The Electric Piano Forum

Repairs, Maintenance & Upgrades => Preamps, Modifications & Upgrades => Topic started by: joostovermars on December 24, 2017, 09:43:50 AM

Title: Tube pre-amp for Rhodes/Guitar
Post by: joostovermars on December 24, 2017, 09:43:50 AM
I have built a tube preamp for my Rhodes Mk1, mainly to be able to connect it directly to a powered monitor.  It is based upon the tremolo channel of the Marshall 18 Watt combo (see schematic) and uses a single 12AX7 tube as a gain stage and LFO, modulating the kathode of the gain stage. It gets its power from a normal 12 V 1A DC adapter that feeds the filament of the tube as well as a voltage pump that generates 220 V B+.
It works pretty well and gives plenty of gain to drive a power amp. Tremolo is nice! I prefer this bias modulating circuit over the neon bulb/LDR method.
Wishes:
-Large El. cap on the 12V DC input to filter the rubbish from the cheap adapters.
-tremolo kill switch
-tone switch to obtain a more mellow tone for guitar. It is very bright at the moment, which can be compensated for by the tone control, but voicing could be more variable.
-when I ever build a version 2.0: a kathode follower stage to lower output impedance. That takes a second tube and probably a 1,5 A adapter. But the output impedance will match the input impedance of modern powered monitors better.

Well, wanted to share this with this forum. A nice "dark days before X-mas" project!
Title: Re: Tube pre-amp for Rhodes/Guitar
Post by: bourniplus on December 26, 2017, 04:34:28 PM
That's really awesome, thanks for sharing. The build quality looks top-notch. If you ever record some audio (to compare with / without the preamp), I'd really like to hear that.
Title: Re: Tube pre-amp for Rhodes/Guitar
Post by: pnoboy on January 03, 2018, 06:20:29 PM
This post is not a reflection of the skill of the OP, but I have listened to my Rhodes through both solid-state and tube preamps, and after considerable experimentation, I have concluded that a tube preamp adds essentially nothing to the Rhodes sound.  Any observed difference in tube preamp sound, assuming that you're not over driving the tube and creating distortion, is caused by small differences in the frequency response of the particular tube preamp and the particular solid-state preamp being compared.  Once the frequency responses are made equal, differences in sound disappear.  Of course, one could design a poor quality solid-state preamp or a poor quality tube preamp, but am I referring to well-designed examples of both.
Title: Re: Tube pre-amp for Rhodes/Guitar
Post by: joostovermars on January 05, 2018, 11:15:41 AM
True of course #pnoboy, but it was great fun to build  :P Nevertheless, the high input impedance & the short signal path do contribute something I assume.
Title: Re: Tube pre-amp for Rhodes/Guitar
Post by: d-rock on January 11, 2018, 12:13:01 PM
This is super cool. Thanks for sharing.