Rectified Voltage

[Smokebreak]

Ive been referring to this current draw calculator lately for help with PT selection for different designs :

http://www.dreamtone.org/Calculate_Current_Form.htm

It also shows rectified B+, which can come in handy.
However, I plugged in the numbers from the Rocket voltage chart, with 250-0-250, and calculator says 340VDC for B+, where Rocket voltage chart lists 290. Why is there such a large discrepancy between the two? [/url]

[JazzGuitarGimp]

Could this be the difference between a SS rectifier (higher B+) and a tube rectifier (lower B+)?

[Smokebreak]

I don't believe so. I plugged in GZ34 in the calculator, just like the Rocket has. I just found this handy table too

https://tubeamparchive.com/files/transf ... es_120.xls

It lists plates of 250-0-250 at 325 for GZ34, which is closer to inline with the calculator's 340 B+ , given B+ will be a bit higher than plates.

[EtherealWidow]

Maybe this is the unloaded voltage? Unloaded a 250-0-250 PT would be about 350 (250x1.4) For loaded voltage, using silicon diodes, the B+ will be around 300 (250x1.2) If using a valve diode it will be right around the original 250.

Here's the article for FW http://www.valvewizard.co.uk/fullwave.html

Here it is for FWB http://www.valvewizard.co.uk/bridge.html

Content is roughly the same.

[Smokebreak]

Maybe this is the unloaded voltage?

This could very well be the case with the calculator. ..I'm not sure I follow the 250AC to 250DC tube rectifier bit though. In my experience DC voltage on the cathode is always higher than AC on the plate , no?

[tubeswell]

Calculators like this only give you a ballpark. If anything its predictions are a little on the high side AFAICT (I plugged in a few values of amp builds I've done and knowing what the actual voltages are, this calculator is giving 20-30V higher)

[Smokebreak]

And in the case of the rocket, about 50V! To cloud things further, if I understand correctly, when considering a "loaded" PT, it seems good practice to never "fully load" a PT, more like 2/3 loading(150mA PT for 2 el84/6V6 seems standard practice), so estimations get even more "ballpark" .

For the sake of discussion, let's say you were building a 2x6L6 amp, and wanted 400V on the plates(exactly!), biased at 70% max diss. Excluding loaded rated PTs(Edcor,Pacific), how would you go about selecting a PT for any given rectification device? Let's assume wall voltage is steady 120, and that 5-10V difference made a huge impact on tone. Is ballpark as good as it gets?

[EtherealWidow]

For most hobbyists ballpark is good enough. I mean, the resistors alone in a typical amp have a +/-10% tolerance anyway.

[randalp3000]

I've found the same thing with my EL84 builds. 300-0-300 230ma transformer should give me 390v with a 5AR4 but I keep getting around 345v.

I just got a new Hammond rated at 287ma, will this cause less voltage drop or should it just run cooler?

[martin manning]

I don't think 5 or 10 volts out of 3 or 4 hundred will make any noticeable difference... it's only a couple of percent. The other thing to keep in mind is that the transformer voltage is probably going to be given either fully-loaded or unloaded, and people are always talking about hitting a given voltage at idle, which is a part-load condition. Also, the rectified DC voltage for a given AC voltage is dependent upon the load and the filtering, so there are other variables involved.