Question About Transformers

[Stuart Polansky]

I'm going to build an amp for my son. I have two complete sets of transformers, output and power. One from an HH Scott 290 and one from an Eico HF87. Any idea which might provide higher quality audio? The amps are both rated for ~50 watts or so. Both used EL34 output tubes. I'm gonna be building Oddwatt type amps again, so I can use EL34, 6L6, KT66/77 types or 6550/KT88/90/120/150.

I couldn't find specs for the transformers. In my present Oddwatts, I'm using Stereo 70 outputs, but with 8 ohm loads on the 4 ohm taps. Amazingly, even with 180mA standing current (Class A), the outputs stay cool to the touch. My point being that current won't be an issue, and I'll select different taps as needed for best performance.

The Scott is often compared to Marantz amps of the day (early 60's). The HF87 is also well known for decent performance.

Thanks for any input about the outputs.

Stuart

290.PNG (394.34 KiB) Viewed 36802 times

[Stuart Polansky]

HF87 schematic. I couldn't post it in the previous post for some reason.

Attachments

[SoundMods]

This old Geezer understands that EICO and Heathkit designed kits around major brands. They were both known for their output transformers in terms of audio quality.

Since both the H.H. Scott and the EICO amps were designed for 6CA7 (EL-34) tubes I doubt you could go wrong with either choice.

Flip a coin.

[FerdinandII]

The Eico uses a voltage doubler for the B+ supply, which is not the best.
170V @ 0.9A
6.3V @ 7.6A
75V for bias
The Scott 290 has a beefy full-wave design.


https://audiocircuit.dk/downloads/hhscott/HHScott-290-pwr-sch.pdf

As for the outputs, the Eico has Ultralinear taps similar to your Dynaco. The Scott is straight Pentode operation.
The exact OPT aren't listed here, but it's a good cross section of OPTs (for your next restomod... LOL)

https://www.one-electron.com/Projects/trans_p1.pdf

[Grover Gardner]

What Bruce said. The HF-87 was only 35 wpc, but UL. The Scott will probably give you more power but in pentode mode. The Eicos are probably a bit "sweeter". Also, the Eico power tranny is surprisingly small and runs pretty hot. Both are nice output transformers, I don't think there's any need to quibble about that.

[Roscoe Primrose]

Just build them both, and let us know

Roscoe

[Stuart Polansky]

Thanks to you all.

Based on conversations with David M., I'll build the Eico/Mullard circuit for my son. The only significant changes being a CCS in the phase inverter tail with equal value plate loads; powering the 12AX7 and 6SN7s from a dedicated DC power supply; ultrafast diodes; and upgraded cap values and types (DC Link in the power supply for instance).

Yes, Grover, I'll leave plenty of room around the power transformer, and take the heater loads off of it, onto a second transformer.

Now, to lay out a PC board!

Stuart

[SoundMods]

Now, to lay out a PC board!Stuart

Party on!

[FerdinandII]

The DynaMutt PC boards used to be a good option for that circuit. I'm not sure if they are still available.

https://music-electronics-forum.com/attachment.php?attachmentid=27479&d=1392301326

[Grover Gardner]

Don't forget the 750pF cap across the "upper" OPT primary. I assume it's there to equalize the winding capacitances (or something like that).