By Dan Torres
Tiny Tone Power. (Part 2)
Last month I started an in-demand project, building a tube amplifier with very low power. I am trying for 1 or 2 watts, maybe less.
The amp will be a single ended class A all tube amplifier and a lot of fun to build.
One of the things to get out of the way first is that tube amps do not price out at “watts per dollar” when you get to power this low.
That is, this small amp with 1 watt will not be 1/50th of the price of a 50 watt amp. Even though power is low, the amp still requires a full power supply - filter capacitors etc, tubes, tube sockets, pots, preamp tubes, chassis. All these parts are the same as used on a large amp.
The only part that goes down in price very much is the output transformer.
So why make it? The asked for goal is an amp that can be cranked up in the living room, and please the owner with true all tube tone, sustain, power tube distortion and all the great tonal reasons tube amps are so preferred. The other major use is in the studio – obvious – easy to isolate while still providing a full-house tone.
This is a work in progress. I am building the amp as I write about it.
The first part of building an amp is design of the power supply.
I have two power supplies for this amp. One without a rectifier tube (Economy) and the other for enthusiasts who demand a rectifier tube.
Notes on the rectifier tube. The famous rectifier tube “sag” effect may not be very dramatic in this case, as even the little 5Y3 tube will be understressed in this amp. The sag only happens if the rectifier tube is “pushed” hard by current draw under full power – 1 watt isn’t much power.
The rectifier tube extra costs are:
#1. A transformer with a 5 volt 2 amp rectifier tube filament
#2. A rectifier tube
#3. Rectifier tube socket
#4. A chassis with room for the socket, tube etc.
#5. Not too significant as a hobby project – more time to build.
Ok, Check out the power supply drawing.
I have left the ground switch in this drawing, but for safety it should really be omitted from the amp – so many hobbyist complain about not having it with so much concern it is illustrated. For a safer amp leave the ground switch out of the circuit and omit the .047 capacitor that is part of the switch.
The drawing shows proper power supply polarity. Be sure to put the white and black wires in the correct places, and the correct fuse.
A power transformer has a “primary” and often several “secondary” windings.
The power from the wall 115V AC is connected to the primary of the power transformer.
The secondary has “stepped up” voltages for the tubes, and stepped down voltages for the filaments.
The recommended transformers in this case are 550V center tapped, or 250V with a bridge rectifier (economy) Both come out pretty close to the same “plate voltage” – the power for the tubes. (330 standard, 282 economy.)
You don’t need to get into all this math stuff if you don’t want to. The plans will account for everything as I write the project.
The transformer for the standard model has to have a 5 volt 2 amp filament winding for the rectifier tube. The circuit is pretty simple, and the amp can use other tubes, such as the 5AR4 for more power (357 plate volts).
The other secondary is the 6.3 volt 2 amps needed for the power tube and preamp tubes.
The power supply is standard filter capacitors and resistors, but you can see some part value differences between Standard and Economy.
This is due to the LIMITATIONS of a tube rectifier.
A 5Y3 rectifier can only handle a certain amount of capacitance at its output, 20 mfd is the recommended value – 23.5 mfd in this case. Two 47 mfd caps in series.
The solid state rectifier can handle any capacitance. Higher capacitance provides more bass (if your output transformer is good enough) and more voltage – power.
The economy circuit uses two 100-mfd caps in series (50 mfd total.) It could use much higher values 220 mfd, 500-mfd etc. But it would make the power supply too “stiff” for a guitar amplifier, also we don’t need the extra power – we are trying to get rid of power.
The resistors in parallel with the first two series filter capacitors are to even out any inequality between the two caps. They also discharge the circuit when the amp is turned off. Leave the standby switch ON when you turn off the power. Then the resistors will discharge the circuit in about 5 minutes.
The output transformer is attached to the output of the rectifier – all noise reduction has to appear before this point. That is why the filter capacitors are so critical.
The power tube will be wired in triode so there is no need for a “screen grid” supply, just two tubes, a driver tube, and the preamp tube.
A single ended output transformer has only four wires, pretty easy from there – but transformer selection and its primary impedance will be quite a job – I have about 5 different ones to try in this circuit for the best tone at low volume.
Ok, next month – getting those tubes in place.
Any and all work should only be attempted by a qualified tube amplifier technician.
Any suggestion, idea or instruction involving any electronic device should only be carried out by qualified personnel. None of these instructions are implied or intended for non-technicians or persons unqualified to work safely with high voltage AC and DC.
Do not attempt any electronic work without the proper tools, materials, workspace, an isolation transformer, a "variac" type variable transformer and correct training in electronics.
Consider this paragraph to be inserted every 5 or 6 words in this article!
If your amp tech is still unsure, please see the full warning included on pages 4 and 5 of "Inside Tube Amps" by Dan Torres.
This article is for entertainment only. Any additional help will require the amplifier to be in our shop for service.
Copyright 1999 by Dan Torres
All rights reserved
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San Mateo CA 94402
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Ok, here's the same warning rap. But read it every time.
Warnings and disclaimer
Tube Amplifiers, all components and related products are electrical products with extremely high voltages that if mishandled or if used carelessly or for improper purposes, can cause life hazards and serious personal injuries. Such equipment is dangerous even when turned off or unplugged.
You shouldn't be working inside the amp unless you know what you are doing.
Don't work on an amp plugged in.
Always discharge the filter capacitors before starting work. If you don't know how, contact us for a reprint on how to do it ("What the Hell are filter caps?")
Solder neatly, and use the smallest amount of solder you can.
Be careful, but have fun.
"Inside Tube Amps" by Dan Torres the Highly Popular Book on tube amps, is available from Amazon.com Barnes and Noble, direct from the author, your favorite hip music store, and lots of your favorite suppliers. Check it out.
Dan Torres is one of the most sought after tube amp experts in the world. Dan is the author of the definitive book on Tube Amp Technology "Inside Tube Amps."
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