Low-Wattage Amps with Clean Tone

[tdfbsi]

I have been interested for some time in designing low-wattage amplifiers that have a rich tone that remains clean through most of the amp’s volume range. After a great deal of tinkering and a bit of good fortune, I found that using a low-gain tube such as a 12AU7 in cascode configuration for the first gain stage followed by a 12AU7 paraphase inverter to drive a cathode-biased, push-pull pentode output section allows me to meet my stated design goals while keeping the overall circuit design simple and cost-effective. Similarly, using a 12AU7 cascode front end followed by a low-gain triode stage from a 12DW7 to drive a single-ended pentode output section yields a substantially clean amp with a 5-10 watt working range. While I am not a skilled guitarist, I always play test my designs, primarily with single-coil guitars, to listen for note bloom, chime, and harmonics at different volume settings. My tests have shown, at least to my ear, that a cascode front end as opposed to the more customary initial triode gain stage enhances the dynamic range of the amp and produces a broader and more even sound spectrum. And while paraphase inverters are not common these days, I like them for their simplicity and the desirable harmonics that they introduce into the signal chain. Ditto for the use of cathode-biased output sections. I should add that for my sound testing I use a Mojotone Lite cabinet fitted with a 10” reproduction Jensen C10Q speaker or a Mojotone Lite cabinet fitted with a 12” Celestian Neo Creamback speaker.

I have built a series of amplifiers based on the above paradigm and wanted to share the designs with readers of The Amp Garage to use as they might like, including to modify and hopefully improve them. Generic schematics for push-pull (PP) and single-ended (SE) amps are attached below along with a document listing the specific component values and typical operating parameters for amps with different pentode output sections. Based on my testing, incorporating TMB tone stacks and the like into the signal path (and providing for gain recovery) tends to diminish the richness of the tone compared to the simple circuit designs presented here. (DISCLAIMER - While I have found these amp designs to perform quietly and reliably, I make no warranties as to their performance or suitability for others’ applications. If you choose to use them in whole or in part, you do so at your own risk.)

The push-pull design has dual channels that can be played separately or in a blended configuration, which yields higher gain and more drive as one would expect. Channel 1 uses a 12AU7 cascode as the first gain stage, while Channel 2 uses a 12AX7 with paralleled triodes as the initial gain stage. Plugging into Channel 1 allows the user to play exclusively through the cascode channel or, when switched on, through both channels. The channels can be blended over a wide range via the separate channel volume controls. Plugging into Channel 2 allows the user to play exclusively through the paralleled triode channel. Channel 1 is voiced for a broad frequency response and has a dedicated treble roll-off type tone control, while Channel 2 is deliberately voiced for a brighter sound that compliments Channel 1 when the two channels are blended. The component values for the cascode and paralleled triode stages have been selected to give comparable gain. I have provided component values for amps with 6K6GT, 6V6GT, or 7591A output tubes in the attached document.

The SE amps are single-channel designs that utilize the second triode of the 12DW7 as an output tube bias-modulating tremolo driver. I have provided component values for SE amps with an EL34 or 6V6GT output tube. You can substitute higher gain 12A_7-type tubes for the 12DW7 if more drive is desired. However, using a 12AU7 in the V2 position will not provide enough gain for the tremolo oscillator to work properly. For the design using an EL34 output tube, I sometimes wire a second octal socket in parallel in the output section, using a shared cathode resistor but separate grid stoppers and screen resistors. This allows you to utilize two 6V6GT’s in parallel, single-ended output mode in place of a single EL34 tube. No other wiring changes are required to use this configuration.

Feedback is welcome. Thanks for reading.

[Colossal]

Welcome, and thanks for your contribution. Nice work! I am a big proponent of those very simple topologies as well. Good examples include the Matchless Spitfire, Marshall 1974 18W and 18W Lite IIb variants, Komet Songwriter, AC30/4 (EF86 -> LTPI). All great sounding amps (IMO) with minimalist controls.

[tdfbsi]

I realize that I stated the wrong value for R15 in the single-ended 6V6GT amp variant described in my previous post. R15 should be 10K/2W rather than 2.2K/3W. The schematic and corrected component value table are attached.

[tdfbsi]

My apologies everyone. I found another error in one of the value tables. The schematic and corrected component values table for the push-pull 7591A amp are attached. The values of R23 and R24 were wrong in the original posting. The correct values are 2.2K/3W and 10K/2W, respectively.

[10thTx]

This amp has only 7 watts and has a clean tone in push/pull configuration

https://el34world.com/Forum/index.php?topic=27966.0

Soundclips to the amp:

https://el34world.com/Forum/index.php?topic=27994.0

With respect, 10thtx

[tdfbsi]

If you want increased gain, more volume, and earlier breakup with the push-pull designs, the easiest modification is to employ a 12AX7 paraphase inverter in place of a 12AU7 paraphase inverter. The figure below shows the component values that I use for a 12AX7 paraphase inverter. They were taken directly from the original 1950 Brimar data sheet for their 12AX7/ECC83.

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