DIY 26 Tube Preamp

I have almost completed my 26 tube preamp. I only have the RCA connectors and volume control attenuator to mount, before it can be fully tested. It is currently using a DIY step-ladder attenuator that would be replaced after the testing is done.

This project took quite sometime since it went through three iterations - the final version comes with Coleman's regulators for the filament supply, K&K Audio's HV Regulator for the B+, and Prometheus Audio OPT. There is a very long thread at DiyAudio that influenced the final design.




I am keen to meet-up with fellow owners of 26 tube preamps in Klang Valley for comparison and exchanging notes. I think mine would be ready in about a week's time for more serious audition.

Pls PM or email via mengtaihoi@gmail.com to exchange notes.

I have finally boxed and tested the 26 preamp,a nd is now driving my 845 tube amp. It replaces the existing 76 tube preamp.

After about 20 hours, the 26 is living up to all the positive comments from the builders in diyAudio - smooth, good soundstage and tonality.



If there are any interested in building your own preamp, PM me for all the schematics and bill of materials.

Good job!

What's the amp at the back? 211 / 845?

PA Opt? I'd recommend Lundahl Opts. Sure you'd be surprised on how good the 26 can be. I've some PA OPT and IT and I wish I'd never ordered them.

Ken
http://www.diy-audio-guide.com

Hi Lhgin,

The amps at the background are my DIY 845 with 12at7, 2A3 driver tubes. It uses a Hashimto OPT (10K) with Prometheus Audio's chokes and power and filament transformers.

I have used PA's OPT for another 2A3 build (nano-C cores) and they are quite OK. The 26 OPT is also a nano-C core OPT. I personally thought PA's nano-C core OPT are quite good value for money.

I have used several other OPT brands - Sophia Electric, Hammond and Hashimoto. They all have their own unique sound. I also have a pair of Lundahl PP OPT that I would use in a 2A3 PP project that I am starting next.

Cool! I dare not really touch anything with >1kV. Kudos to you!

It would be cool if I can visit you sometimes in near future when I go down South.

Hi Mthoi,

Admire your passion and talent to diy the 845. I running on 300B set and would love to own 845 in future.
Wat's that coke can doing under the TT base?

Lhgin - most welcomed to visit me. I am reachable via 012 2236517.

7810 Sam - If you wish to build a 845, I am most willing to assist you in your DIY efforts. The coke cans with golf-balls on-top is used to decouple the TT. I learned from Foo MH (Lenco guru).

I have experimented squash balls, with different brands of golf balls and different cans (cambell soup, carlsberg beer (drank them), chick-peas, etc.). Cant hear much difference.

Hoi,

U r still the most productive Diyer here!!

So u had move the 845 system to a room instead of in living room right?

Hi Mthoi,

Me too a big fan of DHT preamp with transformer output. Planing to built a 26 preamp with Lundahl line output. Perhaps I can get some idea from you.

Which shematic are you using?

Cheers,
Cheah

sss333 - good to hear from you again. Should catch-up some time, call me if you are keen to meet-up.

Noodle88 - Have a look at this URL, I followed the core schematics.

http://www.bartola.co.uk/valves/dht-pre-amplifier/26-dht-pre-amplifier-gen2/

I used a 5u4G rectifier tubes (instead of a AZ1), K&KAudio HV Shunt regulator (instead of Salas) and Prometheus Audio OPT (instead of Lundahl). Nicholas - PA also shielded the OPT with mu-metal. The layout of the 26 tubes are shielded from the power transformers and indusctors by the OPTs. Use of Coleman regulators are a must for the filament supply. Regulated B+ would also help to reduce B+ noise.

I build the 26 in a single chassis and did not have the hum or microphonics that some diyaudio builders encountered (hence 2 chassis, maybe different schematics?). In part, I also decouple the power transformer and inductor with rubber washers and blu-tac.

Happy building. Contact me if you need more help.

After 80+ hours on the 26 preamp, I can only say positive things about the improvements in soundstaging, musicality and smoothness to my existing system.

And it is not that expensive to build - I think I spend less than RM 3K for all the parts (audio note resistors, tubes, obbligato film oil, OPT, regulators, etc.).

Very nice system. mate!

Hi Mthoi,

Yes, me too following Ale's posting. Planing to get the lundahl LL2745 and some powertrans from Thomas Mayer. I will follow Ale's schematic closely, preferably with tube regulator. I also bought Coleman's DHT filement regulator earlier on.
Are you using filament bias on your 26 pre ?

But all this only materialize after I completed my 71a preamp project.

If got time, do meet up for tea + Hifi talk.

Cheers,
Cheah

Hi!

After receiving some parts from Thomas, I can start building my 26 DHT preamp now.





The PSU circuitry is basically same as the 71a preamp. For this Black Gate NH series capacitors are being used instate of oil cap. 

Cheers,
Cheah

Cheah,

Cool... how much kastam tax you kena for it?

Ihgin, 

This round I'm lucky enough to be free from tax !!

Hope the same with my 3 other parcels which are on their way...

Cheah

wow... damn lucky!

good luck on your other parcels!

Great stuff - Cheah.  I am glad that you are building a 26 tube preamp as well. This is a very nice sounding triode preamp.

Which schemtics are you adopting for the build and which LL OPT are you using?

Hi Mthoi,

Basically I'm following Ale's 26 gen 3 circuitry. But with all passive filtering, LCLCLC on B+,
LCL on filament. Will also use filament bias. This time I'm trying BG NH series capacitor for B+ filtering.
Planing to put a 100k step ladder pot for initial stage, eventually I will change it to Slagle AVC. For sure I will put the AVC after the LL2745.

Cheah

This basis of my diy also.  The changes are in using K&K Audio's B+ regulators and Prometheus Audio's OPT.  I used DACT 100k attenuator after the OPT.



I also changed the 10R filament resistor to an Ohmite brown devil 100W (from a dale 50W wire wound - refer to pics).  They are about 8inches long.  That was also a significant sound improvement.  They were from RS Electronics - consider putting a very high quality resistor here.

I have not tried a TVC - maybe at some point I would revisit them.  I was very close to getting a signature TVC from Nicholas before I build this preamp.  After I listen to this preamp, I cant think how a passive device would sound better. Maybe a 26 with TVC that you are planning?

My experience with starved filament was not very successful -  It sounded a bit slow and lazy; good for slow music and vocals. I prefer it better when operating fully at 1.5V.

I also tried various B+/anode voltages (130v - 170V).  I finally settled on 150V and left it there.

Give me a call if you would like to meet-up or to compare notes.  Good to catch-up sometime.

Hi!

Yes, 26 preamp with AVC after the LL2745. These AVC are low impedance, they are design to be put after line output transformers. 
I'm using a pair in my current 12a preamp with good result.
When you have time, do come to have a listen to this 12a preamp. It's have something to tell.

Cheers,
Cheah

Hi !

Some update of my 26 preamp.

The PSU top plate is done with 5mm thick aluminum.





PSU bottom view, Its very similar to my 71a preamp PSU. But this time I'm trying Sikorel capacitor for filament filtering. As for B+, BlackGate NH capacitors will be used. These capacitors are part of my collection.


Another view


Cheers,
Cheah

Hi Cheah,

Very nice! And all the premium parts.  What rectifier tubes would you be using?

Reg.
Hoi

Hi Mthoi,

This one also using 6ax4 TV damper as rectifier.

Cheah

Hi! 

some update of my 26 preamp PSU.







Cheah

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