What is the “World’s Best Cartridge”?

I have one output tube located in the R front corner of on one monoblock that keeps failing in the red glow death mode. This has happened 3 times over the course of years. But of those 3 incidents, two have occurred in the last 6 months. Last time on the bench, I naturally checked the potentiometer that controls bias and the upstream bias voltage circuit and found no problems. Moreover, it worked for 6 months between then and now. Two nights ago, after a 90-minute listening session, I heard crackling on the R speaker and then saw that the tube in that R front position was glowing red. Plus the CCS tube for the input dual differential cascode stage was also glowing red. (Never saw that before.) If you have a genius idea how the input stage CCS could affect bias on only one of 4 output tubes (7241s), I am all ears. (My guess is these were unrelated events.) But otherwise, I have had no problems with 7241s per se. I bias them conservatively at 400mA each. As you know, my amps permit separate control of bias for each output tube, and maybe that’s why I in general do get away with using the 7241. Sound marvelous.

I apologize for the off-topic post, but Ralph is my guru, and he did ask.

@lewm , Good luck getting it straightened out. I am still waiting PATIENTLY for my MA 2s, but I did throw a monkey wrench into the works by asking for a 12 volt trigger on the amps. I think the newer amps bias automatically. I always keep a full set of tubes in reserve. Being neurotic, when one blows I'll change them all. 

Fortunately, the Seta L plus has a front panel gain switch as well as a switch to toggle back and forth between current and voltage mode. But, there is one issue that is a PITA. The flat signal has separate output jacks from the RIAA analog corrected signal. To switch back and forth requires getting at the back panel and moving the XLRs. This makes it more difficult to A/B digital and analog RIAA correction. 

If you have a genius idea how the input stage CCS could affect bias on only one of 4 output tubes (7241s), I am all ears.

@lewm It can't but I imagine you knew that.

Ralph, thanks. Your corroborating my preliminary hypothesis will save time. Getting the very heavy chassis safely down to my basement workshop is my next problem. I’m hoping that the output tube in question was not damaged. Visual inspection shows that none of the 3 cathode wires is blown. That’s encouraging. There’s a problem in the bias circuit for that one position that’s going to be hard to find, because evidently I did not find it 6 months ago when the exact same problem occurred.

Mijo, as I think you know, my amplifiers are essentially older MA2s with huge EI type power transformers that make them very heavy. I think you’ll be getting toroidal power transformers on your new MA2s. Much lighter.

Mijo, as I think you know, my amplifiers are essentially older MA2s with huge EI type power transformers that make them very heavy. I think you’ll be getting toroidal power transformers on your new MA2s. Much lighter.

@lewm To be clear this is the M-240, of which exactly 6 chassis sets were made. It used a lot of MA-2 parts. You built yours up in bits didn't you?