Author Topic: Tube pre-amp for Rhodes/Guitar  (Read 991 times)

Offline joostovermars

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Tube pre-amp for Rhodes/Guitar
« 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!
« Last Edit: December 24, 2017, 09:47:42 AM by joostovermars »
Music teacher, keyboard player
Hammond A100 & Leslie 145 1969 (restored)
Rhodes 73 mk1 1977 (restored)
Hohner Clavinet D6 1973 (restored, almost...)
Nord Electro 5D & Leslie 2101 mk2
Homebuilt Fender super reverb, Princeton reverb, bassman ab165
Hohner Pianet N (restored)

Offline bourniplus

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Re: Tube pre-amp for Rhodes/Guitar
« Reply #1 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.
Mark I - Mark V
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My electric square piano prototype

Offline pnoboy

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Re: Tube pre-amp for Rhodes/Guitar
« Reply #2 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.

Offline joostovermars

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Re: Tube pre-amp for Rhodes/Guitar
« Reply #3 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.
Music teacher, keyboard player
Hammond A100 & Leslie 145 1969 (restored)
Rhodes 73 mk1 1977 (restored)
Hohner Clavinet D6 1973 (restored, almost...)
Nord Electro 5D & Leslie 2101 mk2
Homebuilt Fender super reverb, Princeton reverb, bassman ab165
Hohner Pianet N (restored)

Offline d-rock

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Re: Tube pre-amp for Rhodes/Guitar
« Reply #4 on: January 11, 2018, 12:13:01 PM »
This is super cool. Thanks for sharing.