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Re-doing 6N11(ECC88) SRPP from eBay

July 18th, 2018

 

Another project from eBay. It is an assembled 6N11(ECC88) SRPP board. Who are not familiar – a Chinese vacuum tube 6N11(via eBay) is an equivalent of Russian 6N1 (via Ebay). I read somewhere that Russians transferred their tubes technology in 1940s during the hot communism era and friendship time with other  pro-communism countries. Whatever is the history but I am mainly interested in the pcb itself as it is really good quality and comes with some useful reusable components like the tubes sockets. You can get such assembled PCB for around 12Eur via Ebay. The pinout of 6n11 is exactly the same as ECC88 tube and I want to change few resistors and capacitors in order adopt for ECC88 DAC SRPP output stage.

Details of the board

The original 6N11(ECC88) SRPP board looks like this

Tweaking 6N11(ECC88) SRPP from eBay board top view

Two additional resistors are not show in the circuit (which was given by HK eBay seller), which are located in the top left part of the 6N11(ECC88) SRPP board. These resistors form a voltage divider and the heater’s voltage is attached to the middle point between them.

Tweaking 6N11(ECC88) SRPP from eBay circuitAccording to the seller, 6DJ8 SRPP circuit was designed by Morgan Jones as tube headphone amplifier and it is supposed to drive from 15Ω to 300Ω headphones.

Just few words about the pcb itself. It is a double-sided glass fiber board with the size 110mm × 74mm with thickness ~3mm. I would say quite nicely traced but need a good re-soldering. The board marked as LG-162B. The capacitors, which are included on the board: Rifeking 22uF/250V (shown as C1 and C2); Rubycon RX30 22uF/250V (shown as C3 and C4). I checked ESR of these capacitors with my ESR meter and it appeared that Rifeking’s caps ESR is twice lower. So, I will use them for a while as I do not have any better right no. But looking into the future these Rifeking brand are made in China and some people found them as non-reliable.

ECC88 SRPP reference circuit

If you googled “ECC88 SRPP circuit” you already have end up with huge amount of the circuit. So, the question is which one you should try? Well, there is no straightforward answer as chosen tube’s working point (well different circuits) will influence the sound and produced harmonics profile.  There are other variables which might hugely influence the sound too: 1) input impedance which can a lot influence the sound; 2) input voltage amplitude. I chose to replicate Thorsten’s Loesch’s ECC88 DAC output circuit:

Tweak 6N11(ECC88) SRPP from eBay Thorsten's Loesch's ECC88 SRPP circuit

If we compare the Thorsten’s ECC88 SRPP circuit and the circuit of the 6n11 SRPP board, we would see following differences:

  1. There is RC filter formed from 22uF capacitor and 4.7k resistor in Thorsten’s ECC88 SRPP circuit;
  2. The ECC88 tube’s working point was chosen different and as a result, the cathode resistors are 1k;
  3. Output capacitor size is 0.47uF x 2 =0.96 uF (1uF), which is enough for the power amplifier with 20k impedance input;
  4. There are some other differences in the input section but we will neglect that.

Upgrade plan for 6N11(ECC88) SRPP board from eBay

So, in order to get something “similar” to Thorsten’s ECC88 SRPP circuit’s sound, we need:

  1. Replace the cathode resistors
  2. Replace the output electrolytic capacitors. I chose Audiophiler MKP type (via eBay)
  3. Form a RC filter by adding missing 4.7k and limiting at the same time the supplied current to a ECC88 SRPP stage.
  4. Replace the resistors in top left part with 22k and 47k to get a higher voltage as reference for the heaters voltage.

On the board, it would look like this (the board tracing is only very slightly different than mine):

Tweaking 6N11(ECC88) SRPP from eBay Green LG-162B PCB top view upgrade

and here is the top view of my modified board

Tweak 6N11(ECC88) SRPP from eBay board top view after tweaking

Which ECC88 tubes should you use with this SRPP board?

I would recommend to start with Tesla ECC88 tubes (via eBay) as they are relatively still available for affordable price (sub 20Eur). But these are not JJ tubes, which some sellers wrongly call Tesla tubes. I got a set of Bugle Boy Amperex ECC88 tubes (via eBay) for quite attractive price. Some people prefer Siemens ECC88 tubes sound too (via eBay).

What is left?

To be build a quite and regulated power supply for this board. I already got a R-core transformer for it some time ago. To be continued…