The Newton 300B

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simon
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#196 Re: The Newton 300B

Post by simon »

I found crocodile clips quite useful and have saved my sanity a few times. Connect a clip to the point of interest with the amp off, and the other end to your meter. Power up from a safe distance.

It's not ideal for having a quick poke around a few places, but for just a few places I've found it easier to read the remote end of the Croc clip than poking around inside the guts. Even just having the earth probe connected really helps. You might already do this in which case please ignore!
Ant
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#197 Re: The Newton 300B

Post by Ant »

I (when i muster up the balls to) usually connect a croc clip to the earth busbar or wherever i need that side of the meter and black probe, stick one hand up my back then use the other to handle the red probe.
Switching off between test points and checking the caps have discharged before moving onto the next test point.

Odd thing is that it never used to bother me but it bothers me a whole lot these days. Probably because i have not kept my hand in, havent built an amp or anything else since 2016 i think, since i got the f5
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Thermionic Idler
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#198 Re: The Newton 300B

Post by Thermionic Idler »

Made the final round of changes last Sunday. I think we're done. :D

Firstly the 300B operating point - I did some modelling in PSUD after I'd made my previous changes which led to the operating points sitting about 5% above my target of 350V / 60mA. PSUD predicted that if I added the 68R resistors back into each leg of the rectifier, the operating point would slide back just enough to be bang on target. Fortunately I didn't actually remove those resistors, I merely added bypass wires, so all I had to do to back out the change was to clip those wires out.

The next changes were to the input and driver stage. The research I'd done suggested the 6V6 might be happier with a little more current, so I adjusted the constant current sources to 36.5mA from about 28. Along with this, I swapped out their 300R 5W shared cathode resistor for a meatier 200R 12W one (Mills) and removed the 80uF capacitor from the tail, not actually needed because of the CCS. When I modelled this change in PSUD, it flashed up warnings that the current spikes at switch-on would be a tad too high for the GZ32 because of the increased current draw, so I added a 10W 180R resistor in each leg to smooth those out and bring them under the peak limits. This had the happy side-effect of bringing down the plate voltages on the 12SN7 to about 180 - 190V, around 1.9W plate dissipation per plate, which means I'm no longer limited to the GTA-designated models. I'm pleased with this, they still have a healthy current (about 10mA) but I really didn't see any good reason for running them hotter than that.

The final change was to swap out the 12V power relays for 5V ones ahead of the SilentSwitcher changes taking place next weekend, so we're having to power the amps on manually at the moment.

The voltage measurements confirmed the 300B operating point was about as close to target as is possible with the variability of valves - 350.1V plate voltage on one side at 59.69mA, and 354.2V on the other side at 58mA. IIRC this is one of Thorsten Loesch's recommended configs, and ties in with Western Electric's datasheet. Western Electric engineers knew their stuff. My output transformer is 8k push-pull, so its single-ended equivalent would be 4k, and note that for a 4k load, the lowest figure for third harmonic distortion - which matters in push-pull - is arrived at with this operating point.

The sound is now spectacularly good - Meredith remarked that she could hear the difference quite clearly from the next room. Transparency and depth are really up a notch and everything now just sounds "right" with more clarity in the midrange.

I have to log on to work now, I'll post the final schematic and measured voltages later on.
Deck: Garrard 301 - Audio Origami PU7 - NW Analogue DH3S
Phono: Pete Millett LR, Lundahl SUT, AMB σ22 PSU
Linestage: Bruno Putzeys Balanced Pre + Hans Polak relay mod, LCDuino control
Power amps: 300B PP 'Symmetric Reichert'
Speakers: FF225WK Big Mets
Cressy Snr
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#199 Re: The Newton 300B

Post by Cressy Snr »

Great stuff Dave. It always surprises me, the positive difference a bit of tweaking in the right direction makes. That’s the great thing about DIY; you can voice the project to your room and speakers. I think you’ve proved that what worked for Reichert doesn’t necessarily work across the board. Such is the unpredictability of hi-fi audio. As Reichert is won’t to say, ‘change anything, and it’s your design.’ I guess it’s your design now then. :)
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Thermionic Idler
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#200 Re: The Newton 300B

Post by Thermionic Idler »

As promised some time ago, schematic and measured voltages. I'm not going to say "this thread is now closed off" because inevitably some further change will take place down the road. But I think that's it for a good while.

Firstly the final schematic, with added voltages and currents showing the target operating points (PDF here). And yes, I have noticed that I'm now slightly over 100mA on L1 - I'm not going to sweat it too much as these things have a 15% tolerance for resistance / inductance.

And here are the measured voltages. I think that my 6V6's are a little weak as I'd be expecting to see 245-250V plate voltage with the current adjusted to 36.5 - 37mA. That's pretty close to one of RCA's operating points published here (see Amplifier Class A1 Triode Connection first column) with the same cathode bias resistance (half of 400 ohms obviously as there's two of them). The figures for the 6V6 plate current have been the victims of rounding errors caused by the fact that the multimeter can only give results in whole volts on the 600V scale. I set the currents first by measuring the drop across the 820R resistors so it's actually been adjusted more accurately than that.

Image
Deck: Garrard 301 - Audio Origami PU7 - NW Analogue DH3S
Phono: Pete Millett LR, Lundahl SUT, AMB σ22 PSU
Linestage: Bruno Putzeys Balanced Pre + Hans Polak relay mod, LCDuino control
Power amps: 300B PP 'Symmetric Reichert'
Speakers: FF225WK Big Mets
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Thermionic Idler
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#201 Re: The Newton 300B

Post by Thermionic Idler »

Cressy Snr wrote: Tue Oct 12, 2021 10:01 am Great stuff Dave. It always surprises me, the positive difference a bit of tweaking in the right direction makes. That’s the great thing about DIY; you can voice the project to your room and speakers. I think you’ve proved that what worked for Reichert doesn’t necessarily work across the board. Such is the unpredictability of hi-fi audio. As Reichert is won’t to say, ‘change anything, and it’s your design.’ I guess it’s your design now then. :)
Yep, I reckon so. My key takeaways from this - build it push-pull but bias it like it was single-ended, and take careful note of the recommendations set down by the original engineers in the 30's and 40's. Unstressed valves sound better.
Deck: Garrard 301 - Audio Origami PU7 - NW Analogue DH3S
Phono: Pete Millett LR, Lundahl SUT, AMB σ22 PSU
Linestage: Bruno Putzeys Balanced Pre + Hans Polak relay mod, LCDuino control
Power amps: 300B PP 'Symmetric Reichert'
Speakers: FF225WK Big Mets
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Thermionic Idler
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#202 Re: The Newton 300B

Post by Thermionic Idler »

Following the recent PIO capacitor leakage incident, some research, and a lot of pondering back and forth, I've decided to re-cap the two coupling stages with more reliable film caps. The order to HFC went in this morning. I've also reduced the values to 0.15uF for both stages, as this will more closely match the bass rolloff to the output transformer bandwidth and reduce the risk of it saturating.

For now I've selected the Jantzen Superior Z-Cap - they seem reasonably priced and very well thought of, and also have very tight 2% tolerance so I know they'll be matched.

The 1k + 3nF that Lundahl recommend on the secondary of the input transformer LL1684 was originally implemented with a 3.3nF PIO cap and carbon film resistor - these components will also be replaced with 1% tolerance items of the proper value (Charcroft silver mica and PRP PR9372 metal film as the Takmans were out of stock).
Deck: Garrard 301 - Audio Origami PU7 - NW Analogue DH3S
Phono: Pete Millett LR, Lundahl SUT, AMB σ22 PSU
Linestage: Bruno Putzeys Balanced Pre + Hans Polak relay mod, LCDuino control
Power amps: 300B PP 'Symmetric Reichert'
Speakers: FF225WK Big Mets
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Thermionic Idler
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#203 Re: The Newton 300B

Post by Thermionic Idler »

By the way, I'm not sure what's going on with my image / external links all being broken - if I try to load any of them I get

Secure Connection Failed

An error occurred during a connection to user.fm. PR_END_OF_FILE_ERROR

and I know that 'image' is taking the place of all the pictures I've posted previously. Jeez this kind of thing just annoys the hell out of me.

It looks like an issue at fastmail where I host all the images I share - a service I pay for, incidentally (email and file hosting).

Hopefully this will resolve in due course. I've raised a support ticket about it.
Deck: Garrard 301 - Audio Origami PU7 - NW Analogue DH3S
Phono: Pete Millett LR, Lundahl SUT, AMB σ22 PSU
Linestage: Bruno Putzeys Balanced Pre + Hans Polak relay mod, LCDuino control
Power amps: 300B PP 'Symmetric Reichert'
Speakers: FF225WK Big Mets
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Nick
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#204 Re: The Newton 300B

Post by Nick »

Just a question. How do you measure the voltages to 0.005%?
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izzy wizzy
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#205 Re: The Newton 300B

Post by izzy wizzy »

Thermionic Idler wrote: Sun Jun 12, 2022 9:16 am Following the recent PIO capacitor leakage incident, some research, and a lot of pondering back and forth, I've decided to re-cap the two coupling stages with more reliable film caps. The order to HFC went in this morning. I've also reduced the values to 0.15uF for both stages, as this will more closely match the bass rolloff to the output transformer bandwidth and reduce the risk of it saturating.

For now I've selected the Jantzen Superior Z-Cap - they seem reasonably priced and very well thought of, and also have very tight 2% tolerance so I know they'll be matched.

The 1k + 3nF that Lundahl recommend on the secondary of the input transformer LL1684 was originally implemented with a 3.3nF PIO cap and carbon film resistor - these components will also be replaced with 1% tolerance items of the proper value (Charcroft silver mica and PRP PR9372 metal film as the Takmans were out of stock).
I don't want to rain on your parade coz it's a great build you have there. I have the Superior Z in my speakers but for the more critical path, the mid/tweet, I use the Alumen as it's film/foil construction. I'm not a fan of metallised anything and I think your amp deserves decent film/foil caps not metalised.
Last edited by izzy wizzy on Sun Jun 12, 2022 11:43 pm, edited 1 time in total.
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IslandPink
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#206 Re: The Newton 300B

Post by IslandPink »

I didn't want to press that myself, but that's what I have found, and the main reason I recommend the tin/polyprop Mundorfs. It's rare that anybody listens to me, though ! :)
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Ray P
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#207 Re: The Newton 300B

Post by Ray P »

IslandPink wrote: Sun Jun 12, 2022 10:56 pm I didn't want to press that myself, but that's what I have found, and the main reason I recommend the tin/polyprop Mundorfs. It's rare that anybody listens to me, though ! :)
I did Mark and the Mundorf ZNs are my 'go to' cap in appropriate situations, excellent value for great sound quality IME.
Sorry, I couldn't resist!
Cressy Snr
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#208 Re: The Newton 300B

Post by Cressy Snr »

IslandPink wrote: Sun Jun 12, 2022 10:56 pm I didn't want to press that myself, but that's what I have found, and the main reason I recommend the tin/polyprop Mundorfs. It's rare that anybody listens to me, though ! :)
Yep, I have them in the Mission speaker crossovers where they work really rather well :)
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Thermionic Idler
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#209 Re: The Newton 300B

Post by Thermionic Idler »

4 votes of confidence in the Mundorf ZN - well that's fairly clear-cut! :)

I called HFC this morning and managed to cancel the order before it was sent out. I'm now going with the Mundorf ZN's on the coupling stages as recommended - 0.1uF on stage 1 (12SN7 to 6V6) and 0.22uF on stage 2.
Deck: Garrard 301 - Audio Origami PU7 - NW Analogue DH3S
Phono: Pete Millett LR, Lundahl SUT, AMB σ22 PSU
Linestage: Bruno Putzeys Balanced Pre + Hans Polak relay mod, LCDuino control
Power amps: 300B PP 'Symmetric Reichert'
Speakers: FF225WK Big Mets
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Thermionic Idler
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#210 Re: The Newton 300B

Post by Thermionic Idler »

Nick wrote: Sun Jun 12, 2022 6:38 pm Just a question. How do you measure the voltages to 0.005%?
Can't recall exactly, but I think it may have been that I measured the anode voltage to ground, which would have been an integar value of 425V or so measured on the 600V scale, and cathode voltage, which would have been on the 200V scale and therefore included the first decimal place. The spreadsheet would have subtracted one from the other and produced a result to one decimal place - yeah, likely not that accurate in real life!
Deck: Garrard 301 - Audio Origami PU7 - NW Analogue DH3S
Phono: Pete Millett LR, Lundahl SUT, AMB σ22 PSU
Linestage: Bruno Putzeys Balanced Pre + Hans Polak relay mod, LCDuino control
Power amps: 300B PP 'Symmetric Reichert'
Speakers: FF225WK Big Mets
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