Sizing & Spec'ing Transformers

[BlackCrowe604]

Hi Guys!

So up until this point, I've replaced transformers in amps where an appropriate replacement has already been known, or specified for a kit.

I'm looking to branch away from that. My next build is going to be a SE design with (1) 12AX7 & (1) 6V6. My question to you is, how do you go about sizing a transformer for your builds? I want to start from scratch, and see where I end up.

If anyone would be kind enough to share their approach and considerations I would greatly appreciate it!!

[tubenit]

I prefer PT's with standup mounting rather then thru chassis mounting but have used both. I just like more room inside the chassis of my scratch builds.

I like 300-0-300 PT with 150ma   ................  or 275-0-275 PT with 150ma.  The reason for that is I typically have cathode biased amps where I change between  6V6's and 5881/6L6's.  (sometimes 100-120 ma is OK when I'm just using 6BM8's or 6V6's)   

5Y3GT   300 x 1.1 = 330v    EL84, 6V6, 5881/6L6   
5V4       300 x 1.2 = 360v    6V6, 5881/6L6
GZ34     300 x 1.3 = 390v    6V6, 5881/6L6
solid state  300 X 1.4 = 420v    6V6, 5881/6L6

IF you had a 300-0-300 with 175-200ma then you could possibly include EL34 or KT66 tubes

5Y3GT  275 x 1.1 = around 300v    EL84, 6AQ5, 6K6, 6BM8

**  It is critical that you figure out the heater current of the PT's and the tubes you use also.  Check out the Tube Data 
     Cheat Sheet attached.

Those PT's would cover any tube amp that I would build for myself except the ECL84  So-Low Watt which would need lower voltages on the plates.

With respect, Tubenit

[tubenit]

2nd answer ......................   

What are you building that is similar to what already exists?    Look at the Fender schematic and layout of what is similar that already exists. What voltage is on the power tube plates?  How many power tubes are there?   What are the specs for a "replacement" transformer for that amp design?

3rd answer

What voltage will you need/want on the power tube plates on your scratch build?   What is the combined heater current rating?  What is the combined ma rating of all the tubes?  Do you want tube rectification or solid state?   Look at the "tube data cheat sheet" in the previous post to add up the current draw on the tube plates and on the tube heaters. 

And then double check the cheat sheet with this:  http://tdsl.duncanamps.com/tubesearch.php.

With respect, Tubenit

[tubenit]

Example:

Let's say you are going to build a HoSo56 with an active FX using 6BM8 tubes, 5879 preamp tube, 12AX7 (2nd gain stage & CF), and 12AY7 for active FX.

Heater current:  two 6BM8's (per data sheet) will need around 1.6 amps + 5879 will need .15 amps + 12AX7 paralleled will need .3 amps + 12AY7
                        (series) will need .175 amp.    So we will want a PT that will have at least 3 amps available heater current.

Tube plate current:  6BM8's which are a triode/pentode will take about 33ma each.  So we need 66ma for those.  5879 will need just under 2ma.
                              The 12A_7 tubes will need about 3ma in a preamp position, so 3ma x 2= 6ma.    We're using the 6BM8 triode for the LTPI.
                              So the ma total is 74 ma.   So I am going to probably want a PT with 100ma at least.

Now from looking at the few schematics out there using 6BM8 power tubes, I've made a decision that I want about 305 volts on my 6BM8 pentode plates.    So to get around 305 volts,  I am going to use a 275-0-275 PT and a 5Y3GT tube rectifier which will give me 275v X 1.1 = 303v.

So to build this amp I want a PT that is 275-0-275 & has at least 3 amp heater current rating & has at least 100ma for the tube plates.

With respect, Tubenit

[tubenit]

Now regarding the OT.   I typically just try to find out what other amps using those power tubes are using.  OR .......... I look for a generic OT for that wattage.  So, I might look for a 20w or 18w OT for 6V6's.   And I might look for a 40 watt or 50 watt OT for 6L6's or 5881's.

Since my speakers are 8 ohm speakers, I'm going to make sure the OT has a secondary with 8 ohm tap. I don't worry much about some impedence mismatch.  I am aware others put considerable focus on getting all that impedence to match up just right.  I don't & it has never been an issue for me on any amp.

With respect,  Tubenit

*  After some others have posted,  I will consider moving this thread to ARCHIVES for future reference.

[davidwpack]

Archives yes. And/or this topic: http://el34world.com/Forum/index.php?topic=22555.0

[labb]

This might help.    http://thesubjectmatter.com/calcptcurrent.html

[davidwpack]

Hey thanks! That is helpful. I just bookmarked it.

[davidwpack]

Oi! 10 years of dedication and I'm finally level 2! You guys don't have to buy me any gifts or parties or anything. Is this where I get the Level 2 special 90% discount from Hoffman amplifier orders?

[PRR]

> a SE design with (1) 12AX7 & (1) 6V6

Gee, what an odd line-up. ?? Maybe C-H-A-M-P ?

And that's another thing. AFAIK, most Fender Champs shipped with the *same* PT as the two-6V6 DeLuxe. He could have saved a buck with a smaller Champ-specific iron. But that might mean custom design, and a separate part-number to inventory, warehouse, and replace. Apparently it was economic-enough to use the one PT for both models.

And YOU do not have an arbitrary choice. You do all your fine figuring and specify 47mA exactly. You look at pre-wound parts and find 25mA and a jump to 80mA. Edcor would custom-wind you a part, but probably have to round-up to next available wire gauge and core-stack. And if you guess wrong, the build is disappointing. Often better to quick-guess and buy high.

[BlackCrowe604]

Thanks Tubenit & Labb!

I've been reading Valve Wizard's approach on his page, as well as Robert Megantz's book. I'm here just looking to learn, and appreciate you sharing your advice and approach!

FWIW I found a SE "Mini Plexi" layout online and am curious.

Cheers!

[labb]

Go take a look at these SE amps...http://www.ax84.com/classicprojects.html

[tubeswell]

... a SE design with (1) 12AX7 & (1) 6V6. ...

A tiny Hammond PT is adequate for something like a Tweed Princeton/Champ  http://www.hammondmfg.com/pdf/EDB291AX.pdf

This one has a tapped HT winding (for a 'lower powered' amp sound option) http://www.hammondmfg.com/pdf/EDB290CAX.pdf


The main thing is you need enough current:
1) on the heater winding to feed the total heater demand (plus a pilot lamp if your lamp is powered from the 6V winding), 450 + 300 + 150 = 900mA. 6.3V@2A will ensure solid performance for a 6V6 and 12AX7
2) on the HT winding for the maximum tube current that will be seen. (For a 6V6 in Class A with an optimum load this is about 1.2 x the idle current, i.e. ~45mA, and for a 12AX7 its about 2mA, (3mA tops), so a 50mA HT rating is the bare minimum I'd use. The 'classic' voltage output for Fender Tweed Princetons and Champs is 325-0-325VAC. 310-0-310 will drive the 6V6 really nicely at about 340VDC on these amps.
3) to meet the rectifier filament draw on the rectifier winding (if you want a tube rectifier). For a 5Y3GT or 5AR4, this is 5V@2A.

If you factor in a 10-20% surplus current rating, the PT will run a bit cooler and last longer.