New 845 amplifier

by Andrea Ciuffoli - starting in the March 2007
contact me for any problem or question: webmaster @ audiodesignguide.com

BEWARE! This is an expert-only project because the high power supply voltage, about 900V. Be careful, it can kill you!
Keep always one hand (better if it is the left hand) behind your back.

ATTENTION: This version have some low frequency noise so it is not good for high sensibility loudspeakers (> 90db) so it is upgraded to the new version HERE


The design of this amplifier is 3 years old but only now I have enough space in my house to start again developing new vacuum tube amplifiers.
In 2004 have published its first outline on the page about the SV572-3 SE but now I have decided to use the 845 instead.
I like the SV572-3's but in this last few years they been somehow difficult to find and the 845 are generally preferred by audiophiles.
This project is born so to be simple but with high electrical performances and great sonic results.
The sound quality has been obtain using the best components on the market.


These are the characteristics of this design:


to complete the chassie you need to add only one frame and a cover for the transformers section.



All the audio transformers used in this project are Lundahl products and the distributor list is available here

items models description price (euro)
2 LL1620/90mA Lundahl output transformers (148 euro each) 300
2 LL1671/20mA  Lundahl interstage transformers (68 eur each) 140
2 LL1638/8H 200mA  Lundahl power supply inductances (64 euro each) 130
1 Custom R-core ECU Electronics Industrial co.,ltd (price about 70$ + 150$ shipment) 170
2 534 100ohm Spectrol precision wire wound multi-turn potentiometers 20
2 43 20Kohm Spectrol multiturn cermet trimmer 2
2 534 2Kohm Spectrol precision wire wound multi-turn potentiometers 20
2 845 vacuum tubes  Golden Drangon or Valve Art (from 120 to 400 euro per pair) 120
2 EL34 vacuum tubes Svetlana 52
2 12AX7 vacuum tubes Jan Philips or Mullard 50
2 100uF + 100uF 500V Jensen Electrolityc Dual (price about 44 euro each) 88
2 100uF 500V Jensen Electrolityc (price about 32 euro each) 64
2 220uF 500V Jensen Electrolityc (price about 36 euro each) 72
2 0.22uF 600V Jensen Copper film paper-in-oil (45 euro each) 90
    Varies components + connectors 200
    Total price 1518

Lundahl is one of the world´s leading manufacturer of audio transformers used in many professional studio recording and radio stations so it has great experience and all these products use a custom audio C-core.

About passive components types,  I don't leave many choices:  Caddock resistances on the signal path, Sanyo OS-CON capacitors on cathode and Jensen AudioGrade electrolytc capacitors on power supply.

You can find many components of this project at Audiokit online shop.

I am using a DACT stepper attenuator instead of the normal ALPS or NOBLE solution, because the the sound is clearer.

The Custom R-core has been custom design with 2 secondaries 6.3V 5A necessary only if you want to use the SV572-3 instead of the 845.

About wire forget the teflon ! use only stranded tinned copper wire with pvc isulator.

A very good quality stranded tinned copper wire could be buy directly from E-Z-HOOK that carries an extensive line of fine stranded and extra flexible wire.

I have used the model 9506 with WVDC (Working Voltage Direct Current ) of 5000V for all the main power supply connections, the model 9504 for the signal and the model 9510 for the output loudspeaker connections.

All the wires near the tube socket have been cover by Sterling tubes in order to avoid that the high temperature will destroy them.

As visible in the photos to fix some components I have used some good PTFE Insulated Twist Eye Terminals by Grennan Audio.

For the R-core power supply transformer contact:

ECU Electronics Industrial co.,ltd
NO. 669 Changjiang West Rd,
Huadian Building Hefei, Anhui, 230031 P.R.China
Tel:0086 551 5311411
Fax:0086 551 5324417







Here are some plots generated with my Clio system by Audiomatica.

With a driver stage of this type (180Vrms with only 0.23% THD) I expected a great performace.

The distortion spectrum is perfect driving with this amp. a
5 ohm load until 17W and the output impedance is less than 1.5 ohm.

So the result is a perfect amp. and I am sure there are very few 845 amp. around with this performances.