Output Transformer formula?

[Raoul Duke]

Question about determining output transformer primary impedance. Originally posted in Dumble Files but thought this might be more of a general/technical question. Happened to come across this on an amp manufacturer’s YouTube channel and thought I’d get some opinions.

Is this formula legit?

(B+ voltage squared)/(Plate dissipation x number of power tubes)

Example: a 50w (6L6GC x 2) build using a known PT and circuit that consistently produces a B+ of ~455vdc-

455x455 = 207,025
35 x 2 = 70
207,025 / 70 = 2,957.5; So the correct primary impedance would be around 3k.

Is it really that simple? I imagine that for any build there would be a range that would work; i.e. 3 to 4.5k ?

Thanks for any insight!

[wpaulvogel]

That would be incorrect. For one because your plate dissipation is incorrect. You said a 50 watt amp. And 6L6 isn’t 35 watts dissipation. Plate dissipation for this calculation isn’t about maximum power tube dissipation anyway. That number has nothing to do with how much AC output power an amp with whatever tubes can produce. 6L6 with 3k and 455 volts won’t make 50 watts. Stick to known good figures such as 4k with 6L6 or learn to figure with load lines for descent expectations. 6L6 with 4k and 455 volts will generally make about 42-43 watts clean rms.

[Stevem]

That wattage rating of any tube is DC parameter, not AC voltage which is the audio output.

Go to the Tubes & more site a buy a copy of the RCA rc30 tube manual, it’s worth every penny as they say!

Here’s a example from the tube data section of the on line Duncan tube amp site.

[Phil_S]

RC-30 free download:
https://www.google.com/url?sa=t&source= ... zt6GfDD5AD
Not as nice as having the paper, but it is the same content. You can always print the pages you need. There are many pages of tubes you'll never see or use. Also, you might find a used copy at a very low price if you do a search for it.

[martin manning]

Is this formula legit?

(B+ voltage squared)/(Plate dissipation x number of power tubes)

Example: a 50w (6L6GC x 2) build using a known PT and circuit that consistently produces a B+ of ~455vdc-

455x455 = 207,025
35 x 2 = 70
207,025 / 70 = 2,957.5; So the correct primary impedance would be around 3k.

Is it really that simple? I imagine that for any build there would be a range that would work; i.e. 3 to 4.5k ?

It is if you understand what was assumed in deriving the equation and use it correctly.

For a "50W" 2x 6l6 amp at 450V B+, it gives 450V^2/(2 x 30W) = 3375 ohms (30W is the data sheet Pa max for 6L6GC)

That impedance defines a load line with peak anode dissipation of 200% for a P-P amp, i.e. the load line is tangent to the 200% Pa max curve on the tube's Ia vs. Vg-k plot. 200% dissipation is possible because:

1) each half of the power section only works on half of the cycle,
2) the dissipation is lower everywhere else on the load line, and
3) there will be some sag in B+.

IMO that value should be treated as a minimum a-a primary impedance. If you back out the Pa max along the load line for the typical 4k primary on a Fender 2x 6L6, you get 25W, which is a bit more conservative. That puts the load line at 169% of the 30W 6L6 Pa max. Some Marshall amps famously pushed the theoretical Pa max over 200%, and had to rely on a large B+ sag to stay out of trouble. Note this is all strictly about plate dissipation, not output power.

The equation works for single ended amps too, and it will give the load impedance for 100% peak load line dissipation, where the load line is tangent to the 100% Pa max curve, with a nearly center-biased operating point. For example a Champ with a 6V6 at 322V Va-k (that's 350V B+, 340V Va, and 18.2V Vk, from the Fender '57 Champ schematic), and 12W Pa max would need an 8k6 load.

[Raoul Duke]

Ok, this is good stuff for a new guy trying to learn.

Paul - I see your points when taking the info at face value and understand where you’re coming from regarding sticking to proven rules of thumb. Thanks to you, Stevem, and Phil for pointing me toward a tried and true reference. I’ve already started checking it out and will be picking up a hard copy whenever I can find a decent one.

Martin - as always, I appreciate the extra effort connecting these dots and providing context as to how/when it can be used. Very informative and it gives me more to dig into.

Very much appreciated guys! Thanks again!

P.S. - Paul, I still have Allison and she’s sounding fantastic! That amp is a multi-sensory treat to play that I will never part with. Super touch sensitive and one of the few amps I’ve ever experienced that just vibrates the energy around you as you play. Really special amp for certain.

[GAStan]

RC30 Hardcopy Reprint

[Raoul Duke]

Thanks Glenn!

I was poking around e-bay thinking this was long since out of print, lol.

Definitely in my next order.

[Stevem]

Note that it only covers tubes that RCA made and there's quite a few that only GE made that will not be covered in detail if at all in the RC30 book.

[martin manning]

Take a look at the free .pdf download Phil S posted to see if you really want a hard copy. You can get virtually any tube data sheet from https://frank.pocnet.net, and that is what most of the 750 pages of RC-30 contain, and in an abbreviated format. The remaining 200 pages cover circuit theory and other topics, but much of it is television and radio focused.

[Raoul Duke]

Ahh, ok thanks Martin. Good to know.

Based on what you and Stevem are saying - probably better to just grab what I need and print it out.

Thanks again all!

[Stevem]

Of special interest to you if you want to print them off would be pages 31 thru 38.

[angelodp]

This is great info and is helping me understand this element. Can you possibly go a bit further and point out the specific
points of reference on this data sheet? I see 14w Max diss. for the JJ's

load line is tangent to the 100% Pa max curve ? Help me see that.

[Ten Over]

load line is tangent to the 100% Pa max curve ? Help me see that.

You're going to have a hard time seeing that because the JJ curve is actually for 12W instead of the 14W label. With 450V and 12W, R comes out to 16.875k so that 4.219k would be used for the class B load line. The line will run from 450V to 106.7mA and it is tangent to the 12W plate dissipation curve.