The Newton 300B

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Nick
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#121 Re: The Newton 300B

Post by Nick »

My only question is the power required. If 190ma is the total draw, then is it not half that to go into the calculation as each half of the secondary is only supplying current half the time.

Again, the resistor(s) could go in the centre tap as well or instead.
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Thermionic Idler
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#122 Re: The Newton 300B

Post by Thermionic Idler »

Nick wrote: Sun Feb 07, 2021 8:53 pm My only question is the power required. If 190ma is the total draw, then is it not half that to go into the calculation as each half of the secondary is only supplying current half the time.

Again, the resistor(s) could go in the centre tap as well or instead.
Good question. I went back to check the article and here is what it says:
If the transformer alone doesn't have enough resistance to meet this requirement then you need to make up the deficit by adding resistors in series with each anode. These resistors need to have a power rating that comfortably exceeds:

P = (1.1 × Idc)^2 × R

Alternatively, you could use one resistor (with twice the power rating) in series with the cathode.
Sadly the article doesn't specifically define Idc - so I assumed it to be the rectified DC current and plugged that value into the equation.

I guess if it turns out that I only needed to plug in half the value, then no harm done as the power requirement figure would drop, not increase - it would only be a problem if I underestimated the rating of the resistor rather than overestimating it.

EDIT - I think I'll also measure the actual DCR as it's easy enough to pull the rectifier and put a multimeter across the pins. That'll also include the small resistances introduced by the fuse in each leg.
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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#123 Re: The Newton 300B

Post by Thermionic Idler »

Resistors ordered, along with some more spare fuses and a "true RMS" multimeter - at some point I want to experiment with the 6V6 operating points and I figured having a second meter would be useful for that anyway.

The DCR on the larger transformer secondaries measured pretty much bang on the datasheet value, but the smaller secondary had about +5 ohms per side (still within tolerance) so I was able to drop the additional resistance to 39 ohms from 47.

Still waiting for the rest of the heater drop resistors that were supposed to be delivered last Friday - now three days overdue. F***ing Parcelforce! :banghead:
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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#124 Re: The Newton 300B

Post by Thermionic Idler »

All outstanding deliveries now arrived. Also got some more GZ34's, this time from Brimar to help out their project. Saturday's maintenance window is planned out. (I bought my 300Bs from these guys as well).

It'll just be about fixing the input stage heater voltages to the right values and adding the required resistors into the HT supplies so I don't fry any more GZ34s. Once done I'll re-measure all the voltages so that I can analyze at leisure where I am w.r.t. operating points, and whether any subsequent tweaking is needed there.
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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izzy wizzy
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#125 Re: The Newton 300B

Post by izzy wizzy »

If it's any consolation, my Farnell order is all over the place. Some arriving in a day, parts of it more than week later, some of it ... well who knows apparently despite being in stock. Big checking job to go through.
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#126 Re: The Newton 300B

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izzy wizzy wrote: Fri Feb 12, 2021 8:36 am If it's any consolation, my Farnell order is all over the place. Some arriving in a day, parts of it more than week later, some of it ... well who knows apparently despite being in stock. Big checking job to go through.
Ouch. Yeah I didn't have any issues when ordering the main amp parts in autumn last year, just these ones - maybe a combo of the weather and COVID. At least if I decide to tweak the operating points, the bits (cathode R's) I'll need for that will be from HiFiCollective, they're usually pretty good.
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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#127 Re: The Newton 300B

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Time for another report. I must admit I'm looking forward to a weekend where I can sit down and listen to these amps instead of having them upside down on the bench undergoing surgery.

Anyway, I have now soldered in several new resistors. I did one amp yesterday and the other today.

1R in each of the secondary legs from the input section filament transformer (6.3V-0-6.3V). Now the 12SN7 and each 6V6 are getting precisely the correct heater voltage - well, to within 0.05V anyway (measured 12.55V across one, and 12.62V across the other).

0.12R on the input stage GZ34's to trim them down from 5.55V. Both rectifiers are now at about 5.26V under load, well within the 10% tolerance stated on the datasheet. The smaller power transformer is closer to the rectifiers than the larger one, I suspect the difference was caused by the longer length of heater wiring from the large transformer - it has to pass 1.9A of current.

68R's between the fuses and each anode of the output section GZ34, and 39R's in the same place on the input section GZ34. The secondary resistance should now be a few ohms above the datasheet-specified 125 ohms.

Nevertheless, I still got a hiccup when I powered up the first amp. I'd inserted two of the new rectifiers I'd got from BTP, and about 40 seconds in I felt a small half-second vibration. The B+ on the output stage was nowhere near where it should be, and one of the HT fuses had gone. So another rectifier arc-over or short. All was well after I replaced the fuse and substituted another GZ34, and the failed one seemed to have rather wonky looking anodes, so I can only put that down to an early-life failure - I can't think of any parameter I'm exceeding now. And there were no problems on Amp B which I performed the mods on today (Amp B seems to be the 'lucky' one, the issues I've had have always been on Amp A).

I've re-measured all the voltages and have linked to the results below. As expected, the biggest change was on the 300B plate voltages, which have fallen by just under 30V or so, and the current has fallen back to 80mA give or take a bit. Not enough to make a big song and dance about, I don't think. The B+ on the input stage has dropped about 10V, and it seems that bigger differences result from variances between valves, so I'm not at all worried about that.

There's a postscript story on Amp A. Note the 6V6 plate voltages, the negative phase voltage seems to be a fair way down compared to the rest. When we were listening last night, about an hour in I was aware of quite a significant hum (100Hz?) that had developed on Amp A. I replaced one of the 6V6's with a spare and the hum went away completely, so I suspect if I measured the voltages now, they'd be more in balance.

I do have some electrically matched Reflektor 6V6's coming from Ukraine (they were cheap and seem to be well thought of so I thought I'd give them a shot). Once those get here I may have a play with the 6V6 operating points, easy to do with the adjustable CCS. I'm pretty happy with where the 300B and 12SN7 points are, though.

Image
Last edited by Thermionic Idler on Mon Feb 15, 2021 1:23 am, edited 1 time in total.
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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#128 Re: The Newton 300B

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The last post was made just after I'd completed the surgery and powered the amps up in the system, leaving them with some music from the server to run them back in and make sure the rectifiers behaved themselves. So I'd not had a chance to listen to them properly.

After a good listening session, all I can say is.... wow.....wow.....wow.....

All remaining traces of harshness and stridency (I'd say 80% was eliminated by correcting the 300B heater voltages last week) have completely gone. You can listen to anything on these amps no matter how badly recorded it is. Seems indirectly heated signal valves are also sensitive to the correct heater voltage. The soundstage has widened considerably, and the sense of depth in the sound stage now seems more 'correct'. Some instruments which sounded overly prominent before, now assume their rightful place. Overall, everything now sounds more 'right', and has musicality in spades - we completely lost track of time listening to records.

It seems to me that three things changed, and I don't know for certain which of these was most influential. 1) the 12SN7/6V6 heater voltages as mentioned before, 2) the lowering of the 300B plate voltages and current, 3) adding the HT resistors. My completely unscientific view is that (1) had the greatest influence, possibly followed by (3). The current pulses on the cap input power supply, which couple back into the power lead and the rest of the amp chain, would presumably have been smoothed out quite a bit with a higher impedance input to the rectifier anodes, reducing the switching 'hash' considerably.

I'm unsure about the influence of the 300B operating point changing. I know that Lynn O. likes to run them hotter than this, but on the flip side I've read quite a few posts from people who found the tonality just got better by turning the wick down a bit voltage-wise. I think the Mets are sensitive enough that I don't need to worry about requiring mega power, I reckon anything over 20 watts should be fine. The combination of the high impedance OPT (8k) and 80mA plate current should be enough to keep the bass under control - and it has tightened up quite nicely since I built it, and things have bedded in. Plus the slightly lower voltage translates to more relative head-room in the driver stage, and a shade under 30W plate dissipation is good for valve lifespan.

Playing with the universal loadline calculator seems to suggest that if I drop the current on the 6V6's to 22mA but with a plate voltage of 275V, that whilst the second order harmonic distortion rises a little, the third order figure drops to virtually zero. (I did this with a very high figure entered for reactance load so that the loadline went horizontal, reflecting the plate CCS's). With the valves in push-pull, it's the odd order stuff we're really trying to minimise, so I'm thinking this might be worth a try next.

As I said previously though, I think the 12SN7 and 300B operating points are now dialed in quite nicely, so I'm going to leave those alone.
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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#129 Re: The Newton 300B

Post by Cressy Snr »

Always nice, this DIY lark when a plan comes together. :)
Harshness and stridency is the worst thing imaginable, and it is an utter pain in the arse to deal with and suppress.
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#130 Re: The Newton 300B

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Cressy Snr wrote: Mon Feb 15, 2021 8:40 am Always nice, this DIY lark when a plan comes together. :)
Agreed, I'm chuffed to bits with this. There seems to be this perceived wisdom that 300B amps are slow and soggy - this is exactly the opposite of that. It's lightning-fast and out-OTLs the OTL for transparency. The 300B is a good tube let down by a lot of bad design - you pretty much have to put a small power amplifier in front of it to get it to do its thing properly. Why all these commercial outfits carry on trying to drive them with wimpy tubes is beyond me.
Cressy Snr wrote: Mon Feb 15, 2021 8:40 am Harshness and stridency is the worst thing imaginable, and it is an utter pain in the arse to deal with and suppress.
Indeed, what I was hearing matched up with another comment I'd read from someone with a DHT heater overvolt, so I knew I had to put that right before anything else. What's good about boards like this is being able to learn from other's experiences. That issue with the HT secondary not having enough resistance for the rectifiers was something I'd definitely missed in the initial design, and wouldn't have identified that as the cause of the arcing without yours and Phil's input. And I think it was another story (can't recall who had the issue) about a power transformer either melting or catching fire that made me think it might be wise to add fusing to the HT secondaries per Merlin Blencowe. I have to say, with a wastebasket full of dead fuses, I'm quite glad I did!
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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#131 Re: The Newton 300B

Post by Thermionic Idler »

Hmm, I don't have a lot of confidence about these NOS late 70's Reflektor 6P6S's - or 6V6-ski's - that arrived today. It's odd, all the information is still there but it feels like a sort of Soviet greyness has descended over everything. I'm going to give them another evening of run-in to see if they wake up a bit - there are no technical issues, they seem very well made and are very quiet - but they also seem to have sapped all the musicality out of everything, like Dementors.

Oh well. Was just seeing if I could do matched NOS 6V6's on the cheap. No, no I can't.
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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#132 Re: The Newton 300B

Post by Thermionic Idler »

There is clearly a benefit to be had from properly matched 6V6's though, at least in terms of hum. Now, there isn't any, whatsoever. I cannot hear anything other than complete silence at the speaker. Quite a thing when you consider the power supplies don't actually do that much smoothing (I think there's a full 2V of ripple on the output stage and not much less on the driver stage), and all filaments are AC heated.

I really need a decent tube tester.
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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#133 Re: The Newton 300B

Post by Nick »

Thermionic Idler wrote: Tue Feb 16, 2021 11:11 am I really need a decent tube tester.
If you want to match valves, the uTracer is (IMHO) better than just about any other tube tester (well ignoring a Tek 570. but the uTracer will do more than that for small signal valves).
at least in terms of hum
And of course the hum removing action is the same process that removes even order distortion.
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#134 Re: The Newton 300B

Post by Thermionic Idler »

Ah yes, I've been looking at the uTracer. Also the eTracer which has enough -ve bias to measure the 300Bs but it's ... a bit spendier.

The Raytheon 6V6's went back in tonight, I couldn't take any more. Aaaaaaand as if by magic... the music's back! :D

I thought it would be something I'd have to listen carefully for. Nope, even Meredith who was in the kitchen area prepping the chicken and not listening particularly closely, could hear the difference in the first few seconds.

So now to find a matched set of Raytheons. Although I'm not sure whether to stick with those or try Sylvanias, which I've always liked for *SN7 valves. The problem is the 6V6 is used primarily as a guitar amp valve, so most of the opinion out there is from guitarists operating them at the ragged edge.
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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izzy wizzy
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#135 Re: The Newton 300B

Post by izzy wizzy »

Thermionic Idler wrote: Tue Feb 16, 2021 7:06 pm Ah yes, I've been looking at the uTracer. Also the eTracer which has enough -ve bias to measure the 300Bs but it's ... a bit spendier.

The Raytheon 6V6's went back in tonight, I couldn't take any more. Aaaaaaand as if by magic... the music's back! :D

I thought it would be something I'd have to listen carefully for. Nope, even Meredith who was in the kitchen area prepping the chicken and not listening particularly closely, could hear the difference in the first few seconds.

So now to find a matched set of Raytheons. Although I'm not sure whether to stick with those or try Sylvanias, which I've always liked for *SN7 valves. The problem is the 6V6 is used primarily as a guitar amp valve, so most of the opinion out there is from guitarists operating them at the ragged edge.
Good result. I'm no guitarist but don't those folk like tone? So not a bad recommendation for starters? Not all are on the "ragged edge" AFAIK.
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