HRM "Phoenix" style Preamp

[ElectronAvalanche]

Fellow Ampgaragists!

After a long hiatus, the Dumble bug bit me again and I had a spare 19" 1 Unit enclosure lying around since 2015. I also sold all my Dumble ODS amps over the last few years, since I did not really use them anymore. Still I missed the Dumble ODS sound and the one remaining clone I built also almost ten years ago (SSS 002) might be a nice power amp platform for a stand-alone preamp.

The circuit I used is mostly a plain vanilla 220k/3k3-150k/2k2 afair with HRM followed by a Dumbleator. I used some parts I had still in my box (the big blue HRM tonestack caps are WIMA I used in my very first Dumble (also a standalone preampt) that sounded real nice (at least in my memory). Caps are Philips F630 and Sprague. Some Draloric resistors here and there and I think Beyschlag. For the Dumbleator part I simply used a tube town kit (cheap and small).

OD entrance is 25k ( with 22k tail replacing the 4k7), and I paralleld the 220k with a 39k giving me 33k. Mid Boost, Rock/Jazz, bright with and Accent (bright switch of the Master, which is really the Return Level control) as triple throw (2 values and off). Heaters are DC (12V) for which I simply used readily available boards from some vendor (I needed to up the filtering to 2200uF to get rid of a nasty 100Hz hum). No FET, no FET sim circuit (yet).

HRM controls are on the front panel with the excption of the Mids pot which is still a trimmer (I think mids do not need to be changed that much when dialing different gain structures). Trigger is the OD entrance pot (as mentioned 25k with 22k tail), Taper (HF and LF for High Frequency and Low Frequency) is Treble and Bass of the HRM. I used shielded cables from and to the HRM pots. Voltages are as per #183. Lower voltages (180V on V1a e.g. were too spongy), so I changed the resistors in the filter section accordingly. Transformer is 270V (500mA I think) AC secondary and 14V AC (2A). The 14V feed both the 12V regulator board and the 5V regluator board.

Lettering is water-slide decals. I primed the frontplate with some semi-gloss clear, then applied the decals and then another coat of semi-gloss. Only difficulty is placing the decals so that everything comes out nice and straight. Might consider doing white decals on the black back side.

Really nice to have a stand-alone preamp to play around with different power amps. It is a nice match with the Dirty Shirley clone I built. I have yet to test the preamp with the SSS power amp. My SSS has a Dumbleator so I will go: Preamp Dumbleator Send to SSS Dumbleator Return.

The name Phoenix is only my homage to the late Mr. Dumble. I have no idea what circuit the original Phoenix really was.

Best!
Electron

[Reeltarded]

I generally like preamps, and yours looks great!

[dorrisant]

Very nice! Very inspiring... I thought of doing similar with a low wattage power amp, possibly into a dummy load so it could take advantage of the PI balance throwing harmonics.

[rccolgan]

this is so cool! My next project is this! Which PT are you using?

[ElectronAvalanche]

Thanks for the kind words guys!

@Ryan: the pt which has 270v ac (80ma) and 14v ac (2A) is from a german parts supplier. 49€.

https://www.musikding.de/Toroidal-trans ... V-270V-14V

I also ordered a custom wound 290v, 14v, 6,3v which has not arrived yet, but the musikding pt works perfect with 3 tubes (12V DC heaters) and relay.

I might consider doing a non-hrm version as well.

Preamps come together really fast and do not take away that much space (keeps the wife happy )

Electron