[Mags]

Hi,

I'm in the process of building a 26 DHT tube preamp from scratch and I'm thinking of the options for volume control and source selection. Your autoformer based remote control system is interesting as an "ultimate VC and source select" option for me. I particularly like the availability of 7dB of gain in addition to attenuation as the 26 only has a mu of 8.3, so the VC can help boost low output sources and still allow me to transformer couple the preamp output to get a low output impedance from the preamp.

Are there any watchouts in regard to using the TAP system with a DHT active preamp? Most of the applications i've seen are for passive preamps.

Any info to help me make my decision is greatly appreciated!

[John Chapman]

Hello!

Should work just fine. I have had lots of guys build actives with these things lashed in as attenuators - Jeffrey Jackson uses them in all kinds of DHT custom builds he does for guys. Only downside is the darn cost..... Feel free to call to discuss specifics of exactly what bits would suit your project best. I am at (604) 560-2959 much of the time.

Thansk!

John

[Mags]

Thanks for the reply, John. The cost is not an issue if they are truly the superior option.

Two more questions if you would.

I'm using grid bias with a 9V battery - since these are autoformers I will still need the cap to block the grid DC, correct? As I understand it if they were transformers they would block the DC without the need for a cap.

What is the range of input impedances as the volume control goes from low to high?

Right now i'm still in the building phase and will initially work with a stepped attenuator I have on hand, but once all the parameters are optimized I'll be looking for a top notch remote-control solution.

[John Chapman]

Hello!

Yes - you will need to block dc feeding them. Unless designed specifically to take dc on the primary (not sure if I have seen any designed like this) even TVC's would also need to see no DC feeding them.

Dave Slagle would know the input impedance question exactly but basically I like to keep feeding impedance (from the source) under about 1K or so. It's pretty rare to find a source much higher than that anyway so it is easy to live with that spec.

Have fun with the project!

John

[Mags]

Thanks for the info, John!

I'll be in touch after I'm done shaking everything down.

[happyrabbit]

DHT 71A.....  Who's ur daddy    

Dwight

[John Chapman]

Hello!

Awesome and thanks for that pic! I had not seen one built into those boxes yet - nice fit and looks great.

Have fun with that one!

Thansk!

John

[Mags]

DHT 71A.....  Who's ur daddy    

Dwight

Looking good, Rabbit. Mine will be in a copper chassis I'll bend up from .08" sheet copper, with 0D3 and 0A3 glow tube regulators for each channel fed via CCS, with CCS on the 26 plates, Rod Coleman filament supplies and battery grid bias. Output via Lundahl Iron (probably LL1676 in parafeed with Duelund caps).

Plan to build the pre on a copper subchassis with shock-mounted teflon sockets to tame the 26 microphonics. Have all the parts, now time to start building the prototype.

[Mags]

So I finally Completed the 26 Preamp! Here it is:

[Mags]

And an Internal Shot of the Preamp box:

[mgalusha]

Mighty sweet looking. 

[harry604]

Hi MAGS and MGALUSHA, congrats to your DHT 26 preamp. Very well done! I am looking to build one as well, and consider the design from Kevin Kennedy (Sound Practices on-line article). What I would like to implement is a Autoformer Input rather than the 100k potentiometer. You both seem to use the same autoformer, hence my question: Can you share your design considerations and experience with me? Do you have schematics you can share?
Thanks a lot.
Cheers,
Harald

[JoshK]

Since this thread has come up.  I have yet to start (but plan to soon), my preamp.   I have a fully balanced version of the TAP preamp in the larger chassis and plan to marry it with Kevin Kennedy's  Raleigh line stage using the balanced AVCs in lieu of the ladder step volume control. 

My best guess for configuration would be like this:
Input selection controlled by TAP >  input transformers (Raleigh) > Balanced AVCs > Active tube stage (Raleigh) & Transformer coupled output.   However, one could put the AVCs before the input transformers or even after the output transformers.    Not sure which is best, but I am leaning toward after the input transformers as above. 

[harry604]

i am not familiar with the Relaigh preamp from Kevin. What I know is the DHT 26 preamp, which does not use input transformers but a 100k pot input to the 26. since I saw the two preamps above using both 26 DHT and the Autoformers from Bent, I was asking how they wired up the inputs.

[JoshK]

Sorry man, didn't mean to make out like I was answering your question (I wasn't), just sharing that I am on an analogous quest.

[harry604]

...good to know we are on a similar quest ... could you pls point me to the source of the Kennedy Raleigh preamp you mentioned? I can´t find any reference of it on Kevins Homepage. Thx.

[JoshK]

http://www.raleighaudio.com/index.html

[harry604]

thx.