Introduction

I first experimented with directly heated power triodes (DHT) several years ago when I built a 2A3 single-ended triode (SET) amp with a 6SL7/5691 driver. The sound was magical and I could listen to music for hours without fatigue. The music transported me. I sold that amp to someone who was equally transported by the music. Not long after, I set out to build another SET amp with a directly heated power triode, but with a tube that some consider the best sounding DHT ever produced, the 45. The 45 is truly an amazing tube and if you choose to build an amp with this tube you will not be disappointed. The sound is breathtaking! I decided on a monoblock design (separate power supplies and chassis for each channel) and the 6J5 driver.

The Circuit

6J5 Driver

The 6J5 is the single triode version and electrical equivalent of the venerable 6SN7. The single triode is perfect for a monoblock design where only one stage of amplification is desired. It is very linear tube with a low output impedance and a modest amount of gain (~20). The 6J5 can easily provide enough gain with an input voltage of ~ 6V peak to peak (~2.1V RMS) to drive the 45 to full output power. The monoblocks do not have volume attenuators since they were designed to be used with my DHT pre-amp, which can easily provide the needed input signal voltage. The 6J5 comes in a variety of styles from the "coke bottle" glass tube (visible in the above photo) to the metal shell types. Here is the datasheet.

If you have looked at my other projects on this website, you probably have noticed that I am fond of using a single triode for amplification along with cathode bias and a constant current source (CCS) for the plate load. There is a simple elegance to using a single stage for amplification! I also like using the TL431 shunt regulator to bias the cathode because you can easily adjust the grid-cathode bias and along with an adjustable CCS circuit, you can experiment or fine tune the operating point of the triode. You can read more about these circuits on my website here and here. I am also fond of using the LR8 regulator since it is easy to work with, reliable, and gives ~60db of  attenuation of any remaining ripple from the power supply. You can also read more about the LR8 on my website by checking out my DHT pre-amp and 6C6 phone pre-amp. Here is an excellent blog post by Fred Musset on operating points and the 6J5. I have tried a couple of operating points (all above 5mA and grid-cathode voltages >5V) in this circuit and I can't hear any difference because I think the 6J5 is just a very linear tube in the regions I tested (as evident by its plate curves). I also want to try some metal shell 6J5s just to see if they sound different.

45 Power Triode

The output from the 6J5  is resistance-capacitor (RC) coupled  to the grid of the 45. If you follow these guidelines, the effective frequency response with a 0.22uF capacitor is ~33Hz with a -3dB frequency of 3.3Hz. You can certainly experiment with the value but the monoblocks seem to have plenty of low end. I am using the Russian made K42Y-2 paper in oil types and they seem like they have benefited from some break in time. The amp sounds amazing but I may try my favorite coupling capacitor in the near future, which is the Auricap by Audience. The 45 datasheet gives excellent details for suggested operating points and there are also plenty of 45 amp designs on the internet that can be used for guidance as well. The first thing I did was get some nice output transformers so that the 45 could really shine, and by nice, I do not mean expensive! I think some of the best power and output transformers, in terms of cost and performance, are made by Edcor. I wanted an output transformer that had a frequency response at least from 20Hz to 20KHz and sufficient primary inductance to give extended bass response. I also did not want to drop $1000 or more for a pair of transformers! I opted for the CXSE25-5K. These transformers are almost 10 pounds each and are way overkill in terms of current and power handling specifications but with a primary impedance of 5K (improves linearity) and a primary inductance of 50H (gives more extended and solid bass), they allow the 45 to shine brightly. Also, they cost less than $100 each! In all fairness, I have never heard a SET amp with the 45 and expensive output transformers (Tango, etc.), so I do not have a proper reference point for comparison! All I can say is that the Edcor transformers paired with the 45/6J5 tubes in a RC circuit sound truly amazing.

According to the datasheet, the maximum power output from a single tube in Class A operation is 2 watts with a plate dissipation of ~10 watts, which is the maximum rating for the 45. Generally, I like to run output tubes at no more than 80% of their maximum plate dissipation rating just to have a bit of a safety margin. The operating point of the 45 is 263V plate to cathode voltage (317V-54V) and 30 mA plate current, which gives a plate dissipation of ~8 watts and produces ~1.5 watts of power output. With efficient speakers (>97dB), that is more than enough power to play music above normal listening levels.

Here is the circuit schematic:

Here is a close-up picture of the circuit. The 10W resistors could probably be replaced by 5 watt resistors on the PCB :

The Power Supply

Each monoblock is powered by a Glassware Audio Design PS-1 power supply, which has a circuit with a high voltage current regulator (IXYS 10M45S) paired with a low-voltage regulator (LD1085) which supplies the B+ and a low voltage regulated DC circuit which supplies the tube filaments. You can read more about the PS-1 here. I have used the PS-1 in other projects with great success.  I should point out that the stock PS-1 is designed for a maximum output voltage of 300V, so I had to replace a few of the stock parts with parts that have higher voltage ratings since the output voltage is set at 395V. The output voltage is set by the voltage divider formed by the 33K and 47K 5 watt resistors (see schematic below). I replaced the stock electrolytic capacitors in the high voltage power supply circuit with the 500V Nichicon GN type and the bypass capacitors with a 630V Solen PPE MKP type. I would also recommend replacing the stock HER108 high voltage power supply rectifiers with the IXYS DSEI-12A fast recovery diodes. The transformer I used (Edcor XPWR124) puts out ~445v DC with the ~45 mA total current draw of the amp, so there is ~43v drop across the IXCY 10M45S dissipating almost 2 watts of power. The power supply is reasonable quiet with a calculated output ripple of ~4mV RMS into the LR8 regulator that feeds the CCS for the 6J5. The LR8 is good for about 60dB of ripple rejection and the PSRR of the CCS is good for another 40dB, so the residual ripple in the output of the 6J5 is calculated at ~4mV/100 x 1000, which is equal to ~0.04uV RMS. This is pretty darn quiet and indeed the only noise you can hear at the speakers is "tube noise" and you have to put your ear right up to the speaker to hear it. The RC filter that feeds the transformer and the 45 has a ripple rejection factor of ~20, so the calculated ripple at the transformer primary is ~4mV RMS/20, which is ~0.2mV RMS. The noise should be somewhat attenuated at the speaker because the output transformer "steps down" the voltage. The function of the audio output transformer is to transform the high impedance of the output tube to match the much lower impedance of the speaker. The impedance ratio of the transformer is equal to the square of the voltage gain or loss in this case. The impedance ratio of the output transformer is 5000 ohms/8 ohms, which equals 625. The square root of 625 is 25, which is the voltage loss. For example, if the 45 is supplying a 100V AC signal to the transformer primary, the AC signal across the secondary will be 100/25 or 4 volts. The ripple voltage of ~0.2mV RMS across the primary will get "stepped down to 0.2mV/25 or ~0.008mV RMS.

I wanted to use both the low voltage secondaries on the Edcor transformer and since I pretty much use regulated DC for filament heating on all my projects to eliminate or minimize hum, I had to employ a voltage doubler circuit for the 6J5 filament power supply. The 6J5 filament is rated for 6.3V at 0.3A.  A full-wave voltage doubler essentially "doubles" the output voltage (output voltage = secondary voltage x 2.8) but with a large penalty for rated output current ( output current =  rated current of the secondary/3.6). The 5V secondary is rated at 2 amps, so the maximum output current is ~0.6 amps, which is plenty for the 6J5. The voltage output at the regulator is ~13V, leaving ~6.7V of headroom and the power dissipation through  the regulator is a reasonable ~2 watts. An option with the PS-1 board is to bias the filament output with a voltage divider from the B+, which is highly recommended since this tends to reduce noise. For this pre-amp, I set the bias at ~46V (the heater-cathode voltage difference cannot exceed 90V DC). Noise output is ~ 19mV RMS, which is fine for an indirectly heated cathode. The schematic is below. 

Here is the power supply schematic:

Here is a photograph of the power supply along with the Glassware Audio Design House Ground and Schaffner EMI filtered power entry module (FN281-04-06):

The Filament Power Supply for the 45

Here is the power supply schematic:

Here is the 6J5 filament power supply schematic:

The other low voltage secondary on the Edcor power transformer is rated for 6.3V at 4A and is center tapped, which is perfect for powering the 2.5V/1.5A filament of the 45. I employed the same filament power supply design I used for the DHT pre-amp, which is adjustable, simple, quiet, and reliable. For the circuit design there was no real need to reinvent the wheel since Pete Millet already has designed a regulated DC filament supply design.  I use his regulators in other projects and they perform very well. I modified his circuit so that I could use the high current, low dropout 5-pin Micrel MIC29302WT regulator (TO-220 package) instead of the 3-pin LD1085 regulator. Other than the regulator the circuits are very similar. This regulator has a dropout voltage of only 250 mV at 1.5A of current and the output noise voltage is specified at 260 uV RMS with a 33 uF output capacitor. It is rated at 3 amps maximum current. In the future,  I think I will also try the 5-pin Linear Technology (Analog Devices) LT1764A regulator, which shares the same pinout and features but has a lower output noise voltage at 40 uV RMS with a 10 uF output capacitor. The low dropout voltage is useful because you can reduce power dissipation by reducing the input-output voltage differential. I used a full-wave center tap configuration, which produces a rectified voltage of ~3.4V at 3 amps. The 45 filament is rated at 2.5V, which gives a voltage differential of ~0.9V or 900mV. This gives 650mV of headroom for the regulator and the regulator dissipates only ~1.3 watts of power. I should point out that the dropout voltage is temperature dependent, but with 650mV of headroom there is no danger of the regulator getting close to its dropout voltage because the heatsink will keep everything running smoothly. Indeed, I have not had any issues with this circuit. I also want to point out that if you used the LD1085 regulator, a full-wave bridge configuration would have had to be used resulting in ~7.8V at the regulator input and a power dissipation of ~8 watts! With large filament currents, the use of a low-dropout regulator results in significantly less power dissipation. See schematic below.

Here is a photograph of the filament power supply for the 45. The ground lift switch is also visible, which prevents ground loops when attached to other equipment, resulting is less audible hum at the speakers:

Here is a photograph of the filament power supply for the 45. The ground lift switch is also visible, which prevents ground loops when attached to other equipment, resulting is less audible hum at the speakers:

Here is the schematic for the filament power supply for the 45: Note: the 500 ohm trimmer resistor and the 121 ohm resistor positions should be reversed. This is because the reference voltage of 1.24V is between the adjust and ground pins. With a 121 ohm resistor there is a fixed load current of 10 mA (1.24V/121 = 10mA). With the current schematic, the load current is 10mA for the 2.5V filament voltage. The minimum load current specified in the datasheet is 7mA for up to 3A. It works fine since the filament current draw is 1.5A. However, I will replace this circuit with an updated filament power supply in the near future.