Expected B+ fully loaded and biased

[pjd3]

Hello,

I was hoping for a quick sanity check here. Thanks,

This is my first diode rectification tube amp. I have Musical Power Supplies PT @ 330-0-55-330vac and a pair of JJ 6V6's Push-pull.

Unloaded, I was seeing about 467vdc at the first filter cap. Following the full set of tubes put in (3 12ax7s and 2 JJ 6V6's) and biasing at around 65%, the B+ has dropped down to around 435vdc. On one hand, it was relieving to know that I wouldn't be over-dissipating screens, I admittedly was not expecting the B+ to drop that much. Maybe down to 450 or so but, 435vdc just seemed like notably more than my other builds that used a tube rectifier.

Does this seem to be in an acceptable range for B+ voltage drop considering?

Thanks very much,
Best,
Phil D, (pjd3)

[johnnyreece]

Is this the PT330Mi? Are you using both sets of primaries? The data sheet shows the black and white primary connected and the brown and blue connected. If only using one set, I could see that over-stressing that PT.

[pjd3]

HI johnny,

It is the PT330Mi and I did use both primaries, wiring it up as diagramed for 120vac operation.

Best,

Phil Donovan

[Stevem]

I don’t see where that company offers is 330/55/330 like you say you have.

I see a 325/55/325.

If you have clip leads then set your meter for amps.
Lift the B+ center tap and put the meter in serise.

Can that PT you have supply like 120 ma I guess?

What does your meter show?

Most PTs will have a 10 to 15% regulation factor once there loaded to the max rated amperage.

330 vac rectified thru diodes will give you 462 vdc, cut that back by 10% when the PT is dishing out 120 ma and now your vdc is 416.

[pjd3]

Hey Stevem,

Yes, the PT is 660vac across the coil and specs 130mA.

The model is found on the "Products" tab, the 2nd entree from the very bottom of the page.

Thank you,

Phil D

[pjd3]

Stevem,

According to your tolerance figures, I'm within a reasonable amount of voltage drop. I just wanted to make sure since it seemed a bit lower than what I am used to.

Best,

Phil D. (pjd3)

[pdf64]

https://www.musicalpowersupplies.com/ap ... 281%29.pdf

[katopan]

We say diode rectified gives 1.4x AC voltage as the DC, but this is unloaded. My rule of thumb for valve amps incorporating the first filter cap ripple has always been 1.32x. 330 x 1.32 = 436VDC so in my books you're spot on where it should be.

[R.G.]

We say diode rectified gives 1.4x AC voltage as the DC, but this is unloaded. My rule of thumb for valve amps incorporating the first filter cap ripple has always been 1.32x. 330 x 1.32 = 436VDC so in my books you're spot on where it should be.

Decent rule of thumb for tube rectified supplies. The one-step-finer rule is 1.4 * AC minus the rectifier drop and any resistive losses in the transformer.

Here's some things I posted here on transformers:
https://ampgarage.com/forum/viewforum.p ... f479a274c0

I did some more posts on tube vs semiconductor rectifiers as well:
https://ampgarage.com/forum/viewtopic.p ... 28#p472728
The bottom line is that tube rectifiers have a big forward drop and can be modeled as a semiconductor diode plus resistances to any degree needed. At "typical" guitar amp currents, maybe 100ma to 500ma, they're going to drop 30-60V off the AC wave peak.

Currents into the first filter cap can only happen when the AC mains voltage through the rectifier is bigger than the first capacitor voltage. For reasonably low values of ripple, this only happens leading into the peak of the AC voltage, so current goes into the cap in short pulses near the peak. That means that all the current for the outgoing DC has to be done in a short pulse every peak. So the peak current must be several times higher than the DC current out. That means that the rectifier voltage drop is even bigger.