Bob-I wrote:I'm fairly convinced that some of the magic bloom is caused by the power amp pulling the B+ rail down briefly. I've tried a preamp like this, exact voltages as my amps, and run into one of my amp's power section. It felt totally different.
I would have the same impression. I have had a lot of requests from players for preamps, and I wouldn't mind having one myself to replace my agin Rocktron Chamelion in my recording desk.
Power supply sag and recovery time are a BIG part of the dumble equation.
I would suspect that messing with the divider chain and filter cap size you could approximate the same powre supply "feel".
It is fun to watch the rails as you play on a scope, they are ALL OVER THE PLACE...
Another bizarre idea would be to "modulate" the rail using a FET as a pass transistor and use the autio to "correct" the rails. In effect negative power suppply regulation.... Might also be able to incorporate some electronics from a compressor to shift and delay the recovery time on the rails. Lots of ideas and too little time... Shit is gonna fly when I retire...
Hmmmm, time for some spice work...
Located in the St Croix River Valley- Afton, MN
About 5 miles south of I-94
aka K0GWA, K0 Glas Werks Amplification www.glaswerks.com
My first move is to make the OD to HRM mode not sound like the drive-up speaker at Arby's.
If the dual rails kill the magic, I could just pull the HV off the main amp and into the preamp. I'd have to rearrange the rail a little and probably pull the juice off B+3. Most of the organ amps I've hacked did that. There was one main power supply that could run several preamps -- usually using some pretty flimsy connectors and wires.
Some thought would need to go into more a portable cable and fittings but it wouldn't be an all day project. In my head I see a sturdy switch that would either leave things as is or run power to the preamp and lift the 150k FET-replacement tail resistor. Food for thought, sh
For the last couple years I've been circling the idea of creating preamps to mimic the various Dumble generations that would work with a single power amp. Lately that has turned more towards introducing discrete preamps that would work with a complete amplifier. The heavy, expensive parts are pretty similar in these amps and a working alt preamp would give options without the heft.
The schematic below is a winter project that is designed to work with a 2nd gen 50-watt head. This particular preamp is an HRM. I have a 100-watt 102 as well but that has the loop built in and wouldn't align with this box.
On the schematic, the input to the preamp goes to one of two relays in parallel. In the normally closed position that feeds right back out to the head. In the normally open position it goes through the preamp. The other DPDT relay controls the output. The normally closed position takes the send from the amp and sends it to the Dumbleator. The normally open position takes the output from the OD relay to the loop. I am using two relays to ground the unused preamp but that might be overkill.
The rest of the preamp is pretty familiar. I didn't put a bright on the clean master in this schematic but that is an option with this circuit since the #2 lug isn't grounded. I plan to build this in a 2.5" high rack box. It would fit in a single-height chassis except for the extra HRM tone-stack knobs clutters up a single row of controls. If I used a daughter-board or put them on the main board 1U would be fine. I've had excellent results with the Antek toroidal power transformers. They have one at 0-280VAC with two 6.3V windings that is perfect for this application.
I haven't lurked much here lately but over the years I haven't seen a gizmo like this one. I'm concerned about introducing ground loops. The plan is to use the same power strip but there will be a lot of wire between the chassis grounds. Not so touchy at the pre/power amp junction but susceptible to hums and squeals early on.
If anyone has an experience or opinion they want to share, I'd be glad to hear about it. If this project is a "go", I'll catalog the experience and post it as I go.
The gizmo is done -- and it works. I was able to get the clean and the normal OD to sound a lot like the 102 I was testing it on/through. The HRM sounds like it should so far but I've only played it in the basement with one guitar. It will take some further tweaking to understand in a place where the sound pressure doesn't make the fluorescent lamp shrouds rattle.
The big change from the originally-posted schematic was that I thought I'd need to damp the signal from V2b to the HRM when both the normal and HRM ODs were being used. Turns out, it needed some help. I put a parallel resistor into the 470k/500p network. Were I to do this again I'd put in a dedicated new entrance with its own pot.
I didn't have that much trouble with sag. If in further testing I feel I need to, I'd either pull the B+ directly off one of the amp nodes or just rewire the toggle to let both preamps play and choose which was active at the next pole (like it is now). The plan was to make this work with any amp that had a passive FX loop so the B+ idea wouldn't be practical for multiple amps.
In truth, this was an interesting project but not really necessary unless I had a Marshall or maybe an Ampeg that needed a very different preamp choice. Adding a different preamp doesn't make a D-based amp that more versatile. Still, I do this for fun and I had fun. One possibility would be adding a speaker emulator and going directly into the board. I'll play with that.
I hope this was interesting and good luck with your own projects, sh
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THANKS for sharing the project! It was an interesting project and a worthy effort.
One possibility would be adding a speaker emulator and going directly into the board. I'll play with that.
I tried a d-inspired preamp design (D'Mars ODS with an active FX) into a PA system. I wasn't satisfied with it. It was just OK at best. I got convinced that the LTPI and power tubes all play a major factor in the tone.
