MKTP1 Vacuum Tube Modular Preamplifier
(EH 6922 tube)
In my previous review for Miu-Tech of Hong Kong, I built and reviewed their MRA DIY Portable Amplifier which is a great little DIY amplifier kit. In this review however, I am taking a look at their hybrid headphone and preamplifier, the MKTP1.
A rather large box for a tiny little amplifier…
As you can see from the open box picture, the amplifier is well surrounded in molded fiber board. A hefty power supply and an RCA to 3.5mm TRS audio cable are also included.
The only thing that didn’t come with mine was an AC cord, which is not a big issue; the AC adapter has a standard IEC C14 socket that takes a C13 computer cable and I don’t think there is anyone out there that does not have a few spare C13 cables in a drawer.
The workmanship you find after you get the amplifier unpacked is excellent–the machined aluminum front plate with recessed TRS jacks, an aluminum volume control and hex screws holding it to the case. The TRS jacks are recessed in larger holes to accommodate plugs with bigger ends like the Grado mini adaptor cable I use on the DT880′s. Everyone who sees the amplifier finds the acrylic tube protector a nice touch.
The same attention to detail is also evident in the back panel, a Line Out TRS jack, RCA Line In jacks and the power adapter connector.
Part of the fun of owning a tube amplifier is the ability to experiment with different tubes that are available on eBay and from other sources. The tubes you can roll in the MKTP1 are a little limited compared to a full size tube amplifier, but there is still a great variety of available compatible tubes that you can try.
Tube rolling is not for everyone. I got involved in it with my Little Dot MK IV amplifier and the LD Tube Rolling thread on Head-Fi. Chasing the little “Fire Bottles” can be an expensive hobby if you allow it to be as there are always “Better” tubes that are just out of reach. The sellers on eBay know how much demand there is for popular NOS tubes. They also know quite well how much these tubes can be sold for; it is like anything, the higher the demand, the greater the price. Occasionally some have found a very good deal on excellent tubes that the seller either didn’t know what they were or the auction was badly worded etc… But you have to scour eBay globally and possibly haggle with the sellers and at some point in the game, the work involved acquiring these tubes outweighs the sonic benefits gained.
When the MKTP1 arrived I listened to it for a few days in its stock format, which I must say is very enjoyable. Then the case was removed and the rolling began; I have a few compatible tubes and quite a lot more that are not. Well I rolled them all with a few minor modifications, I don’t recommend this as it is not really advantageous to roll tubes that you can’t just plug in, adjust the bias trimmers and enjoy. I rolled the incompatible ones just to see if this amp would run them and I was very impressed with the results.
At one point I had a Reflektor 6H9C octal socket tube running just for a “lets see if…” scenario, it worked with modifications and is not really a tube you would use.
The MKTP1 is not a rolling friendly amplifier compared to others that are on the market. The board has to be removed from the case and you have to have a voltmeter to measure the bias voltages with the new tube installed and the MKTP1 powered up without the case.
*Note When working with regular tube amplifiers there are always LETHAL VOLTAGES present that will kill you instantly. The MKTP1 has none of these voltages so you can change tubes and run the amplifier safely without the case installed.
To change the tube or the opamp you first need to remove the acrylic tube protector from the top of the case. It is a little hex head screw but the 6mm Torx screwdriver will fit perfectly.
Then you can pull the stock tube straight up and out.
After the tubes are removed:
- Remove the volume knob and unscrew the nut & washer from the volume control shaft.
- Remove the 4 corner screws from the back plate and slide the board out of the case.
*Note The 7806 voltage regulator on the bottom side uses the case as a heat sink so the case can get rather warm after a long period of use. When changing a tube you won’t have the board out of the case long enough to cause the regulator any trouble with no heat sink.
Get a jewelers flat blade screwdriver and your voltmeter handy, set the voltmeter to DC Volts if it is not an auto range model the scale should be 20V, now look at the following board layout.
Depending on which tube you are installing, you might have to move the jumper at the top of the picture. If you are installing another 6DJ8 tube it should already be set on the 6DJ8 side. If you are installing a 12AU7 tube then you need to move the jumper to the other pair of pins. The following voltage measurements are the same for both tube types.
Install your new tube and power up the amplifier. Your headphones do not need to be plugged in to adjust the bias.
Push the Negative or Black probe into the board via marked with a Ground Symbol in the picture.
Push the Positive or Red probe into the top or bottom via surrounded by the white silkscreen box next to the tube socket, one via is marked with a 1 the other with a 6.
Measure the DC voltage on each hole and adjust the A trimmer pot for via 1 and the B trimmer pot for via 6. The voltage reading should be the same value for both sides and be between 6 and 8 volts according to the online manual for the MKTP1.
Opamp rolling is the same as it was in my previous review of the MRA DIY amplifier except in the MKTP1 there is only one opamp.
*Note The NE5532 that comes stock with the MKTP1 is an industry tried and true opamp that is used in many high end audio products, so changing it out would just be a mater of personal preference rather than performance.
After everything is changed and the measurements are done you can test it to see if there are any problems by plugging in a set of headphones and your source–if it sounds okay then put it all back together in the case and enjoy.
Electro Harmonix 6922EH
This is the stock tube that ships with the MKTP1, still made to this day in Russia at the ExpoPul Factory (Reflektor) at Saratov, Russia. You can see by the date code on the above tube it was manufactured approximately 1 year ago (April 2013) and is a new tube not NOS. The ExpoPul factory makes two-thirds of the world’s vacuum tubes used for music, Tung-Sol’s, Sovtek’s, Svetlana’s and Genalex Gold Lion’s all these tubes are manufactured at Saratov under the management of the New Sensor Corporation.
The sound of the 6922EH tube is very clean. It is a good all round “neutral” tube that is very popular and used by many manufactures of high end audio gear. These could very well be some of the reasons miuaudio chose the Electro Harmonix 6922EH as the stock tube for the MKTP1. It is inexpensive, has a great neutral sound, readily available and very popular.
Mullard ECC88 (6DJ8)
This tube was, I believe, manufactured in Britain in 1964. The code on the bottom of the tube is “B4L1″ someone can correct me if I am wrong. The ECC88 went in nicely and a little bias adjustment later I was listening to the Mullards’s detailed, warm and musical sound. This is a good tube to use if you can get one for a reasonable cost.
Voskhod 6Н23П (6N23P)
This tube was made at the Voskhod – Tube Plant in Kaluga Russia around April of 1978. This is one of my favorite Russian versions of the ECC88/6DJ8 type tube. It has the most bass compared to the other two tubes and can be a little too dark for some listeners or setups.
*Note many tubes were tested in the MKTP1, I don’t have any compatible 12AU7′s but I did have a pair of Sylvania 5751′s (12AX7) that I tried and did get to work after a few modifications.
The MKTP1 inital setup was with my FiiO X3 and beyerdynamic DT880 Pro’s. The MKTP1 can drive the DT880 with authority and the volume control at 10 o’clock provides a very respectable listening level.
The album I use for a lot of my testing is Jethro Tull’s “The Best of Acoustic”, 24 tracks of Tull’s best acoustic songs. This is an excellent album for the DT880′s as they faithfully reproduce the instruments in realistic detail with a nice soundstage. The acoustic guitar work and Ian Anderson’s flute on the track “Salamander” will give you chills.
The MKTP1 has a neutral sound, not overly warm or bright. The stock 6922EH is a tube used in many high end preamps so it doesn’t colour the sound in any way. It provides an excellent level of richness and detail to the MKTP1 that one would expect in higher end systems. I like my headphones neutral as well as my amplifiers unless I want that bass thump of EDM and then I have Ultrasones for that purpose.
My DT 880′s are always plugged into my Little Dot MK IV, which is an amplifier that punches well above it’s price point with some cheap replacement tubes. If I didn’t have a regular OTL tube amplifier for the beryer’s then I would be very satisfied with the MKTP1 as my main desktop amplifier. I have used this amplifier steady for the past 3 weeks I have done a minor modification to support one of my favourite tubes from the 60′s, the Westinghouse 6BQ7A which I find very euphonic in it’s presentation, smooth and musical.
The miuaudio MKTP1 is a very proficient tube hybrid amplifier that competes well with more expensive tube amplifiers, it will drive 250 Ohm headphones with authority as well as lower impedance headphones. (Sounds great with the Koss Porta Pro’s as well.)
If you are looking for a hybrid desktop amplifier with a small footprint, great sound and is versatile with Line In & Out, then look no further, the miuaudio MKTP1 for $107 USD from the Miu-Tech store will meet your needs.
I would like to thank Ivan Lai for the review sample and to thank you for taking the time to read this review.
Constructive criticism is always welcome.