Mains Transformer for EL34PP

[AlmostGotIt]

Hey Guys,
I'm brand new to the site. I've been looking at the EL34 PP amp in the article by Claus Byrith (http://www.lundahl.se/pdfs/claus_byr...fier_30wpp.pdf). Instead of just choosing the same mains transformer as him (Lundahl LL1669A), I wanted to get an "equivalent" tx from Hammond. I've done alot of reading on tube amp design (your "online tube learning for newbies" section is great!!), but there hasn't been alot on sizing mains transformers. From the EL34 datasheets, it looks like a PP pair of EL34s in UL mode could draw up to 150-200mA (so a stereo amp would draw 300-400mA) (I know there are other draws on the HV line, but this is the majority I believe). But Hammond tranies kinda max out a 200-250mA. Now is my 300-400mA estimate way too high (and I need to do more reading), or are Hammond tranies catering to mono-block design?

[SY]

The article you linked to states that he is dealing with monoblocks. For a stereo amp, you'll need double the current, not just in the HV winding, but the heater windings, too. Oversizing a bit is not a bad idea.

[amperex]

After downloading the massive 34 page file, the transformer is spec'ed at 350 VCT, 6.3 volts & 130 volts with a 240 volt primary. Using a HV bridge & the center tap for a second lower voltage B+ supply, a full-wave CT circuit is not an option without two transformers.

For a stereo amp I would specify 350 vct @ 600 ma, 6.3 vac @ 8 amp & 130 vac @ 150 ma. The 130 volts is feeding a huge 220uf cap, thus the high current spec of the 130 volt winding.

Hammond does not manufacture any iron that will work. A company in the USA that will manufacture one-offs to spec is Heyboer in the state of Michigan (616) 842-5830. They ship to Canada.

[AlmostGotIt]

Thanks for the replies. Sorry about the 34 page download. I'm spoiled with a high-speed connection.

I'm more concerned with HOW you guys come up with the current requirements of PP output stages. I haven't found any direct papers or discussions on this topic (but there are alot on single tube class A), so I'm trying to piece it together from articles like the one by Claus Bryith. I don't just want to copy, I want to learn.

So how do you guys calculate HV current required for PP output stages???? Are there any threads that I missed or articles that you guys learned from? Or am I reading the EL34 datasheets correct by saying a PP pair of EL34s in UL mode could draw up to 150-200mA? Thanks.

[cerrem]

The input cap size does not dictate the winding current selection....
As the cap gets bigger the PEAK current goes up....but the conduction angle is reduced, therefore the current pulse gets more narrow... ie, smaller Duty Cycle.... The current ratting of the transformer is in direct corelation to it's rise in tempertraure above ambient 25 C.....Since Wattage is with respect to TIME...the ON time of the current pulses are reduced as peak current goes up...The overall Energy remains the same...

The RMS current demand of the PP circuit remains the same...

Chris

[mr mojo]

Hey Almost!

I just completed my first DIY tube amp and used Heyboer to custom wind my power and output xfrmrs.

Top-notch folks, best customer service BAR NONE and very good iron at a very reasonable cost.

I suggest giving them a call and telling them what you need. I had them wind a 22 lb. monster: 800vct @750ma, 5v@10amps and 6.3vct@10amps. Iron works perfectly and doesn't even reach room temperature-even after a few hours use!

IIRC cost was @ $250.

They also custom wound faithful copies of Dynaco a-420s, 6-60k response and rated for 70 watts. Since during testing after construction they passed an undistorted sine at 4 HERTZ! I think they made a fantastic pair of outputs.

Ask for Alden or Phil.

Best,
mr mojo

[richwalters]

Originally posted by mr mojo
Hey Almost!

Iron works perfectly and doesn't even reach room temperature-even after a few hours use!

That's the correct result....even motors rated at the correct load shouldn't refuse the backhand touch. Towards physics, for best efficency iron losses should equal copper losses and youv'e got that.

You mentioned undistorted sine at 4 Hz....-could you tell us how much power you got out ?

richj

[AlmostGotIt]

Hey mr mojo
What did you make for your first tube amp? Is it true - does it sound even better when you make it yourself?!?!?!

What info did you give the guys at Heyboer? Did they figure out the specs from your circuit? Or did you have to tell them the voltage/current ratings? If so - how did YOU figure them out?!?!

I've heard of Heyboer a few times on this forum - sounds like great transformers (no pun intended).

Thanks,
almost.

[mr mojo]

Hey Almost!

Well, I think it sounds better than any Ive heard so far-but then it's safe to say I'm pretty "biased."

Here's some pix:

First DIY amp pics-as promised!

Don't be at all afraid of ordering custom iron-that way you get exactly what you want and don't have to make compromises!

As for info they'll need:

They'll need to know mains voltage in. In the US it's 120, but you're in Canada? so I don't know what you folks use.

If you're gonna go stereo rather than mono, then after speccing the mains you need to give them specs for secondaries.

I took a peek at your circuit and if you're building a stereo unit you'll have to power a pair of 5AR4s-just like my amp-dual rectifiers are pretty sweet!

Most of the time the 5AR4 plates run @ 400v, so on the high-voltage secondaries you'll need 800 volts center-tapped.

As for HV secondary current, a pair of 5AR4s can generate @ 300-320ma. IIRC and each pair of EL34s should draw 60ma in PPAB1 or 125-140ma in AB1 UL.

So, you could spec the HV at @ 350ma You'd be OK, but just barely. If you've got the space, money and back muscles, I'd double it.

So, HV secondary becomes 800vct(IIRC 820vct listed on your circuit, but I'd go with 800 so if your mains voltage is a little high, which almost all are, you won't go over the 410v per side-but it's your amp so it's your call!) at a minimum of 350 ma.

7-800 ma would be even better-less sag and better voltage regulation and less heat.

Now, you'll also need a pair of rectifier filaments on another secondary winding as well.

A single 5AR4s filament will draw 5v and 1.9amps. You've got a pair, run in parallel, the 5v stays the same, but the current doubles to 3.8. A safe number would be 5 amps. Once again, rate it higher and it'll run that much cooler.

As for your tube filaments, they're 6.3vct-(in your schematic it's 3.15vct; which you choose depends on how you run your filaments) and 1.5amps per EL34 and .3amps for each ECC83(12ax7) I don't know what the EF88 is rated for.

So, for the current ratings for the tube filaments, simply add 'em up: 1.5(4)+.3(2)=6.6 amps for the EL34s and 12ax7s.

I'd guess the EF88s might be .3 amps like the 12ax7s-but that's only a guess. If so, adding an additional .6amps brings the grand total to 7.2 amps.

I'd go a minimum of 8 amps-bearing in mind the filament amp draw for the EF88s is only a guess.

So, You'd need a power xfrmr with 120 or 220v mains in-depending on what you use in Canada.

800-820vct HV secondary rated at a min. 350ma.

5v rectifier winding at a min. 4 amps.

And a 6.3vct or 3.15vct filament winding at a min. 8 amps(depending on the current draw of the EF88s.)

Unless I missed something on the power xfrmr listed, that's about it.

If you go through Heyboer they'll do a good job of walking you through all this as well.

HTH,

mr mojo

[mr mojo]

Rich,

Max measure power output was 34.6wpc!

Some pix if you're interested in a peek:

First DIY amp pics-as promised!

Best,
mr mojo