Digging deeper into impedance issues and capacitor leakage

Thanks for your February 2010 article (“Can You Turn That Thing Down?”) that discussed removing one set of output tubes of a Twin Reverb, or using a switch to disconnect their cathode connections, to lower the amp’s output. The article also discusses removing one speaker from the circuit, increasing the output impedance, and connecting the screen grids to the plate of the output tube.

If one set of tubes are pulled, the output impedance doubles, does it not? If that’s so, then removing one speaker would double the output load and reflect the new output impedance value. The output transformer matches the impedance of the output stage to the load, and the ratio of the windings remains the same. So would that keep the impedance balanced?

I’m one of those old, frustrated BSEE [Bachelor of Science in Electrical Engineering] people that knows he didn’t get out of college what he went there for. I’m working on my old Twin and want to decrease weight, as well as output power, as much as possible.

I also know that capacitor leakage can be an issue, as an amp I built out of spare parts had to have all the caps replaced. Do the non-electrolytic capacitors of the last decade or so still degrade and leak like the old paper insulators? I don’t have a lot of equipment to troubleshoot with and have a ton of distortion that sounds to me like bad caps. I replaced all the electrolytics about six or seven years ago and the amp has no hum, but it hasn’t been used much in 25 years.


Hi Gerald,

Thanks for reading the column. You have a couple of questions here, so let me address the impedance question first.

You are correct that when you remove one set of output tubes from a Twin, you are effectively removing half of the load from the primary side of the output transformer. In a perfect world, you would like to maintain balance between the transformer’s primary and secondary sides. In order to do this, you would need to reduce the load on the secondary by half. This can be done in two ways. On a transformer with multiple taps, you can simply select the next lowest impedance setting. Say your rig consists of a 100-watt head and 16-ohm cabinet. If you removed two of the output tubes from the amp, you could simply select the 8-ohm impedance setting. Now your 8-ohm output has a 16-ohm load, which is half the impedance load it would normally see.

This also matches the two-tube load on the primary side of the transformer, which is half of the load it would normally see, so the primary and secondary loads are balanced.

On an amplifier such as a vintage or reissue Twin Reverb, you do not have the option of selecting output impedances. In this case, in order to match the primary and secondary loads, you would need to reduce the load to half of the original load. This can be done in one of two ways. You could replace the two existing 8-ohm speakers with two 16-ohm speakers. With the speakers wired in parallel, as they normally would be in these amps, the resulting load would be half, or 8 ohms. The other way would be to disconnect one of the stock speakers, effectively reducing the load to 8 ohms. (In your letter you referred to this as “doubling” the load, but it is actually cutting the load in half.) Keeping in mind that in reality, removing two of the amp’s output tubes only reduces the output by approximately 40 percent, and assuming that the remaining speaker could handle the amp’s new full output, you would have the optimal scenario.

Moving on to your question regarding signal caps: The newer-style signal caps do not tend to be as leaky as the old paper-foil caps, but yes, some still can become leaky. The biggest offenders in my experience are what are known in the field as the “chocolate drop” caps, used in late-’60s and early-’70s Fender amps. When these become leaky, they can cause the amp to become very anemic and distorted.

The quickest way to measure for leaky caps is with a voltmeter. Most times, signal caps in a tube amp will have one side connected to a high-voltage source, such as the plate of a tube. This side, of course, will have substantial DC voltage applied to it. To check for leakage, you need to measure the DC voltage on the opposite side of the cap with reference to ground. Most caps will have some form of measureable voltage on this side of the cap, but if you begin seeing voltages that are in the hundreds of millivolts to the 1-volt or more range, I would consider the cap to be suspect. Replace the cap and retake the measurement. If it is relatively the same, the voltage may not be leaking through the cap. Its source may be some other part of the circuit and the original cap may be just fine. If the voltage is substantially reduced, continue this procedure through the amp and replace as many caps as necessary. Once you do this, you will probably have a much better sounding amp.

Hopefully that will give you a lean, mean Twin Reverb machine.

Jeff Bober
Jeff Bober, one of the godfathers of the low-wattage amp revolution, co-founded and was the principal designer for Budda Amplification. Jeff has just launched EAST Amplification. He can be reached at pgampman@gmail.com

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