While waiting for crossover components, speaker drivers, filament reg parts, and output transformers, I have been working on the "revamp" of my 6A5 amp.
I love the sound of this amplifier, but there are a few things I would have done differently, so I've decided to streamline things.
First off, the power tubes. The battle of the 3W IDHT titans, R120 vs. 6A5G. I spent a few days evaluating these tubes, with speakers and headphones, and my ears told me the same thing in both cases, that I prefer the sound of the 6A5G. The R120 has an interesting sound. Not quite as airy as the 6A5G, more intimate staging, emphasized low end with dynamic impactful bass, the upper midrange is somewhat forward, very nice detailed treble, it has a very fast and dynamic sound, an attention grabber. No doubt this sound has its place and another person might prefer it, but the airy, inviting, relaxed sound of the 6A5G is more to my taste. It has its fair share of dynamism and low-end kick, but just the right amount, the R120 borders on aggressive.
So, the 6A5G stays, the R120 goes into the tube archives until I dig it up again. This has consequences for the amplifier revamp. Way back, I had wanted to use voltage regulators on the 4V drivers of this amplifier, the reason being that some of the tubes in my collection have 1A heaters, others 0.65A, being able to freely swap back and forth between the two was appealing, and it still is. But alas, the 6A5G has one major drawback - due to its internal center-tap, each tube requires its own dedicated heater winding.
With two windings for the power tubes and one for the tube rectifier, that leaves one for the drivers, which will not work with my voltage regs. Changing to R120 would have allowed me to parallel the heaters and use an artificial center-tap, freeing up a second winding for my driver tube voltage regs. But the 6A5G is sticking around, so the regulators are not making it in the final build (I have weighed other options - dedicated rectifier transformer, nixing the rectifier outright, etc. - just letting it go). No big deal, a nice to have, not a need to have.
Next issue to address is thermals. Plan is to redo the chassis, add additional ventilation, redistribute/remove specific voltage dropping resistors, and change chassis-mount resistors to air-cooled through-hole. I am also leaning toward removing the combination socket and committing to 4V recitifers, giving up the 5Z3. I've found swapping rectifiers in this amplifier has subtle-to-undetectable effects on the sound. The best is the U18/20, I can't see myself using anything else at this point, FW4-500 as backup. I've changed the passive supply from a CLCLC to a CLCLCRC, the CCS loaded drivers have been moved from the first tap to the last, a more conventional layout, with plenty of headroom to drive the 6A5G to clipping and cover the droput voltage of the CCS.
Finally, I changed my grounding scheme from a conventional star ground to a distributed star ground. This is a hybrid between star and bus, in which there are local star nodes that are daisy-chained from input to output. There is a single chassis connection on the power supply cap where the two channels converge. This made no measurable difference in power supply noise, but it is more neat, and call me crazy but I feel like it improved the clarity. Hard to say, I'll just go with it
So the current amp is now something of a test bed for the finalized design that will go in a new box. These changes aren't essential, but the amp will run cooler and I feel better about the supply layout and grounding scheme. If you feel better, the amp sounds better
Three night shifts, then maybe I will be rebuilding the Snells, really looking forward to it.