The Newton 300B

[izzy wizzy]

More listening into the early hours tonight, so I'm going to be like a dead duck in the morning - again. I really need to get this thing back onto a space near the bench with the base off so that I can use it as a visual reference to quickly get the second one finished, but I can't stop listening to it. Maybe mono is where it's at...

One of the things I'm noticing is that you don't have to listen to reference recordings to hear its benefits. Everything sounds right - we had some old Kay Kyser recordings playing earlier - these were either 70's or 80's transfers onto LP which pre-dated all the awful intrusive digital "clean-up" efforts - and they somehow just sounded right. Maybe it was the signal chain consisting entirely of 30's era valves - who knows.

Tomorrow - tomorrow I'll get it back on the bench.

I feel your pain. Good PP amps are rather fine

[Thermionic Idler]

Here's a full interior build pic, actually in focus this time. There is one item outstanding and that is to wire in the power relay at top right, so that it can be remotely powered up / down from the preamp.

[Ray P]

Cracking build Dave

[Thermionic Idler]

Cracking build Dave

Thanks Ray. I think I succeeded more or less in optimising the signal path lengths and layout, being able to plan it all out beforehand in CAD helped, although I did end up making a few tweaks at build time. It helped that Lynn laid out the ground paths in his schematic, and I followed them to the letter as he knows a lot more about good grounding than I do! It paid off, these things are quieter than the Beasts - at least the first one is.

With a heavy heart, it's now back on the bench. I want to document the measured voltages - I do remember all the voltages on the output stage were within a volt or two of the Spice model. On the driver stage, Spice predicted 517V or so HT whereas it was about 504V measured, with values down the chain reflecting the slightly reduced value. The HT was supposed to sit around 500V anyway, so I think that's all good.

So I just need to progress the other one and then we'll see how the stereo performance is. I think these actually could replace the OTL's, Meredith was swooning in front of the Ella Fitzgerald LP I dug out, and I found the sound utterly addictive. One bonus is that re-valving costs are about half that of the OTL's (unless I upgrade the 300B's to Shuguang Treasures or something), and I don't have to rely on one manufacturer - it uses valves that are still being made except for the 12SN7 - and if the world runs out of those all I have to do is tweak the heater wiring a bit to convert it for 6SN7. The other bonus is that the electricity usage, even with the valves deep in class A, is still about half that of the OTL's (I think).

[izzy wizzy]

I'm sure Lynn would love to see your build and hear your thoughts on it.

[Thermionic Idler]

I have posted a reply on the diyaudio thread where he originally shared the Symmetric Reichert circuit, in the hope he may spot it.

[Dave the bass]

Cracking build Dave

It might just be a dry joint.

[Thermionic Idler]

Quickly powered it back up on the bench as I wanted to actually record the voltages and also re-run the scope tests to measure the gain.

Here are the voltages, the first figure is that predicted by Spice and PSUD, the second is the actual:

Output stage B+ 467V / 471V
300B Plate Phase + 460V / 467V
300B Plate Phase - 460V / 467V
300B Cathode (between hum bucker and 500R cathode resistor) 78V / 77V

Input stage B+ 517V / 510V
between CCS and 820R: 293V / 312V +phase / 313V -phase
6V6 plates: 269V / 286V +phase / 289V -phase
6V6 cathose: 17V / 18V

12SN7 plate resistors: 494V / 485V
12SN7 plate + phase: 220V / 212V
12SN7 plate - phase: 220V / 213V
12SN7 cathode: 5V / 5V

Whilst measurements only tell part of the story, here's what a 10kHz square wave looks like, as captured by the Analog Discovery device:

[Thermionic Idler]

Some more pics of Channel A final build after setting up on a shelf behind my desk / bench under some strip lights, to refer to when building the second. The signal section:



and the power supply section:



Here's where I'm at on Channel B, transformers all fitted, and this time around I'm going to tie off (insulate and secure) the unused wires BEFORE installing anything else - one of the most awkward final jobs on Channel A was trying to do that after the PTFE boards and filament transformers were mounted in place. There are a lot of unused wires (the 6.3V secondary, the 5V centre tap, plus several primary taps to allow a whole range of input voltages to be set). The build of Channel B will still go left to right, so with the mirror imaging this means the power supply gets done before the signal section.

[Thermionic Idler]

Also, if anyone is having issues viewing the images can you please PM me, as it seems some configurations might be blocking them. Ta!

[Ray P]

Your images work fine for me Dave.

[pre65]

Your images work fine for me Dave.

And me.

[Cressy Snr]

And me

[Thermionic Idler]

Heater wiring - probably my least favourite part of any build, now done. Also the transformers are all connected up except for the HT secondaries (red and red/yellow wiring). The unconnected black wires are from the two chokes.