Can your amp take the heat—literally?
Let’s continue our conversation about filter chokes, from last month’s column.To my ears, filter chokes sound smooth and very clear. They open the sound when used in the power supply and hold up under heavy use. It’s important to remember that the power supply components (rectifiers, filter caps, filter choke, dropping resistors) are what we’re listening to and feeling in an amp. Once your guitar pickups connect to the first preamp tube grid, everything else is a copy of that signal via modulated power supply voltage/current. Each gain stage creates an analog of the signal it’s fed, so the power supply plays a massive role in the sonics and touch-sensitivity of any amplifier.
Most modern higher-power amps will use a filter choke in the power supply between the power tube B+ voltage and the next stage down the line, which is the power tube screen supply. Then the voltage flows through dropping/feed resistors to the preamp filter caps.
What about just using a power resistor instead of a filter choke? Resistors are much cheaper, and that’s why you see them in low-budget, typically low-power amps from the ’40s through the ’70s. Some of these amps are great sounding, with raw, primitive bite and caveman-like swagger: Valcos, small tweeds, Silvertones. Here’s a rule of thumb: Generally, for the purest sound you want a filter choke, but opt for a resistor between the plate and screen supply if you want a sound like those bargain-bin classics. That sounds does provide a fantastic vibe in a mix!
After completing a new build, I like to run through the circuit with a pen in hand. I start from the input jack and move with the signal path to the first gain stage, making sure it’s correct, and then I continue to follow the signal path as it goes through the amp all the way to the output jack. You can find a lot of mistakes that way.
My first build was from the tweed-style schematic in the Angela Instruments Catalog that I mentioned in last month’s column. It used a tube rectifier and an off-the-shelf “universal” power transformer. This power transformer had a lot of windings, so you could build many types of amps using it. That gave me plenty of room to get in trouble, it turned out.
Before I explain how, it’s tube anatomy time. Vacuum tubes have either directly or indirectly heated cathodes. With directly heated cathode tubes, the cathode and the heater/filament are the same element inside the tube. (The filament is the part of the tube that glows and gets the tube hot enough to work.) Indirectly heated tubes have a separate filament and an independent cathode element inside the tube. This helps with noise from the heater winding and provides a slow start, as the cathode needs to heat up enough to conduct current. 5Y3 and 5U4 tubes are directly heated, while the 5AR4/GZ34 is indirectly heated. The latter’s cathode is internally connected to one side of the heater.
Often windings on power transformers have a center tap, which is a wire in the middle of the coil. Typically, this is connected to ground for lower noise and more efficiency with balanced/full wave operation. My universal transformer’s 5-volt tube rectifier filament supply winding had a center tap that I connected to ground. Seemed like the correct thing to do, since the other windings had center taps connected to ground.
I’d turn the amp, which used a 5AR4 rectifier tube, on … and 30 seconds later the fuse would blow. I rechecked all the wiring and it seemed good, but again and again the fuse blew after 30 seconds. Talk about depressing. The next day I started looking at every schematic I could find, eventually noticing rectifier filament windings on the old Fenders did not have a center tap to ground. They had a “floating” AC winding.
Was this it? I snipped the 5V winding center tap and threw the switch. Success! What I learned is the cathode of the rectifier is the voltage output point for the power supply. If a tube rectifier’s cathode is connected to the heater and the heater supply is center tapped to ground, then you’re shorting out the power supply. Got that? The fuse blew once the rectifier heated up enough to conduct current. Remember, rectifiers turn our AC to DC for the amp to run on, but only once the rectifier tube is hot enough to work. (Note: There is one common tube rectifier that is indirectly heated with no internal cathode-to-heater connection that is safe to run with a grounded center tap filament winding. It’s the EZ81/6CA4, found in the Vox AC10, Marshall 18-watters, etc.)
If this seems a little overwhelming, rest assured that building your own amp makes everything about amplifiers less intimidating, and your skills and knowledge will grow as you go.