Stromberg Carlson Tone Stack Mod?

[jonathan_kea]

Hello All,

I've converted a Stromberg-Carlson New World AF-502 to a guitar amp by replacing the phono input with a standard 1/4" jack. It actually sounds pretty good at bedroom volumes...

However, the tone controls are somewhat perplexing. When i dial the bass past noon (50%) there is a drop in volume and treble, but no perceptible boost in bass.

The input for the SC goes directly into the volume pot before the 6AV6 pre-amp tube. Would it be better to have the input signal go directly into the 6AV6, then into a Baxandall (or?) Vol, Treble, Bass?

Any recommendations would be greatly appreciated...

[shooter]

It actually sounds pretty good at bedroom volumes...

Bass take more power to "make a change"  open it up to everyone's gone but me volume and see how it reacts

[sluckey]

I suggest removing C2 and R4. They're not good for guitar. If happy nothing else to do.

However, that preamp is designed for a hot ceramic phono cartridge so the 6AV6 provides enough gain. But it's not enough gain for a guitar pickup. I would totally remove the 7-pin socket, ream the hole to fit a 9-pin socket for a 12AX7. Then totally remove that tone stack. Now rewire the 12AX7 just like a Princeton Reverb preamp. Huge improvement.

[jonathan_kea]

Sluckey, that's a great idea! I'll try that. Curious would C2 and R4 provide any benefit if I were to use this as an MP3 player...?

Shooter - this thing sounds pretty rancid when cranked, but the bass control still behaves the same - starts to reduce volume once turned past noon.

[sluckey]

would C2 and R4 provide any benefit if I were to use this as an MP3 player...?

Yes. That circuit was commonly called a "loudness" control, usually switchable. It improves bass response at low volumes.

[chocopower]

Another easy one:


Cut C8 (or just desolder one end) and raise R9 to 100k (or add a 56k in series to ground)
Bass control should work much better for guitar that way.

[Backwoods Joe]

Like the the (resistance reading Chart),

[jonathan_kea]

Hey Ya’ll,

Wanted to let you know I completed the two mods above today and it sounds much better - a bit more trebly and overall much better tone.

The tone controls still prefer to be in the middle position, and I’m starting to wonder if the pots are just made that way. Might switch them out if time permits, or do the 12AX7 mod in the future...

Thanks for the tips!