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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 1 response by gs5556

gs5556
1,242 posts
 

A loading impedance of 15K will not damage the preamp or its tubes. All it will do is impact the lower frequencies and you will not get optimal performance from the preamp. My guess is that AR uses a low value coupling cap on the output for sonic reasons and is using the 20K minimum input impedance of the connected amp to make up for it.

 

Tubes going bad could be a symptom of high heater voltage. Check your wall voltage. If it's greater than 120V it could be a problem for tube gear, especially if their transformer specs are based on a 115 volt primary.

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