Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Colas LeGrippa on December 27, 2011, 05:15:43 pm

Title: vibrato circuit wanted !
Post by: Colas LeGrippa on December 27, 2011, 05:15:43 pm

Hi,

Got an extra 12ax7 on a chassis and I'd like to use it for a vibrato. It is a SE amp. 1 preamp tube, CF and one power tube. Never built a vibrato circuit before.
Any simple schem I could copy from a classic amp ?

Thankss

Colas

Title: Re: vibrato circuit wanted !
Post by: stratele52 on December 27, 2011, 05:31:03 pm

Look on a Fender scheamtic, small amp

Title: Re: vibrato circuit wanted !
Post by: HotBluePlates on December 27, 2011, 05:47:33 pm

Fender VibroChamp (http://www.el34world.com/charts/Schematics/files/fender/fender_vibrochamp_ab764.pdf).

If either of your gain stages don't have a cathode bypass cap (or if it's less than 10uF), that helps. If your amp doesn't have a feedback loop, just pretend that the 47 ohm resistor on the schematic isn't there.

Title: Re: vibrato circuit wanted !
Post by: Colas LeGrippa on December 27, 2011, 06:00:04 pm

cool, thanks , I'll get back to you with final result ( if it works :icon_biggrin: )

Title: Re: vibrato circuit wanted !
Post by: tubeswell on January 03, 2012, 01:23:35 pm

FWIW this time last year I did this SE amp (see schematic) with a trem circuit using 1/2 a 12AX7 for the LFO and a IRF 820 source follower to drive the (parallel) output tube grid(s).

As it has cathode-biased output, the depth control doesn't really kick in until the pot is about 1/2 way round the dial (because the auto-bias function of the cathode biasing tends to 'fight' any change in bias. But when the depth pot is turned up it sounds good).

Strat with a Celestion G10 greenback (using 1 x 6L6 in the output section and 5U4G rectifier) soundbyte link here.

http://music-electronics-forum.com/attachments/12710d1297150553-super-ac4-g10-6l6-5u4g-lo-ht-trem.mp3 (http://music-electronics-forum.com/attachments/12710d1297150553-super-ac4-g10-6l6-5u4g-lo-ht-trem.mp3)

Another longer soundbyte of various trippy trem settings here (me accompanying myself)

http://www.nzguitars.com/forum/download/file.php?id=8343 (http://www.nzguitars.com/forum/download/file.php?id=8343)

Title: Re: vibrato circuit wanted !
Post by: DummyLoad on January 03, 2012, 01:53:47 pm

use a 6AU6 - doesn't require funky RA pots.

http://www.el34world.com/charts/Schematics/files/danelectro/dano_commando.pdf (http://www.el34world.com/charts/Schematics/files/danelectro/dano_commando.pdf)

--DL

Title: Re: vibrato circuit wanted !
Post by: panhead on January 05, 2012, 11:52:48 am

The tremolo channel of the 18 watt Marshall is interesting. It adds a resistor between the plate and plate resistor of the gain stage and injects the plate signal from the oscillator between these resistors. It looks like the oscillator is modulating the plate voltage of the gain stage.