6J4 6AU6 EF94 6P6P 6V6 Preamp

Click pics to enlarge. The roomy interior is a DIY tweaker's paradise. The preamp is all grounded to the chassis. Note that, as one member of diyaudio commented upon (link in text), the Earth of the IEC socket is not connected (lower left). The left knob is the selector, which is soldered to a small PCB. At the left of the PCB, you can see resistors directly coming off it (to the 6J4 together with the nearby caps). At the bottom of the pic, you find at the right the 4 RCA inputs. to the left is the single output, and you can see its grey shielded cable crossing the entire chassis to the front (where it connects with the volume pot). 

Review: 6J4 6P6P Preamp, Part I

(Important) Addedum 2/16/23: Due to new development, I'm currently using another preamp. So, I opened this one up and contemplated doing some upgrade. After studying it for a while, it dawned on me that, unlike what the schematic says, the volume pot is NOT placed after the selector, but rather at the very end, before output! In fact, you can see that from the pic above, and I have amended the text below the photo. This is highly unusual, and I asked the seller why, and he replied that having it at the end lowers noise (not surprising, given that the gain is so high and the 6V6 is a power tube that can output a few watts!). Hmmm, while that may be true, I wonder if it has sonic costs. Those savvy with circuits, please post comments if u can!

Part II (cable change); Part III (a little tube rolling)

After I bought bought my first Chinese Amps and deemed them satisfactory, I decided to buy a Chinese Preamp. Since I’m using low-powered SE amps, I need a Preamp with Gain. There are those who will take issue with this, but it is my firm belief that a Preamp with Gain gives one better oomph (a better First Watt). With few exceptions, I’m not for Unity Gain or Passive Preamp (including most TVC). I also much prefer to have the Preamp at my fingertip. This often mandates the use of long interconnects to my amps (or long loudspeaker cables), and the TUBE Preamp at least partially offer some form of buffering. It didn’t take long for me to settle for this one.

https://m.tb.cn/h.UmNicJc?tk=vEnvd6J6979

Ergonomics and Function This is a fairly basic line stage preamp. 4 Inputs and a pair of Outputs. Selector and Volume Knob. It is tube rectified (Chinese 6Z5). BUT, the tube complement is unusual, to say the least. Each channel employs 1x 6J4 and 1x 6P6P (both Chinese). Fully assembled, it costs here in China RMB 658 (USD 97) delivered.

Why I Picked This One There are numerous preamp offerings on Taobao, from very cheap DIY boards (LITE was one of the first ones to popularize this) to pretty expensive ones. Here are my criteria:

• As I have all kinds of Preamps (but not in China), including premium ones (say, Yamamoto, Kondo, Shindo), I didn’t have undue expectations nor want to spend much money. The price is right.

• I shun Preamps that use popular tubes like 12AX7(U7), 6DJ8/6922 and 6SN7 etc. The reason is because their old stock tubes cost too much and since I have plenty in NYC and HK I’d not want to buy more of those tubes here in China. That immediately takes out a large swarth of offerings featuring most of the Chinese (and Russian) 6Nx types, many of which have no real Western exact equivalents anyway. For this preamp though, I determined in advance of purchase that a bit of meaningful tube rolling can be had for not much (see section below). In any case, I'm increasingly not fond of 9-pin tubes - larger octals etc offer better sound.

• I am a great fan of the 6V6. Being entirely not Triode, this preamp has a fuller sound than those that use the tubes mentioned above. Mind you, not fat, but just fuller. In many ways, this preamp has a circuit that is closer to an amp.

• I also shun any replica types, and that further trims the offerings. I have the real Marantz 7, McIntosh C-22 etc, and have had experience with many others (like mediocre Matisse, the circuits of which are strangely popular in HK and China), why bother with replicas? I also tend to shun certain topographies, like SRPP (which, as an exception, I do have in the form of my Wavac PR-X2). Add all that together, and not many are interesting to me.

Click to enlarge. From Top, SMPS, Reisong Transformer, TDA-1305 DAC and 6X5 rectified, 6AU6/6V6 Preamp.

General Sound I have used this for months. It runs very quiet. Even with stock tubes, the sound was very good. I’m not going to write too much about it since, doing a bit of research for this article, I found there is a very good article from Nimophile (Singapore). I find his report to be well written and concur with most of his findings. I’d like to add that since there is not much regulation, although the preamp has plenty of gain, in a sense it’s not particularly “dynamic”. Don’t get me wrong, there is nothing wrong with its dynamics, just that it doesn’t have the usual jumpy and over-eager quality of high gain preamps, which can be annoying; rather, even when loud it maintains a basic smoothness. Also, since I use SE amps, my preamp volume setting is unlike his (around 9 o’clock). It should be emphasized that lowering the feedback resistor value to lower gain increases feedback, which would usually be detrimental to the sound (usually less detailed). Unlike what he mentioned, I don’t even feel a tinge of grain in the treble, but then unlike him I’m using a tube amp and he an ICE amp.

Tube Rolling As Nimophile mentioned, tube rolling improves the sound meaningfully. The 6J4 has exact equivalents in 6AU6/EF94. I got a pair of Mullard (cheaper than Siemens) EF94’s for RMB 82 (USD 12). 6Z5 is the same as 6X5. I have quite a few of those in HK, left from my days with ANUK kits. But I bought one here with broken guide pin for RMB 36 (USD 5.3). Both changed the sound for the better. As for 6P6P (6V6), as a fan of 6V6, I knew for a long time that Russian ones are nothing special (current ones are better; older ones crap) and Chinese ones better. Nimophile found out the hard way but I totally would concur with his findings. When I get back to HK, I shall bring back old-stock US 6V6s, which is emphatically a cut above.

This Preamp is sold on many platforms: AliExpress, Amazon, Ebay etc, under various names and sellers. The Preamp is also sold on TB as a kit, but since assembly charge is low lazy me opted for the finished product. During my research, I found out this kit has been around for a long time (just Google “6J4 6P6P [or 6V6] Preamp”), and some of the issues have been discussed in various DIYaudio threads (here and here). It’s worthwhile reading those. I’m not sure whether the kit is still available in the West, but it is on TB.

Issues First, the RCA Sockets are not of the best quality (common grievance). I have one pair of particularly irksome MIT AV cables that have RCA connectors with Visor Grip. When pulled out, they ruined one of my 3.5mm to RCA adaptor (here) and also took out the collar (ground) of one of the RCA sockets in this preamp. Exasperating! Second, for some reason the IEC socket is kinda loose fitting in my sample, and I have lost power a few times; no great harm but irritating.

DIY and Upgrade For those with Soldering Skills, I’d recommend getting the Kit. I’d probably like to change the volume and selector pots, at least a couple of the RCA Inputs. I’d likely upgrade also the shielded signal cables to great-value dirt-cheap Canare L-2B2AT (review to come) or, even better, Belden 8451 (I have never formally reviewed it, but it appears in many of my mod articles, just search the blog for "8451"; also, just recently found it on TB, and bought some, review to come). The caps and resistors look pretty good to me. When I get the patience and urge to repair the ruined RCA connector, perhaps I'd implement some of these measures. But after finding out what I detailed in the Addendum above, I'm kind of at a lost...

Conclusion This Preamp is highly recommended for its value and sound.

The schematic below is old and may not exactly conform to current offerings. Certainly, as you

read in the addendum, the volume pot is actually at the very end, before output.