one channel super Reverb tremolo conversion

[KimboDb]

Hello I'm just wondering if it's an easy conversion to convert the tremolo circuit to what Fender used from what the Hoffman board uses. I know I'm getting it wrong but I guess Fender used a roach transistor or circuit, and the Hoffman circuit fluctuates the bias. I have the ab763 one channel and the tremolo is lackluster.

[tubeswell]

The stock AB763 circuit does use a roach trem. These tend to produce a very 'on/off' type of tremolo, unless you spend a lot of time modding the LDR driver circuit to get it working smoothly. They are also very prone to ticking. This is to do with the demands of getting the neon lamp to smoothly vary in its intensity. Fender tried different variations on the LDR driver stage at different points in the evolutionary history of this type of trem. You can examine the differences by comparing a bunch of BF and SF schematics from different eras. You can use a LED instead of a neon lamp driven roach, and IIRC Merlin Blencowe discusses some examples (I think, oddly enough in the 'signal switching' chapter?) in his 2nd edition pre-amps book. (I don't have my copy with me at the moment).

Bias-vary trem is used to great effect in the Princeton Reverb and in the Brown Vibroverb (6G15) and in the Brown Tremolux and Vibrolux, and the Tweed Tremolux. With proper bias adjustment, these can be made to sound intensely hypnotic. The key is getting the output from the LFO stage to drive the output tube bias right to the edge of cutoff, but without clipping the output stage's grid (which otherwise results in 'helicoptering'). Some experimentation is required to fine-tune it; e.g. try experimenting with modifications to the capacitor values in the LFO stage's RC network (e.g. changing one of the 0.1uF caps to .02uF, or changing the .02uF cap to a .03uF will change the envelope/shape of the LFO wave-form and trem speed). Also try adjustments to the LFO buffer stage driver intensity (by changing the voltage divider ratio between the cathode follower stage and the intensity control). Also try altering the output tube bias. Don't try all the changes all at once though - its a thing best tried one-at-a-time. If using bias-vary trem in an amp with a LTP inverter, it also helps if you have the LTP biased in the centre (i.e. not too hot or not too cold) as either extreme (particularly over-warm bias) affects the A/B duty cycle of the output stage under heavy signal conditions, which in-turn messes with the bias-vary trem effect as you crank the amp.

The blackface Vibro-champ uses a bias-vary driven tremolo, but it wiggles the bias of one of the pre-amp stages. When properly set up, this also produces a very nice tremolo.

The Fender Brown harmonic tremolo is very nice too, but is not so much about amplitude modulation as about harmonic modulation.

[super&plexi]

Would  Fender Brown you are referring to be the say 62like a pro Maybe? I ask because one like that came into our shop wayback, and I remember plugging inand thinking it was very phase shifter likequite unlike the tremolo I was used to. then I heard it againon a 4tube (pwr)Brown era solid-state rectified amp I had for a while, Maybe vibrAlux(e)?

[tubeswell]

Would  Fender Brown you are referring to be the say 62like a pro Maybe? I ask because one like that came into our shop wayback, and I remember plugging inand thinking it was very phase shifter likequite unlike the tremolo I was used to. then I heard it againon a 4tube (pwr)Brown era solid-state rectified amp I had for a while, Maybe vibrAlux(e)?

Yep for most of the Brown amps with harmonic trem, there's an 'earlier Brown circuit' and a 'later Brown circuit'. For the Pro these are the 6G5 and the 6G5a respectively. Each uses a slightly different circuit variation to get the harmonic tremolo effect. The 'a' variant came out in 1962.


But not all Brown amps have the harmonic trem. The Brown Vibrolux (6G11 and 6G11a), Brown Tremolux, (6G9b), and Brown Vibroverb (6G15) all use a bias-vary trem wiggling the 6L6 grids, as do the Brown Deluxe (6G3) and Brown Princeton (6G2) with their 6V6s.

[sluckey]

Hello I'm just wondering if it's an easy conversion to convert the tremolo circuit to what Fender used from what the Hoffman board uses. I know I'm getting it wrong but I guess Fender used a roach transistor or circuit, and the Hoffman circuit fluctuates the bias. I have the ab763 one channel and the tremolo is lackluster.

The conversion is doable although it may not look pretty on the Hoffman board. Some would say it's easy, others would find it difficult or impossible. It would require a knowledge of both circuits and the ability to work from a schematic.

The intensity of the bias vary tremolo effect will be affected by the bias setting. It may be possible to get a stronger tremolo effect just by fiddling with the bias pot. And there are several documented methods to increase the tremolo signal in the Hoffman circuit. I suggest exploring these methods before attempting to convert to the real AB763 tremolo circuit.

[stringbender.dk]

Hi!

I just read your conversation. it is very inspiring.
I just wanna know - is it easy to replace the neon light tremolo with a LED?    and does this mod need some other resistors or caps, to replace the neon light with the LED????

thanks, and happy christmas!

[tubeswell]

You can vary the brightness of an LED by varying the voltage across it. http://www.learningaboutelectronics.com/Articles/How-to-vary-the-brightness-of-an-led


So connect the LED to an LFO output (as long as the LFO output voltage range is appropriate for the LED). Then add a LDR next to the LED, and you have LED-roach trem!

[PRR]

> vary the brightness of an LED by varying the voltage

Yes; but LEDs are hyper-sensitive to small voltage change. The usual path is to start with more than 2V of LFO signal, put a resistor in series with the LED, so LED *current* is semi-linear to LFO voltage.

[stringbender.dk]

Thank you very much  :)

And the tremolo will be smoother then the usual "on/off, on/off" sound with a LED then with the photocell, or LDR?

I have a super reverb that a tech have converted to a bias type tremolo. it works, but the intensity is not that deep as I want. I wish it could be like a brownface vibrolux   :)
We tried to do some mods to it, after some advise from here, but it was not that good. the speaker moved forward an backward and made a loud noise...  maybe the tech did something wrong, I dont know...

but he made it useable...   but then I read that someone have replaced the ldr with a LED and made a smoother tremolo sound in that way....  :)    but I haven't found any schematics or layouts for it.... 

I need something to come with, before I can get a tech to wire it.

But thanks for your answers, I really appreciate them

[sluckey]

Read this...

     http://el34world.com/Forum/index.php?topic=22774.msg243496#msg243496