Dynaco ST-70 Series-3, by Radial Engineering

[emrr]

http://www.dynaco.com/inside-story.php

 

[scott2000]

Sexy

 

[DaveP]

Thanks Doug,

Very interesting read

DaveP

 

[Speedskater]

I notice that he writes about an US 115 Volt AC line.  In the day and age it would be better to design for a 125 Volt AC line.

The last NEC code books referred to it as a 125 V line. 115 V is ancient history.

An overvolted power transformer is not a happy transformer.

 

[emrr]

That's funny.  Not sure it's any big deal though. 

 

[DaveP]

Dual voltage transformers sold in Europe are 115/230V

DaveP

 

[Gene Pink]

I notice that he writes about an US 115 Volt AC line.  In the day and age it would be better to design for a 125 Volt AC line.

100 percent agree. At my house it is 123-124V.

About 15 years ago, a friday night in the entertainment district in downtown Austin, TX, there was a whole block of bars and clubs blacked out on 6th street, utility guys crawling in and out of manholes, I asked one that was just standing around "What's the power distribution down there, 2400 or 120/208?" He looked at me funny and said "You mean 125/216."

Ahhh. The slow creep-up, I call it. We were 110, 115, 117, 120, and now 125-ish.

Gene

 

[DaveP]

Is this because you have different voltages in different states, that is to say you don't have a national grid?

DaveP

 

[PRR]

> The last NEC code books referred to it as a 125 V line.

Citation? I do not recall the NEC taking a stand on voltage except </>300V. But I only have five older copies here.

> Is this because you have different voltages in different states, that is to say you don't have a national grid?

We have 3 grids: east, west, and Texas. Yes, Texas is a grid to itself. (There's also a part of Maine fed from Canada not the US, though further west the Can and US grids interconnect.)

But on the way down from 100KV there are many multi-tap transformers and typically voltage regulators. Long lines are NOT simple dropping resistors. End-point voltage is a complex thing.

On my street it is fairly simple. No large (factory) motors. Transformers over-provisioned against growth that did not happen. Not-long runs to a sub-station which appears to regulate against the sometimes huge drops from dam and distant gas burners. AFAIK the high line runs 13KV and a pole-pig at each couple houses drops that to 125V+125V fairly solid at the street. However my house is a long way back from the street. I get over 124V when load is light, under 110V when the dryer and water-heater run. This was installed by the builder, and is dubious, but not worth upgrading.

OTOH, stuff happens. I worked in a city with two feeders. After a hurricane, a backhoe (JCB) took-out one of the feeders. The city was re-tapped down to ~~110V to limp along. My office complex had been nomially tapped at 117V and was now 108V. The substations were in bad places, a plan to improve both began, so we worked that way for about 5 years. Mostly everything ran fine. However I was building/testing an experimental tube amp, all my numbers were low.

 

[Gene Pink]

Is this because you have different voltages in different states, that is to say you don't have a national grid?

DaveP

Many grids in the usa, not frequency synced, but I believe the voltages are all similarly above the 120V standard, as most I have measured run high. Admittedly not a large sampling.

https://en.wikipedia.org/wiki/Continental_U.S._power_transmission_grid

Interesting thing in there, "variable frequency transformers" for connecting different grids that aren't synced. New one on me. Clever.

https://en.wikipedia.org/wiki/Variable-frequency_transformer

Constant power loads (motors, switching supplies) when fed a higher voltage will draw less current. Less current means putting off expensive upgrades to the grid.

Jpeg:
Left meter is line to neutral, and right meter is neutral to ground in a half-tansed attempt at taking a kelvin measurement. Ground and neutral are connected at the panel, no current in ground (hopefully), then G-N measures the voltage drop on the neutral from panel to this outlet being measured. Neutral and hot are the same size and length, should be the same V drop for the hot. So (N-G x 2) + L to N should be what it would measure in the panel, or ~124.6VAC.

Gene

 

[pucho812]

in places like L.A. we got daily fluctuations based how many people turned on their hvac that day. On super heavy usage days we can get the occasional brown out... 

 

[Speedskater]

> The last NEC code books referred to it as a 125 V line.
Citation? I do not recall the NEC taking a stand on voltage except </>300V. But I only have five older copies here.

It's in the ratings for components. (from a 2011 book)
Article 406.3(B)
Receptacle rating.  .........not less the 125V or 250V...............

 

[JohnRoberts]

Is this because you have different voltages in different states, that is to say you don't have a national grid?

DaveP

The voltages can vary by neighborhood, now if the different states had different mains frequency that could suggest different isolated grids.

There are a small handful of different grids in the US and their interconnections are high voltage DC so the mains frequency in CA can be different than NY, in the short term, but long term I suspect the mains frequency is pretty stable.

JR

PS: I've told this story too many times, but one evening (last century) I noticed that my incandescent lamps were unusually bright. Since small variations in brightness should not be noticeable I dragged out my VOM and measured 140V on my nominal 120V outlets. Long story short a bump load leveling transformer at my local substation a couple miles away, was stuck on boost, and voltage would have kept climbing as load dropped off that night, if the utility didn't unstick it. The utility worker didn't believe me at first, but it was a slow night so he drove out and confirmed the over voltage with his own meter.

 

[PRR]

> It's in the ratings for components.

Precisely. The receptacle must be rated 125V, even though the utility may give you 120V, 115V, even 108V.

Attached is from my 2008 NEC, but I found other confirmation that this section got renumbered.

Questions.

Many-many receptacles are sold as "120V". I don't care to pull one out of the wall and see what the actual rating is.

Nobody believes a "125V" part will spark arc and fry at 126V. But what about JR's 140V? I'm not real worried, but how much up-side is tested into today's "125V" parts? Or 40 year old parts in our homes?

 

[JohnRoberts]

> It's in the ratings for components.

Precisely. The receptacle must be rated 125V, even though the utility may give you 120V, 115V, even 108V.

Attached is from my 2008 NEC, but I found other confirmation that this section got renumbered.

Questions.

Many-many receptacles are sold as "120V". I don't care to pull one out of the wall and see what the actual rating is.

Nobody believes a "125V" part will spark arc and fry at 126V. But what about JR's 140V? I'm not real worried, but how much up-side is tested into today's "125V" parts? Or 40 year old parts in our homes?

The issue with outlets is contact current (and heating). Spacing wrt voltage is probably high enough to ignore the typical lightning hits to (or near) outside power lines.

Speaking of spacing, back in the 80's a lawyer called me up to hire me as an expert witness when his client claimed that voltage from his home power drop (typically 240V) jumped some distance to his aluminum ladder and injured him.  I told the lawyer that the distance a 240V electrical source could jump is measured in thousandths of an inch if that ( much less than a spark plug gap). 

I didn't get hired for some reason. 

JR

PS: I expect 120v outlets are not rated for 240V just to make sure no bubba's use them that way...(and hilarity ensues ).  I have replaced a couple,  several decades old outlets because A) contacts became hot (high resistance) and B) became intermittent. 

 

[PRR]

The 125 and 250 (and 208, 440, 600...) numbers come from NEMA. The intent is that you can't plug a 120V device into a 240V outlet. The numbers are purely nominal, to indicate service type, not actual or maximum voltage.

The NEC is as puzzling as ever. There are no NEMA receptacles under 15A. Ah, NEC is not endorsing NEMA. You "could" use devices specified and Approved (specific meaning) from other systems. In fact I do not know that a UK receptacle would be instantly banned from US work (it has a UK rating of 16A). On inspection, it lacks any US/CAN-based ratings so could be banned just for that. Same reason I can not use a cigar lighter socket for a house outlet: not tested for the purpose.

 

[Speedskater]

While 'UL' does permit other connection systems in high tech lighting systems, I don't know if NEC will allow them in outlet boxes.

Note that you plug a cord into a receptacle and the receptacle is mounted in an outlet box, not withstanding the some manufactures are now labeling their receptacles as outlets. 

 

[alexc]

Interesting to see how the pricing will be  - if it's competitive, I could see a lot of success for this modern re-imagining of the venerable Dynaco ST-70.

Looks like quite a few improvements made - with some nice speakers and a really nice preamp/eq it would pretty much answer a lot of needs.

Still, in today's world of many options, price is a big component. Radial makes some very nice equipment; I would expect it would be priced 'accordingly'.

For the diy-er, I think 'parts builds' will remain the best option to affordable, high quality tube-ism.

 

[emrr]

I swear the price was in there somewhere, don't see it now.  I recall $3500 US. 

 

[mjrippe]

I swear the price was in there somewhere, don't see it now.  I recall $3500 US.

I hope you are mistaken, Doug.  AFAIK a vintage Stereo 70 only goes for ~$750 in working order.  No way will they get over 4x that price.