eldartford
4,262 posts
 
Mattybumpkin...If amplification is the problem I really think the power amp is probably the place to look. EL34 and KT88 tubes are pentodes, and I bet that there is local feedback from the transformer to the output tubes. "Zero Feedback" probably refers to "global" feedback.

I suggest that you try a SS amp, or one of a few tube amps that are flat to 100KHz (but these are not low feedback).

For some things, 39 is over the hill.
eldartford
4,262 posts
 
Trelja...While I cannot disagree with your preference for low (or zero) feedback amps, I have difficulty believing that the difference you hear derives from greater bandwidth. Reduction of feedback will change the sound so that it reflects the characteristics of the amplifier, (most likely the output transformer) rather than the input signal. I would expect the bandwidth to be less with low feedback, and this may emphasize harmonics in the 12 to 18 KHz range that you may find pleasing.
The ability to vary the feedback is interesting. Do you have, or can you generate frequency response plots for various degrees of feedback?

Also, is the amp really "zero" feedback? No local feedback around stages, or "ultralinear" transformer windings driving output tube grids?

A schematic would be nice.
eldartford
4,262 posts
 
Mattybumpkin...A word of encouragement!

You, like eveyone else, have lost, or will lose high frequency hearing. When tests are run, (at least on me) sine waves of various frequencies are used. I cannot hear a sine wave much over 10 or 12 KHz, but I can clearly detect the action of a supertweeter or a filter operating up around 20KHz. My theory to explain this is that the ear senses rate of change of pressure as well as pressure change, and the steep wavefront of the HF sound is sensed, even if the frequency is not. Hey..it works for me.

Also, loss of HF hearing does not, at least for me, affect spatial perception, eg: imaging.
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