Micropower subminiature tube guitar amps with reverb and vibrato
Author: Dmitry Nizhegorodov (dmitrynizh@hotmail.com). My other projects and articles
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1. Intro
2. Designs
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2.1 4-tube push-pull amp with paraphase splitter, reverb and vibrato.
2.2 5-tube push-pull amp with concertina splitter.
2.3 6-triode push-pull amp
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3.1 Plate curves and models
3.2 Power dissipation
3.3 Selection of tubes, operation points
3.4 Clipping/Cranking
1. Intro
The goal of these SPICE projects is to come up with one of several candidate schematics for subminiature tube-based micro-power guitar amps that are feature-reachm namely contain tremolo (a.k.a vibrato) and reverb; a tone stack and some multichannel capabiliyes are desirable too.Three candidates emerged from what appeared to be three ill-shaped but distinct flocks of design ideas; All three are presented here in the next section; all three designs are moving targets.
2. Designs
This section contains complete, functional amp designs alone with brief description. The next section contains additional data, measurments, simulation comparison, discussions.All designs assume that twin-triode subminiature tubes 6021 or 6021W are used. Other tubes can be used instead, with great (and greater) success, as mentioned in the discussion section.
the designs differentiate mainly in poer amp area, which in turn infuences other parts.
All designs at the moment share the same, tube-based, 3-triode spring reverb module and the same 1-triode-based tremolo (vibrato) oscillator.
All designs were tested and tuned in SPICE and in flesh (but not necessarily with subminiature tubes - check the discussions section for details) and most took many iterations.
2.1 4-tube push-pull amp with paraphase splitter, reverb and vibrato.
This design has the output stages originally influenced by popular DIY projects with self-split output push-pull triodes. Yet it has become a paraphase output stage design, which measured, SPICE-simulated, and most importantly, sounded better than self-split one.
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max open gain @ 1khz in 8 ohm .................120 max undistorted power @ 1khz in 8 ohm, mW .... 650 max distorted power @ 1khz in 8 ohm, mW ..... 1250
2.2 5-tube push-pull amp with concertina splitter.
this design grew from a flock of vintage tube push-pull amp schematics all featuring bias-modulation vibrato and reverb that is *not* standard fender splitter-mixer with 4.7meg/10p gain-soaker node in the middle. Gibson GA-19RVT is the best representative. Gain is the theme of this design. Concertina splitter fits the bill, as it is ~ 2x more gain than a typical differential stage.
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max open gain @ 1khz in 8 ohm .................220 max undistorted power @ 1khz in 8 ohm, mW .... 800 max distorted power @ 1khz in 8 ohm, mW ..... 11250only 9 triodes are shown. the 10th can form an extra gain stage, see the discussions section.
2.3 6-triode push-pull amp
with fender tone stack, vintage fender-style sugnal mixer and typical fender power stages with presence control. In other words, this is a derivative of classic fender vibro reverb toplogies with differential power driver stage, bias (not LDR) vibrato and the onmipresent reverb splitter/mixer node found an all fenders except GT16.
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max open gain @ 1khz in 8 ohm .................250
@ 100hz and 2khz.................700
max undistorted power @ 1khz in 8 ohm, mW .... 800
max distorted power @ 1khz in 8 ohm, mW ..... 1250
only 11 triodes are shown. the 12th can form an extra gain stage, see the discussions section.
3. Discussions
3.1 Plate curves and models
Any in-depth analysis of the schematics is very difficult without a look at plate curves
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Lastly, composite push-pull curves:
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3.2 Power dissipation
A few words on power dissipation: subminiature tubes a much less prohibitive regards power limits than normal tubes. It is not recommended to overheat or abuse them in any other way. 6021 and 6021W double-triodes add some extra confusing as different vendors and years of production provided different power dissipation limits. Roughly, all datasheets except RCA and Sylvania 1957 6021 specify 0.7W per triode as limit. RCA and Sylvania 1957 specify 1.1W per triode. Russian tubes such as 6n16b, 6n17b specify 0.9W per triode.
3.3 Selection of tubes, operation points
These schematics were optimized in SPICE with 6021 in all positions; While 6021 appears to be a good "universal" triode, its versatility is a compromise: its Rp is somewhat too high for output/reverb stages and its mu is somewhat too low for gain stages. Hence an amp built with, say 6n16b in output/reverb stages and 6n17b in gain stages will function better, providing more total gain and better driving ability. Only a few parts need be tweaked to accommodate such tubes; output stage cathode resistors need be lowered by ~ 30%, cathode resistors of the gain stages can stay the same or be slightly lowered, and those plate resistors that are under 100k increased to 100k. A few minor tone adjustments may be needed for the grid shelving networks to reflect higher gain, higher Rp and higher Miller capacitance.
3.4 Clipping/Cranking
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this one needed around 2x more imputs swing (dip at 1k at tone control
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this help to understand how the vaule of presence control pot affects tone (frequency responce). Th plot curves are for different positions of the pot - at 0, 25, 50, 100 percent.
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4. References
Author: Dmitry Nizhegorodov (dmitrynizh@hotmail.com). My other projects and articles