Kegworth Show Information

The information for this year's show is now live at www.thehifishow.com or by clicking on the show banner below.

NON EXHIBITOR ROOMS NOW OPEN - Special Offer....Stay Saturday, join in all the build up excitement on Sat night and then gain FREE access to the show on Sunday

LAST MINUTE WAM HIFI SHOW AVAILABILITY - ONLY A FEW EXHIBITOR ROOMS LEFT - ONLY 5 SLEEP ONLY ROOMS LEFT - ONLY 3 BIG SYSTEM SPACES LEFT

Brumjam

01A Preamp Builders Unite!

Recommended Posts

I've replaced the FT3 caps with some Jupiter cooper foil wotsits to see if I can hear a difference.  The new pair of 01A valves I ordered have also been installed.  I've still got more noise from the left channel so I will go through and check my wiring.  

Share this post


Link to post
Share on other sites

Since I made the 01A pre, I have been using the type 26 PSU with a switchable output.  But I decided to make a dedicated PSU, with matching birds eye maple veneer.  I simply copied the other PSU, so the voltages would be the same.  But being senile, I wired the umbilical socket like the plug; that is a mirror image!  When I got weird voltages on the Coleman filament regulator I did a continuity check and discovered my mistake.  Assuming I knew what I was doing was obviously wrong.  Luckily the HT fuses had been pulled, so that was OK, 115V at the anode.  I PMed Rod Coleman, who gave me a test procedure, and it was then that I realised I had been using the wrong value of resistor for the dummy filament.  Changing to the correct value (17 ohms)  gave me the right voltages, so no harm done.  Except to my self esteem!

I haven't connected it all up yet, but it should be OK now.  I just need to make a second umbilical cable, and then I will have two completely separate DHT pre-amps.

I just need to make a new PSU for my Concordant Excelsior clone and I will have 3 valve pre-amps.  Just because. :D

46764586212_e9b21a3e5d_c.jpg

  • Like 1
  • Upvote 2

Share this post


Link to post
Share on other sites
Super Wammer

Nice work Richard......which DHT pre do you prefer or is it a case of they just give different presentations? 

Share this post


Link to post
Share on other sites

I like the slightly softer sound of the 26, but the HF roll off in my room is a bit too much.  In a brighter room it would probably be fine.  The hum I get with the 01A, which is cured by using a TVC on the output as the volume control. makes a direct comparison difficult.  When I used the TVC on the 26 it softened the sound more than I liked, but it makes the 01A useable.  Now it has its own PSU I will be able to chase down the hum without losing the use of the 26. The 01A has too much gain for my power amp (1V for 250W output!), so any hum is amplified too much without the TVC on the end to attenuate it.

At the moment I am using the analogue pre-amp section of the Benchmark DAC, which is pretty good in its own right. Later today I'll probably connect the 01A and see what happens. :nerves:

Share this post


Link to post
Share on other sites

Hi Richard,

they look lovely, i've always had a penchant for a birds eye maple finish.

Share this post


Link to post
Share on other sites

Thanks.  I had a cheap local supply, so I gave it a go.  Just need to respray the lacquer on the pre to match the sheen on the PSU.

And it still works!  I just tried it; the only possible fly in the ointment is microphony.  If I tap the chassis I can hear ringing through the speakers. which may impact the sound at high volumes.  We'll see.

Share this post


Link to post
Share on other sites

Stunning work (as usual) Richard.  My 01A pre is poorly as a result of connecting my DAC output to the pre output instead of the input :doh:.  I’ve not checked what damage has been caused yet.  Hopefully whatever I’ve done is limited to the gyrator boards.  Lots of distortion on both channels.

In the meantime I’m enjoying my 4P1L pre which is a much better match for my VFET power amp.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Recently Browsing   0 members

    No registered users viewing this page.