How should I choose transformers if I want to clone an Eico HF81 or a Dynaco SCA35 ?

[pete1729]

I'd like to try my hand at DIY and build an EL84 based integrated amp with a phono section. I recently finished rehabbing a Lafayette KT236, and promptly gave it to my girlfriend. Although I've never heard one, The EICO HF81 is well reviewed and seems a likely candidiate as does the Dynaco SCA35.

I have no idea how to choose a power transformer or output transformers. Where do I start?

I'm just at the dreaming stage and would like to get an idea about what the project would cost.

 

[Shadowdog]

The clones of the Dynaco SCA/ST-35 Z-565 OPT's are affordable , measure exceptionally well in distortion specs. and bandwidth at full power as well as the vintage as they are exact copies with the same amount of interleaving . The Triode Electronics and Dynakits have different secondary outputs of 4 and 8 ohms verse the original ones 8 and 16 ohm secondary output leads. Magnequest's clone is exact in every to the original with optional Ni lams. They are finished to a higher std. also, and little pricer.

PS trans can be supplied by the first two mentioned companies also. You can also get very affordable suitable PS trans. from Edcor with different voltage specs for different needs. Hammond also makes suitable PS trans.

Check out Output Transformer tests in the yahoo Eico group and Tronola site.

The Eico HF-81 would be better to clone due to the design of the PS and tone controls, but the Dynaco clone Z-565 OPT's would really add to the amp and they are readily available. They are also physically much bigger and heavier with more interleaving.

If you don't need all the tone controls it would be easier to clone the Dynaco ST-35 with the original 7247 driver splitter tubes or use diytube's version which splits that tube into the 12AU7 (or better 12BH7) and 12AX7 which it is internally. You can P2P both versions or buy PCB's and just add a volume pot.

Whatever design you choose add Dave Gillespie's affordable EFB mod for better sonics and output tube life. You also gain to 2-3 W/CH for a full 17-18W/CH.

For the phono preamp you can buy a PCB for more advanced designs than the Dynaco SCA version such as the Marantz 7C or wire it P2P. See classicvalvedesign for PCB boards and info.

 

[vinylkid58]

I have no idea how to choose a power transformer or output transformers. Where do I start?

I'm just at the dreaming stage and would like to get an idea about what the project would cost.

To clone a Dynaco ST35, you're looking at just over $500 for the kit. The transformer bundle alone is ~$240. I wouldn't try to build an integrated amp with a "built in" phono stage as a "first" project. An amplifier just doesn't require a complex regulated power supply that a decent phono stage would need. If you build a phono stage in a separate chassis, you will have a much quieter preamp, that is easier to troubleshoot. Start with a simple project first.

http://store.triodestore.com/diy35kit.html

jeff

 

[Br1ck]

I agree, a scratch build can be done, but if you have never built an amp I'd get a kit.

Tools I've used for scratch build: drill press, stepped drill bits, hole punch, Dremel with cutter disks, drum sander Dremel attachments, etc

Kit: soldering iron, screwdrivers, wire strippers, pliers.

The kit will look great. The Scratch build will be rewarding once you get it working.

 

[jaymanaa]

The active tone circuits are what makes the 81 sound good, but the iron is the key to the Dynaco. Go with the Z565s and you won't be sorry.