London Power's "PA"-series Tube Power Amplifier Kits

[makinrose]

Hi Guys

If the protection diodes are effecting the sound there is a serious flaw in the circuit - but that is not unusual for mesa products.

Every component has parasitic elements, and for diodes it is their capacitance and leakage current. A decent diode has no leakage current until very near its breakdown, and in any case, this should be microamps, nothing significant to the tube output stage. The Mesa tech is either mistaken or ignorant of the facts if he thinks the diodes change the sound.

Resistors in the same position have no impact on the sound. They have no erratic behaviour nor do they do anything abrupt. Frankly, at full output, the sound will be so loud through common MI drivers that aural compression will keep you from hearing that sound accurately, meaning that IF the diodes were to alter the sound there is no way you could tell. This is hypothetical because in the real world the diodes do no such thing to the sound.

There is nothing special about the diodes used here, either. Ideally they should be fast, so go for the UF version rather than 1N, if possible. Current handling does not have to be high but you'll see both 1A and 3A types used. Leave the leads long if you want the diodes to survive many incidents (if there are so many incidents that the diodes conduct something must change about how you use the amp).

I did find the notion a little strange when he told me that....   One my reactions was the same as yours: with 100W amp and EV12L how would you even hear it if it only happened with the amp cranked?

  It was a MKII which as you probably know is an utter nightmare to work on. It was designed in such a way that accessing the solder joints was hard and components are difficult to remove since they bent the lead over and left them pretty long. Even getting the chassis out the tiny cab was a hassle.  I've joked with other people that Mesa has few people on staff whose job it is to make their amps hard service.

At any rate I'm definitely going to add those resistors to new builds! Thanks for all the insight.