OTL Asylum OTL, Output Transformerless Amplifier User Group.

---

[ Follow Ups ] Thread: [ Display  All  Email ] [ OTL Asylum ]

---

[ Alert Moderator ] [ Edit ] [ Delete ]

---

Re: PS transformer made for an OTL?

0

Posted by Paul Speltz on June 2, 2001 at 21:32:24

In Reply to: PS transformer made for an OTL? posted by Dan on May 30, 2001 at 21:46:31:

I have mentioned this idea to both Mark and Dan in the past, and now I will pitch it out here for fun. I don't know how well it will work, but I want to try out the concept soon. It is a two tube per channel OTL that can drive about 20 watts into about 40 ohms. One gain stage tube which can be a 6SN7, 12SX7, or like Dan said a 6H30pi with a fair amount of current running through it. Plus one 6336A/B in a circlotron output stage.

Here is the twist. Take the Voltage Strap idea of the Super Linear Cathode Follower (SLCF) circuit and apply it to the circlotron triodes. The voltage strap is a HEXFET supplying the plate with voltage that follows the movement of the cathode. I think that this will remove the need for the Cascode gain tube and the Cathode Follower driver tube. Here is why.........so I think:

1) The Voltage strap will drop the 6336 "idle" plate voltage (maybe even in half) which will allow the 6336 grids to be biased at -10 to -20 volts instead of the typical -30 to -40 volts. Now not as much voltage swing is required at the grids of the 6336. So not as much gain is required and the cascode tube is not needed. The Voltage Strap on the output triodes allow for a much lower plate voltage, yet keeps the tube from getting voltage starved when outputing a lot of power. The Voltage Strap can also be adjusted to set the amp's bias and balance.

2) With the cascode tube removed, the gain stage has much lower impedance especially with maybe 10mA instead if 1.5 mA of current. This along with the fact that 6336 are not specified to go into grid current, there is only 1 grid to drive, and grid capacitance is nilled because both the cathode and the plate are following the grid, leaves me to think a CF driver tube is not needed as well.

I am most interested in seeing if the Voltage Strap affects the output impedance of the circlotron. I am hoping it will lower its impedance. Mark warns that I will loose linearity running lower plate voltage, I am hoping I will gain linearity with the addition of the Voltage Strap. After all the Voltage strap is half of what is called a super LINEAR cathode follower circuit.

Dan's 12 ohm speaker impedance will need to be multiplied by 4 with the "ZEROs" autoformers to look like 48 ohms. Of course the ZEROs will act as a DC current bypass (0.33 ohms DCR) in the event of a 6336 short and protect the speakers.

I have mounted two IRF730 HEXFETS and an octal socket to an aluminum plate with hopes of testing the concept some time. I think it will work well.

Paul Speltz

• SLCF Circuit. Voltage Strap is the top IRF730   (Open in New Window)

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

VH Audio

---

[ Top of Page ] [ Contact Us ] [ Support/Wish List ] [ Copyright © 2025, Audio Asylum, all rights reserved ]

---

[ General ] [ Speakers ] [ Tubes ] [ Vinyl ] [ Digital ] [ Hi-Rez ] [ Video Asylum ] [ Cables ] [ Tweaks/DIY ] [ Music ] [ Films ]

---