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

EDIT: files delated, see updated files later in the thread

[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!

[Turret]

I made some changes to the PSU around the relay supply. I cancelled the doubler and redesigned it for a regulator with a 12V transformer in mind. I'm still a bit unsure about how to wire the status lamp to the relay supply. It should be the easiest part, but I'm feeling a bit rusty with this. I would prefer to avoid injecting any junk into the heater supply. The lamp is rated for 6.3V. Should I use a divider after the regulator or wire it before the bridge? What do you suggest? Any tips would be appreciated.

EDIT: Elevated heater supply is still there, I forgot to kick it

EDIT: files delated, see updated files later in the thread

[martin manning]

I marked up the accurate 124 schematic drawn by member icracer to reflect the current TAG standard ODS eyelet boards and chassis. I believe everything you are looking for is included there.

[Turret]

I marked up the accurate 124 schematic drawn by member icracer to reflect the current TAG standard ODS eyelet boards and chassis. I believe everything you are looking for is included there.

Indeed! Am I going mad? I'm sure this schematic wasn't there before

Thank you.

I'm not sure if there is any interest in that, but I will be building my #124 on a Vox style tagboard and terminal boards. Once my layout is done, and verified, I'm happy to share it

[martin manning]

I marked up the accurate 124 schematic drawn by member icracer to reflect the current TAG standard ODS eyelet boards and chassis. I believe everything you are looking for is included there.

Indeed! Am I going mad? I'm sure this schematic wasn't there before

It wasn't... I just created it yesterday ;^)

[martin manning]

I'm still a bit unsure about how to wire the status lamp to the relay supply... The lamp is rated for 6.3V. Should I use a divider after the regulator or wire it before the bridge? What do you suggest? Any tips would be appreciated.

Use an LED with the appropriate current limiting resistor.

PS The mid boost in your preamp schematic is not drawn correctly.

[Turret]

Good catch, I had the C6 wired incorrectly, is this better?

For the pilot light, can I use 6.3 Fender style lamp with a different (higher resistance) dropping resistor?

[martin manning]

Good catch, I had the C6 wired incorrectly, is this better?

Yes that works.

For the pilot light, can I use 6.3 Fender style lamp with a different (higher resistance) dropping resistor?

You could use a 6.3V incandescent (#47) lamp like Fender, and add a dropping resistor, but that requires 150 mA. If you want that look, there are LED equivalents that operate on 12V, like this: https://www.superbrightleds.com/ba9s-le ... amt-Single

[Turret]

Good catch, I had the C6 wired incorrectly, is this better?

Yes that works.

For the pilot light, can I use 6.3 Fender style lamp with a different (higher resistance) dropping resistor?

You could use a 6.3V incandescent (#47) lamp like Fender, and add a dropping resistor, but that requires 150 mA. If you want that look, there are LED equivalents that operate on 12V, like this: https://www.superbrightleds.com/ba9s-le ... amt-Single

This is exactly what I needed. I couldn't Google that!

I have to shoot off to work, but I will have more questions, if you dont mind. Also, you are more than welcome if you want my source files. I drawing is done in Diptrace, its freeware (with some limitations)

[martin manning]

...I will have more questions, if you don't mind.

Sure. It'll be interesting to see what you come up with.

Also, you are more than welcome if you want my source files. I drawing is done in Diptrace, its freeware (with some limitations)

Thanks, but I'm a KiCad guy ;^)