DIY KT66 PP amplifier

This is a DIY KT66 PP amplifier that I built two years ago. The schematic is based on British Leak's products. The driver and phase inversion section uses Leak TL50+'s design with EF86 as the driver and 12AT7 as the phase splitter. The power amplifier section uses TL12 Point One's design. I tried using GZ34 as rectifier and finally decided to use 5R4 instead.

The power and output transformers are Chinese made. KT66s are Chinese made too. Can't afford to buy Genelax used or NOS ones. The sound is just so so, a bit sloppy compared to my another EL84 PP. I like the look though. I will replace the Chinese made output trannies with a pair of Japanese Tango when I have time. But it's not likely to happen this year.

JBL 4310s

Got a pair of JBL 4310s from eBay a couple of years ago. They were in poor condition when arrived. A cabinet was cracked during shipping. The grey surfaces were painted in black ugly. One 2212 had rubbed coil and I finally had to replace it with a spare 123A-1. Despite the poor condition, they sound quite nice to my ears. Compared to my bass shy Klipsch Heresy, these 4310s can render more details and of course have better bass.

When driven with my DIY KT66 PP amplifier, the combo can handle rock music much better than the KT66 with Heresies. I can finally feel the energies of Red Hot Chili Peppers, Pink Floyd, Jamiroquai, Jimi Hendrix, and even Metallica from the combo. Although far from satisfactory, these 4310s seem to have more potential than the Heresies.


Tango U-405 PP Output Transformers

Ordered a pair of Tango U-405 PP output transformers for my next project, KT66 / EL34 push-pull mono blocs. I planned to use the schematic of Leak TL12 point one as my reference. May change the gain stage from EF86 to 12AX7 or ECC85.


In love with Joanna Wang

The other day, my wife borrowed a CD from one of her colleague and told me this girl, Joanna Wang was one of the hottest singer song writer from Taiwan. At that time, I mistoke this Joanna Wang with another similar singer called Susanna Wong and thought, 'Susanna Wong is actually not that good.' Of course I noticed I was wrong when looking at the CD cover. Joanna's debut album, , is so damn good. Her voice is so devine that can easily pierce through one's hearth. There are a few cover songs in the album, including Spandau Ballet's greatest hit, True. Check her out from her official website: http://www.sonybmg.com.tw/pop/joanna/musicplayer1.htm


Modifying crossovers of my Heresies

I just read some posts from Klipsch forum that it's about time to change those capacitors in Klipsch Heresy I. These posts made my heart itchy that I finally bought 4 Jensen 2.2uF 100V from a locally tube audio supply shop. After I put in those Jensen, I didn't realize there's any audiable difference. Damn! "Maybe the caps need some time to run-in..."

The owner of the shop also encouraged me to own a pair of Tango U-808 output transformers. The opinions on the web also confirmed the quality of these trannies. So, may I will get one next month...


My single-ended EL34 tube amplifier

Since I was addicted to the sound produced by the cheap Motorola singled ended EL84 console amp, I decided to build an SE amp with more power. At that time, I had a few Chinese Shuguang EL34s on hand, I decided to go for an EL34 SE.

The chasis was made in China, purchased from a DIY supply shop in Sham Shui Po, Hong Kong. All trannies were Hong Kong made. Dirt cheap. I thought, "This EL34 is just a start. When I have gained more experience, I would eventually build a 2A3 SE."

Power tranformer:

Output transformers:


Trannies installed:

Filament leads soldered:

The outlook seemed not bad:


With tubes

These were the parts take from another tube amp:

It didn't take too long to do the soldering:

At first, 12AT7 was used as the gain stage, with 36K plate load resistor:

It could also take KT88. Of course not optimized for that:


Repairing Ampex Preamp

This Ampex preamp was obtained from eBay. It was normal when I got it. But one day when I switched it on, no sound was coming out. Since then, it was placed in the corner. As I am unemployed these days, I got time to take care of it.

Yesterday, I took it out and examined it for a whole day, did some stupid wrong things before I figured out that the power transformer was blown. And my cat made me dropped and killed my Sanwa multimeter. I got to buy a cheap Chinese made digital AVO for replacement.



So, after having a good experience with my Motorola EL84 SE, I decided to deconstruct the Mr. Liang's KT88 and use the components to build a new SE amplifier. The parts that I could use were the Jensen coupling capacitors, Jensen electrolytic capacitors, some 6DJ8s, EL34s, KT88s, 6L6s, ALPS potentiometer, RMG resistors, etc.

I then bought a pair of 20W EI 2.5K / 3.5K local Hong Kong made single-ended output transformers, an EI power transformer and a Chinese made chassis. That was my second SE amplifier with no negative feedback. I tried using paralleled 12AX7, 1/2 12AT7, 1/2 6DJ8, 6C45PI to drive the power tube including KT88, EL34 and 6L6.

The results were not very satisfactory. Then, I swapped the pair of trannies with a pair made by a Chinese tranny guru. Since then, I started to realize how important the role of the trannies is in a tube amplifier. The difference was so obvious that the machine sounded completely different! What an open sound stage that was.

At that moment, I was using the 6C45PI with a lot of driving current. The only thing that I didn't satisfied was the dynamic.

Therefore, I started thinking maybe one stage of amplification was just not enough to drive the power tube good. With this idea in mind, I changed the driver stage from one to two, using 1/2 6DJ8 to drive 1/2 6CG7 and then coupling to the power tube. And the result was just good. And that is the circuit I'm still using at the moment to drive my pair of Klipsch Heresy. And the power tube that I like most in this machine is KT88.

I got two pair of 6B4Gs in hand and the project would start in the near future. I have quite high hope towards 6B4G.

Push-pull vs Single-ended

Before I used the Mr Liang's KT88 PP amplifier, I was using Musical Fidelity A2 to drive a pair of Dali Royal Menuet. I didn't satfisfy with the bass performance of MF A2. I thought the KT88 PP could give me better bass response but it didn't.

After listened a while, a net-friend motivated me to do some mods. He said the output transformers of my amp were the weakest links. As a newbie, I didn't believe him. And I decided to do my own way. Firstly, I removed the PCB and completely rewired the machine with point-to-point soldering, as I read from the web that point-to-point would make the sound better.



Secondly, as told by the net-friend that RMG carbon composite resistor was the best choice for tube amplifier, I changed all the resistors to RMG. Thirdly, all the red WIMA coupling capacitors were changed to Vitamin Q. The first phase of mod was basically done.


And the sound... not much different. Why?

Then, I suspected the stock Chinese tubes were not up to par. So, bidded some GE 6DJ8s and 6CG7 to replace the Chinese 6N11 and 6N6. The sound "changed". It's a little diffrent, but not necessarily in a good way. Was that because of the push-pull design?


In the mean time, I kept reading tube related websites, and started to have fantasy towards single-ended amplifier. And I acquired an ugly small SE Motorola EL84 console amplifier from eBay. Each channel used 1/2 6DJ8 to push an EL84. And the sound was just amazing! The mid and the high were just so seductive that just blowed my KT88 off.


From that moment, I was convinced that SE was my cup of tea.


My tube journey started from a cheap Chinese KT88 amplifier

That morning, I logged on to the audiophile forum as usual and I watched the discussion about a Chinese manufacturer called Mr Liang's Audio. They said the KT88 push-pull amplifier was okay for a price under US$200. So, I bought it. Since then, I was addicted to vacuum tube!