What tremolo circuit should I use?

[überfuzz]

I'm in the middle of designing a new amp. At the moment I'm thinking, tremolo, yeah why not. It's just that I don't know what circuit to use. Prior to this one I've only used fenderish with hartley oscillation, dumping signal to ground. I kind of like it, but if there's any other way to go I'm all ears.

One idea I had was fenderish but with transistors. Should work with the heater potential, right? Is DC in heaters a NO NO..?

Ah well, if anyone like to bash my idea or give me some pointers...

[tubeswell]

I love bias-vary trem on fixed-bias output tubes. See the 5G9 circuit for a 6V6 amp, and 6G9B or a 6G16 for a 6L6 amp.

If you want to do an SS cicuit to save on heater current, you can sub IRF820 mosfets directly for each triode tube stage (see attachment for a proven circuit where I have used IRF820 a a source follower). RG also has a circuit on his site for a BF style trem for pre-amp wiggle (with roach) using IRF820s.

I have also attached an example of a pre-amp trem wiggle circuit if you want to try that. Its not as intense as bias-vary trem at the output stage tho'.

Then there is the 'harmonic vibrato' circuit from the Fender Brown (6G4 or 6G12A etc) amps. This is nice sounding trem.

Then there is the Magnatone style, the Vox AC30 style and so on, and they are all really nice too.

[überfuzz]

Thanks for the input! Should work great, I just have to decide what circuit to use...

[Tom Phillips]

I’d also suggest that you listen to some examples of the various types of tremolo to help you decide which sound you like best. If you can’t find live examples to audition then you can search for demos on YouTube. Following are two examples that I found. I’m sure there are many more but you have to wade though quite a few useless search hits because, in part, of the public’s confusion about the terms “tremolo” vs. “Vibrato.”

LDR (bug) vs. Bias Wiggle (bias modulating) Tremolo - YouTube (Fender light bug type vs Bias modulation type)
victoria amplifier tremolo demo - YouTube (Discussed in Victoria Amp company terms but is Ourput tube bias modulation vs. driver tube bias modulation vs. Fender three tube modulation of the Fender blonde & brown amps which is described well by the presenter)

Cheers,
Tom

[oc disorder]

Just thought I'd point out that one of the recent Fender Twins (Pro series-Concert/Pro Reverb & Twin)

http://support.fender.com/schematics..._schematic.pdf

which for memory made in China with a deeper cabinet (and ridiculously heavy to lift ! ) have a version of the output tube bias modulation.

I remember as I had a bias problem caused by one of the sections of the dual gang Intensity control.

After cursing, then figuring it out and repairing I tried the amp out especially the trem and was pleasantly surprised ! For memory seemed smoother and deeper...

Just thought I should point it out for those who haven't encountered these "newish" pro series.

Just checked the date on the schematic 2000 ! ... hmm oh well maybe not so new... O.C.

[überfuzz]

I'm closing in on a nice tremolo, I think. Before I say yey, I need to know what voltages a Tremolo Roach needs to run properly. Any insight on that..?

[Tom Phillips]

You need to be more specific. Voltages at the roach or the B+ running the oscillator or?
A general answer is that any normal tube amp design will have a power supply with a B+ rail that will support the Fender BF roach type tremolo circuit.
If you download one of the Fender reissue schematics (Such as Deluxe Reverb or Twin Reverb) they will have circuit voltages posted and you can checkout the voltages in the tremolo section.

[überfuzz]

I'm opting to use a mosfet. It's not as straight forward as tubeswell stated, so the circuit is only based on the ab763 tremolo. The voltage I'm interested in is the voltage driving the light bulb, or is it a LED maybe.

[jpfamps]

The Fender BF optocoupled circuit uses a neon bulb.

Neon bulbs require a minimum voltage (the strike voltage) before they illuminate. This is usually around 70V for a neon bulb, and thus the requirement for a significant voltage swing from the oscillator in this type of circuit. Once the neon illuminates, the voltage required to keep the bulb illuminated is somewhat less than the strike voltage.

The non-linear nature of the bulb's response to the voltage across it (and indeed the non-linear nature of the LDRs response to incident light) will affect the dynamics of the tremolo effect.

You can of course use an LED based optocoupler, in which case you would only need a relatively small voltage swing and this would probably be better implemented with solid state parts running at low voltage. Peavey used this approach in early versions of their Delta Blues amp.

Whether they still do this now I don't know as optocouplers have fallen foul of the EU RoHS regulations.