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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…

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skucie
57 posts
 

As I understand it, the only risk of going below that 20K amplifier input impedance is that the bass may roll off prematurely.

I do not believe it's related to your power tube issue.

lewm
13,336 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. But 1:10 does not represent a wall or cut-off point, the fraction of signal energy transferred from one component to the next is a continuum on that ratio. 1:10 usually preserves more than 90% of the signal energy. It goes down from there as the two impedances become more and more equal. At 1:1 ratio, 50% of signal energy would be lost to ground. In the case of these two components, I doubt that the problem with the Voltage Regulator tube has anything to do with the impedance match, which is actually acceptable, even at 50Hz.

ghdprentice
7,320 posts
 

Definitely contact Audio Research… last time I had a question I called. They were very helpful. I don’t remember the tech guys name now.

I owned a REF 5SE… I changed one tube after 2,000 hours only because it was recommended. I upgraded to the REF 6SE before any other tubes needed changing.

Definitly something is wrong.

dover
2,265 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.

itsjustme
682 posts
 

what gs5556 said.

1V/15k ohms = .00007 amps = .07mA = teensy weensy

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