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JBAA - High Fidelity Tube Junk Box Audio Amplifier

 Bruce Heran   USA Flag   To email Bruce, type out the email address.

JBAA - High Fidelity Tube Junk Box Audio Amplifier

All DIYers have junk boxes. I suspect that mine is above average. It ought to be as I have been designing and building gear for a really long time. Mine is even larger because I have all the left overs and culls from Oddwatt Audio. Still the allure of building something from either salvaged or surplus parts is quite high among most DIYers I know. So what better way to use some of the parts than building a really high quality tube audio amplifier. To be sure there are lots of tube amplifier projects out there. This one is different as the design, while similar to some of my other projects, notably the PoddWatt, it is able to use a huge variety of tubes and other components. I won't say it is a generic design, but it probably comes close. The actual amplifier in the photos is one using 12AX7s as amplifier / drivers and 6005s (industrial 6AQ5s) as the outputs. This amplifier delivers a very clean 3 Watts RMS with about one more a bit less clean on top. Distortion and noise are below 1% at 1 Watt and response is within 0.1dB from 20Hz to 50kHz. It is only down by about 0.5dB at 10Hz and 60kHz. The upper -3dB point is an astonishing 100kHz. I did not include a volume control, but one could be added at the inputs. Replacing the 100K fixed resistor with a 100K control would be my choice. Now back to the junk box concept. You can use any of the tube combinations listed in the section Suggested Tube Types below for both the output and driver tubes. All work well and there are likely some others, particularly if you use the industrial versions. I have tested it with most of them. The more powerful ones like the EL84s will deliver approximately 5 Watts RMS output. The less powerful ones like the 6GV8 will only deliver about 2.5. The basic design is a push pull ultra linear class "A" type. A true pentode version would deliver a little more power output, but have about 0.5% more distortion. I did not consider triode modes, but they are a possibility as well. However the output tubes are required to provide some gain and very low gain triodes (like 6AS7) will not work. The driver stage is a SRPP followed by a self-inverting output stage. The use of the constant current source in the cathodes of the output stage allows for single ended drive. It looks strange, but trust me it works extremely well. I have used it in numerous DIY and commercial designs up to push pull KT120s and it is nearly bullet proof. The output transformer is used to invert the signal to maintain proper phase relationships. That way the amplifier is non-inverting and can use a minimal amount of negative feedback to insure stability above the audio band. The negative feedback is optional and is not used to "clean up" the signal as in many other designs. It has essentially no effect on the sound. I do recommend it as the amplifiers (depending on tube types) can easily be influenced by high frequency EMI.

JBAA (Junk Box Audio Amplifier) with 6Y6GA and 6SL7 tubes

Photograph 1: JBAA (Junk Box Audio Amplifier) with 6Y6GA and 6SL7 tubes


JBAA (Junk Box Audio Amplifier) Details

There are a few things that are important in the design and could possibly derail an otherwise good build. The LM317s will generate heat and need to breathe. I mount mine through a hole in the chassis and use a small heat sink. Be aware that the tabs on them are not at ground potential. They are actually at about 12-18 volts DC. Certainly not harmful, but if you ground them the amp will self-destruct. I used solid state rectifiers and suggest you do the same for simplicity. A tube version would work fine but you would need to increase the secondary voltage of the power transformer to cover the drop in the tube rectifier. I used DC on the heaters, partly because the power supply boards had that provision and it is slightly quieter. AC heaters are fine but remember in either case the heater circuit cannot be grounded. Doing so will cause the driver tube to fail. My experience with all "totem" pole designs is that one of the tube cathodes will be at about one half the B+ voltage. If you ground the heater circuit the differential between the heaters and that cathode will be high and my experience has been that anything over about one half of the tube specification for that tube will result in eventual failure through internal arcing. To overcome this problem a voltage divider from the B+ is attached to the floating heater circuit and raises it by about 60-70 volts DC. There is virtually no current flow in the divider. Another thing to be aware of is that I use a three wire power connection with hot, neutral and earth ground for the AC mains connection. The earth ground connects directly to the chassis. This allows it to protect the user from any sort of internal equipment failure and it can act as an EMI shield. I recommend steel chassis as they are the best for that. The amplifier signal ground is not directly connected to the chassis. It connects via a type X2 capacitor and parallel resistor. This arrangement avoids ground loops between other components and the amplifier. A nose to tail parallel pair of rectifiers can also be used in place of the X2 capacitor and resistor. I personally have not found that to be as good though.


Components

The power transformer can be any one that can supply about 360 volts center tapped for full wave rectification or if you use a bridge rectifier around 180 volts. For tube rectifiers I would use about 400-430 center tapped. The total current needed for the tubes is about 200ma (DC) and to have a reasonable margin 250-300 should be available. The one I used had a special heater secondary of 10.9 VAC. It was a "kit" left over part. (Edcor carries it and will sell it to individuals) When rectified and filtered this provides a nice 12VDC for the heaters. Approximately 2 amperes are needed for the 6005s as I used a series arrangement for them. Using an actual 12 volt secondary will necessitate a dropping resistor that will vary in value with the choice of tube types. Something in the 0.5 to 2 ohm range is typical. It will dissipate a goodly amount of heat so placement is important. Your power transformer should be able to deliver at least 1.5 times the needed heater current.

The output transformers I used came from Edcor and are type CXPP-MS-10K. They are a bit pricey, but like I said my junk box is really large and well stocked. Any output transformers with 8-10K primary impedance that can handle 100ma balanced (50 per side) or more will be fine. I used the ultra linear configuration. The U/L taps can be anything from about 23% to 43% and not make a great deal of difference. Notice that the design uses the output transformers to invert the signal. It results in the overall amplifier being "non-inverting". If you check the phase shifts you will see this works. If you use negative feedback and get it backwards the amp will oscillate.

The quality of the coupling capacitors is very important to the sound of the amplifiers. You do not need to use "boutique" capacitors, but good ones from established manufacturers are recommended. I would suggest ones from, Panasonic, WIMA, Vishay, Jantzen Audio, Audyn, Sprague, Solen, and similar ones. The Russian K40Y-9 paper in oil ones are a little more costly but do well in this design. The 200 volt size in them is sufficient. Equally the quality of the output transformers is important. The ones shown are from Edcor (CXPP10-MS-10K) and a bit pricy but IMO worth it. The less expensive GXPP10-6-10K are actually nearly as good and only reduce the top and bottom ends slightly. Hammond ones are available as well. All other components are generic in nature. I prefer one half watt metal film resistors (except as noted on the schematic) but the choice is yours. The LM317s do need to have a modest heat sink.

JBAA Amplifier using 6005 and 12AX7 tubes

Photograph 2: JBAA Amplifier using 6005 and 12AX7 tubes with some of the tube types you can use


Building the Junk Box Audio Amplifier (JBAA)

The design lends itself to either PCB or point to point wiring. I used both in the amplifiers shown in the photos. My power supply was on a PCB left over from a commercial kit, but the remainder of the circuitry was point to point. It was not particularly elegant, but well into the spirit of DIY. As always, I recommend using a chassis about 1.5 times the size you think you need. It makes for easier builds, easier trouble shooting (stuff happens from time to time) and better component cooling.

Warning: This project uses potentially lethal voltages and should not be undertaken by anyone who is not familiar with working with such voltages or may not be comfortable with projects that entail such voltages.

JBAA (Junk Box Audio Amplifier) Schematic

Figure 1: JBAA (Junk Box Audio Amplifier) Schematic

The actual build is not particularly critical and the layout is pretty flexible. You need to use good wiring practices and soldering technique. I find that most build failures are because of soldering issues. A "cold solder joint" will mess up an otherwise good build and often be hard to find. I am all for protecting the environment, but I only use lead based solder. I have seen too many DIY builds that were messed up by lead free solder. It certainly will work, but takes more skill and more heat to get right. I use a hot iron (900 F) to insure good connections. I have found that high heat for a short duration works far better than lower heat and a long duration. Components are less likely to be damaged that way and connections more likely to be good. Some things to consider in the layout and build are:

  • Shield all low level signal carrying conductors over about 5 cm long.
  • Use a substantial signal ground buss or star arrangement.
  • Keep signal wires and components away from other wires and components that carry power, particularly if it is AC. Watch for sensitive tubular signal capacitors. Some do pick up hum and noise from external sources.
  • Place the driver tubes away from the power transformer. The output tubes usually don't matter.
  • Be sure to ground the transformer cases to the chassis by scraping away the paint.
  • Place power and output transformers at right angles to each other.
  • Remember that the LM317 regulators are used as constant current sources (CCS) and not as voltage regulators. See my guide about The Care and Feeding of LM317 and LR8 Integrated Circuit Regulators, Particularly in Valve Circuits for more information.
  • Beware of ground loops. If you don't understand what one is and how they work do a search for it. The subject is too extensive to be included here.
  • I like to use several colors for the wiring. Not because it looks pretty, but because it makes it easy to see how things are connected and easy to trouble shoot if need be. Just be consistent.
  • Remember that tube socket pins are numbered from the key or gap in a clockwise manner from the bottom side.
  • The quality of the final sound is highly dependent on the quality of the coupling capacitors and output transformers.

JBAA (Junk Box Audio Amplifier) Power Suplly Schematic

Figure 2: JBAA (Junk Box Audio Amplifier) Power Supply Schematic

For additional valve amplifier design and construction tips, see my blog entry about design and construction tips and suggestions for vacuum tube amplifiers. Also, I have also posted some suggestions for a tube amplifier wiring color code.


Junk Box Audio Amplifier Operation and Final Checks

When you are finished with the build, set it aside for a day or so. Then check it over for errors. Insert the tubes. I next suggest that you hook up at least one meter and the best place (if you only have one meter) to see if everything is OK is across one of the sense resistors. If the number of millivolts is correct there or can be adjusted by the balance control to be correct then it is likely that the amplifier will work. Do both channels. The tubes are pretty robust and will tolerate a fault for a few minutes. If those measurements are not correct you need to look for a wiring error or a component incorrectly installed or of the wrong value. If the values are OK then I would measure the DC voltage at the middle point of each SRPP. Use either the cathode of the upper triode or the anode of the lower one. The voltage at either point ought to be about one half the voltage applied to the upper anode. It can vary by as much as 25% depending on the matching of the two sections in the tube. If you use separate tubes like a 6C4 for each half then they should have similar results. You can do all these tests with or without a load on the amp. To finish the checks you need to add speakers of appropriate impedance and feed in a signal. Depending on the choice of tubes you can use many sources to do this. I have found that the output from portable players, preamps, and pretty much anything that will allow you to control the volume will work fine. If you build an amplifier with the 100K ohm volume control you can use nearly any source that provides over about 100 millivolts of signal. The actual gain will naturally depend on what tubes you use.

Budget Variation of JBAA built on a S-5 chassis with 6GV8 tubes

Photograph 3: Budget Variation of JBAA built on a S-5 chassis with 6GV8 tubes (many mods to PCB)


Suggested Tube Types

For the output stage: Grouped by the cathode resistor in the LM317 circuit

  • Group 1 with 18R resistor 6V6, 6AQ5, 6005 (Photograph 2), 6Y6G
  • Group 2 with 15R resistor EL84, 6BQ5, 6F5P and 6P15P-EB, 6Y6GA
  • Group 3 with 20R resistor 6K6, 6W6, 6GV8, 6BM8, ECL82, ECL85

For the driver stage: 12AX7, 12AU7 (lower gain variation), ECC82 (lower gain variation), ECC83, 5751, 12SL7, 6SL7, triode sections of some of the above multi section power tubes.

You can use similar tubes with European or Russian designations for any of the listed types. You can also use other tubes you may have that have similar dissipations for the output stage and other small signal triodes for the driver stage. As it ought to be clear now, nearly any combination of tubes will work. I personally like the sound of the EL84s and 6Y6 the best in combination with the 5751 and 12SL7 respectively.


Measured Performance - 6AQ5/5751 Tube Version of JBAA

Actual tubes used: GE JAN 6005W and Sovtek 5751
Response: 20Hz to 20kHz +0.1/-0.25 dBv at 1 Watt RMS into resistive 8 ohm load
-1 dBv points: 10Hz and 41 kHz
S/N wideband: 89 dBv

Distortion + Noise at 1 Watt RMS into resistive 8 ohm load
50Hz 0.70%
1000Hz 0.46%
10kHz 0.40%
20kHz 0.84%

Test Equipment:

  • HP 331A Distortion Analyzer
  • Velleman Dual Channel USB PC Oscilloscope (with built in signal generator)
  • Residual THD+N: 0.20%
  • Noise floor in workshop (on day tested): -98dBv

Listening

The sound produced by any of the combinations will be quite rewarding. It is far better than many other small tube amps and will easily drive many speakers. I recommend only using ones that have sensitivity higher than 90 dB/W if you use the amplifier in a larger room. For close up listening any speakers will be fine. Full range speakers are the best as the bass response is deep and excellent

Good listening
Bruce