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Voltage regulators and input impedance

I have an Audio Research Ref 5SE pre running into a Modwright KWA 100SE power amp. The input impedance of the Modwright is listed as “15K at 50 Hz” and the specs on the Ref 5 suggest “20K ohms minimum load”. 
 

So here is my question: I have gone through 3 6550 tubes in the Ref 5 (which AR says is used as a “voltage regulator” in the power supply) in the past 3 years - sovtek, tung sol, svetlana, all new. The last one lasted under 500 hours. I am thinking of potential causes, including excessive heat (the location needs better ventilation), something wrong with the preamp, etc, but I am wondering about that input impedance on the power amp. Am I running the Ref 5 into too little of a load? Would that put undue strain on the power supply tube, causing it to fail prematurely? Would love to hear from someone who knows this stuff better than I…

ilikemiles

Showing 2 responses by dover

dover
2,299 posts
 

Why don't you fire off an email to Audio Research - their customer service is exemplary.

Also check with Modwright - it may be very easy for a local tech to increase the input impedance of your power amp to 20k without impacting the amp. If this is possible the overall sound may be substantially improved through better matching.

dover
2,299 posts
 

Well, it’s not that the bass will roll off, although I guess it would at a barely measurable rate since the midrange input Z is stated to be 23K ohms, as that once you get below a ratio of 1:10 for output Z vs input Z, you start to lose more signal voltage to ground, which could eventually, as the ratio gets progressively lower, result in noticeable loss of gain.

That is absolutely correct - when I distributed tube gear in the 80’s we provided a modification service to customers power amps to increase the input impedance to provide an optimum load to match the tube preamps - a significant benefit in terms of "apparent gain" and overall sound quality.

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