Tube DIY Asylum: Valve Substitutions - DON'T.... by Thorsten Do It Yourself (DIY) paradise for tube and SET project builders.

For Sale Ads FAQ / News / Events

Posted by Thorsten (M ) on October 4, 2005 at 07:18:45

In Reply to: Latest version of my 7788/7721 LCR 600Ohm RIAA Stage posted by Thorsten on October 2, 2005 at 15:11:26:

Folks,

Let me absolutely clear here.

The circuit works the way it does because of using what works.

You can use the D3a or E810F in the second stage, if you get your wiring right you can swap them in/out, but the D3a has more gain, so using the E810F gets maybe 2db less gain.

The D3a in the circuit as shown biases to around 80V/16mA, which gives around 2K output impedance and a gain of 60 (35db) based on the curves.

In the first stage only the 7788/E810F has the required transconductance (if anyone knows a low noise small signal Pentode with > 50mA/V transconductance @ 30mA anode current pleae tell me).

The E810F is used into a 600 ohm load, with around 60V on the screengrid and around 70V on the Anode and around 30mA Anode current to get maximum transconductance, which should be in the 55mA/V region. That gives a total gain of the first stage 33 or around 30db.

Using a valve with less transconductance like the D3a or E55L rapidly looses gain, the actual measured Phonostage I have at hand showed 43db overall gain with less than 0.5db difference between the channels and a pretty flat RIAA (my 600R Modules are the first pair, current ones reputedly are much flatter, they limit the RIAA response flattness).

I hope those additions make clear what goes on.

Ciao T

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

Schiit Audio

---

[ 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 ]

---