Ask Amp Man: From Silverfaces to Silvertones
Fig. 1. On most older Fender combo amps, the output transformer impedance is optimized for the combo’s particular speaker load, so hooking up an external cab causes an impedance mismatch.

Making sense of the external speaker jacks on vintage Fenders—and resurrecting a cool old Silvertone.

Hi Jeff,

I have a 1975 Fender silverface Twin Reverb. It has two 8-ohm speakers wired in parallel, a 4 ? load. It also has an external speaker jack wired in parallel with the internal speaker jack. But the labeling on the chassis and schematics seems inconsistent and unclear. If the internal speaker load is 4 ohm and I connect a 4-ohm external load, the output transformer sees a 2-ohm load. Or if the external load is 8 ohm, the OT sees 5.6 ohm. Is either of these loads safe? Keep up the good work!

Ron Rumsey

Hi Ron,

Why Fender included external speaker jacks (Fig. 1) on its early combo amps is a bit of a mystery to me. On most Fender combo amps (except for later types with impedance switching), the output transformer impedance is optimized for the combo’s particular speaker load. A Deluxe Reverb output transformer, for example, would have an output impedance of 8 ohms in order to generate its full 20-watt power into the 8-ohm load of the single 12" 8-ohm speaker. On a Twin Reverb, there’s a 4-ohm secondary on the output transformer to develop the full 100 (or 80) watts across two 8-ohm speakers wired in parallel (for a 4-ohm load). A Super Reverb has a 2-ohm secondary winding to develop 40 watts across four 8-ohm speakers in parallel (a 2-ohm load).

Fender seems to have overdesigned their output transformers in the past. Thousands of them have been running into improper impedances for decades, hanging in there
just fine for the most part.

Fender’s external speaker jack was a nice feature, ’cause we all know more cabinets are better! But using it to power any other speaker would cause an impedance mismatch. Say, for example, you connect an 8-ohm cabinet to the extension speaker jack of a 100-watt Twin Reverb: This presents the amp with a total load of 2.66 ohms, which makes the output power drop due to the impedance mismatch. (From experience, I can guess the power decreases by about 30 percent.) It also increases the impedance load on the output tubes, causing more wear. So now you’re left with a Twin Reverb with an output of approximately 70 watts, with 46.7 watts powering the combo speakers and 23.3 watts powering the extension cabinet. Your sound may be better dispersed and, depending on the efficiency of the external cabinet, may even seem louder, but the bottom line is there’s an electrical mismatch. I must say, however, Fender seems to have overdesigned their output transformers in the past. Thousands of them have been running into improper impedances for decades, hanging in there just fine for the most part.

On the other hand, avoid impedance mismatches when it comes to Marshall amps. Fair warning! I hope that sheds a little light on the mismatch mystery.

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Fig. 2. The four 100 µF 150V filter caps in a vintage Silvertone 1484.

Hey Jeff,

I’m in the process of bringing an old Silvertone 1484 back to life, and I came across your Premier Guitar post. Thanks for sharing your expertise! I have a quick question about one of your comments. You said that replacing the four 100 µF 150V caps in the voltage-doubler circuit could generally improve performance and increase voltage or capacitance values slightly. But how much is “slightly?” I’m having a hard time finding 100 µF 150V caps for less than $20 apiece, and I’m curious what my alternatives might be. Thanks for any help you can offer.

Jonathan Clay

Hi Jonathan,

Thanks! I’m glad you found my column helpful to your Silvertone resurrection. Regarding the capacitor values and ratings in the power supply: You can always go higher in voltage than what was originally installed in the amp. Capacitor design has come quite a way since those amps were designed and built, and capacitors can be substantially smaller these days.


Fig. 3. These replacement 100 µF 350V filter caps are more readily available from amp parts suppliers.

For an alternative to 100 µF 150V caps (Fig. 2), consider the more readily available 100 µF 350V caps (Fig. 3) from such suppliers as Mojo Musical Supply. They should fit fine and work well. (It might be more difficult to find slightly higher capacitance values such as 125, 150, or 200 µF, as these are relatively uncommon values.) I hope they give your Silvertone a killer tone!

Photo 1

We’re almost finished with the aging process on our project guitar. Let’s work on the fretboard, nut, and truss rod cover, and prepare the headstock for the last hurrah.

Hello and welcome back to Mod Garage. This month we’ll continue with our relic’ing project, taking a closer look at the front side of the neck and treating the fretboard and the headstock. We’ll work on the front side of the headstock in the next part, but first we must prepare it.

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Diatonic sequences are powerful tools. Here’s how to use them wisely.

Advanced

Beginner

• Understand how to map out the neck in seven positions.
• Learn to combine legato and picking to create long phrases.
• Develop a smooth attack—even at high speeds.

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Knowing how to function in different keys is crucial to improvising in any context. One path to fretboard mastery is learning how to move through positions across the neck. Even something as simple as a three-note-per-string major scale can offer loads of options when it’s time to step up and rip. I’m going to outline seven technical sequences, each one focusing on a position of a diatonic major scale. This should provide a fun workout for the fingers and hopefully inspire a few licks of your own.
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