Where to tap off of the Fender Twin Pre Amp circuit?

Keith Hilton · From: 248 Laurel Road Ozark, Missouri 65721

Fender does not have a pre-amp out. I have a 1995 Custom Fender Vibrosonic tube amp. 100 watts, built like the Fender Twin. Where do you tap off the circuit to create a pre-amp output. I suspect that tapping into the circuit might change things. What should one do to prevent change,loss, or improper bias? Didn't someone tell me there are companies who are making pre-amps similar to the pre amps that are in all the old Fender tube amps? Wonder why Fender never put a pre amp out connection in the back of their tube amps?

Jerry Erickson · From: Atlanta,IL 61723

Keith,
Fender usually puts a master volume control right before the phase inverter tube. That would probably be the best spot for a preamp output. On the BF style amps there are 2-220K mixer resistors that go to a .001 coupling cap and then to the PI.

Dan Tyack · From: Olympia, WA USA

Keith,

If you are looking for a direct output to get the full 'vibe' of the twin, I don't think you're really looking for the preamp output. I think that Paul Rivera had it right on the later fender amps (80s-early 90s) with a 'line out', which comes after the power amp section.

Jay Ganz · From: Out Behind The Barn

Yeah, but wouldn't that be a power amp out then Question

You'd be feeding a high wattage signal into the mixer, effects, or whatever. Can't be. Rolling Eyes

Some sort of attenuator would be needed in that case.

Dave Mudgett · From: Central Pennsylvania and Gallatin, Tennessee

Like Dan, I prefer the sound after the power amp. Yes, you need to drop the signal voltage to line level.

Gerald Weber wrote an Vintage Guitar Magazine article about this in the 90s, and I think he put that in his amp book. This approach directly taps the speaker connection. Here are a few links to rehashes of that type of thinking, there are more:

http://www.harpl.com/199619624.html

http://www.rru.com/~meo/Guitar/Amps/Kalamazoo/Mods/ezlo.html

http://www.tdpri.com/forum/amp-central-station/68891-simple-line-out-circuit.html

The exact values of the resistors depend somewhat on personal taste plus the nominal speaker impedance. One can easily rewire the extension speaker jack and not drill holes in the amp. The third article suggests alligator-clipping the dropping circuit to the speaker terminals, useful for a non-permanent installation.

I did something similar to this on a beat up but great sounding little Princeton Reverb a long time ago so I could use that to slave a larger amp when I wanted the pushed-Princeton sound at a higher volume. I thought it made the slave sound pretty good - not quite the same as the Princeton, but I found it useful.

Of course, YMMV and all that. Smile

Keith Hilton · From: 248 Laurel Road Ozark, Missouri 65721

Dave I will read the 3 articles and let you know what I think later today. Is Paul Rivera still building the Rivera amp? What is a link to his web site? Did Paul Rivera come out of the Fender Custom shop in Oregon?

Pete Burak · From: Portland, OR USA

Here is an article on Pre-amping a Fender Twin:
http://www.dozin.com/gear/preamp/mod.html

Bari Smith · From: Spartanburg SC USA

www.torresengineering.com $15.95 for Fender kit...1/4 out w/adjustable level!!!! Very Happy

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Brad Sarno · From: St. Louis, MO USA

Keith, here are 4 preamps that at their core are exactly based on the Twin's EQ/gain preamp section.

Mesa Boogie Studio Pre
Alembic F2b
Revelation Tube Preamp
Tonic Tube Preamp

If you look at the Twin's schematic, right there before the 12AT7 phase splitter section, there is a .001uF cap that is carrying the whole preamp signal. You could tap right there after the cap. You may want a bigger cap value there to give full bandwidth to your next stage in case it's a lower impedance that you're sending it to. You definitely want to be AFTER the cap because the high plate voltages are on the signal path right before that cap.

Brad

Mark MacKenzie · From: Franklin, Tennessee, USA

Brad, what type of cap do you recommend here? Is this the place for an Auricap type or Orange drop 716?

Thanks,
Mark

Brad Sarno · From: St. Louis, MO USA

Given those two options, I'd lean to the 716p Orange Drop because it's a film&foil type cap. The Auricap is just metallized film, and overpriced in my opinion. The Auricap is a nice, neutral, and very good cap, but I just tend to like foil caps better for tube and steel guitar applications. Foil, to me, has a more 3D, silky detail in the highs. A more direct sound. It's pretty subtle and euphonic when splitting hairs between one good cap and another, but foil tends to win out for me. 716 Orange Drops are simply just really good sounding caps for guitar in tube amps. They tend to lean toward the more forward, brightish end of the spectrum. If you want the lush depth and detail of a good foil cap, but want a mellower, warmer tone try the Angela tin caps. www.angela.com

Brad

Jim Sliff · From: Lawndale California, USA

Be wary of any Torres kits. You can buy the same parts for a few bucks and find instructions on the internet...and I've seen a few of his kits with instructions that were either not for the amp being worked on, wrong, or illegible. He also seems to have no qualms about mods that require drilling holes in collector's items....

As far as why there is no preamp out on Fender's vintage-style tube amps, that's because there was no preamp out on amps in the 60's/early 70's. It wasn't needed....and I'm not sure why you'd want one now - if you want a direct-inject low impedance signal to run to a board, it really should come after the power section; running a preamp signal to a mixing board normally results in pretty bad tone - you lose all the "guts" and heart of the amp. If you want to run two amps, just use an a/b/a+b switch or a stereo chorus (and don't use the chorus effect) as a splitter. Also, running a preamp out into another preamp causes a whole different series of issues - you're creating a series of gain stages that can be hard to control.
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Olli Haavisto · From: Jarvenpaa,Finland

There a few good speaker emulator DIs that connect between the amp and speaker.
You still have to use the speakers for load,though. Hughes& Kettner and Palmer are two manufacturers that I know of.
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Olli Haavisto
Finland

ajm · From: Los Angeles

Keith: Just exactly what are you trying to do?

Our answers would be better if we knew this.
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Artie McEwan

Bill A. Moore · From: Silver City, New Mexico, USA

I posted this info. in the other thread concerning output impedence. The 94 Twin OT, pt.#002-6478-000, has a seperate winding that outputs to an XLR connector. It has the added advantage of 3 taps for 4,8,or16 ohm output to the load, and is wired to 2 jacks for paralell or series connection to the main speaker. The guru shows this xformer for 60.00.

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