SE DC 6V6/6AQ5 HiFi Amplifier

LOW DCR SE Tetrodes with Feedback

This schematic represents a directly coupled SE 6V6/6AQ5 amplifier with local and global feedback.


The power supply filter is a LOW DCR LCLCRC type featuring Triad and Edcor chokes.

All chokes are 21 ohms  DCR (direct current resistance) or lower. HiFi but not Ultra Fi in terms of DCR.


The rectifier is a 25 amp SS FWBR.  The correct B+ for this circuit is derived by adjusting a variac.


Basic circuit measurements are listed below:


THD @ 1W = 1% , 2W = 2% , 3W = 4% before clipping. 

THD measurements were taken at 1KHz.

Bandwidth @ 20Hz = -.6 dB , 20KHz = -2 dB

OPT : Heyboer 5K/8  14W

Construction of the prototype on a simple wood breadboard.


This breadboard project amp is currently installed in my Altec horn system. 

Subjectively, the sound is very good. 



DT 667

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Screen Grid Regulator

Shunt Voltage Regulator with Zener Diodes

This is a simple shunt voltage regulator for the screen grids of SE DC 6V6/6AQ5 finals.  


There are a series string of (3) 1N5380 120V 5W Zener diodes bypassed by a 22uF cap from g2 to ground.  The 2k ohm current limiting resistor drops 40V from the B+ node.


I have the regulator circuit wired up and ready for preliminary testing.


This circuit has been published on Audio Asylum Tube DIY forum with a few guys leaving comments about using glow tubes as voltage regulators.  


That is an idea I tried a lot in the past, but I don't have enough room for more tube sockets on my small format breadboard, so solid state components have been utilized.


Update: 6/28/2019 - the regulator circuit had its initial power up. Dropping about 20VDC across the 2k ohm current limiting resistor.  The screens now sit about 10VDC below the plates on each channel.


Now for some subjective listening on my Altec horn system.


After a long and miserable week at work, much needed relaxation tonight.


DT 667   June 2018