Bias vibrato questions

[Groover]

I like the way the output bias vibrato (as used on most brown Fenders and in BF & SF Princetons) sounds, and that the volume variation seems centered at the same level as the signal without the vibrato on - ie no percieved drop in level, as there is with the opto vibrato in larger BF/SF amps.

So what are the drawbacks to that design. I figure there must be some since Fender moved away from that design.

Any reason it is a bad idea to convert a SF Deluxe, Pro, or Twin Reverb to that vibrato circuit?

[Enzo]

Why Fender did something is irrelevant. Perhaps they found the optical system cost less. Maybe it was a matter of how it fit on the part board. Maybe they found their return rates higher. Maybe it required a bit more attention setting it up befoe shipping. WOuldn;t want to add X hours to the labor acount. .But your amp is not a statistic, yours is just one example. What do you care if a dollar is added? Fender cared about a dollar, because after 10,000 amps, those dollars added up to a salary for one of his engineers.

The system worked and you liked the sound.

[Groover]

So it's not so inherently hard on the tubes or something as to be undesireable from a technical standpoint, then?

[Enzo]

I don't recall anyone ever griping that his model so and so bias yanker was going through tubes.

And if your tubes last 967 hours instead of 1000, does it matter in the face of getting the sound you want? Not saying that happens, just asking "if?"

[GibsonLover]

I agree that why Fender did something is irrelavent. Thank God they reached their zenith with the Blackface series, which I love, but was probably an accident of cost reduction in reusing circuits. After all, look at the '59 Bassman - It was a disaster as a bass amp. Also the Brownface Harmonic tremolo - they actually changed the circuit, adding a tube, and only using half of it. There goes my impression of Leo as a bean counter.
I ran across one reference as to why the output bias modulation went away, and may be valid:
These older types of Tremolo circuits eventually got replaced by a photo-cell arrangement after becoming a customer relation's nightmare for tube amp companies ... if a class-AB output stage is improperly biased then the bias modulating Tremolo circuit can either have little effect on the output stage or it can drive the power tubes into meltdown when the "intensity" control is maxed ... this may be a reason why some amp manufacturers insist that people NOT modify or install variable bias in their fixed-bias amps (but that's another issue) ... if your amp starts acting funny when any of these Tremolo circuits are operating then it's time to either re-bias the power tubes or replace them with a new set and bias ...
That was copied from an interesting link:http://www.lynx.bc.ca/~jc/tremoloCircuits.html
Given the warning, you see that circuit often on 6V6 outputs and less on 6L6. I'm doing a Vibrolux build right now and am considering the updated Custom Shop Vibrolux Reissue version noted in the link.

[martin manning]

Leo's tremolo and reverb work ranks him as a pioneer of electric guitar effects in my estimation.

There is some commentary on the various Fender tremolo circuits in "The Soul of Tone," the recent tome on Fender's amps. Nothing about problems with the bias modulating trem, but there is the suggestion that the "harmonic vibrato" eventually disappeared due to the parts count and especially the number of tubes.

MPM

[Groover]

Thank you GibsonLover for that great link about tremolo. Pretty much covers it!