1W Dumble Practicalities

[JM-1]

I thought I'd build an amp with:

Power Amp Section from Rob Robinette's Blackvibe MICRO 12AU7
- 1 watt
- Cathode biased
- 12AU7 power tube (in push-pull)
- 12AT7 phase inverter
- Presence control
- Master volume (after phase inverter?)

and marry that with a Dumble 124 preamp section.

The front panel however needs to take
- 8 potentiometers
- 1 jack input and
- 3 toggle switches

So it looks like it would need to be a full-size amplifier which, when considering the 1W output, seems a little bit silly.

Before I shelve the project, are there any practical ways of making this smaller please?

Expert opinions deeply appreciated.

RR763_BLACKVIBE_MICRO_Schematic_small.png

[JD0x0]

I don't think you could get much smaller, unless maybe you go with a tightly spaced PCB setup, but even then, with proper spacing for the tubes, it'd be fairly large.

Also consider the fact that ODS amps generally do not get their distortion from the power amp, and the master volume, IME, is very effective at bringing the volume down, even on a 50-100W setup. At some point you get diminishing returns, as you need some headroom if you want your 'clean' channel to stay clean. Why the post phase inverter master? This would potentially give you some phase inverter distortion, but again, not really needed seeing as the preamp is where you generally get distortion on these amps, and they are no slouches, especially with the OD trim turned up. I'd think you'd just be limiting your cleans even more and possibly muddle up the nice OD tones by throwing additional phase inverter and output clipping on top of it.

[WhopperPlate]

Why can’t you squeeze everything onto some tiny boards and mount them vertically ? Like a dumbleator, that will reduce the necessary real estate.

Or maybe just some point to point tag/turret strips mounted on the sockets .

Unless you need the layout to be exactly dumble these are definitely a space saver .

Also , with small enough potentiometers , or dual concentric, you can squeeze a lot more into a panel layout . Maybe find a slightly taller and less wide chassis and make two rows of controls .

You could probably squeeze it all into a larger pedal sized enclosure if you tried hard enough .

[GAStan]

Maybe take a look at the schematic for Mojotone's Studio 1 Watt kit.

https://www.mojotone.com/Mojotone-Studi ... lifier-Kit

It can use either a 12AU7 or 12BH7 tube that pulls double duty as PI and Power tube. Super simple power section that would be good starting point to tailor to your needs.

Mojotone also has an OT for these tubes (1580) with 3 impedance outputs, amount of impedance depends on the tube you're using.

Using a single tube would decrease the space needed yet retain your preamp. NFB could be still be used.

In case you're wondering, yes I do have a Studio 1 Watt, first amp I built. Really enjoy it.

Glenn

[JM-1]

Thank you, friends for your very kind and helpful replies.

I’m based in the UK, and so getting a kit from Mojotone would be prohibitively expensive.

But I was thinking of putting it in a much smaller chassis with the following layout.

IMG_6761.jpeg

Tell me please, would you guys sacrifice a non-buffered passive effects loop in favour of external foot-switching? Or would you prefer to have an active effects loop with send and return level controls?

I pretty much never use the effects loop, unless I don’t like the sound of an amplifier and wish to insert a graphic equaliser there.

I place a microphone in front of the speaker and run it to a stereo effects processor.
100% wet modulation effects (chorus, reverb, and delay), then run to 2 full range stereo speakers on either side of my 1×12 cab.
(I got the idea from watching Larry Carlton)

IMG_8583.jpeg

[GAStan]

JM-1 I was not suggesting you buy a kit, but to look at the schematic for it to look at the power section. The schematic can be downloaded freely from the link I provided. It is a simple power section that could easily be put after any preamp for a low power amp.

On the effects loop personally I prefer the footswitch over the loop. I can always use my effects before the amp if I need them.