How to determine B+

[PlantEater]

I notice the peak voltage from my Ampeg V2 power transformer is 626V while the B+ is around 550V DC.  How does one calculate this?  Internal PT resistance, OT resistance, etc?

Thanks

[shooter]

How does one calculate this?

very few do, it's quite complex and for tube amp build design, not really worth the learn'n curve, or re-learn'n what's forgot 


there are so many examples of things already proven that stealing is lots faster, easier, and less prone to calculator errors

[PlantEater]

I was thinking more form basic principles for a rectified power supply and a resistive load under idle, no input. Since B+ doesn't equal Vpeak, there must be a voltage divider at play - OT and PT windings, effective Tube resistance.  Just trying to understand why my B+ is so much lower than spec,40v - something wrong or just  within expected variation?

[PRR]

Schade published the complete study:
Schade, O. H., Proceedings of the I.R.E, 31, p341-361, (1943)
https://diyaudioprojects.com/Technical/Tube-Power-Supplies/

In the late 20th century, Duncan's got popular:
https://www.duncanamps.com/psud2/

[PlantEater]

Great resources. Thanks!
Didn't realize one had to consider the primary winding resistance in the mix.

[acheld]

That's why it is called a "circuit."

I know it is obvious, but it took me awhile to figure that out . . . 

[Willabe]

Different tube rectifiers drop different amounts of voltage too.

A PT that has high mA rating will run higher dcv than an under rated PT. It will sag under idol and even more with full load.

Bias the output tubes hotter, the dcv will drop, bias the output tubes colder the dcv will rise.

[PlantEater]

  I'm a slow learner but fast forgetter - little capacitance, little resistance.

Makes sense biasing hotter would draw more - more drop.  Trying to figure out why my B+ is 40V low.

[shooter]

Trying to figure out why my B+ is 40V low.

40v(DC?) is less than 10% of your stated 550vdc.  aged tubes drawing a few extra mA each will drop volts
drifting/wrong Resistors will drag on volts.  Leaky caps will drag volts.


the main values you want is PA tube dissipation and PA tube bias.  those 2 values goes a really long way to understanding volts "loss"




does it sound good?  IF yes;
play til it breaks, study when not playing so when it breaks you're set.

[PlantEater]

Exactly. It was cold bias that made me wonder about low B+. Bad distortion at low volumes - replaced bad 7027a's with Ruby Chinese 6L6's which pretty much cleared that up, but fixed bias was a few volts high and tube dissipation was only 5W.  Lowering bias resistor from 75K to 65K helped as did replacing most first stage of filter caps - up to around 9W (waiting on more 40uf's) Don't want to lower bias resistor too much according to the Valve Wizard http://www.valvewizard.co.uk/bias.html

[Willabe]

..... but fixed bias was a few volts high and tube dissipation was only 5W.  Lowering bias resistor from 75K to 65K helped as did replacing most first stage of filter caps - up to around 9W (waiting on more 40uf's)

Are those GB or GC 6L6's?

Either way 9w dissipation is way too low. 

How are you determining the 9w dissipation?

[PlantEater]

Bias is actually now up to 15W with first stage filter caps replaced. Waiting on 2nd and  preamp filter caps to come in. For power I measure each OT leg winding resistance and measure voltage across to get current and multiply by plate voltage.

Tubes are Ruby 6L6GCR about 15 years old -