Vibrato Circuit Mod

[CJ]

first step, find beats per minute of classic vibrato tunes.

 

[CJ]

notice that most songs are in the 400 range, probably dimed out the speed knob so build the circuit with a couple of slow ranges for Buffalo Springfield and Credence, and add 4 more ranges around 400 BPM so we can cover everything and add a resistor ladder for fine tuning as it is desirable to match up the speed carefully, so we set the coarse ranges with caps on a Lorlin 2 x 6 (one section not used) and we set the fine tuning with resistors on another switch and add an on-off since everybody puts intensity on 10 so no need for a pot, after a couple of weeks of tweaking here is what we got: (note: 0,0218 sb .218 on r

 

[CJ]

notice we are running 6BQ5's in the Princeton now and they sound great in this circuit, also we changed the bias circuit to a cathode resistor as the Amperex tubes seem to sound better this way, so adding vibrato to a cathode bias push=pull was a challenge, as luck would have it, someone brought a Hohner Contessa amp into the shop which uses vibrato with a push-pull cathode bias so we borrowed that circuit , if you go through the schematics of every Fender amp made, you will see very few amps with push pull self bias vibrato circuits, there are a few Gibson amps that use it with a slightly different circuit, , so now we have 36 presets matched up perfectly to the tunes we like to play, all repeatable for live shows as long as the drummer can dial in the right tempo, if not, we can fine adjust the speed, an online metronome was used after we got tired of counting out the beats to a stop watch which will put you to sleep in a hurry, we had to parallel some caps to get the ranges right, now i know why Tube Tech uses two caps per freq range on their EQ's, so they can get consistency in their products as caps sometimes come in all over the place , i suppose you could use poly-carbonate caps for temp stability if so inclined, that is all,

 

[Gus]

Very cool

 

[Bobby Baird]

I could never get my bias-modulating trem to work on my brown face 6g16 vibroverb. I think the voltages may be to high compared to back in 63'. May pick up a variac.

 

[fazer]

You may need to bias output tubes cooler for the tremolo bias voltage to have more effect. Included a pic of my vibroverb original.

 

[fazer]

I added a bias pot and adjusted resistance values to allow more adjustments. I’m traveling this week. Or I would post voltages

 

[Bobby Baird]

Yeah I have the bias adjustment pot installed. I can lower the bias all the way still not getting in motion. Another problem when you do lower the bias it staves the output tubes of power and the sound is a little cold. I believe I run my 6l6's at 34mA from memory. Sounds better to me.

 

[fazer]

I’ve always liked hot bias on 6L6’s. But the only way I know is to lower the bias current if you want depth in the on off volume.

 

[fazer]

That’s a cool circuit CJ. I have a Standel amp from 60’s. I might try that. It has EL84 outputs cathode biased. I wonder about using a MOSFET instead of a tube for the oscillator: I have a circuit that I think is from mosfet follies.