For some time, I have been reading about different preamp designs, and the idea of a 26 or other DHT design sparked my interest. I came across Kevin Kennedy's design posted about 4 years ago, and the excellent decription and design seemed the right way to go. Also, a friend of mine had Kevin design a type 50 amp and the design was very thorough. So, I contacted Kevin and had him update his design.
The description, in his own words, "I designed and built the original version of this pre-amplifier about 8 years ago when I could no longer contain my rampant curiosity regarding the possible performance advantages of DHT’s used for this purpose. My original intention was to build something simple and inexpensive to allow me to investigate the possibilities. The V3 version is the third iteration of the design and features some minor improvements in the audio design and a significantly improved power supply design. In addition this version now supports the direct use of 01, 12, and 26 family DHTs with just the flip of a couple of switches."
The most difficult aspect was deciding on and finding some suitable plate to line transformers. I initially obtained some UTC A-25s and also some Magnequest B7-15K permalloy transformers. I had thought that the difficult to find and expensive UTC HA-133 Permalloys used by Kevin were too expensive.
The unit was constructed by Rene Rivo and shown at the November Manila Audio Show hooked up to some terrific horns and also a DIY turntable and arm. My same friend liked the design so much that he purchased the unit witht he A-25s. So now, given how well everything turned out, I acquired a set of the HA-133s, The system sounded great and the preamp was dead quiet. Here are some pictures of the outside and guts, showing the design and neat implementation.
The description, in his own words, "I designed and built the original version of this pre-amplifier about 8 years ago when I could no longer contain my rampant curiosity regarding the possible performance advantages of DHT’s used for this purpose. My original intention was to build something simple and inexpensive to allow me to investigate the possibilities. The V3 version is the third iteration of the design and features some minor improvements in the audio design and a significantly improved power supply design. In addition this version now supports the direct use of 01, 12, and 26 family DHTs with just the flip of a couple of switches."
The most difficult aspect was deciding on and finding some suitable plate to line transformers. I initially obtained some UTC A-25s and also some Magnequest B7-15K permalloy transformers. I had thought that the difficult to find and expensive UTC HA-133 Permalloys used by Kevin were too expensive.
The unit was constructed by Rene Rivo and shown at the November Manila Audio Show hooked up to some terrific horns and also a DIY turntable and arm. My same friend liked the design so much that he purchased the unit witht he A-25s. So now, given how well everything turned out, I acquired a set of the HA-133s, The system sounded great and the preamp was dead quiet. Here are some pictures of the outside and guts, showing the design and neat implementation.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
Thanks for posting this Dana. I'm delighted to see how nicely it turned out. Wish my prototype was nearly as pretty.. 😛 Nice job..
I think the UTC HA-133 are near impossible to beat in this application, think you will like the result even more.
The prototype is the longest lived component in my entire system (going on 9 yrs) and the eventual replacement will probably be quite similar to the V3 shown.
I think the UTC HA-133 are near impossible to beat in this application, think you will like the result even more.
The prototype is the longest lived component in my entire system (going on 9 yrs) and the eventual replacement will probably be quite similar to the V3 shown.
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Nice work bro. Good choice of caps. Those m-caps rocks.
Kevin is the man for 26 Pre. His tips for 26 tube preamps are hard to beat.
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Hi, Anybody have a link or can post a schematic to version 3 or is that what is on the KTA site? I have the parts to build one, I was goint to use a shunt reg to feed each channel and use a current reg for the filament.
Daniel
Daniel
Hi Dan,
The very similar V2 version is available via Positive Feedback Online which you should be able to link to via my site. I'm not offering the V3 design free for public consumption at this time.
The major difference between the V2 and the V3 is in the support for several dht at the flick of a switch, however these types are also supported by the V2 design - it's just not designed for on the fly tube rolling between 01, 12, 26 types. Incidentally I am using the original V1 to this day! (It's very good, the newer versions are just a bit quieter and more flexible.)
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Hi Kevin and thanks for the reply.
I also like your 300b PP amp design; I first saw it in VTV many years ago. I like the way you design amps with regulated supplies; the best of old and new.
I actually have parts for many different projects but not the time to make them. I need to spend less time and effort getting parts and just start building with what I already have! As I get older, this becomes more and more obvious to me.
Daniel
I also like your 300b PP amp design; I first saw it in VTV many years ago. I like the way you design amps with regulated supplies; the best of old and new.
I actually have parts for many different projects but not the time to make them. I need to spend less time and effort getting parts and just start building with what I already have! As I get older, this becomes more and more obvious to me.
Daniel
Thank you for the gracious compliment, but all the credit belongs to Kevin for an as usual great design, and the builder, Rene Rivo, in Manila. I can do some building, but my skills come nowhere near his level. Everything he has done for me has been faultless and very reasonably priced.
Thank you for the kind words GTHICM! It was a pleasure building this for you! Our next one would be just as good, if not better! 😎
Well, how does it sound? I looked into this design myself, but it's beyond me to build it. I may start with just a stepped attenuator. I wonder what a well designed preamp does to the signal that a passive volume control would not - does it change the sound? I am very tempted to add a tube preamp to the chain just to experiment with it, but that seems like a rather expensive experiment...
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