|
Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders. |
|
|
|
Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders. |
|
In Reply to: I am much fascinated posted by sser2 on April 28, 2017 at 15:41:30:
Thank you for your appreciation.
By the way, if you take a second look at the schematic you will see the GNFB which is taken from an auxiliary winding which is connected to the input cathodes (there are about 12 dB of GNFB).
As a matter of fact, the output transformers are the two toroids in the center of the second picture. They are standard 2X115V/2X6V 50VA 50-60Hz.
One of its outputs delivers the GNFB signal as auxiliary winding allows to utmostly decouple the GNFB signal from the non linearities of the loudspeaker impedance (if any).
In the schematic there is another trick: if you take a look to the 829B cathodes (3E29, GI-29 or GU-30 tubes were also successfully tried) you will see that 15V DC are connected through a voltage divider.
This allows to adjust the DC current balance of the output tubes by trimming the grids voltages around a few volt of nominal DC pedestal.
However, as there is only a single cathode per tube, I had explored two alternative procedures for monitoring the DC current imbalance (without using an amperometer).
You can either use your ear to trim the cathodes' voltage until you will no longer hear any residual hum (when using a small loudspeaker next to your ear) or alternatively look (and minimize) the on-screen line harmonics when using a PC equipped with some suitable spectrum analyzer SW.
As I recycled the case of an old tape mass storage of a SUN workstation (the grey metal case that structurally supports the wood panels) I had freely available 12.6 V DC for the 12AT7 heaters and a well filtered 15V DC for use.
In the latest picture (of the amplifier with four 829B tubes) there are two switches (even though barely visible).
They were introduced to select 14dB or 8dB of GNFB.
In fact, as I routinely listen to classical music in streaming via my phone, I needed those 6 dB of extra gain to reach 95 dB of ambient SPL by using my 89dB/1W/1m four way speakers.
Last but not least and for sake of completeness this is the ambient output spectrum (captured by the internal mic of my notebook) with the switch in the 14 dB GNFB position.
This post is made possible by the generous support of people like you and our sponsors:
Follow Ups
- RE: I am much fascinated - 6AS7_6SN7 01:21:01 04/29/17 (0)
Top of Page ]
[ Contact Us ]
[ Support/Wish List ]
[ Copyright © 2024, Audio Asylum, all rights reserved ]
