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DHT preamp with 1920s UX-171A RCA Balloons.
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Author:  mwhouston [ 23 Mar 2020, 06:10 ]
Post subject:  DHT preamp with 1920s UX-171A RCA Balloons.

On an Oz forum a member was asking me if I would build him a DHT preamp from a schematic he had a link to. The schema had used an active load supplied by a FET. I felt the mix of sand and glass wouldn’t bring out he best in the DHT. He dropped the idea and I took it up.

I needed a schematic and Bruce of oddwatt fame (on a request) was happy to come up with a design. Nice and simple and no sand. Once I had the design I ordered some more Hammond wood cheek chassis and started collecting parts. Fortunately the preamp can use the same external PS I use for my 6SN7 preamps with some under the bonnet limiting resistors. This way I can swap out preamps and keep the one external PS. The tubes are 1920s UX-171A RCA balloons. They have nickel plates.

I’m currently build two very big SS power amps for others. A 300/500W Holton and a F5 turbo. Ones is 99% complete the other about 60%. Before parts started arriving I got into the preamp. See the images for an assembled but not wired preamp.

Finish is Hammertone blue, step attenuator and ceramic sockets. More to come.
Attachment:
A96CAE56-BB85-4E6C-9524-4597C2A50EF9.jpeg
Attachment:
7D246A9C-124D-445A-ABFA-0B0A4BD35AAE.jpeg
Attachment:
CDA6A9AC-ABB6-4D49-9E4B-978805134D3E.jpeg

Author:  Woodo [ 24 Mar 2020, 21:30 ]
Post subject:  Re: DHT preamp with 1920s UX-171A RCA Balloons.

:thumbsup: very pretty. Happy with how it sounds?

Author:  mwhouston [ 24 Mar 2020, 21:34 ]
Post subject:  Re: DHT preamp with 1920s UX-171A RCA Balloons.

Woodo wrote:
:thumbsup: very pretty. Happy with how it sounds?

Not wired.

Author:  ILoveHiFi [ 25 Mar 2020, 02:23 ]
Post subject:  Re: DHT preamp with 1920s UX-171A RCA Balloons.

Stepper pots with hifi resistors are deffnatley good start to any hifi pre amp biuld

Author:  mwhouston [ 25 Mar 2020, 02:30 ]
Post subject:  Re: DHT preamp with 1920s UX-171A RCA Balloons.

ILoveHiFi wrote:
Stepper pots with hifi resistors are deffnatley good start to any hifi pre amp biuld

Agree.

Author:  Woodo [ 26 Mar 2020, 01:24 ]
Post subject:  Re: DHT preamp with 1920s UX-171A RCA Balloons.

mwhouston wrote:
Woodo wrote:
:thumbsup: very pretty. Happy with how it sounds?

Not wired.
my bad, should have read more carefully!

Author:  mwhouston [ 26 Mar 2020, 01:27 ]
Post subject:  Re: DHT preamp with 1920s UX-171A RCA Balloons.

Woodo wrote:
mwhouston wrote:
Woodo wrote:
:thumbsup: very pretty. Happy with how it sounds?

Not wired.
my bad, should have read more carefully!

Working on two big power amps for others. Should get time soon.

Author:  mwhouston [ 06 Apr 2020, 21:50 ]
Post subject:  Re: DHT preamp with 1920s UX-171A RCA Balloons.

Got the preamp finished about four days ago but it had a terrible hum. Tried different earthing arrangements but nothing worked. Then moved the 24 step stepper to the output after the output cap and 90% of the hum dropped away.

Extremely transparent presentation definitely worth the effort.

Specs;

HT = 100V
Vf = 5V
Ip = 2mA
Bias = 3V
Attachment:
323B8FB5-37A7-4EF4-8953-14D18288E5F9.jpeg


Ceramic sockets, 1uf PIO output cap, 24 step stepper.

Author:  mwhouston [ 06 Apr 2020, 21:58 ]
Post subject:  Re: DHT preamp with 1920s UX-171A RCA Balloons.

Schematic;
Attachment:
A8E2EDD7-FA98-4ECA-BE96-5003D03B84EC.jpeg

Author:  Woodo [ 07 Apr 2020, 15:54 ]
Post subject:  Re: DHT preamp with 1920s UX-171A RCA Balloons.

Very nice. Would a ground lift switch from the 4s preamp help the hum?

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