The #124 build

[Turret]

Hello everyone,

Good to be back after a long break.

First of all, I would like to thank the Admin for re-establishing my account. It has been over 10 years since I built anything significant, and equally long since I participated in DIY forums.

One of the main reasons I'm reaching out is that I could use an extra pair of eyes on my project. But before I continue, I also want to express my gratitude to everyone for the valuable resources available here.

I’m preparing for a #124 build with a friend as a bonding exercise, and I'm very excited about it since it's been a while since I worked on a project.

It will take some time before we gather all the necessary parts, so I've decided to focus on the documentation in the meantime. I have redrawn the schematic to make it easier for me to read while also incorporating a few gentle modifications:

- I've changed the mains capacitor from 330uF to 220uF, as I can no longer find any 330uF axial capacitors.
- I'm canceling the 50W/100W switch.
- I've marked the capacitors with their respective outside foil.
- I've elevated the heater supply.
- I've moved the pilot light to the relay supply PSU (though I'm very unsure about this).

I would appreciate it if you could take a good look at my schematic and provide any feedback. Ideally, I hope you find it useful.

Thanks!
T

[xtian]

Nice drawings!

You want the main fuse on the hot wire, not neutral.

The bias design is such that if the wiper on the bias pot fails, your tubes will red plate. Better design is if pot fails, full -ve voltage passes, leaving the power tubes cold.

Your NFB lead should be connected directly to the 8-ohm secondary so that it does not change when you change the impedance switch.

You might like to add 1R bias sense resistors on the power tubes' cathodes for easy checking bias current.

[Turret]

Nice drawings!

You want the main fuse on the hot wire, not neutral.

Good catch, on it

The bias design is such that if the wiper on the bias pot fails, your tubes will red plate. Better design is if pot fails, full -ve voltage passes, leaving the power tubes cold.

I'm not sure if I understand, would you mind to elaborate? Generally I understand, but not sure how to rectify it?

Your NFB lead should be connected directly to the 8-ohm secondary so that it does not change when you change the impedance switch.

You absolutly right, I missed that. Wouldn't that be 4R? I though the #124 is 4R load into 8R speaker

You might like to add 1R bias sense resistors on the power tubes' cathodes for easy checking bias current.

I was thinking about it, I'm using the Eurodubes Bias Meter, and I wasn't sure if it's needed. I will add it, though

Thank you

EDIT: changes updated, minus the bias pot

[Turret]

Merlin to the rescue!

https://www.valvewizard.co.uk/bias.html

Is that what you mean?

[martin manning]

Have a look at the ODS eyelet board .pdf here: https://ampgarage.com/forum/viewtopic.p ... 08#p365408
There is a fail-safe bias supply mod on page 5.

Yes, the GNFB should be tied to the 4R speaker tap. There is no pressing need to elevate the heater circuit as there is no cathode follower with risky Vh-k, and heater hum is not an issue.

[Turret]

Have a look at the ODS eyelet board .pdf here: https://ampgarage.com/forum/viewtopic.p ... 08#p365408
There is a fail-safe bias supply mod on page 5.

Wow, that is resourceful! Thank you for sharing your plans with us. I will dip-dive ASAP

There is no pressing need to elevate the heater circuit as there is no cathode follower with risky Vh-k, and heater hum is not an issue.

I didn't know that before. I'm allergic to noise, and I try my best to minimize it. That's why I've decided to relocate the pilot lamp circuit. Any suggestions you have would be greatly appreciated!