Stereo Single Ended KT88 triode/tetrode amplifier

[Karel Mars]

Where did you find those covers Karel?

transformer cover

 

[Karel Mars]

After a week of listening to the SET KT88, we decided that the sound could be bettered, but how? Tried 6J7 with 100K feedback resistor from KT88 anode to 6J7 screen grid. Improvement, but so-so. 6J7 in triode mode; also so-so. So the 6SL7 came forward as the next possible candidate. We contemplated wiring one 6SL7 stage as a cathode follower. The next development was to improve the cathode follower. A 6BL7 came to mind. Same pinout as the 6SL7, but much lower plate resistance and transconductance; much better for a cathode follower. The 6SL7 triode DC connected to a 6BL7 cathode follower works very good.

For the moment the SET KT88 runs with a 6SL7 triode voltage amplifier, 6BL7 cathode follower into the KT88 triode. The circuit also sounds great with 40% UL KT88 SE. (I have a switch next to the KT88 to change modes)



These 6BL7's look good!

 

[Karel Mars]

Emil helped me measure the SET on his beautiful audio lab. This design wil be at the expo too.

 

[kolakidd]

I can get these covers. Now to consider the potting compound!

View attachment 6030

@Karel Mars where did you find these covers?

I got some off Aliex a few years ago, but a more convenient source would be welcome.

 

[kolakidd]

Oi. Scratch that. Brain dead Saturday morning me didn’t see that there was a page 2 with the exact same question.

 

[Karel Mars]

The client took delivery of the SET and indicated that they are happy with the product.

Now we are building #0004 SET with rounded transformer covers!

A question: What type of SET would you like to see at the Expo in October?

1. KT88 or other beam tetrode type with tetrode/triode switch
2. DHT type with 2A3
3. DHT type with 300B
4. DHT type with 811A

 

[Family_Dog]

That looks great, Karel, well done!

I'm a KT88 lover, so there is my vote

-F_D

 

[Stefan]

I have no real experience and, therefore, no informed preferences, but why not a Kt150? I have considered a few new suppliers, and the KT150 seems to receive quite a bit of attention from designers and the market in single-ended designs. Plus it looks the part ...

 

[Karel Mars]

I have no real experience and, therefore, no informed preferences, but why not a Kt150? I have considered a few new suppliers, and the KT150 seems to receive quite a bit of attention from designers and the market in single-ended designs. Plus it looks the part ...
View attachment 6167

I had a look at the specifications of the KT150. Same impedance output transformer, but gapped for 120mA! That is a biggie!

If I roll outputs for KT150, they will stand about 150mm high.

 

[Karel Mars]

Here are some photos of the Expo amp build

 

[Family_Dog]

Looking very good Karel!! I suspect it sounds as good as it looks


-F_D

 

[Karel Mars]

Next up is an order for a design that is size constrained. Due to the specifics of the client's hifi rack, this one can only be 150mm high, feet and transformer covers included. The footprint has to be 360mm x 300mm.

After some calculations, it became apparent that no 300B is going to fit into that space. Only the KT88 fits (if we recess the tube socket with 5mm). If we allow 10mm for rubber feet, and 40mm for the chassis, it leaves us with 100mm on top. We decided to use 96mm laminations for the core. With covers the size is exactly 100mm.



This design will also have a mesh cage for the tubes, but tubes only. The transformers will not be in the mesh cage.

We found 35mm VU's that also fits the bill.



Updates to follow.

 

[Family_Dog]

Great stuff Karel, I love watching build threads and your designs are so immaculate!


-F_D