For months I have been trying to diagnose the cause of the crackling and what many, including myself, thought was extreme tube rush or a leaky capacitor or a poor ground in my new SR-45s. Leading to extreme frustration and many nights tearing the amps back down, swapping components, rerouting ground paths, wild grid stopper experiments, pulling transformers to test in other amps, hand wrapping various shielding around the wires and disappointment. The amps just sounded like an angry bowl of rice crispies.
After nearly a year of tinkering I gave up and decided to finally rebuild my Paramount 1.1s with my NOS nickel pinstriped TFA-2004s and EXO-03s. And while I was at it, I rebuilt the N801A crossovers for my Alter Magnificents. I had bought all the parts 5 years ago and never got around to it because I bi-amped them. But I wanted to see how the new Pmounts would sound by themselves.
So after spending an unreasonable amount of time polishing the EXOs, lots of drilling and filing to squeeze all that iron in, I arrived at this:
The A2 voltage is a bit high, 485v, leading to a plate voltage on the 2A3 of 277V when adjusted per the manual. So I trimmed them to bring the plate voltage down to 255V and hooked them up with the new N801s. And they sound wonderful. Though I'm still debating dropping the B+ a tad.
They however don't have the bass punch on the Altecs i was used to. Plenty of tone, but little tactile punch. So I recapped an ST-70 from my pile of ST-70s to rebuild, and figured I'd biamp the Altecs again with the Pmounts on the horn. Swapped everything around again, plugged them in and...........even angrier rice crispies. Unlistenable.
It wasn't the SR-45s. It was the Nakamichi EC-100 active crossover. Which I never thought to check because, well, I didn't build it. Something in the high pass filter is toast.
Threw the SR-45s back in full range. They sound gorgeous.
And luckily, as an audio component hoarder, I have a bunch of unbuilt Nelson Pass 6-24 active crossover boards and components I picked up about a decade ago to replace the Nakamichi. But I never got around to it.
So now the shuffle continues. We'll see if the Pmounts stay 2A3s and end up the horn amp for the Altecs. With the ST-70 on the 416s or end up getting converted back to 300B with the SR-45s on the horns. Too many large, new caps that need to burn in to make a decision yet.
And I am off to rebuild my first Bottlehead kit, an original Foreplay, my second BH kit, Paramour 1s and finish designing the actively rectified RC coupled capacitance multiplier power supply I've been working on for a PT-7 based 45 amp. I've got it down to less than 1 Millivolt of ripple. But it has a tendency to ring when the TX saturates. Blowing the active components in the process. So I'm going to test some tiny LC snubbers out to see if that will calm it down. And I need to find a way to measure sub Millivolt ripple. My current testing equipment has too much bench noise to see it amongst the garbage floor. May also try LC coupling the multiplier, but I would have to wind a custom inductor for it. And I'd rather not at the moment.
Thanks again to PJ, PB, Doc B, Eileen and everyone at Bottlehead for providing me with decades of fun problems to solve.
