Pro 6G5-A Vibrato circuit Q

[octal]

Gents,

I've been looking at the brown "harmonic vibrato" circuit for one of my freak builds. Is the cathode follower buffering the output of the trem oscillator really necessary? It pushes the tube count up to two and a half. The cathode follower is driving a 4.7Meg resistor (admittedly, into a capacitive load) but I find it hard to imagine that it would make much difference to omit it. Yea or nay?

Maybe Fender wanted to pad the tube count on this top of the line amp to make it seem more impressive?

The 6G12 Concert also uses a harmonic vibrato with only two tubes, but the circuit looks less elegant (the phase split of the vibe signal is done sort of like a paraphase inverter as opposed to the cathodyne circuit in the 6G5-A)

I welcome any and all thoughts!

[octal]

Aah, I didn't realize that the difference between the two styles of vibrato was related to the revision number, not the model - i.e. The 6G12 concert uses four half triodes, the 6G12-A uses five half triodes, same thing with the pro comparing the 6G5 to the 6G5-A.

I'm thinking that Fender must have changed the circuit for a reason, though I still suspect that my hybrid idea (the 6g12A circuit with the trem osc cathode follower omitted will work.

[Enzo]

The LAST thing in the world Leo Fender would have done would be to add an unecessary tube to "pad the tube count." No one would be impressed anyway. Leo would mostly go the other way. he would eliminate whatever he could to keep the COST down.

[octal]

That's a good point. On the other hand, Leo was all about reducing costs to make money, so if "padding the tube count" helped give him a sales edge and netted him more money, who knows what he would do? I do imagine that the "harmonic vibrato" was probably deleted in the blackfaces for cost/reliability reasons.

I have some Fender ad copy from the blackface era and every amp model has the tube count mentioned- "Fifteen tube performance" and such. Of course, almost nobody nowadays would be impressed by a high tube count but I suspect that the More=better=it goes to "11" angle played well back in the day.

Any thoughts about the circuit itself? I just can't fathom why a single triode oscillator needs a cathode follower to drive a load completely isolated by a 4.7 meg resistor!

[GibsonLover]

I'm with Enzo on this one. I've posted before asking about that. Since the circuit was updated with -A soon after it came out, a tube was added, AND half was not used, I gotta believe it was necessary. My question was more directed at the ReVibe circuit, which copied the earlier version, yet I don't see people with ReVibe's complaining about their oscillator dying. Another point may be its performance - a slower speed needs more amplitude and/or less loading on the LFO. Who knows.
If you find yourself one triode short, drop an IRF820 in there. It's just a cathode follower, and it's not in the audio path. Same with the cathode follower used on bias modulated outputs.

[tubeswell]

I welcome any and all thoughts!

Hi octal, FWIW I had a query about a similar circuit a while back here;

http://music-electronics-forum.com/s...ad.php?t=10295

(Don't know if that helps)

[octal]

Thanks to everyone for your responses. I'll probably build it stock and then try jumpering out the cathode follower to see what happens.

Nathan