[John L Stewart]

At 130 views & counting it seems like we need a follow on. So here it is with more power. This was originally published in the Glass Audio Projects Book, April 2002, along with 16 other vacuum tube designs. The space on AudioBanter is tight, so some of the information will appear in a following post.

MORE POWER FOR THE AFFORDABLE SE AMPLIFIER

While working on the earlier Affordable SE Amp (GA 4/00) it occurred to me there might be an easy way to get even more power with only a slight increase in cost.

All single ended amplifiers need to run in Class A mode if they are to deliver good quality sound. In order to do that they need to be capable of dissipating considerable heat. Tubes in the list for the original low cost SE Amp barely fit that requirement. However, several compactrons with a beam tetrode & high mu triode in a single envelope are available on the NOS market at reasonable cost. See Table One. They have power handling ability at about 50% better than those on the previous list. To further this project, I selected the 6LU8.

CIRCUIT FEATURES

1) Operation in Triode or Ultralinear mode.
2) Operation with or without feedback.
3) Power frequency hum cancellation.
4) Start/Standby & Run Modes.
5) Excellent performance.
6) Low cost has been maintained.

TRIODE OR ULTRALINEAR?

The original amplifier used a low cost Hammond 125E for it’s output transformer. That transformer has a center tap which could be connected to the screen of the beam tetrode, thus providing a degree of ultralinear operation. I arranged layout of the various parts so that a comparison of performance could be made between triode & ultralinear topologies. Referring to Figure one see that the screen grid of the beam tetrode is shown unhooked. You can make a connection either direct to the plate (triode operation) or to the 125E center tap (ultralinear).

CAUTION- Don’t use a switch to make the circuit change. Energy stored in the magnetic field of the 125E tries to keep current flowing. You could cause a failure of the insulation in the transformer because of the large induced voltage caused by switching. When you make the change your amplifier should be completely disconnected from the power source.

At midband the resulting amplifier delivered 5 watts in triode mode & 6 watts when connected for ultralinear operation. Refer to Table Two for a summary of performance of the various configurations.

THE MODIFIED AMPLIFIER

As shown in the schematic the amplifier is connected so that all of the gain is used by feedback. If you would like to run without feedback then unhook R4 from terminal one of the 125E & connect to COMMON. You will also need to remove C3 & R5 from the circuit & connect C2 to the junction of R6 & R7.

The coupling cap between the voltage amp & the power amp becomes more critical as more feedback is used. You are normally told to use as large a coupling cap as practical at this position. However as feedback is increased
a hump will appear in the low frequency response. I measured up to 4 db rise at 30 to 40 Hz. C3 & R5 provide a step in the low frequency response. This allows more feedback to be used without instability.

An important addition to this amp are three grid stopper resistors. This tube has very high transconductance & will likely oscillate on it’s own if given a chance. I knew I had problems when my FM receiver was interfered with
while the amplifier was running. I observed spectrum up to 150 Mhz. Grid stopper resistors lower the Q (Quality Factor) of the circuit according to the formula

Q = (1/R)  SQR ( L/C )

As little as 100 ohms in a conductor interferes with the RF to the extent that it is eliminated. The audio is left intact.

The remaining LF time constant in this amplifier is the filter section of the power supply. Many experimenters either ignore this one or simply follow the lead of others. However, it is in the audio path & does have considerable effect on the final result.

For triode operation connect an 8 ohm loudspeaker to terminals 2 & 6 of the 125E. If you would like to use the ultralinear mode, then you can connect an 8 ohm loudspeaker to terminals 3& 6.

THE POWER SUPPLY

You will need more power for this version of the SE Amp. Here it is supplied by a Hammond 271X transformer. Filtering is provided by a PI section consisting of a Hammond 156L, 5 Henry choke & a pair of 100 microF caps.
This time the caps are rated at 450 volts working. Refer to Figure 2.

The filter section is resonant at 10Hz. That has an effect on the low frequency performance of the amp. A simple regulated power supply would eliminate this problem. Perhaps next time.

A current sampling resistor of 10 ohms is in the center tap lead of the HV winding. Using a X10 probe I measured peak current of 250 mA on the scope. Average current was 70 mA.

The pilot light has been moved to the 5 volt winding to somewhat relieve load on the 6.3 volt winding. However, total load is well within the capabilities of the 271X. I measured input to the power transformer to be
46 VA* in the run mode. In standby it was 20 VA. The 271X is rated for 63 VA input.

A Start/Standby mode is provided by the 3PDT, center off switch S1 sections A, B & C. Diode ring D3 thru D6 sets the heaters to about 4.9 volts when S1C is open. C105 & C106 are Mallory part number UN103M. These caps are Underwriters Laboratory approved for across the line applications.

POWER FREQUENCY RIPPLE CANCELING

In a previous article (GA 3/98) I had described a simple addition to the power supply circuit which would almost eliminate the power frequency hum component. That would be 60 Hz in North America & 50 Hz in Europe & elsewhere. This component of hum results often when a centertapped transformer is used in the power supply.

Two halves of the HV winding may not be balanced for resistance. For example, in the Hammond 271X the two sides of the HV winding measured 167 & 142 ohms. The power frequency ripple happens to be in the same range as the resonance(s) of most loudspeaker systems. This sometimes causes problems in systems using SE triode amps. Here is a practical application of that circuit.

You will find the fix in this amplifier easy to make. The 5 volt winding is connected in series with the center tap of the HV winding. The extra 5 volts will be series aiding for one side of the HV winding & series opposing for the other side. There are no polarity markings on most power transformer leads. On the first try there is an even chance the hum will get worse.

Your best check for the proper connection is by using an oscilloscope. Connect the scope probe to the ungrounded end of the 10 ohm resistor (TP A) in the center tap return lead. Proper connection of the 5 volt winding results in the current pulses all having about equal amplitude. If you have connected the 5 volt leads the wrong way, alternate current pulses will have somewhat different heights. Refer to Figure 4.

When connected properly I measured a 12 dB reduction of the power frequency ripple as compared to the normal hookup. It cost us nothing.

If you don’t have an oscilloscope to make this adjustment, two other kinds of measurement are possible. A peak responding AC voltmeter such as the HP 410A, 410B or 410C could be used to measure the voltage peaks produced across the current sampling resistor. Correct connection of the 5 volt winding leads is that which produces the smallest voltage. This will vary depending on your circuit but will be in the range of 2.5 volts.

Finally, you can build your own simple peak responding voltmeter. See Figure 4. Most any signal or power diode will work. From my junk box I tried a 1N4007 as well as a very old 1N56A. The 1N56A is a Germanium device so has a smaller forward drop. It worked best. The external VOM of 20000 ohms per volt was set to the 3 volt range. Connect the 5 volt winding leads to give the lowest reading when connected to the test point on the 10 ohm sampling resistor. Indication will be about 2.5 volts.

LINE STAGE

When you use the feedback version of the amplifier you will find the gain to be somewhat low. In order to get enough gain most experimenters will try three stages of gain. Instability often results when feedback is connected. Even the famous Williamson amplifier exhibits some low frequency problems since there is too much gain & phase shift inside the feedback circuit.

A simple line stage can easily fix this problem. Refer to Figure 3. This is an application were a pair of feedback amps are connected to provide enough gain without instability. As shown the line stage has a gain of 8.

CONCLUSION

Was the project worth the time & effort required? I would say yes. For a very reasonable cost the amateur can get some experience of what vacuum tube audio is really about.

In my various designs I try for synergy. That is to say the various parts should somehow compliment each other so that the final result will easy to implement, safe & reasonable cost. All designs will have some tradeoff’s.
The designers job is to separate those which are important from the rest.

The primary objective I had in mind with the Affordable SE Amp was to illustrate what is possible on a tight budget. I wanted to avoid exotic & otherwise expensive components which seem to be running rampant in the present vacuum tube audio revival.

For those who would care to spend a little more money the most effective improvement they can make to this design is a better output transformer. The Hammond 125E was not designed with high fidelity in mind. I would recommend the Hammond 1628SE. You will get a large improvement in the low frequency response. Distortion at all levels would be reduced since the 1628SE is specially designed for single ended applications. Refer to Table 3 for some comparative distortion measurements made at 100 Hz.

*VA- Volt-Amperes