10 lbs of stuff in a 9 lb toroid amp

[mesherm]

My latest amp that I stubbornly refused to buy a bigger chassis for.
I used an 8" x 12" x 2" Hammond that I had on hand.
The Trafomatic PT is one I procured on ebay for not much money althought it took a while to get it. Same with the OTs which are Amplimo 3A524s. The PT is 360-0-360 rated at 300ma, no bias taps however.
It will probably kick out more since I am not using the 5v winding.
I was going to use double 5AR4s but there just wasn't room so I used diodes instead. I had to use a CL90 as a dropping resistor after the diodes to tame the B+. After the diodes and CL90 I used four 100uf 400v caps in series-parallel so I have essentially a 100mf 800v cap. Then I use a C354 choke for each tube pair into each side of a JJ 100/100mf 500v cap.
The driver is a VTA board so all the filter caps and stuff for the driver is taken care of onboard.
I shoehorned in a small 50v tranny for bias voltage.
The Amplimos have no UL tap so I am running triode mode with the screens connected through a 100 ohm resistor and a UL4007 diode.
I used 6CA7EHs because they were less expensive than the KT88s I was going to use. I will swap the KT88s from my other amp at some point if need be.
The first thing I noticed when I played music is the bass...lots of bass punch compared to a stock ST70. It sounds great and I am listening to it, burning it in as I type.
http://webpages.charter.net/mesherm/mm-1.jpg
http://webpages.charter.net/mesherm/mm-2.jpg
http://webpages.charter.net/mesherm/mm-3.jpg
http://webpages.charter.net/mesherm/mm-4.jpg

[sorenj07]

Wow, that project looks really sweet! Congrats on the awesome build! My favorite part about toroids is the complete lack of coupling it has on anything (don't have to worry about putting anything near it, or it near anything, except maybe tubes). What driver board is that? Anyway I'll let you get back to burning that baby in. Any idea on wattage?

[mesherm]

The driver board is one of Roy Mottram's tubes4hifi boards.
I like them for home built amps because all the filter caps are on board and you just have to feed it 380-420 volts or so from your B+. It also has all the bias circuit stuff onboard. So one just has to worry about feeding the power tubes.
The Amplimo's are rated for 40 watts. I'm sure the PT can handle that so the limiting factor would be the tubes. Using diodes to couple the plates to the screen grids along with the 100 ohm resistor increases the power in triode mode so with the 6CA7s or maybe KT88s I could hit 40 w per channel. That was my goal in using a C354 choke for each channel.
I can't really crank it that high in my apartment though.

[EWBrown]

I thought I did pretty good building a DIY35 with Rev D board onto a 9X10X1.5 chassis, but this one definitely gets the blue ribbon first prize for lots of watts on a small chassis... Very nice job!


I got a couple of the smaller Trafomatic 190VAC @ 200 mA and 26VAC @ 700 mA power "donuts" from Boris Sasic, for use with various K502 / 16A8 projects. These were reasonably priced, around $30, and there is a potted and shielded version for $10 more. Most of the PTs and OPTs he offers on Ebay or his Engineering Vistas site are made to order, the Trafomatic factory is in Serbia, so there is some delay in delivery, but the little ones are usually on hand, in stock..

/ed B in NH

[Shannon Parks]

I swear that that first pic has an optical illusion of sagging from all that iron! :)

[mesherm]

Surprisingly its not that heavy. I can grab it by the PT and carry it around with one hand. After about 3 hours of listening I can report that the sound is fantastic. It beats any amp I have including my DIY35. The bass is great and the highs are flawless. Cymbal hits are distinct and crisp. I played some acoustic J. Tull that had glockenspiel in it and it brought tears to my eyes it was so clear and sparkly. Then I palyed Satriani's Surfing with the Alien and I thought he was in the room with me. The imaging is THAT good. I didn't use any designer parts. The coupling caps are just Orange Drops 715p and 716P. Standard metal films from Mouser.
I can only assume its the Amplimo output trannies that sound so good but at 46 Euros each they were hardly designer. I plan to swap the KT88s from my other amp in this evening although I can't see how it could sound much better.

[EWBrown]

Trafomatic also makes an ST35 and ST70 set of transformers, the ST35 looks like this: (Obviously not a standard layout for the tubes).



I'm tempted...

/ed B in NH

[TomMcNally]

Take a Dremel to Baby Blue and you can have that radial layout ! Yellow_Light_Colorz_PDT_07

[mesherm]

Well I have had a chance to do some tube rolling the last few days on this amp.
The KT88EHs I had hopes for did not sound good at all. The bass was muddied and the highs too shrill.
I tried a set of SED 6L6s I have had almost forever and they sounded better than the KT88s.
Then I dropped in a set of JJ KT77s. I will leave them in for a while as they sound great. So the best tubes so far have been the KT77s and the 6CA7EHs. I am running both types at 50ma a tube.
The KT88s I was running at 65ma. Is it possible that toroids saturate more easily and thats why the KT88s sounded bad?

[EWBrown]

That could be the culprit, the KT88s were drawing too heavily, and saturating the OPTs.

The new JJ KT77s are supposed to be "the cat's meow" as far as a EL34 / 6CA7 replacement.

Be warned, the early released KT77s have very skinny pins, they won't make good (if any) contact with older or used octal sockets. The latest release KT77s have the normal "fat" pins, and work very well. I've loasded the 77s into a heavily modded ST70 and they do sound very good!

/ed B in NH

[EWBrown]

I sent off for a set of those "hockey puck" ST35 trannies, but I doubt that I'll do any dremel surgery on the blue board... They cost bit more than a Triode "Dynaclone" set of ST35 iron, but this should be an interesting experiment, and I like the look. I've heard good things about these Trafomatics, we'll see if these reports are justified.

Another advantage of this style of trannie is that it doesn't require six drilled and punched holes to mount each one, just two holes each,(or even just a single hole each, with a little creativity)! Yellow_Light_Colorz_PDT_05 Yellow_Light_Colorz_PDT_06 Yellow_Light_Colorz_PDT_07 And that's a good thing... Yellow_Light_Colorz_PDT_11

/ed B in NH

[mesherm]

The toroids definitely mount better than the normal four eared variety. One bolt per tranny. They weigh less and are smaller also.
If I had had a little bigger chassis I could have mounted all the trannies inside for an ultraclean look. I might have had room for the two 5AR4s I was intending to use also. Oh well...just have to wait for a future project.

[EWBrown]

Toroid trannies are supposed to be "self-shielding" in that the magnetic field (allegedly) doesn't escape the core. So there should be less problem with AC "hum" magnetically coupling from power to output trannies.


I got a couple of step-up/down isolation toroids at a swapmeet last year, they have two 120VAC and one 240VAC winding, and are good for 500VA, so I figure, at typical tube amp power consumption, they could be wired up with 120VAC primary and the 240VAC and the other 120VAC windings connected in series for 360VAC. Going one step further, the center hole is pretty large, so additional windings could be added, say for 6.3VAC and 5.0VAC filament windings. Figuring out the "turns per volt" isn't difficult, I figure, try 20 turns and see what it produces... I'd SWAG somewhere between 5 and 10 TPV, it's easy enough to try out.

/ed B in NH

[erichayes]

Hi All,

My experiences with Electro Harmonix KT88s in both my 50 and 100 watt prototypes (which use OTs either cloned from or based upon the Eico HF- 89's OT and the Dynaco A-450) parallel Mesherm's: nebulous low end and fingernails-on-the-blackboard highs. And I run my KT88/6550s at 72 mA per tube. When Bud and I were souping the amps up last year, that was one of the first things we discovered--two otherwise identical amps, one with JJs and the other with EHs, one sounded good and the other sounded bad. Swapped the tubes out, and guess what?

Just made me an even more rabid JJ KT88 disciple.

Toroids made with ferrite cores saturate quicker than regular transformers, but they're used for RF, not AF. If the core is made of the standard silicon steel alloy, it should have pretty much the same characteristics as a regular EI core.

[mesherm]

"...nebulous low end and fingernails-on-the-blackboard highs"
Yep, Eric that describes it exactly.
The funny thing is the same KT88EHs sound great in triode mode in my other amp with Hammond 1650G OTs. The 1650G impedance is listed as 6600 while the toroids are 4000. I would think the KT88s would be ok with the lower impedance but maybe not. The difference was immediately apparent though...probably those fingernails scraping..lol