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You really need to do this :)

Posted by Jim Doyle (A) on October 24, 2005 at 13:30:18

In Reply to: Re: 6B4 in an ST-70 - YES - YES - YES posted by zacster on October 24, 2005 at 09:23:18:

What you need:

1. A Quad of Sovtek 6B4Gs
2. A pair of these resistors:
http://www.hndme.com/storehuntington25watt.html
3. One extra, outboard 6.3V filament transformer (6V 3A)

You need the extra filament tranny to power the filaments of your
6DJ8's.. Why? First off, the DHTs are going to be cathode biased
at center tap for about 60V. Second, signal current is going to be
riding the filament current in common mode. So, the 6DJ8's need
their own isolated power source. DHTs want their own dedicated winding, all the time. Since you got 4 6DJ8's, you need about 2A.
You might look for a 6V wall wart to do this job nicely!

The EL34 sockets will need zero modification. Unlike EL34 triode
wire modes, there is no unsoldering and resistor action for the
6B4G.

No additional bypass cap is needed for your cathode bias R. Why?
You are interested in Class A1. The only reason for putting a capacitor there would be for doing Class AB. Why in the world would you want to do Class AB with DHTs ?

If you can dial down your fixed bias close to 0V, you may not need
to unsolder the -Vg supply from the grid leak/coupling capacitor network on pin 5. Otherwise, you will have to disconnect the -Vge
wire. (no biggie)

Sovtek 6B4Gs are rated for 450V, so they should be OK with a Sovtek
6B4G. If you use a 5AR4 and the typical 430V of the Dyna ST-70 B+
supply, then use a 750 ohm cathode R. This runs the 6B4G at 50mA.
If you use a 5U4GB, the ST-70 supply is going to give you 370V.
In this case, use a 560 ohm cathode R. Dont use a NOS 6B4G unless
you are at 360V B+ or less! The Sovteks are good stuff.

-- Jim

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