[Rusty Jefferson]

No recommendations for alternative iron unless the vendor has one. It would likely require changes to the feedback circuit that would require some knowledge/skill to perform. There are reproduction ST-35 transformers being made and perhaps the vendor simply felt they didn't justify the cost or weren't available when they designed the kit?

Since the amplifier is autobiased the largest issue using the toroidals is eliminated. It may be worth reaching out to the vendor to ask about output transformer upgrades. If the circuit is close to original, it might be reasonable to swap in the reproduction OTs since you've already invested quite a lot in other components most people building these wouldn't.

[planet10]

Transformers were spec with the ST35 kit.

The Dyna OPTs are very good, and modern replicables typically a bit netter. I have 2 of those plus 2 SCA-35 power transformers (and a nice chassis) for a dual mono EL84 PP that leverage swhat was learned with our cott build.

Warning: not all that pretty ;^)  … until you close your eyes.



dave

dave

[planet10]

Also curious Dave, is there a bad thing about 18g solid for internal wiring?

How much current does it have to handle. I’d use 24 or 28g solid cryogenic wire.

Take 5 (in Ottawa?) is a good wire source.

Can you post the AN schematic. Not a huge fan of their work.

dave

[planet10]

My EL84 amp is probably my 3rd favorite amp of the ones I own right now

My 2^nd favourite. I have at least that many amps.

dave

[nlitworld]

So I did some digging and was curious on opinions. I found that Edcor and Hammond both have a comparable OPT based on specs of 7.6K primary, 40% UL tap, 4,8 output. The Hammond only rates for 30-30K output which is a bit interesting. I think the Edcor looks like the better unit of the two, but is it actually $350 better than the Antek I already have?

Hammond 1650FA
Edcor CXPP25

[planet10]

I would skimp on everything else — except polys i the PSU — before the OPTs.

It might still be possible to get O’Netics OPTs. Lundal would be my second choice.

dave

[nlitworld]

Yeah Dave, unfortunately everything else has already been purchased including the Anteks. First place I checked was Lundahl and it looks like they don't make an OPT in the specs I'd need. Perhaps they do custom orders, but I just don't see going down that rabbit hole right now. I think I will assemble as is and perhaps reopen this idea down the line if I really feel something is lacking.

[FullRangeMan]

Alright Rusty,
Your comment got me thinking, if I were to upgrade OPT what would be a good option? Like I said, I'm not really knowledgeable enough on transformers to know what's what. I'm sure I could find something with similar specs but if you know of something better sonically that isn't hateful pricing, I'll give it a look.

Current OPT is Antek MP-25W76

Product Brief:
Output power                     25 Watt
Turn Ratio                          30:1
Ultra Linear tap                40%
Primary impedance           7.6K ohm
Primary DC resistance      191 ohm
Output impedance             4 & 8 ohm
Output DC resistance        0.3 & 0.5 ohm
Max DC plate current       125mA
Frequency response          10Hz - 20KHz
Suggest Tubes                   807, 5881, EL34, 6L6, 6V6
Weight                              3.5lb
Dimensions                      4.0" d x 2.0" h

Too bad you wont use an EI OT, it allow you to choose the lamination type and shape, Toroidal OT use standard silicon-steel tape.
Edcor and Hammond use the regular quality M6 for low price, but at Hashimoto you can get Hi-B lamination EI or C core.
http://www.tube-amps.net/Hashimoto_Products.htm

[nlitworld]

Too bad you wont use an EI OT, I allow you to choose the lamination type and shape, Toroidal OT use standard silicon-steel tape.
Edcor and Hammond use the regular quality M6 for low price, but at Hashimoto you can get Hi-B lamination EI or C core.
http://www.tube-amps.net/Hashimoto_Products.htm

I guess it's just my inexperience with listening to multiple types of tube amps that have me questioning whether splurging for an EI core is really that much better. I want to build this thing right from the ground up, but please understand I'm splurging to get this far and build out this kit finally. Mortgages suck, and that payment never goes away.  Plus Lundahl and Hashimoto don't make a 7.6k primary OPT so anything from the boutique guys would be custom order or redesigning the circuit that I have no major great understanding of the math behind. Remember, I'm an amateur tinkerer and like to build stuff but I need strict, clear instructions.

I'm asking honestly now, what about an EI or C core is so much better than toroidal? Is it the core material that strictly makes the difference? Is it the winding wiring, or is it just because EI core are what people have used for 100 years and that's "how it's supposed to be"? What are the possible sonic improvements I'd be missing out on, and how much better could they really be?

[Tyson]

IME, toriodal output transformers in tube amps lead to a faster, more dynamic sound than IE or C core.  Which, again, IMO, pairs very well indeed with an EL84 tube.  I think you are going to be VERY happy with your amp.

[Rusty Jefferson]

I feel kind of bad about planting the seed about the OTs. Didn't mean to sow doubts about this build.

I have several friends who are great amplifier designers and builders. I asked them independently about your amplifier and they all knew and respected the vendor selling the amplifier kits. The fact all their other amplifiers (and the original ST-35) are designed around traditional EI transformers says something. Perhaps it's just cost/benefit this amplifier was designed around the toroidals. Reach out and ask the vendor about it.  They will (hopefully) tell you why they chose to use toroid transformers and put your mind at ease about their use or confirm there would be a benefit to using a particular EI output transformer. Nobody will know better than them, I'm sure they have investigated reasonable options. They may in fact feel there is no benefit to using an EI OT. Again, you don't want to modify the design without their help and the knowledge of how to do it. Changing a transformer will require other adjustments to the circuit, you can't just drop one in.

[nlitworld]

IME, toriodal output transformers in tube amps lead to a faster, more dynamic sound than IE or C core.  Which, again, IMO, pairs very well indeed with an EL84 tube.  I think you are going to be VERY happy with your amp.

Exactly the sound I'm looking for. Fast, punchy and dynamic. I have subs filling in all the bottom end and musical preference is more punk/metal than smooth jazz. Punchy drums and guitar tone are priority #1 and 2 and exactly why I'm excited about the EL84.

And Rusty, don't sweat it. I'm excited to build this out and just looking to make sure I don't overlook anything obvious. I like rabbit holes of finding out better gear and what makes things sound different. I've never dug into transformers before so I was really quite intrigued by the "what if". I have a lot more to read up on, but I think I'll do it while listening to this amp and their toroids for now. All good   

[Tyson]

Exactly the sound I'm looking for. Fast, punchy and dynamic. I have subs filling in all the bottom end and musical preference is more punk/metal than smooth jazz. Punchy drums and guitar tone are priority #1 and 2 and exactly why I'm excited about the EL84.

And Rusty, don't sweat it. I'm excited to build this out and just looking to make sure I don't overlook anything obvious. I like rabbit holes of finding out better gear and what makes things sound different. I've never dug into transformers before so I was really quite intrigued by the "what if". I have a lot more to read up on, but I think I'll do it while listening to this amp and their toroids for now. All good   

Only thing I'd strongly encourage you to chance is the coupling caps.  The PIO's tend to melt in that amp.  Ask me how I know that, lol.  Use these instead https://www.hificollective.co.uk/components/miflex-kfpm-01-copper-foil-capacitors.html

Those also will give you a faster, more transparent sound because of the poly while still keeping a nice glow on the mids from the copper.  And most imporantly, won't spew brown goo all over the insides of your amp.

[nlitworld]

Was it heat or electrical demand that cause them to pop like an overcooked hot pocket? My thought was to try them with keeping tubes exposed to limit any hear demand, and still using proper 630v rated caps. If it's pure electrical torture then perhaps I should investigate further before finish soldering?

[Tyson]

I'm pretty sure it's heat.  I've had them melt inside other amps that were very hot too, like the 6C33C amp I have.  On the other hand, in my 300b amp, they are fine.

[nlitworld]

Yeah I remember you mentioning before and that spurred the thought of going custom chassis and removing any heat source outside. I also went for proper 630v caps instead of trying to squeeze out with 600v just in case it was pushing them closer to electrical limit.

The Hammond chassis I'm using will be upside down (lid on bottom) plus the caps will be dangling down below everything else. Board will be offset about ½" from chassis to tuck small resistors topside and everything else hanging underneath. All tubes and transformers will be outside chassis. Fingers crossed we don't have a Kilauea situation, but you can say you warned me if things do get oozey. 

[nlitworld]

So progress has been made, and also stalled out. Was able to get 95% of the board populated with mountable parts, but I missed one set of the Amtrans resistors in the input tube section. They can get popped in easy enough at the end, but that section of the board will be TIGHT! I'll pop coupling caps in there at the end as well, since I need access to those resistors.

Anyway, here's photos. My butt is sore from sitting on a stool all day.