Audio Projects – 25 W Stereo Amplifier

This page includes supporting information for the 25 W stereo amplifier project described Chapters 10 and 12 in The TAB Guide to Vacuum Tube Audio, along with commentary from the author. This page picks up where the book left off.

Shown on the right is the second-generation 20 W stereo amplifier described below.

25 W stereo amplifier

Note 1

The basic 12.5 W amplifier circuit of the final design implementation includes solid-state rectifiers for the B+ power supply. Mounting for these devices is accommodated on the PWB, as described in Chapter 10 and Chapter 12 of the book. At the reader’s option, it is possible to use a vacuum tube rectifier in place of the diodes. A 6CA4 tube functions well and matches the 9-pin style of the other tubes used in the amplifier. The 6CA4 is still being manufactured and is readily available. No changes to the PWB are required, other than routing leads that would go to the diodes to the tube socket. The original RCA design described in Chapter 10 of the book calls for a 5BC3 rectifier tube. This device will certainly work fine in this application, but it appears to be available only from NOS and can therefore be somewhat difficult to find. If a rectifier tube is used, be certain to route the B+ transformer secondary leads to the PWB and then, using jumper wires, to the rectifier socket. This is necessary in order to provide an input for the bias power supply. Operation of the circuit without proper bias will result in component failure.

Note 2

In the description of the 12.5 W amplifier in Chapter 10, the option of using a 6U8A tube in place of the 7199 as specified was discussed. The 7199 pentode/triode performs very well in this amplifier. The problem, however, is that the tube is no longer in production and finding NOS devices can be difficult (and expensive). As described in Chapter 10, it is possible to substitute a 6U8A so long as certain socket pin connection changes are made. The necessity to reroute three pins makes such a change difficult after the amplifier has been built, particularly if a PWB is used.

An interesting option involves a plug-in adapter available from at least one vendor that makes the necessary pin changes between the chassis socket and the tube. The adapter is sold by Antique Electronic Supply (part # T-7199-ADT). The adapter is designed specifically for 6GH8A to 7199 applications. The 6GH8A is a medium-mu sharp-cutoff triode/pentode, as is the 6U8A. Fortunately, the pin changes needed to use a 6GH8A as a substitute for a 7199 are the same as for a 6U8A.

The author has tried the adapter on a 6U8A as a substitution for a 7199, with good results. Listening tests revealed no obvious differences between the tubes. Bench measurements followed.

25 W stereo amplifier on bench

Measurements were taken on one channel of the amplifier with the 6U8A installed using the plug-in adapter. No significant differences were observed in frequency response, THD, IMD, or noise. The photo on the left shows the amplifier on the bench under test. The photo on the right shows the plug-in adapter. You will note that aesthetically the device may leave something to be desired in applications where the tube is visible. Still, it is an easy and inexpensive modification, and it avoids the difficulty of finding the 7199 type. The cost difference between a 6U8A and a 7199 is quite significant (in one case nearly a 10x difference). The larger question, however, is how long the 7199 will continue to be available—at any price. Therefore, having a backup-device type is important.

6U8A tube adaptor

Following the successful tests of the 6U8A substitution for the 7199, tests were made on the 6GH8A as a substitute for the 7199, again using the 9-pin socket adapter. Listening tests were conducted that compared one channel with the 7199 and the other with the 6GH8A. No significant differences were detected. Bench tests showed performance comparable to the 7199 and the 6U8A.

It should be noted that some users have a preference for the sound of the 7199 over the lower-cost substitutes. The listening tests by the author as described here were not exhaustive. All other things being equal, the author recommends using the 7199. The problem, however is the difficulty of finding the 7199. At some point device availability must take precedence over preferences.

Note 3

The Bill of Materials for the 20 W stereo amplifier described in Chapter 10 of the book specifies a rating for circuit breaker CB1 of 5 A. This is actually higher than needed. Something in the 2 to 3 A range would be quite adequate. Builders may wish to try a 2 A device, such as the TE Connectivity W58-XB1A4A-2 (Allied Stock #70199386). If nuisance trips are encountered, a 3 A device may be used instead, such as the TE Connectivity W58-XB1A4A-3 (Allied Stock #70199433). Apart from the fixed load of the tube filaments, the bias setting on the final tubes is the major determinant of average power draw of the amplifier. At a bias setting of –30 V, the amplifier draws about 900 mA with no audio input applied.

As described on the Regulated Power Supply page, builders may want to change the value of VR1 to something higher than the 2.5 ohms (cold) specified in Chapter 12. VR1 is a power varistor intended to limit current inrush when the power supply is first switched on. Once the supply has reached operating temperature, VR1 is taken out of the circuit (shorted) by RYL-1. A slightly higher value for VR1 will provide greater benefit. A value of 5 ohms (cold) works well with this circuit (GE Infrastructure Sensing #CL-40, Allied stock #70181325), limiting the measured inrush current to just over 3 A. With a 10 ohm (cold) device (GE Infrastructure Sensing #CL-60, Allied stock #70181341), the inrush current is reduced to about 2A. For builders who use a 2 A circuit breaker for CB-1 (which is recommended), the 10 ohm device is a good choice.

Note 4

Users may hear shortly after the amplifier is powered on a slight buzzing noise from the chassis (not from the speakers) for a few seconds. This is normal. As the amplifier warms up, the surge-limiting power thermistor bypass circuit of VR2 and C11 ramps up the voltage to relay RYL1. During a brief portion of this ramp, the relay may vibrate slightly; however, within a few seconds the relay will close. The speed of this action is a function of the ambient temperature.

Also, it may be observed that when the amplifier is first turned on, a faint hum can be heard from the speakers. This hum will go away within about 30 seconds of power on. Because the amplifier power supply uses silicon rectifier diodes, the B+ voltage is available immediately upon power up. This can allow any residual supply hum to be heard in the speakers. As the output tubes warm up and begin to conduct, the residual hum is canceled out by the push-pull output tubes (the residual noise is 180° out of phase). After the amplifier has warmed-up, the typical noise floor of –80 dB to –90 dB is achieved.

Note 5

An interesting condition can be observed with this amplifier when measuring square wave performance. Certain settings of the volume control provide for better square wave traces. To illustrate: Apply a 1 kHz signal to the input with the volume control fully clockwise. Adjust the generator to produce 8 V rms output at the speaker terminals into an 8 ohm load. Observe the trace displayed on the oscilloscope. Some ringing can be seen on the leading edge of the trace. Now adjust the volume control so that the ringing is minimized. This is usually around the 3 o’clock position. Readjust the signal generator to produce 8 V rms output at the speaker terminals. Note that the oscilloscope trace is clean.

The measurements documented in Chapter 12 of the book were based on this technique. Fortunately, very little listening is done at full-open volume control settings. This situation is a function of the characteristics of the input pentode stage, and was observed with both a 7199 and 6U8A tubes. In addition, a similar condition was observed with the 50 W stereo amplifier, which uses the same input pentode arrangement.

Note 6

Another version of this amplifier was produced that incorporated a few small changes that were useful additions to the basic design. This amplifier is described as a 20 W stereo unit. Although each channel is capable of 12.5 W power output at low distortion, performance over time is unpredictable at maximum output. For this reason, the amplifier's stated rating is conservatively specified.

Version 2 of 25 W stereo amplifier

Version 2 of the amplifier, shown on the left, added headphone jacks to the front panel and a speaker on/off switch. The speaker switch is used to mute the output at the speaker terminals when the headphones are used. To avoid running the output transformer secondary wires from the rear panel to the front panel, and back to the rear terminals, a pair of relays was included. The speaker on/off switch on the front panel switches a 5 V dc source (taken from the VR1 relay bypass circuit) to control the relays. When the relays (one for each channel) are energized, the 8 ohm secondary of each output transformer is applied to 8 ohm 10 W resistors.

The added headphone output was a feature suggested by a friend who wanted to use the amplifier for headphone listening, as well as for speakers.

The other major change, cosmetic in nature, was the addition of a Plexiglas cover over the amplifier, as shown. This gives the amplifier an interesting look and tends to protect the tubes from casual contact.

Circuit board for second-generation 25 W amplifier

A modified circuit board was used that incorporates quick-disconnect terminals for connections to the transformers and other chassis-mounted components. The goal was to reduce the number of off-board connections in order to simplify construction.

The new PWBs feature a ground plane over the component side with a solder mask and silk screened legends. Two boards are used for the implementation; the power supply components are installed on only one board. In addition, a pair of speaker terminal PWBs is used to terminate the output transformer secondary windings.

The input stage uses a 6U8A pentode/triode, replacing the 7199 tube used in the original RCA circuit.

The automatic protection board developed for the Dorm Amp (see the New Projects Page, Dorm Amp) was incorporated in the Version 2 20 W stereo amplifier. As described on the New Projects page, this circuit board includes several system-protection features including: automatic shutoff, over-temperature shutdown, and B+ under-voltage shutdown. Because the physical layout of the 20 W stereo amplifier and the Dorm Amp are essentially identical, it was possible to assemble another auto-protect PWB and install it in the 20 W amplifier with no modifications.

This project underlines the value of having a documented, set layout for components. Among the benefits are device interchangeability and upgrade options.

Note 7

The cost of building any project is an important consideration for the audio enthusiast and hobbyist. The focus of the first-generation 25 W stereo amplifier was to optimize the various elements that went into the project. Cost was a secondary consideration. For the second-generation unit, an effort was made to accurately determine the bill of material (BOM) costs and the time needed to build the amplifier.

The total BOM (including shipping expenses) for the second-generation stereo amplifier was $1,936. The major cost centers included:
• Electronic components = $510
• Transformers = $500
• Printed wiring boards (PWB) = $455
• Plexiglas cover = $125
• Tubes = $110
• Decals = $75
• Front panel = $100

Builders could eliminate the PWB cost by using hand-wired terminal strip construction. Note that two PWB designs are used in this amplifier—one for each channel and one for each speaker output circuit, for a total of four boards. Other potential areas of cost reductions include the back and top-side decals, and the Plexiglas cover.

The total time required to build the second-generation stereo amplifier—from ordering parts to completing performance measurements—was approximately 21 hours.

The BOM cost for the optional auto-protect board (described elsewhere) was $686. The cost breakdown was roughly 50% for parts and 50% for the PWB. Construction time was about 2 hours.

As with any electronic product, costs fall rapidly as the volume (number of units produced) increases. For a one-off project, however, there are are not many ways to reduce the cost without reducing the feature set.

Note 8 (January 2017)

Continued work to improve the 20 W stereo amplifier has resulted in an updated second generation amplifier. Detailed step-by-step documentation is provided in a 194 page User and Assembly Manual that is available for download. If you are familiar with the Heathkit assembly manuals of the past, the approach taken with this new manual should seem comfortable. For those who want a printed version, it is available for purchase on Lulu—a web site for specialized, print-on-demand documents. (There is charge of $14.95 for the printed manual; see "20 W Stereo Amplifier: User and Assembly Manual" on the Lulu web site,)

In addition, a ZIP file is available for download that will assist in building this amplifier. The ZIP file contains the following individual files:

• Schematic diagrams of the amplifier and auto-protect circuits as Acrobat (".pdf") files
• Bill of materials for the amplifier and auto-protect circuits as an Excel file
• Printed wiring board layout files for the amplifier, speaker terminal, and auto-protect boards as ".pcb" ( ExpressPCB) files
• Front panel layout as a ".fdp" ( Front Panel Express) file
• Chassis layout, bottom view, as an Acrobat" file
• Chassis layout, top view, as an Acrobat file
• Drill pattern for the chassis as an Acrobat file
• Layout for the acrylic trim piece as an Acrobat file

Note that the files above are provided as-is. Every effort has been made to make sure they are complete and accurate, but no warranties are expressed or implied. Builders are encouraged to double-check the information contained in the above files prior to proceeding. For the front panel layout, users can customize the text as desired; e.g., "Built by John for Mary, January 2017." All files are provided for personal use only. No further distribution is allowed.

The current version of the 20 W amplifier builds upon the previous versions, adding some new features and simplifying construction. A description of the latest design can be found in the User and Assembly Manual, and for the sake of simplicity will not be repeated here. The estimated cost to build the amplifier is $3,900 for parts, and about 25 hours assembly time. As noted previously, cost reductions are possible, depending on the preferences of the builder. For example, the BOM cost can be cut almost in half by eliminating the add-on circuits (preamp power supply, auto-protect board, and auto-off board), which then places the BOM cost in the same range as detailed in Note 7 above..

Considering the price tag, this project may be outside the range of some builders. However, you may find that elements of the available downloads will be useful in other projects, or wish to adapt some of the circuits for your own projects. The available documents are, thus, offered as a starting point for future efforts.