The Newton 300B

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Thermionic Idler
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#151 Re: The Newton 300B

Post by Thermionic Idler »

Thanks Steve.

Forgot to post a link to the final schematic - PDF here.
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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Irene Idler
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#152 Re: The Newton 300B

Post by Irene Idler »

Mr. Idler posted photos of these on Facebook and one of my Texas friends asked for the schematics and wants to build his own now. He's a NASA engineer (he's designed and built stuff that's now flying in space) with his own wood/metal shop, so I expect he's up to the challenge. His wife may not forgive us for introducing him to another time-consuming hobby, but I'm sure once she hears them she'll be delighted.

I bet his won't be as pretty though. Cressy did brilliant work on the wood. I didn't think I'd like the look of these as much as I did the Beasts, but they're stunning -- and they sound even better now that Dave's tweaked them a bit. The level of transparency is absolutely perfect; I've heard systems that sounded airy and insubstantial, but these have all the presence you want, along with absolute crystal clarity. Admittedly I'm biased, but Mr. Idler does amazing work.
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Ray P
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#153 Re: The Newton 300B

Post by Ray P »

Irene Idler wrote: Sun Feb 28, 2021 6:51 pm Admittedly I'm biased, but Mr. Idler does amazing work.
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Thermionic Idler
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#154 Re: The Newton 300B

Post by Thermionic Idler »

Powered up the system last night to watch Line of Duty, to find almost no sound from the left channel amp. Well there was a sound, a very quiet sort of farty rendition of the signal.

Popped off the cover this morning to find both HT fuses open-circuit, so looks like another GZ34 bit the dust (even though I fixed the issue with the power supply resistance being too low). Replaced the GZ34 and fuses and all was well. Quite glad I designed in that fusing (I followed Merlin Blencowe's method).

It's a damned shame that NOS GZ34's are so depressingly expensive - with the design using four that's just not a viable option, but these new production things seem a bit on the fragile side. The ones I got from Brimar come from China I think. I ought really to have done the sensible thing like Steve and use TV damper diodes, but I wanted the amp to use current production so designed it for GZ34, and with two octal sockets I'm kinda committed now. Starting to wonder if I shouldn't go back to Sovtek.
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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#155 Re: The Newton 300B

Post by Thermionic Idler »

Ruling out NOS GZ34's for now due to sheer cost, I've decided to try substituting 5U4's in the output stage, to see if I can get around this issue of new-manufacture GZ34 unreliability (perusing various forums suggests that pretty much all current production is questionable).

The 5V secondary is rated to 3 amps so no problem with the extra filament draw. I've sent off for a pair of Svetlana winged C tubes from Ukraine, they are inexpensive and seem to be well thought of, and in this application should be coasting.

I'll probably also try substituting my existing Mullard GZ32's back in on the input stage as the current draw there is only 95mA, well within its capabilities.

Both these actions will reduce the HT voltage a bit, I guess we'll see what that does to the sound. TBH I'm unsure where the operating points are at this point, having moved from a place that had 245V line voltage to one where it's 235V - oh and we also have solar panels here so I probably ought to measure the voltage off-grid (night) and off-panel (day)...
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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#156 Re: The Newton 300B

Post by Thermionic Idler »

I just found this little nugget which was quite interesting:

https://www.dhtrob.com/overige/idh_vs_dh_rectifiers.php

so he seems to think it's best from a reliability perspective, to have DH rectifiers matched with DH valves and vice versa, because the warm-up times will be in sync. According to him, my design (IH rectifier, DH output valves) may be causing cathode stripping on the output stage rectifier for a few seconds at switch-on and that may be why I had the failure this week? Possibly new-manufacture rectifiers are more prone to it than old-stock?

Or is it all bollox? I know that Rozenblit for one, doesn't think cathode stripping is a thing.

Regardless, rather cunningly the split power supply does actually allow me to match the rectifiers to the valves in that manner. Nice! :D
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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#157 Re: The Newton 300B

Post by Cressy Snr »

Funny you should mention DH rectifiers Dave. I’ve been pondering the same issues as you. It might be nonsense, this cathode poisoning thing, but I’ve been mulling over fitting either a Type 80, or a 5U4G in my rectifier position. Most 45 amplifiers, including the well-respected Yamamoto A-08 seem to use use 80s or 5U4Gs as their rectifiers.

I may have a go with a 5U4G tomorrow and see what happens.
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Nick
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#158 Re: The Newton 300B

Post by Nick »

may be causing cathode stripping on the output stage rectifier
They are rectifiers, so by definition they spend half their working life not conducting.
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#159 Re: The Newton 300B

Post by Cressy Snr »

@Dave.
Hi Dave,
I’ve just finished modifying my 45 amp to take 5V rectifiers and have put in a 5U4G of the same kind you have ordered.
It sounds excellent. I don’t think you’ll be disappointed.
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Nick
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#160 Re: The Newton 300B

Post by Nick »

I have spent a bit of time looking around. I can't find any authoritative text that indicates that "cathode stripping" as described in that link occurs, the only reference I can find I related to high voltage tubes, and some to grid emission. Exactly how much force do you think 500v will apply to a cathode? I suspect that laying the valve on its side will apply more force to the cathode coating.
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Thermionic Idler
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#161 Re: The Newton 300B

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Nick wrote: Sat May 29, 2021 12:45 pm I have spent a bit of time looking around. I can't find any authoritative text that indicates that "cathode stripping" as described in that link occurs, the only reference I can find I related to high voltage tubes, and some to grid emission. Exactly how much force do you think 500v will apply to a cathode? I suspect that laying the valve on its side will apply more force to the cathode coating.
Probably a red herring then - this rings a bell actually, I vaguely recall reading somewhere that yes, it's an issue, but one that only transmitter engineers need to worry about. I guess I was just casting around to ascertain whether there were any factors contributing to this failure after only four months in service other than "valves fail" - I cannot think of any data sheet parameter that I am even close to exceeding.

I'm still going to re-jig the circuit for the 5U4 though, I think I'll be able to get away with bypassing the power supply resistors I added to stop the GZ34's arcing, because the 5U4 calls for less power supply resistance, and that should help offset the increased voltage drop. I'll need to check the numbers to make sure though.

EDIT - looks like I would still be JUST over the limit if I took out those 68R HT resistors - I still need about +7R on the left channel and +5R on the right channel. It's close! Parallelling some 12R wirewounds with the existing 68R's (= 10.2R) should do the trick, and then it'll be easy to set it back up for GZ34's in the future if I need to.

Last night I substituted my two coke-bottle Mullard GZ32's back into the input stage and the sound seemed to open up a bit as a result.
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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#162 Re: The Newton 300B

Post by Thermionic Idler »

I've been doing some more reading to try and bottom out my understanding of power supplies - I've even ventured into the Radiotron Designer's Handbook. Along with this, more modelling in PSUD.

So this business of having to have a certain amount of power supply resistance for capacitor-input - the research I've done suggests that this required resistance really isn't fixed to just the voltage - it can vary with the value of the first capacitor in your power supply as well. The figure to actually pay attention to is the "Transient Peak Plate current per plate", which is specced as 3.7A for the 5AR4.

I think the 5AR4 datasheets generally assume the first capacitor to be 60uF, and modelling in PSUD confirms this - it peaks at 3.12 amps on the first couple of cycles if the PS resistance is set to the specified 125 ohms. Drop this to 60ohms and the amps go up to 4.76 - well above the design limit.

However, if I drop the value of cap 1 to 5uF as it is in my amp, then even with 60 ohms resistance, the peak comes out at only 1.74 amps, which is quite a ways inside the design limit. Changing to 125 ohms - the effect of adding my resistors - and the peak only falls to 1.56 amps. Not a whole lot of difference really.

All this suggests that the limiting resistors I installed in February are probably not needed because of the capacitance being so small, even for the GZ34, and can certainly be done away with for the 5U4. At least if the tubes are performing to their datasheet specs(!)

I think what I will do though, is implement Nick's suggestion of a couple of 1N4007's on each leg going into the rectifier. It seems quite a few people have done this and basically report a win-win - the modification is sonically transparent but also there are fewer rectifier failures. Some seem to think it's actually the PIV (peak inverse voltage) that the Russian and Chinese rectifiers have trouble with, and the 1N4007 mod takes that out of the picture.

I think this post sums things up quite nicely:

https://www.diyaudio.com/forums/tubes-v ... ost5152731
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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Paul Barker
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#163 Re: The Newton 300B

Post by Paul Barker »

Not meaning to sound smug, but this is all new science based, or rather required, because of poor modern rectifior valve construction quality and ignorance of first cap requirements.

Ive never broken a valve rectifier over the short 23 years in the game. But in the conditions Ive never exceeded first cap rule or any other parameter, and ive only ever used nos original valves made when the charts were published. the manufacturers around then made the rectifiers fit for purpose, no ambitions plans or workarounds for sub standard production.

Never broken a rectifier valve. Most direct heated but not all.

Ive given some of my builds a real hard time; unwittingly exceeded specs of many valve especially 811a shunt regulators, cooked them rather hard. But they were cheap. had choke input transformers short on one of my 212 builds. Blown a black gate wkz, watched balloon px25’s fragile old filaments pop on turn on. Nick and I deliberately exceeded spec on gm70’s.

But Ive never exceeded spec of a rectifier and never blown one. Maybe you guys arent buying fit for purpose rectifiers. Ive used a lot of times u19’s. a lot of lower powered designs had 5u4’s and equal amount potato mashers.

Make sure youre chokes are twice as big as you think you need. wound to take twice the voltage you think you’ll expose them to. Over rate youre caps. Watch out for damage to valve shunt regulators. Those are the unreliable items that will combust in a split second. The shunt valves give you fair warning. The chokes and caps dont. Neither do the rare valve filaments. puff out like a light bulb.
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#164 Re: The Newton 300B

Post by steve s »

Must say I too have had little trouble with gz 32/34/37 /5u4 rectifiers.
An 8 uf 1st cap or usually much less. ( That's my tuning cap to get the voltage I want )
And as for the resistance, what I add is only to get equal resistance on the tx winding, there's usually enough resistance in the winding itself.

But I always use the old valves..
I have a decent stock of gz32 and 37s ? Let me know if you are stuck
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Thermionic Idler
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#165 Re: The Newton 300B

Post by Thermionic Idler »

steve s wrote: Wed Jun 02, 2021 12:14 pm I have a decent stock of gz32 and 37s ? Let me know if you are stuck
Thanks Steve - if you have any Mullard coke-bottle GZ32's that might be useful, as I only have the two and no spares - though hopefully they'll last a good long time especially after the 1N4007 mods are done. They seem to do a nice job of running the input stage, where the current demands are that much lower.
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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