Audiophile “Shiny Eyes” PSE al DHT Tube Amplifier

All 4P1L DHP triode connected, Lundahl transformers IT/OT

I am happy to present my second tube amplifier in the “Shiny Eyes” series. It preserves the look of the series, with cherry stained hardwood cabinets and gold anodized aluminum top plates by Proto Panel.

This amplifier features the same tube for all audio stages, the Russian direct heated pentode, triode connected : 4P1L. It is an amazingly linear tube in  triode mode. The sound of this tube is similar to the sound of 300B, 2A3 and some would say that is surpassing the famous ones. The amplifier is designed with only two stages coupled by an interstage transformer. Both Final Stage and Driver Stage are single ended and run in filament bias mode. Excellent filament regulators by Rod Coleman isolate the filament from power supply. Interstage and output transformers are high quality Lundahl. High voltage supply is using damper tubes for rectification and CLCLC for filtering to a extremely low ripple voltage. In addition, a shunt regulator supplies power to the driver stage tube.

The amplifier consists of two enclosures: one is the power supply (weighting about 55 lbs.) and the other one is the main amp (weighting 32 lbs.). They are connected by two 7 wire 18AWG cables, that bring power to the main amp. The power supply is running slightly warm (is designed with a good power reserve), except for the rectifier tubes (they are placed in  the middle and protected by chokes and transformer covers). The main amp is running hot in the middle section where many slots are provided to release the heat developed in the filament bias resistors(a total of 62W). Heat is raising by natural convection provided by over 300 holes on the perforated enclosure’s bottom. The amplifier was tested for at least 5 continuous hours at time and presented minimal drift. The fact of having to waste power in the filament bias circuits is counterbalanced by an amazing sound. The driver stage tubes are a bit microphonic. This was “tamed” by starved filaments, suspended sockets, rubber grommets and rings.

“Shiny Eyes” PSE was broken in for at least 50 hours using small reflex cabinets Mark Audio’s Alpair 12P full range driver. ( I have an extra pair of cabinets that are currently listed for sale on eBay).

To find out more about this amplifier and the speakers you can visit my blog website at https://simplepleasuretubeamps.wordpress.com/ .

Specifications:

1. Distortions : 0.5% THD at 0.5W, 0.7% THD at 1W and 1.55% THD at 3W, all into 8ohms
2. Frequency response:  25Hz-30kHz, -1.5 dB
3. Voltage Gain: 4.7 times or 13.5 dB
4. Input Sensitivity : 1Vrms at 2.75 W into 8 ohms
5. Input Impedance: 50kOhms
6. Damping Factor: 4 for 8 Ohms load
7. Mains Voltage: 120V at 60Hz

Features:

A. Power Supply

1. Quality designed for high reliability and pure sound
2. 5 transformers and 4 chokes
3. Extremely low ripple on high voltage rails ( dead silent on 92 dB speakers and 104 dB headphones
4. CLCLC filtering, LCLC separate for each channel
5. Main power transformer by Edcor, high quality designed for tube amplifiers
6. Individual power transformers, split bobbin, and raw power supply for each stage/channel filament
7. Polypropylene in oil motor run Epcos capacitors ( except first capacitor which is Solen fast)
8. Full wave tube rectifiers
9. Fuses: 3A slow blow main transformer, 2x2A slow blow filament supply
10. Ground earth chassis connected to main’s earth for safety and to star signal ground
11. Lighted Main Switch
12. Two 7 pins 18AWG military grade female connectors (one for each channel)

B. Main Amplifier

1. Six tubes all of the same type 4P1L (4П1Л)on highest quality ceramic sockets
2. Two interstage transformers type Lundahl LL1692A
3. Two output transformers type Lundahl LL1664/70mA
4. No capacitors in the signal path (except the last power supply ones)
5. SSHV regulators for driver stage high voltage supply
6. Filament bias with Rod Coleman’s regulators
7. Takman resistors except for filament bias ( final stage is wirewound Ohmite, driver stage is wirewound Mills)
8. Wiring signal path with teflon silver plated
9. Original Alps blue velvet potentiometers
10. High quality Neutrik RCA isolated inputs
11. High quality speaker binding posts
12. 200mA round panel meters measuring total high voltage current per channel

I love the sound of my amplifier ( all design challenges paid off). This a low power amplifier ( 2x3W), providing a transparent gorgeous sound. However, 3W stereo of single ended tube amp will sound much more powerful than a solid state showing the same power specs. This amp, being single-ended must be used in combination with a high sensitivity  (> 92 dB) full range driver speakers in order to reveal its true identity.

It is hard to describe the sound of this amp for those who never listened to a quality single ended tube amplifier. It has a heavenly aura, harmonic subtleties and a timbre that will give instant goose bumps, and you will definitely fall in love with it.

Thank you for taking your time to look at my creation. Feel free to ask questions.

Best regards,

Radu

 Comment: This is one of the most impressive looking amps I’ve seen in terms of build quality – pretty stunning work. Congratulations!

So on to the circuit. I’m a big fan of the 4P1L and have 150 of them, so I’m future proof. I intend to use this tube for quite a while. Sound is up there with the best DHTs like 2a3, 10Y, 46, 300b. It has its own quality – very clean with good dynamics and bass, and extended treble. It’s clear rather than mellow like the 26. It’s big advantage apart from being the cheapest of all the good DHTs is that it can be used in filament bias, and that’s how it’s used here. All the other tubes above are either possible but challenging to use in filament bias (46, 10Y) or just not suitable at all (2a3, 300b). Filament bias allows you to use a small cathode resistor like approx. 15 ohms in this case, and no bypass cap. I believe coupling caps and cathode bypasses degrade the sound to the point where it’s better to design them out, and that’s what is done here.

My own PSE 4P1L is very similar to this, and I’ve seen other similar designs, though probably very few have been built yet all in filament bias. I believe this is the way forward. I used similar transformers – LL1660/18mA (ideally should be 25mA) and LL1620/80mA. These are good enough and better than many others. Whether they shape up to Hashimoto, Tamura or Tango I don’t know. I’d also like to try James, Silk and a few others.

Bearing in mind that the 4P1L has a healthy gain of around 10, what we have here is a 2 stage amp that can drive reasonably sensitive speakers with 2v in from a DAC. In my case I have Alpair 10 full-range units which are fine in a small-medium room with orchestral and jazz. Now that’s something – a 2 stage all-DHT amp with no coupling caps or cathode bypass caps and tubes you can buy by the dozen

Sound quality? I’m a professional musician and I love this amp for its clarity – the detail surpasses anything else I’ve built including several 2a3 and 300b amps. Clear inner parts in complex orchestral textures, lyrics you can suddenly hear and understand. In addition the tone and timbre are very close to original acoustic instruments to my ears. The voice is exceptionally clear, and the grand piano is very realistic – you can hear the keyboard noise, the hammers and the overtones.

this is an amp which demands the best possible power supply and you have to consider that the filament supplies – one for each tube – are effectively signal supplies and demand the same level of sophistication. The signal passes through the cathode resistor, which is in the filament circuit. Don’t even think of skimping on the power supplies – think chokes and regulators and also motor run caps in the PSU. I use a DN2540 before a voltage reg tube for the input section.

I urge more people to build this. It would be hard to make the amp look as nice as this one, but a more modest build with the same parts will open your eyes to what’s possible in terms of clarity and timbre in a medium power SE amp. This isn’t a traditional 300b SET. For me it’s better than that.

Andy Evans, London UK

Time: January 28, 2014 at 4:09 pm

Thank you Andy for the kind words, the best description of the amazing sound and for reassuring the readers that our quest was well worth to take on.