Fender Harmonic Tremolo -unused 1/2 12AX7- swap with 12DW7?

[Brinkman]

So it looks like the 12AX7 (V5) in a five-triode harmonic tremolo circuit can be swapped with any noval dissimilar dual-triode that has a 12AX7 triode at the first section, pins 6-8.

My question is, could I use a 12DW7 which has 1/2 a 12AX7 for section 1 and 1/2 a 12AU7 for section 2? Could I then use the second section as a cathode follower to drive the signal in a preamp out capacity?

I know this would require some extra switches and wiring but it would add some functionality to the unused Ext. Speaker output jack.

Is this instance I would be modifying a vintage amp that has already had the cabinet “modified” (combo turned into a head). So the horse has already left the barn.

Any thoughts?

[Brinkman]

Anyone?

Surely someone has done something similar.

[Willabe]

Surely someone has done something similar.

No, I don't think it's an absolute that someone has done this. I don't think many of those amps 3 x 12AX7 harmonic trems were made. I've been here ~14 years, have read close to every post and no one has asked about doing this that I recall.

In this instance I would be modifying a vintage amp that has already had the cabinet “modified” (combo turned into a head). So the horse has already left the barn.

Not completely, chassis still unmolested, yes? No holes drilled?

I know this would require some extra switches and wiring....

Now your talking about messing with the head, drilling holes for switches. It's your amp, but just because someone put the amp chassis into a head cab doesn't mean the horse is out of the barn. I wouldn't mess with that chassis. You can get a new cab for that amp from different places that will be to original spec, size/covering/grill cloth/handle. What happened to the original cab?

..... and 1/2 a 12AU7 for section 2? Could I then use the second section as a cathode follower to drive the signal in a preamp out capacity? ......  add some functionality to the unused Ext. Speaker output jack.

Running the vibe signal out the ext. speaker jack which is next to the main speaker jack might not work. Might cause parasitic oscillation problems. Not good layout and lead dress design.

And if you did this what would you do with it? Just run the vibe into a different power amp? Why, the Fender power amp is fine for that preamp.

[Willabe]

You might be better off selling that amp, so someone could save it by getting in a correct combo cab.

And you could take some of the money and build a Re-Vibe. It's a Fender brown fave harmonic trem/vibe with Fender tube reverb in 1 chassis.

I'd build sluckey's. It's proven layout, with several guys having built it;

https://www.sluckeyamps.com/revibe/revibe.htm

[Merlin]

I'm confused, the cathode follower is part of the tremolo, how would you be using it for a preamp out, too?
BTW, you don't actually need a cathode follower for a line out, you can use a passive divider

[Willabe]

I'm confused, the cathode follower is part of the tremolo, how would you be using it for a preamp out, too?

You already know this, The 3 tube harmonic vibe uses only 5 triodes, he wants to use the unused/left over triode for a CF out on the ext. speaker jack. 

[Brinkman]

Okay change of plans. Not buying the head after all; I got a bandmaster chassis and brown faceplates instead. The above question still stands and i do appreciate your concern about the posterity of vintage instruments.

[Willabe]

..... and 1/2 a 12AU7 for section 2? Could I then use the second section as a cathode follower to drive the signal in a preamp out capacity? ......  add some functionality to the unused Ext. Speaker output jack.

Running the vibe signal out the ext. speaker jack which is next to the main speaker jack might not work. Might cause parasitic oscillation problems. Not good layout and lead dress design.

And if you did this what would you do with it? Just run the vibe into a different power amp? Why, the Fender power amp is fine for that preamp.

Functionality to the unused ext. speaker jack? Why? Loads of amps out there that have an ext. speaker jack that guys never use. And now a days with people playing softer, many don't need an extra speaker cab. 

[Brinkman]

Running the vibe signal out the ext. speaker jack which is next to the main speaker jack might not work. Might cause parasitic oscillation problems. Not good layout and lead dress design.

And if you did this what would you do with it? Just run the vibe into a different power amp? Why, the Fender power amp is fine for that preamp.

Because I might be playing into headphones on account of living in an apartment (again).

And you could take some of the money and build a Re-Vibe. It's a Fender brown fave harmonic trem/vibe with Fender tube reverb in 1 chassis.

It was thinking about building revibe that got me into this. But I already have a reverb tank. I might not live in an apartment forever, so the power amp would be nice to have. Besides, it allows me to sell the excellent condition brown ‘62 Princeton that I’ve been holding on to for the last 20+ years.

[Brinkman]

Feeling better about deviating from the fender schematic on a clone scratch-build. I had to use a bandmaster chassis and faceplate/backplate but the output transformer will be a clone of the Vibrasonic 45550. The speaker cabinet will be ported, using a JBL D130 that I already purchased. Other mods will be a bias adjustment pot and James style tone controls.

[Willabe]

Why the James TS?

Putting in a James TS will lessen the amps overall gain. How much, I don't know.

[Brinkman]

No good reason. When I was researching my build someone on another forum was praising the sound of a Vibrasonic with a “baxandall tone stack.” That piqued my interest.

[Willabe]

Baxandall is different than James, look them up for the differences.

[Brinkman]

I put it in quotes because baxandall was used when I came across it but it wasn’t an active circuit they were referring to, but a passive one.

[Willabe]

I put it in quotes because baxandall was used when I came across it but it wasn’t an active circuit they were referring to, but a passive one.

I think, a James TS is passive, a baxandall is an active TS circuit.

Tone Stacks - Part 2 - James & Baxandall | PedalPCB Community Forum

[Brinkman]

Yes, that’s my understanding as well. And if you want to really split hairs, neither baxandall or james were the first to publish their namesake circuits.

[Brinkman]

So it looks like the 6G8 twin and 6g14 showman used james-style tone stacks in addition to the five-triode harmonic tremolo also used in the -A circuits of the “professional series.” I’ll probably copy that preamp circuitry and marry it to a single push-pull pair of 6L6GCs and a clone of the 45550 output transformer, similar to the output stage of the Vibrasonic schematic.

Otherwise, my only real deviations would be the adjustable bias and, per the topic of this thread, using a 12DW7 in place of a 12AX7 to drive a “preamp out”. I have all three of Merlin Blencowe’s books; I’ll consult those when it comes time to designing the cathode follower.