It’s been a while since I wanted to design and build an SET using the famous 6C33 triode. I also liked the idea of having a “winter amp.” It will warm up the room during cold and snowy winter days.

Why monoblocks in this case? I chose to build monoblocks to have better heat management control. In addition, this choice allowed me to maximize the performance of the power supply.

The power supply is made of several blocks:

1. High voltage for the power tube. This uses an Antek 200VA toroidal power transformer, Sic diode rectified and CLCLC filtered using two, screw terminal, 1000uF/500V capacitors.
2. High voltage for the driver and gain stage using a separate 30VA power transformer.
3. Raw negative supply and Coleman fixed bias regulator.
4. DC regulated supply for the 6SN7 filament.

The signal circuit uses a 6SN7 and 6C33. The first triode of the 6SN7 is a gain stage and the second triode is the driver. They are capacitor coupled to the 6C33, and a -7.5dB GNFB is applied for controlling the distortion and damping factor.

The power triode is loaded on a Hashimoto H-20-600 output transformer for an exquisite sound. The triode has a mixed bias using partially self-bias and partially fixed bias. There is a multi-turn precision potentiometer. A 500mA Hoyt USA panel meter helps adjust the anode current to 200mA.

There is a 135 seconds delay on applying the high voltage to 6C33. That is a necessary practice to protect the power triode from early failure.

All components in the signal path are of high audio quality. The binding posts and RCA are made by Cardas.

Specifications are as follows:

Main Voltage: 120Vac 60 Hz, Power draw: 115W

Main fuse: 3A SB, Anode current fuse: 300mA SB

Max output power 12W

Input sensitivity 0.8Vrms, Input impedance 47kohms

Frequency response: -1dB 20Hz-40kHz

THD: 0.8% at 2 W, 2% at 10W

Damping factor: 5 Size:   12.75x16x11.5 (WxDxH), and 34 lbs

I designed this amp for whoever needs a single amplification device. You can use it with a passive preamp, AVC/TVC recommended, but not absolutely necessary. A good stepped attenuator ( remote or not) will perform excellent too.

This amp is a delight for tube rollers. However, my intent was to offer many choices of appropriate drivers for a better sounding 300B amp. The amplifier is a 3 stage one, having the 300B as power stage, loaded on a Hashimoto H-30-5S. The driver stage has a 4 pin socket to use with the 45 tube, or an 5 pin to 4 pin adapter to use with either 46 or 47 in triode mode. The driver is loaded on a LL2753/30mA and has a switch to toggle between the 45 and the 46/47 drivers. The first stage is a gain stage and has an octal socket for use with the 6J5, or a 5pin to octal adapter for using the 76 super triode. As one can see, there are many combinations possible with the hope that one will find their preferred sweet spot. Sure enough certain combinations will not provide full gain to use with a passive volume control, and a preamp with gain will be required.

This amp was designed to give as low distortions as possible and keep the 300B plate dissipation low enough so the tubes will last along time.

The amp is providing clean 6W. The input sensitivity, worst gain case scenario (76-45-300B) is 1.7Vrms, and best gain case scenario (6J5-47-300B) is 0.34Vrms. The output transformers have 4-8-16 ohms taps so any options can be wired. Having a 5Kohms primary, one can always use 4 ohms speakers on the 8 ohms tap or 8 ohms speakers on the 16 ohms tap and get almost double the power for a bit higher level of distortions. I tested the amp on 4 ohms Buchard S400 MKII used on the 8 ohms tap and is sounds as good as it can be. Sure enough more sensitive full range speakers will be a better match to a low power DHT amp.

The amplifier has 2 enclosures and 2 umbilical cords between the power supply and the main amp. Each enclosure has two nice looking Simpson panel meters. The power supply meters show the high voltage on each channel. The main amp meters show the plate current of the 300B on each channel. These meters are coming in handy to show the proper operation of the amp. Any problems will reflect on these meters indication.

The power supply is dual for the high voltage ( see two toroidal 200VA transformers and two sets of 6CJ3 rectifiers) having a CLCLC filtering using 4 pole Mundorf HV+ capacitors. There are 5 independent low voltage regulated supplies to provide power to all filaments. Therefore the umbilical cords are carrying DC only. The way is designed this amp is super quiet.

This amp is built with lots of TLC and provide an exquisite sound when using the appropriate speakers.

Description

This is a high end, 300B low power headphone amplifier with a 32 ohms output at max 6.75W. It has also 8 ohms speaker output which makes it a versatile amplifier.

The amplifier is uses all Hashimoto transformers and chokes, and it is assembled on a sturdy, high quality, 1/8″ aluminum made by Dave at LandFallSystems, and wrapped on walnut front and sides.

 The amplifier has a vintage look which it is emphasized by a large Bakelite knob and an aircraft dimmable Dialight red panel indicator.

This amplifier was designed with TLC in order to provide best out of the 300B tubes. It is a single ended design and consequently the sound is pleasant, detailed and easy to listen for long hours, with a predominant second harmonic.

The power supply has a powerful 5AR4 or GZ34 rectifier, followed by a CLCLC filtering using two Hashimoto 10H chokes, and two 4 poles Mundorf HV+ high fidelity capacitors. There are 3 separate 6.3Vdc regulated supplies for the 300Bs and C3g tubes, an excellent choice for zero hum. The filament supply for the 300Bs is using super regulators from Belleson.

The audio section of the amplifier uses the C3g pentodes in triode mode loaded on a cascode CCS for minimal distortions. The C3g is used as gain stage/ driver and it is capacitor coupled to the 300B, using two excellent MiFlex KPCU copper foil Poly/Paper in oil capacitors. The final/power stage is using the 300B self-biased using Mills resistors and Mundorf Mcaps capacitors. The 300B is loaded on a specialized Hashimoto headphone transformer.

All the small parts are audio grade and tested for the best sound. I used Takman Rex, Takman Rey, Mills, AMRG, Vishay Milspec and 5W Kiwame for the supply bleeders.

The amplifier is integrated with one RCA and one XLR input, and the volume control is a ladder type TKD CP2511 potentiometer, which is very smooth and clean sounding.

The XLR is transformer coupled to the RCA, using a high quality Jensen JT_11P-1 line input transformers. There is a switch provided to allow using either the RCA or XLR input. The XLR inputs have a ground lift switch which is recommended by Jensen technical stuff to keep it on lift position.

The bottom plate has over 100 holes for natural convection and 4 custom feet using wood and vibration isolators.

The amplifier delivers 6.75 maximum on 32 ohms (14.7Vrms), 4.3W on 50 ohms, and 1.8W on 120 ohms and 0.86W on 250 ohms headphones.

The amplifier will also deliver maximum 6.75W on 8 ohms speakers. There is a switch provided to use either speakers or headphones. However, depending on the grounding scheme of the audio chain it might have no audible effect.

Specifications and measurements

The following measurements are only to give you a sense of the approximate performance. I used a computer software and interface and this comes with some artifacts and noise that are not present when the amp is connected in an audio path. One, if owns high quality audio analyzer, can take more precise measurements

• Input impedance RCA : 50kohms, unbalanced
• Input impedance XLR : 13kohms, balanced
• Frequency response: 20Hz-20kHz @ –0.5dB
• Input sensitivity: 0.44Vrms at 1.1W power output
• Max Input sensitivity: 1.12Vrms at 6.75W power output on 32 ohms
• Gain max 22.5dB (13.3x)
• AC voltage 120V, 50/60Hz
• Dimensions: 19.25″x14″x10″ (W x D x H)
• Weight: 55 lbs.
• Main fuse 3A slow blow

Lead time: 3-4 months.

Please email me to discuss details at : simplepleasuretubeampsRCT@gmail.com

The EML20A is an excellent choice for a single stage preamplifier that can satisfy the most exigent audiophile. This DHT has enough gain to use it with line output transformer and it is indeed, as stated by Emission Labs, one of the lowest distortion tube ever built.

The preamplifier has a separate power supply, which is a great practice inn order to make a quiet DHT preamp.

The power supply uses all Lundahl power transformers and chokes for the high voltage section. It is a CLC  supply with the Ludahl chokes in common mode for lower noise. The rectification is hybrid, using two TV dampers 6CJ3 on the positive and SiC 1200V on the negative.

The power supply is connected to the main preamp chassis through a 6 feet umbilical cord using a MIL spec Amphenol connector. The umbilical cord is carrying only DC and it is screened with copper mesh, which a good practice to have a good S/N ratio.

The low voltage supply for the filaments of the EML20As is on a separate board with its own transformer. There is a third DC supply for the Volume remote control and shunt output delay circuit.

The main preamplifier  uses the excellent high gain EML20A mesh in a self biased setup and loaded on a Monolith Amorphous Core LTA 01/25mA line output transformer (wired 4:1).

The cathode bias resistors are Mills MRA-5 and the decoupling capacitor is Elna Cerafine.

The input is DC coupled and the output is transformer coupled. There are no coupling caps for input or output, an no negative feedback ( global or local).

The volume control is ePOT V3 mini by Tortuga Audio using either LDR module or 60k discrete steeped attenuator module. The display is Oled and remote by Apple.

This is one of the simplest one stage preamps that is using an excellent sounding DHT and extremely low distortions, second harmonic being dominant. The gain is 14dB ( 5x) and with a 2Vrms source will provide 10Vrms at a Zout of 375 ohms. Measured THD was 0.02% at 10Vrms output ( 50Vrms on the plate).

This  headphone amp was due for many years. It was many years ago when I start fiddling with the idea.

After many experiments and measurements, I concluded  that a SET 300B headphone amp is best fit to my sound taste and design preference.

With few things left over or repurposed from previous projects, I’ve managed to finish a C3m – 300B headphone amplifier. It is a one enclosure amplifier using all Lundahl transformers and choke and the long lasting EML300B. If you don’t recognize the C3m in the pictures is because I cut out the aluminum screening can.

The power supply is using a Lundahl power transformer and choke and a hybrid bridge. The rectifier bridge is using SiC 1200V diodes for the negative and two 6CJ3 TV dampers for the positive. The vacuum tube rectifiers are ramping the B+ in a very slow manner ( about 20 seconds). The filtering is C-L-C with 50uF first cap and 220uF second. I am using a Neurochrome Maida 21st Century regulator which provides 385V with extremely low ripple, and protection at 300mA. This accomplished two of many requirements : a super quiet amplifier and the 300B/output transformers are protected. The filaments of the 300Bs are DC powered from a separate supply using its own transformer and it is regulated. Also the C3m filaments are DC powered from a separate supply using its own transformer and it is regulated. I am very careful about the design of the power supply which is a key to a good sounding SET amplifier.

The main amplifier is using the LL2765AM/70mA wired as 12:1 step down. The 300B is biased at Ua=310-320 V and Ia=65-70 mA, dissipating about 22W on the plate. This gives a lot of headroom for this amp and also a long life for the power triodes. The driver/gain stage C3m is loaded on a cascode CCS and it is capacitor coupled to the 300B using very good sounding Miflex caps. Both tubes are self biased and there no NFB applied.

The enclosure is a excellent built and sturdy by Landfall Systems. I used blood wood hardwood for the front and sides. The top plate was redone, CNC at Front Panel Express for a more professional/pleasant  look.

The amplifier frequency response is 25Hz-25kHz at -1dB, the overall gain is 14-15dB and THD with 1Vrms input is 0.02%, delivering 1W on 32 ohms or 100mW on 300 ohms. The FFT, at a 2Vrms output shows only the 2nd harmonic, all other harmonics are hidden bellow the noise floor. This gives this amplifier an extreme SET DHT sound, very clear, pleasant and clean.

The Zout is 8 ohms. The headphone amplifier can also be used as a preamplifier delivering very clean signal to a lower gain input sensitivity amplifier. Given the very low output impedance, it can drive heavy capacitive loads ( long cables/lines) and amplifier low input impedances.

There is no greater satisfaction for a music lover than having an ideal audio chain. This is a 300B amplifier that will not disappoint. It is the result of many years of experimentation and R&D with DHTs ( direct heated triodes).

Why 300B?

• Because it is one of the most praised DHTs of the past.
• Because it can deliver enough power to drive speakers that are not super sensitive and therefore more available.
• Because it is available as new production in many brands and “flavors”
• Because, if well designed, an amp using 300B can satisfy an exigent audiophile

I am publishing this build after the 71A sub-watt amplifier. I love the sound of both amps, as well as few other amazing DHTs such as the 45. Each amp can offer rewarding sound experience as long as it is paired with an appropriate speaker. I want to emphasize that one can’t go wrong when using these old design vacuum tubes.

I’ve always liked the sound of a 2 stage amplifier. The best sounding of 2 stage amps have a DHT for both the final stage and the driver.  In many cases these amps have low input sensitivity and therefore need a preamp with medium or high gain. If the preamp is using a DHT then the whole audio amplification  is DHT. The great advantage of having all DHT amplification is the low distortion where the 2nd harmonic is predominant. There is no need for global negative feedback. The sound will be amplified in a way that is not loosing detail and will be extremely pleasant.

This amplifier is using either the 10Y/VT25 or the EML20A as driver. The driver is loaded on an interstage transformer Lundahl LL2756/25mA. The driver is powered through a CCS ( cascode depletion mode mosfet) that is adjusted to 25mA. Both the driver and the final stage ( EML300B) are self-biased.

The 300B is loaded on a excellent sounding output transformer Monolith Etude1. The output is switchable between 4 and 8 ohms.

Each tube filament is powered by a Coleman regulator that has individual multi-turn potentiometer for fine adjustment. As always there is no “free lunch”. The excellent sounding Coleman regulator needs re-adjustment every time one will replace a tube. Believe me, it is worth the trouble…

The power supply was built in a separate enclosure ( using a Modushop ( Italy) Pesante enclosure placed sideways to minimize footprint). It has 4 separate raw DC supplies for the filaments of the 4 DHTs. The high voltage supply is in a quasi choke input mode and it uses all film caps ASC. The rectifiers are 6CJ3 TV dampers.

The umbilical cord is permanently attached to the power supply enclosure and has a military grade Amphenol 11 pins connector at the amp end. Few years ago I found a nice 1000V NOS large Simpson panel meter on eBay. I am using it to show the B+ high voltage of about 480V. The power supply has two main switches: one for the filament supplies and the other one for the high voltage supply. They are in series and therefore one can’t turn on the high voltage before turning on the filament supply. In the past I’ve used a relay to delay the high voltage. In this case, I prefer manual switching because of the slow warm up caused by the Coleman regulators. Some of the 300B need few minutes to reach steady state on the filaments, such as the EML300B. Turning on the filament supply switch will allow the user to wait a desired amount of time before switching on the high voltage. The high voltage will slowly ( takes about 20 seconds) ramp up. This is beneficial for the life of these expensive vacuum tubes.

The input sensitivity is about 3Vrms for 8W output with the EML20A driver and about 8Vrms with the 10Y driver. I am using a switchable 01A-26 full gain  ( x8  or 18dB)  preamplifier that provides sufficient signal level for both drivers.

I will always refrain to describe how any of my builds will influence the sound. That should be a personal preference. However, the synergy amp -speaker/room is the most important. It definitely sounded noticeable better using Rullit’s field coil speakers in cabinets designed by my audiophile friend Rich, compared to my TangBand tower speakers.

Here are some pictures that will give you an idea of how this amplifier is built:

And here is the power supply:

Seeing an amp delivering such a low power, one might ask the question: ” How low can you go? “. Believe me, I can go even lower.

Here is a 750mW DHT amplifier using the amazingly good sounding 71A. Last year I’ve been asked by Kevin at Glow In The Dark Audio to build such an amp and here is the link to the description and his impressions about it: http://www.glowinthedarkaudio.com/radu-tarta-71a.html.

At that time I was not thinking to build one for myself, but after listening to the amp I’ve built for Kevin I’ve decided to make my own. It is almost the same design with the exception that I’ve used the C3g as driver. The reason I needed a bit more gain was to be able to use it without a preamp.

I’ve used same Sony power transformer and Tamura 5k/8/600 output transformers. I am impressed with the sound of these small output transformers: very detailed and balanced.

The enclosures are made with Cherry hardwood and the top plates are 12×12” sanded aluminum.

The power supply uses the 80 rectifier in a choke input LCLC filter, and separate raw DC supply for the filaments of the 71A and C3g. The umbilical cord is screened and carries only DC and it is terminated on a Mil spec Amphenol connector. I found two vintage Westinghouse panel meter that show the B+ voltage and the total current in mA.

The amplifier is using an active load for the C3g ( DN2540  CCS) and it is capacitor coupled to the 71As. Both stages of the amp are self biased and are using high quality components. The amplifier has a very smooth TKD CP2511 stereo attenuator.

The filaments of the 71A are powered by two Coleman regulators that bring the sound of this little power amp to a next level. There are two multi-turn potentiometers and a panel meter that allow the user to adjust the voltage to 5V for each 71A filaments.

The amplifier can also be used as a high gain ( 30dB) line stage due to a 600 secondary on these little Tamura transformers. This comes handy when one has a very low input sensitivity amplifier. It can also be used to drive a First Watt F4 ( which has unity gain) to its full power.

Due to its careful built and separate power supply this amp is perfectly quiet.

At this level of power output one needs ultra sensitive, easy to drive speakers of 106+ dB.

Shiny Eyes Turbo 45 SET Amplifier

My latest 45 amplifier is a great candidate for a keeper. After a listening session to a Yamamoto 45 amplifier at Rich’s place, and following his thought about more power from a 45 amp that can better drive his Zu audio 99 dB speakers, I started a R&D and breadboarded many of driver choices. The final result was a switchable EML45/45B with 45 driver and 6SN7 gain stage. Despite the efforts to make it as compact as possible, in one enclosure, the final result was a 20.5x13x11″ and 55 lbs monster. The power supply contribute to a good portion of the weight. It has a custom made Heyboer power transformer used for HT with a 750V CT feeding a full wave tube rectifier ( 2x 6CJ3) followed by a 1.5/10uF-LL1673/10H CCMR mode-100uF/550V  Mlytic HV+–LL1673/10H CMMR mode – 220uF/550V Mlytic HV+–. All filaments are DC regulated using LD 1085/V. Separate dual bobbin 56W Triad transformers are used for all 4 of the 45/45B tubes. The 6SN7 filament is also fed from a regulated power supply.

The amplifier is using Electra print 7k/8 ohms low IMD custom transformers and LL2753/30mA as interstage. One can use different type of 45 for the driver such as old RCAs, old globes, EML45. The final tubes can be either any type of 45 or switchable to EML45B.

The wood case is made of exotic bocote wood sealed with 3 coats of shellac and two coats of urethane.

The amplifier sounds excellent using 45 globes for the driver and EML45 for the finals, but is really blooming when using EML45 as driver and EML45B for the final.