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RE: DB110G (Bogen) 1950s.

Posted by ivan_terrible (P) on December 26, 2023 at 11:53:02

In Reply to: DB110G (Bogen) 1950s. posted by ivan_terrible on December 26, 2023 at 11:20:29:

So, to cut a long story short I decided on another upgrade.
Short out the cathode resistor to ground make up a mini PSU with a 110v-10-0-10 transformer and inject fixed bias into it.

Here was the old small anode 6V

And here was the westi 7408

The mod is suprisingly easy, just put a 120k pot across the resulting 30-31V DC and there you go.
As you would expect all you really have to do is imitate the original bias point - around -21>-22V and you have it.

I really wanted to see how much difference it made.

Well.
The difference was truly astonishing.
The power output literally double giving good clean power to well over 10W

Now that was the sound I have been expecting to hear out of old 6V6 amps.
So, if you are still fighting with distortion forget cathode bias.
Get it fixed bias, and if you feel really ambitious drive the grids with cathode followers/mosfets all the way into AB2*.

The 6V6 was made to run in fixed bias AB1 and also AB2 up to 10V positive on the grid.
If you do that*, halve the output transformer reflected load by putting your 8 ohm speaker on the 15V tap, and the 6V6 will be putting out nr 25W.

That of course would mean changing the mains transformer and increasing the reservoir capacitors.
Talking of which the number ONE reason why these amps eventually die are the ancient twist grip dual section electrolytics.

Ask Jan Philipp Wüsten about getting some decent replacements of those.

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