About the heater circuit of Marshall DSL100HR

[The Dude]

IMO, I'd just leave it. A 12AX7 will last at least 10,000 hours. Even if you lost say 15% of tube life, you could play roughly 24 hours a day for a year nonstop. If it's an OCD thing, you could add a bucking transformer (which would also lower B+), add resistors, build a rectifier circuit and run DC filaments, replace the PT with a 120V version, add a pair or two of very high wattage reverse/parallel diodes, etc., etc. IMO, none of these methods are better than just leaving it be and sacrificing a small bit of tube life.

[Mackerel]

OK, thank you bros! ありがとう！

[Greg_L]

With modern wall voltages what they are in the U.S., all of my amps have filaments that run up near 7v. Just yesterday I measured 126.8v from the wall. Texas power grid, when it's not freezing, it works great!

You know what I do about it? Nothing, I just play them.

[Tom Phillips]

This is a long shot but one more thing to consider is the calibration of the meter you used to measure the high heater voltage.
Do you have a way to verify that the reading is correct? Maybe another meter to compare readings.

[Mick Bailey]

Over here in the UK it's fairly common to see heater voltages in excess of 7v. Fender amps especially, where 7.6v isn't uncommon in amps built through the 90s onwards and tube life doesn't seem to be affected much. My thought also is to check the meter to be sure. If it's anything other than a true-RMS meter, or out of calibration, then the measured voltage may not be accurate.

[pdf64]

How about a pair or 2 of back to back diodes in series with the heater supply? See Merlin http://www.valvewizard.co.uk/heater.html
I concur that highish heater voltage doesn’t seem to have obvious immediate / short term consequences. But over the course of his book, I recall Tomer raising a few failure mode or lifetime shortening mechanisms resulting from it http://tubebooks.org/Books/Atwood/To...um%20Tubes.pdf
So it seems that it ought to be beneficial to get heater voltage in line.

[Helmholtz]

How about a pair or 2 of back to back diodes in series with the heater supply?

Would have to be real big diodes here as heater current is around 7A and I'm not sure how to take into account the relatively long turn-on surge.
Maybe a 10A bridge rectifier would do. If wired as shown by Merlin each 2 diodes are operated in parallel.
But might introduce "crossover" heater buzz.

BTW, V1 is heated by 12VDC. If too high, adjustment via R129 would be easy.

[Tom Phillips]

When needing to reduce heater voltage I have historically used either diodes or resistors. Lately, I favor resistors because I can get the exact desired final voltage rather than be limited to the ~0.6V increments provided by diodes and there is no concern about added diode switching noise.