Why did Fender move to optocoupler trems?

[Bluefinger]

This is simply out of curiosity ... why did Fender move to optocoupler tremolos in their amps? I have now converted three of my buddies blackface amps to bias trems and they all are happy as they can be. It uses less components, only half a tube instead of a full and sounds WAY better. Some say it puts stress on the output tubes but I have been using bias trems in my tweeds for a long time and haven't experienced any exessive tube wear. Is there something I am missing?

[Leo_Gnardo]

Good question! Having repaired a number of amps with bias trem, one of their failure modes is having a leaky capacitor from the trem LFO generator, that drives the bias current way high in output tubes. Blinking light trem avoids this. Granted just about all the bias trem amps were 50+ years younger when Fender's decision was made to go blinky-lite, probably fewer failures back then. But that's what I've noticed.

Also Fender had tried their fancy "swoosh" vibrato in their larger amps, took 4 or 5 triodes to do that & it didn't capture the market. They may have noticed Ampeg was installing blinkers in some of their amps & decided to go that route, back to simple amplitude modulation tremolo.

[J M Fahey]

Well, it is crude, inefficient and stressful, besides applies to the whole amp, not just one channel.

Almost forgot: does not inject low frequency oscillator modulation in the signal path.

Technically way more advanced and no doubt Leo liked that.

If you think about it, bias tremolo moved power tubes from starving Class B (or Class C) operation or plain cutoff (I'd love to see the audio waveforms at that point) to redplating overbiasing.

[pdf64]

Yes, just to expand that point, bear in mind that a whole string band would expect to use the bigger amps, probably serve as PA too (hence all the inputs; the -6dB pad from input #2 is likely just a bonus arising from the input mixers and clever use of switching jacks).
So bias trem etc would be useless, to have them all pulsing in unison may invoke sea sickness etc.

[Bluefinger]

ok, that makes sense ... sounds like a good reason for the decision

[Justin Thomas]

Having them pulse in unison is okay. It's when you run four amps at once all with built in tremolo set to different rates, and add a stereo chorus running two and a stereo flanger for the other two, also both running at different rates, that it gets seasickening... Oh, wait... I forgot - two of the amps have reverb, too...
Try it sometime. It's fun!

Amps used: Ampeg Reverberocket 2 (opto), 1962 Concert (5-triode wacky trem), 1969 Dual Showman Reverb (opto), and 1979 Vibro-Champ.

Justin

[rjb]

Cultural Differences

...a whole string band would expect to use the bigger amps, probably serve as PA too...

Here's what I think of when I hear the term "string band".


https://en.wikipedia.org/wiki/String_band

[J M Fahey]

That's because you are way too square.
Being hippie minded, with chest long beard and hair, wearing sandals even walking through knee deep snow, flowers in my hair, wearing carpenter jeans or Indian silk pants, etc. , "string band" means this to me:

sounds cool after your second acid hit or third sausage sized joint.

[52 Bill]

There were and still are a lot of single players that use the normal channel for a vocal mike and the vibrato channel for their guitars.

...the -6dB pad from input #2 is likely just a bonus arising from the input mixers and clever use of switching jacks).

I read something back in the late '60s from Fender that the 2nd input was designed to be used by a second instrument with isolation between the two or alone with a phono player. The reduced volume level could handle the higher output signal and the mixing resistors paralleled to form the correct 47K load resistance.

I think I calculated it out and it came close to 47K. I agree it is a clever use of the switching jacks.

[pdf64]

There were and still are a lot of single players that use the normal channel for a vocal mike and the vibrato channel for their guitars

Yes, Tony McPhee / Groundhogs used to be regulars in my area; for solo gigs in pubs he'd use an AC30 with the vocal mic into the normal channel.
Here's their classic https://www.youtube.com/watch?v=RtUPXKtTT_8

[g1]

I read something back in the late '60s from Fender that the 2nd input was designed to be used by a second instrument with isolation between the two or alone with a phono player.

Recently had an old princeton with original "instructions" in the back. Seems to me the phono player reference was in there.
The original karaoke machine.

[Leo_Gnardo]

Here's what I think of when I hear the term "string band".

That's a mighty nice string band! Check out this little trio:



The guy on the right must have had a string endorsement. Who could afford to keep up with that gadget?

more string bands here, feast your eyes folks!

https://www.google.com/search?q=mand...33DDMQ_AUICCgD

Juan, that band you put up, that's the Incredibly Strange Band. Well at least that's how I'd introduce them in my FM DJ days.

[Chuck H]

In the Teagle and Sprung book I have it's implied that Fender went with the optocouple trem primarily because it offered an independent, non trem channel as pdf64 mentioned (are there points for this?). But there may be another reason...

When Fender started making the BF designs they really started pushing the tubes. Higher voltages on the power tubes to attain more performance was actually a bold, insightful and influential move! Unfortunately it makes the implementation of a bias modulating tremolo more problematic. Since tremolo was still a popular feature in the Fender line (the Bassman is the only amp that never had it by the BF period) they needed a way to implement that effect without further stressing the tubes. Not that bias vary trem won't "work" on a BF amp design, but it's tricky and less than ideal. I've made custom amps with BFish ideology and bias vary trem and I can tell you first hand... With the higher plate voltages and colder bias it doesn't work well if the amp is clipping. Or even pushed close to clipping. You're pretty much limited to lower volume clean tones using bias vary trem with high plate voltages. So, considering the "new" design ideology with the higher plate voltages for more watts, as well as the advantage of independent channels with truly different sounds, it seems natural that Fender would move to the opto trem for all models that could benefit from it. It's worth noting that the Champ and Princeton (their bottom two in the lineup) never changed to the opto circuit. I'm sure it was considered an upgrade from the usual trem circuits at the time for the reasons presented. What is preferred now for it's specific tonality wasn't necessarily what was attractive then for purely functional options!

[Justin Thomas]

I would think, and this is my theory, that as demand for Reverb in Fenders increased (since Ampeg beat them by a full year at least), they wanted a way to have verb & trem, without increasing the tube count. Leo was a businessman, and while I don't think he was CHEAP, he was ECONOMICAL. The trem in the big Brown/white amps by 1962 used <5> triode stages. We a 6G12A for reference. To add another two preamp tubes would have been an un-needed expense. I don't believe that the move was made so they could have a completely independent channel w/out trem, because the brown/white amps HAVE independent channels. And I'd bet there's STILL less total parts count in the Twin Reverb compared to a white Twin...

The opto trem was a way that Fender could cram 2 effects in the same amp, while not having to punch new chassis for more tubes. There are 6 preamp tubes in both a Brown Twin AND a Twin Reverb... personally, I'll skip the verb for that Brown trem...

Justin

[Chuck H]

I gave you a like for your observation of Leo's economy mindedness. He wasn't cheap, but he certainly wouldn't add yet another two tubes (For a total of 13 friggin' tubes in a single amp! I guess that would be "21 tube performance!" in the catalog lit ) when his engineers showed him the opto circuit as an alternative. Spot on.

But I would still opt for the BF reverb over the weirdo 5 triode wiggle trem. Never liked them. Vive la difference