The Tartan Suit - 60W Ripper Design

[moonlighter]

While my "Vermilion Cliffs" ODR project is well underway, I couldn't help but start to imagine what a higher gain Dumble type design would look like refined.

Attached is "The Tartan Suit" which I'm dubbing as an Overdrive Supreme.
It's something similar to Amp Nation's Ampliphonix, but as a true single channel cascade.
Overdrive setting is essentially an Ultraphonix JCM800, and the Clean is a Skyliner.
The phase inverter is a 12AT7 configured as a "choke tail pair", hopefully adding some real chug.

The only problem I can immediately foresee, is having to use 2x 3PDT relays for the channel control, unless I dive into MOSFET switching.

Opinions welcome!

[Ariel Meyer]

What is the design of the choke tail suppoaed to achieve ? Never seen that before, its a choke attaches to the PI?

[moonlighter]

The choke replaces the usual tail resistor in the pair.
What we benefit from it is not having to use grid coupling capacitors because we don't have to bias the grid directly.
A choke has a large AC impedance, but a low DC resistance, so by utilizing it here we get a normal cathode bias (659R in this case), but we still keep our strong cathode-to-cathode signal because the impedance is large.

Another benefit is the cathode isn't raised to a high voltage, so our plate-to-cathode voltage is the same as a normal gain stage.

[martin manning]

You will get a little lower gain and more bandwidth, plus excellent balance. What will you use for the inductor? A small audio transformer?

[Roe]

You will get a little lower gain and more bandwidth, plus excellent balance. What will you use for the inductor? A small audio transformer?

this choke apparently https://www.hammfg.com/part/157G

[jazzbass]

Hi moonlighter,

I am following this and your Vermilion Cliffs project. Nice work.

Franco

[martin manning]

You will get a little lower gain and more bandwidth, plus excellent balance. What will you use for the inductor? A small audio transformer?

this choke apparently https://www.hammfg.com/part/157G

Honestly I don't see much to recommend it. I tried a simulation of it against the Dumble 124 PI and there isn't much difference except for the lower gain. A pound of iron, and way overkill for the 5 or so mA passing through it.

[moonlighter]

You will get a little lower gain and more bandwidth, plus excellent balance. What will you use for the inductor? A small audio transformer?

this choke apparently https://www.hammfg.com/part/157G

Honestly I don't see much to recommend it. I tried a simulation of it against the Dumble 124 PI and there isn't much difference except for the lower gain. A pound of iron, and way overkill for the 5 or so mA passing through it.

We're eliminating two coupling capacitors and the phase shift they cause. Add that it also cuts down the parts count, and that seems like more than enough reason for me. Why not push boundaries? If it weren't for amp builders being inventive this forum would likely not exist

[moonlighter]

Hi moonlighter,

I am following this and your Vermilion Cliffs project. Nice work.

Franco

Thanks for tagging along! Glad to share

[nworbetan]

I've been comparing inverter stage spice simulations the last couple days (definitely going to go look for #124 next), so I spiced this choke tail pair last night, but now that there's obviously some interest I should probably jump on the bandwagon and post about it too.

The last time (and first time) I saw this I didn't bother spice simulating it because I assumed the choke's impedance increasing with frequency would have a much more detrimental effect on the bias current than it actually does. With a 30h choke the bias current is stable enough at guitar frequencies that the distortion it causes will mostly be subtle and easily remedied with a little nfb. The inverted/non-inverted balance is a lot better than I expected it to be. There's only a fraction of a volt of difference between the sides at guitar frequencies, it almost looks too good to be true. The catch seems to be that it doesn't perform as well at low or high frequencies (specifically, the distortion gets more and more noticeable at 20hz and 20khz, but even then it's subtle at usable signal voltages) and hi-fi snobs may not like the way it sounds. But it looks like it'll work "just fine" at reasonable signal voltages and at guitar frequencies.

What does it sound like though?

[moonlighter]

...But it looks like it'll work "just fine" at reasonable signal voltages and at guitar frequencies.

What does it sound like though?

Well, after my ODR is built we'll find out eventually. This is will be next on the list.

[didit]

Hmm --

The 155C - 60H/2750Ω/8mA - on a much smaller core would seem better suited to the application. Curious if that was evaluated in the design process, and rejected.

Best .. Ian

[moonlighter]

Hmm --

The 155C - 60H/2750Ω/8mA - on a much smaller core would seem better suited to the application. Curious if that was evaluated in the design process, and rejected.

Best .. Ian

Too much resistance for the shared cathode currents, it would make the bias too cold.

[moonlighter]

I've added a Depth control and a "More" switch. Bright control on the OD master vol seemed redundant and was removed.
Revision attached.

[moonlighter]

Now I may have gone into the deep end...

In a quest for SLO levels of overdrive, we now practically have a whole Ultraphonix JCM800 as the overdrive circuit, with our clean ODS cascading into it.
The only issue I'm having is I have a spare 12AX7 Half. I've considered an effects loop, but honestly for this amp it'd be better to use a Metropolous board or full Dumbleator.
Maybe use it as a seperate send for the OD? Basically how SSS002 does the Reverb send.

Relays have been swapped for Optoisolators, I'm also going to make an option to leave the ODS stack engaged during overdrive.

Currently this project has consumed my attention over my ODR build, so I may repurpose my acquired parts into building this first and just save my ODR chassis for the future...