Voltage regulators and input impedance

At either 15kOhms or 20 kOhms your current is extremely small. Take a 1Vrms signal your current is only 0.05 or 0.07 milliamps respectively.

Trust me, your amp’s input impedance is not causing your tubes to burn out because of maybe 20 microamps more load.  It is much more likely however to soften the high frequency response a little.

Perhaps there’s a part failure or a bad solder joint in the VR circuit that results in excessive plate dissipation ( too much wattage) across the 6550. You need to have it examined by a competent tech or by ARC directly.

My guess is the impedance issue is a red herring so far as the VR tube failures are concerned. I think you wrote above that ARC specifies an input impedance of not less than 20 K ohms. This suggests that the output impedance of the pre-amplifier is around 200 ohms. (I didn’t look it up.) An actual 15 K input impedance, even in the low bass frequencies, is not going to over stress the preamplifier in any way. And anyway the input Z of the Modwright rises to 23K ohms at 1 kHz which sneaks in over the lower limit.

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.

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.

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.