Calculating loaded B+ values from transformer specs

[beasleybodyshop]

Hello Everyone!

I am piecing together a build and was wondering if someone could possibly check my math a little bit and maybe give me some insight into planning for loaded voltage drop.

The PT I am using has a center-tapped high voltage winding (185 - 0 - 185vAC). Based on Hammond's super useful design guide for rectifier use:

http://www.hammondmfg.com/pdf/5c007.pdf

I could go one of a few different ways. My goal is to drive a quad of EL84s in cathode bias - So if I use a SS rectifier, and following the HALF WAVE capacitor input load diagram, can I expect [370 x 1.41 = 521vDC] unloaded?

Or would it be more realistic to use the V(avg) D.C. calculation and say, [370 x .90 = 333vAC] Unloaded?

I know my PT has a Center Tap, which I could just effectively tie off an not use. But I don't see a way to use a Full Wave rectifier setup and get anywhere near 300+ volts.

And I know these spec values aren't taking load into account - I would assume I use a calculated current draw for a quad of EL84s to figure out what the loaded voltage is?

Any thoughts or help with the math on figuring out the loaded voltage i could expect would be great. I was a History major in school mostly because I failed college Algebra twice so any help is appreciated.

[martin manning]

You have that right. 185-0-185 full wave (or two-phase) gets 185*1.414=262V, or 2*185*1.414 using a FWB to get twice that (523V). Too much for EL84.
If those are unloaded voltages, loaded voltage will be roughly 5% lower than those figures. What you need is an AC30 PT, no?

[beasleybodyshop]

Yeah I suppose that's what I should do I was under the impression that this PT was meant for EL84s possibly just wanted to make sure the math was right.

For the sake of argument, I've dug through some tube data sheets and it seems like this PT would be perfect for the 6AQ5, which is rated for roughly 250vDC plate voltage. Does about 10W in PP configuration. I figure this PT would easily do a quad of these fairly cheap and plentiful tubes for about 18-20 watts. Which would be plenty enough for my project realistically speaking. They spec a lot like a 6V6 from the looks of it:

[martin manning]

Looks like that would work. OT primary Z would be 10k for two tubes in push-pull at 250V Va and Vs.

[wpaulvogel]

What's the current rating for this transformer? What brand?

[Phil_S]

At 250V, there is no meaningful difference between 6AQ5 and 6V6. In fact, the data sheets are interchangeable.

FWIW, I have used a pair of old stock, very strong testing 6AQ5 running 310-315V and they seem to be very happy there. The point is that you have some wiggle room. And, yes, they can be bought inexpensively, but the problem is finding a quad that both tests strong and that are reasonably well matched.

For a reasonable price, you could buy new production JJ 6V6 and get them properly matched. For example:
https://www.tubesandmore.com/products/v ... lectronics
https://www.thetubestore.com/jj-6v6s
https://www.tubedepot.com/products/jj-6 ... acuum-tube
https://www.eurotubes.com/store/pc/6V6S ... ir-p41.htm

For a quad, allowing for a few preamp tubes, you'll need capacity of about 165mA on the secondary HT winding.

Since there is no free lunch -- every decision revolves around mA capacity and voltage, you might as well throw into the mix consideration for a pair of 6L6.

[beasleybodyshop]

Here's the cut sheet for the PT. Doesn't list much in the way of current ratings for HV winding but lists a whopping 7 amps for the heater winding, and 3A for a tube rectifier winding. While I don't know the current rating for the HV winding I would expect that the PT is meant to drive at least a quad of output tubes of some sort.

[beasleybodyshop]

.

IMG_20200126_173951828.jpg

[beasleybodyshop]

I suppose I could also "mismatch" the primary winding a little and change the voltages a bit as well.

I never considered using 6V6s, but if those are on the table then I suppose EL84s are as well? My initial goal was to run a quad of EL84s.

[sluckey]

I'm pretty sure that PT is meant for some amp with a quad of BIG tubes. One phone call to MM will solve the mystery.

[Colossal]

I'm pretty sure that PT is meant for some amp with a quad of BIG tubes. One phone call to MM will solve the mystery.

That looks like a 100W PT for a Marshall voltage doubler circuit.

[beasleybodyshop]

Ok I got off my lazy ass and wired up the PT to my variac and measured 220 - 0 - 220 vAC on the HT winding.

This PT is a Mercury Magnetics one labeled "FM100-P" which shows as a general purpose 100W PT on their website and nothing more. Beginning to wonder if maybe I don't have the right datasheet. Size wise it's at least as big as a Tweed bassman PT so I feel like size is more than plenty for my 20-30 watt needs. Lol

[wpaulvogel]

That’s a 100 watt Marshall Plexi transformer. It’ll work but you can also sell it for $200 any day of the week and buy a more suitable transformer for your application. A Heyboer Trainwreck power transformer or a AC30 power transformer would be much better.