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Post subject: Question regarding the 65 Twin Reverb
Posted: Tue Nov 24, 2009 10:12 am
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Hi all. I am not an amp connoisseur whatsoever, so I am hoping you Fender experts can help me figure this out.

I have been trying to record with guitar but I am confined to small spaces. So, I have been disconnecting the internal speaker of a 65 Twin Reverb to just use the head of the combo through a line-in setup (instead of actually mic-ing the speakers). A friend of mine let me know that doing this could be dangerous because tubed amps are designed to balance ohm levels through the internal speaker line.

I do not know how to research this, or what I would need to know about my specific model, or if there is a way to solve the issue. For instance, I do not know what terms would be used to describe this phenomenon and I don't know how many revisions have been made to the '65 Twin - affecting whether info on any 65 Twin pertains to mine. And also, this same friend had said there are products designed to match the ohm level between the head and speaker, but reduce the volume. Since I turn up the volume to about 5 on this amp when recording, I would blow through 3 apartments in either direction of me if I kept the internal speakers connected.

Any help you can offer will be appreciated.

BrandonB


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Posted: Tue Nov 24, 2009 3:23 pm
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Your friend is right - running your amp as a head without a speaker or resistive load can fry your amp - definitely a no no.

The Twin amp is a big, loud amp. To tame it, you'll need a speaker attenuator or simulator. These present a resistive load to the amp so that it doesn't destroy itself, but reduces the sound to a normal level.

Candidates for a in-line speaker attenuator (speaker out -> attenuator -> speaker). Note these reduce the sound considerably, but don't eliminate it:
Dr. Z's Airbrake - http://www.drzamps.com/airbrake.html
THD Hot Plate http://www.thdelectronics.com/product_p ... plate.html
Weber http://www.tedweber.com/atten.htm
On all of these, you must have a speaker plugged in at the other end. I have a Dr. Z Brake Lite (under 50 watt version of the Airbrake) in my Bandmaster and it's great.

Candidates for a speaker simulator (speaker out -> simulator)
Palmer PGA-04 (http://www.palmergear.com/pga04.shtml) - these are expensive, but if you don't want any sound outside of what you're recording, this will do it.

In-line devices to get tone to tape, but don't reduce the volume - these are similar in hookup to an attenuator, but your guitar is still just as loud). This is not what you want.
Radial JDI DI box
Hughes & Kettner Red Box
ADA MicroCab II

Depending on how often you record vs. playing live will depend on what you want to use. If you always find yourself just recording and not playing live or playing at louder levels, you may want to buy another amp that's lower wattage to get the sound you want. A Twin Reverb is a great amp for playing live or in a studio where you can crank it to 5. For an apartment, you may want a Princeton Reverb (great tone) or even a champ - stick a mic on either of those and you can get great tone at less than eviction-notice levels.

Good Luck!

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Posted: Tue Nov 24, 2009 7:32 pm
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Thanks, that was just what I was hoping someone could tell me.

BrandonB


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Posted: Fri Nov 27, 2009 7:07 am
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From Billm :

"16 ohms isn't nearly enough impedance to cause problems, but a tube amp will survive a shorted, zero-ohm speaker connection longer than it would survive an open connection, with no speaker and no shorting switch on the jack.

The problem with high or infinite impedance is that too large a magnetic field builds up in the OT because there is no load. The field ultimately collapses. Just as there is a step-down from plate voltage (320-340V in a PJ) to speaker voltage (10V or less), there is a step-up in the un-drained voltage that builds up if there's a high impedance on the secondary (speaker) side. So 50 volts there can become nearly 2000 volts that gets kicked back to the output tubes. The result is arcing in the tubes and breakdown in the insulation of the output transformer windings.

The PJ, along with most modern amps, has protection diodes that act like little lightning arrestors to ground out these high-voltage pulses. The best thing, however, is to be sure that you're never playing into an open speaker connection.

Playing into a shorted connection for a while will draw too much current from the power supply and will either pop the fuse or (more likely) burn a resistor in the power supply. The same resistor also tends to get toasted if the screen grid shorts in one of the EL84s.

In any event, the PJ will drive a 16 ohm cab just fine."

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