How to wire the MKIII in cathode bias

by Wieslaw Lipowski Sat Nov 21, 2009 6:20 pm

Wieslaw Lipowski: Posts : 6
Join date : 2009-05-21

Post n°1

How to wire the MKIII in cathode bias

by Wieslaw Lipowski Sat Nov 21, 2009 6:20 pm

I would like to get rid of the fixed bias setup in my old MkIII's and wire them in the cathode bias instead. I hope in this way I won't have problems with setting and readjusting the bias anymore.

I am thinking of using 1K/5W resistors and 220uF/160V (orange Rubicon HSF) electrolytics at the cathodes, and 470K grid-leak resistors. The B+ is stock 480V.

What do you think? Is this worth doing sound and safety wise?

Thanks in advance
Wieslaw

by **Bob Latino** Sun Nov 22, 2009 2:53 pm

Bob Latino

Bob Latino: Admin; Posts : 3276
Join date : 2008-11-26
Location : Massachusetts

Post n°2

Re: How to wire the MKIII in cathode bias

by Bob Latino Sun Nov 22, 2009 2:53 pm

Wieslaw Lipowski wrote:I would like to get rid of the fixed bias setup in my old MkIII's and wire them in the cathode bias instead. I hope in this way I won't have problems with setting and readjusting the bias anymore.

I am thinking of using 1K/5W resistors and 220uF/160V (orange Rubicon HSF) electrolytics at the cathodes, and 470K grid-leak resistors. The B+ is stock 480V.

What do you think? Is this worth doing sound and safety wise?

Thanks in advance
Wieslaw

Hi,

Before you scrap the Dynaco Mark III bias system are you sure that there is not some issue with the stock bias system that can be easily dealt with? If your bias is not stable then I would consider trying the following first before you make the switch to a cathode bias system ..

1. Bias does go up and down as your line voltage changes. Check for variations in your line voltage.
2. A bad rectifier tube can allow the bias to wander a bit. Try a different rectifier tube.
3. A bad selenium rectifier can cause a wandering bias. Most people replace the stock selenium rectifier with a diode.
4. A bad 50 uF cap between the "-" terminal and the second lug on that three lug terminal strip can cause bias issues.

Bob

by Wieslaw Lipowski Sun Nov 22, 2009 3:27 pm

Wieslaw Lipowski

Wieslaw Lipowski: Posts : 6
Join date : 2009-05-21

Post n°3

Re: How to wire the MKIII in cathode bias

by Wieslaw Lipowski Sun Nov 22, 2009 3:27 pm

Thanks so much for your response.

The stock bias system is ok, the voltages are stable. I just want to give my old Dyna MkIII's to a friend who does not know anything about electronics, valves or high voltage, and who lives far away from me. I doubt he even has a voltmeter or something. I consider MkIII's with cathode bias a good idea and a safe way for him to run the amps.

I once saw Jadis EL34 and KT88 amps with cathode bias with 510k grid leak resistors, 500R and 200uF/100V cathode caps. I had an idea I could do the same to MkIII.

Kindest regards
Wieslaw

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