5AR4 Power Supply Question

[JJH0906]

Hey all,

Happy Holidays to you! What a better way to spend Christmas Day than working on an amp build project, right?

I have a question for fellow builders on the initial power supply. I've chosen to use a 5AR4 rectifier and the DC voltage I'm getting out of it seems really high. Attached are the schematic for my power supply: I'm using the 5AR4, a Hammond 290DX power transformer (datasheet also attached) and CLC filter circuit that uses an Edcor 10H/200 mA inductor.

The deal is, I'm supply the power transformer with 122VAC, so very close to the TX rate 120VAC supply. However, I'm getting 725VAC on the full high voltage secondary (~360-0-360), no where close to the 650VAC that the TX is rated for. The VDC is also massive, with the output voltage showing 496VDC at the top of C2 for the B+. Now, none of this is loaded yet so perhaps that's what's throwing me off but this is honestly the first time I've powered up a tube rectifier, so I'm not sure what's "normal".

Any input you might have is much appreciated!

[martin manning]

You've attached 290CX data sheet, not 290DX as you say above. The DX data sheet says no-load secondary voltage is 712.3, so I'd expect something close to what you are seeing.

[JJH0906]

Agh! Busted again! Sorry about that, I had the sheet for the TX I had intended on using.

OK, thanks for the observation. I was just really surprised to see it so high but after I figure out the rest of the power needs, I'm sure that'll buck right down. I don't have a high ohm/watt resistor I could just stick in there but understandably it'll start to trickle down once it gets loaded.

[pdf64]

Compared to solid state rectification, there won’t be much if any voltage dropped until HT current is drawn.
No current, no sag.

[JJH0906]

While we are on that subject, who would like to educate me on what the definition is of + and HT+?

I have the general concept but in a lot of the research I've done, I've never gotten a clear answer on what's what.

[pdf64]

It’s somewhat anachronistic terminology but high tension HT means high voltage. As opposed to LT low tension. I’m not aware of the division between the 2; assume danger level, around 75V.
In valve amps, HT is (almost) always positive, so it goes without being noted.
There’s typically + and - LT supplies for solid state circuits though.
I suppose a fixed bias supply might be referred to as LT-, but it’s not usually done.
North American terminology is B+ for HT and C- for bias. With A being the heater supply. It dates back from the days before mains electricity, when a radio set would have a separate battery (A, B and C) for each supply, with A battery being low voltage high current, B being high voltage medium current, C being medium voltage low current.

[JJH0906]

Wonderful explanation! I'm still confused, haha....

That's what was throwing me off, I would see references to HT+ and it seemed to just be the high voltage (B+, which I have come to learn stands for "battery", from days of yore), but then there are multiple high voltage values, depending on what's going where.

[pdf64]

Yes, HT is non specific, in that in addition to the rectifier-reservoir cap node, there will be multiple other nodes at HT. But current drawn from any of them will result in HT current flow, eg via the rectifier, PT HT winding, 0V common.

[JJH0906]

Ok, I just...I mean.... How does.....

Ha ha! I get it. Sort of ...

[pdf64]

It should be fairly apparent from the context what exactly is meant when HT / B+ is mentioned. If not, ask
The only daft question is the one not asked for fear of looking stupid

[Phil_S]

To build on what was already said, HT means high tension (Brit speak; similar term in French and probably all the Romance languages) or high voltage. A transformer may have two or more windings that are often named by functional intention: Filament, HT (or American English, high voltage), Bias, etc. Stevem is on a one man crusade to use the convention V+ in place of B+. I tend to agree with the idea, but feel like it will never get traction because B+ is so entrenched in the discipline.

B+, as you note means Battery. This is a holdover from the days when DC voltage was typically derived from batteries. These days we understand it to me rectified high voltage from a transformer, thought I suppose it could also be understood as supplied from a battery, depending on context. Plus and Minus are understood as arbitrary conventions.

[Stevem]

Yes, it’s very high time to rise out of the early 1900s terminology!!
Only in terms of tube based electronics is V+ or V- not commonly used.
And there is zippo logic in that to me!

[pdf64]

…
B+, as you note means Battery. …

I don’t think B is an abbreviation for battery per se, rather it indicates the B battery for the HT supply, as opposed to the A battery for the heaters, or the C battery for the bias. See https://antiqueradio.org/bsupply.htm

[martin manning]

I don't think using "B+" is as bad as all that, but using V+ or V- is too non-specific. If I got to choose I'd use Vaa, where doubling the subscript refers to the supply voltage as opposed to the voltage at the terminal (a = anode), the same as Vcc or Vdd is used for the supply voltage for bipolar transistors and FETs, respectively. A negative supply voltage for the cathode (k) would then be Vkk.

[pdf64]

Wouldn’t a negative bias supply be Vgg?

Bells and whistles amps may have several V+ supplies.

It would be nonsensical to refer to the PT’s HT winding as the V+ winding. Particularly as it may also be powering a V- supply.