Home SET Asylum

Single Ended Triodes (SETs), the ultimate tube lovers dream.

[ Follow Ups ] [ Post Followup ] Thread: [ Display  All  Email ] [ SET Asylum ]
[ Alert Moderator ]

III. Output Stage

Output Stage

I have chosen to operate the 2A3 at its maximum rated dissipation of 15 watts. To operate it at lower dissipation, plug in a 5Y3 rectifier to reduce the voltage! The indicated Hammond 125ESE is a popular and inexpensive output transformer; its inductance of around 10 henries is marginal but works well enough for most audiophiles. There are many superior output transformers of 2500-3000 ohms impedance which can be substituted with no other circuit changes.

I've shown a 220 ohm grid stopper. The value is flexible, 47 to 10,000 ohms having been used by others – but don't leave it out, the 2A3 has pretty high transconductance and oscillates easily. Carbon composition is the gold standard for grid stoppers, but there is some evidence that metal or carbon film are acceptable and may be more robust, stable, and easier to find.

I chose a 25 ohm hum pot partly because Radio Shack has had a decent wirewould rheostat of that value for decades. This should be wirewound or other bulk metal because of the high currents flowing through it; other materials are prone to be noisy. You can dispense with it by using the 2.5v transformer's center tap, but in my experience that will produce too much hum unless you have an exceptional 2A3.

The cathode resistors are shown with what I think is the minimum power rating for reliable operation. They will get hot! Note that the popular 25-watt resistors in finned aluminum housings are meant to be bolted to a heat sink of 25 square inches – that's a LOT of chassis area and it will get pretty hot. I prefer the old-fashioned ceramic core wirewound with some air circulation to cool it.

The cathode bypass and ultrapath caps have been discussed with the power supply.

The optional meter shown should be set for a zero indication, so the voltages are the correct fraction of the total. This gives pretty accurate operating points over a wide range of actual power supply voltages. A zero-center meter of +/-1mA sensitivity will cover an error of roughly +/-100v. You can just use a regular voltmeter for periodic trimming. Be aware there are high voltages on both terminals; I don't recommend bringing these connections outside the amp!

This post is made possible by the generous support of people like you and our sponsors:
  Schiit Audio  

Follow Ups Full Thread
Follow Ups
FAQ

Post a Message!

 Forgot Password?
Moniker (Username):
Password (Optional):
  Remember my Moniker & Password  (What's this?)    Eat Me
E-Mail (Optional):
Subject:
Message:   (Posts are subject to Content Rules)
Optional Link URL:
Optional Link Title:
Optional Image URL:
Upload Image:
E-mail Replies:  Automagically notify you when someone responds.

[ Top Top of Page ] [ Contact Us ] [ Support/Wish List ] [ Copyright © 2024, Audio Asylum, all rights reserved ]
[ General ] [ Speakers ] [ Tubes ] [ Vinyl ] [ Digital ] [ Hi-Rez ] [ Video Asylum ] [ Cables ] [ Tweaks/DIY ] [ Music ] [ Films ]