Pavel's Auto Bias on Dynaco Mark III

Question ---- has any body installed Pavel's A.B. Board on a Dynaco Mark III Amp and did it require an 1:1 isolation transformer?

Please let me know.

I think for the safety of the amps you’re going to need one for each.

stewdan wrote:Question ---- has any body installed Pavel's A.B. Board on a Dynaco Mark III Amp and did it require an 1:1 isolation transformer?

Please let me know.

I asked Pavel this question. His answer was "The original Dynaco do not have a DC filament connection to ground. They use a capacitor, which does not matter when connecting the AB module."

I hope this helps.

Hi Bryce  --- thanks for the reply, so you used his AB Board on a Mark iii?  

Which of Pavel's AB boards did you use, the rectangular shaped one or the one with the funny shape that sort of hugs the outline of the driver board??

Did you used the original Dynaco driver board or something else? I know you can used his AB board with the original driver board.

Please let me know,
Stew

Hi Stewdan,

I have not installed Pavel's AB board on a Mark III.

I asking Pavel specifically about the Model AB-D-MKIII and Erhard Audio's comment regarding it: "It is important to note that the AB-D-MKIII MUST use an isolation transformer, as the Dynaco MKIII's filament supply is referenced to ground." Again, Pavel's answer was "The original Dynaco do not have a DC filament connection to ground. They use a capacitor, which does not matter when connecting the AB module."

Pavel's answer is exactly the opposite to Erhard Audio's comment. Take your pick.

Bryce --- for what it is worth, Pavel responded to some questions via a message and sent me this picture of the AB-D-Mkiii and the original dynaco Mkiii driver pcb:


https://gyazo.com/e84b9f56ea15e7c2cc8de1dc1ffb5f88.jpg




The capacitor that he is talking about is between the Red-Yellow and Yellow-Green Wires on the terminal strip.

stewdan wrote:Bryce --- for what it is worth, Pavel responded to some questions via a message and sent me this picture of the AB-D-Mkiii and the original dynaco Mkiii driver pcb:


https://gyazo.com/e84b9f56ea15e7c2cc8de1dc1ffb5f88



The capacitor that he is talking about is between the Red-Yellow and Yellow-Green Wires on the terminal strip.

The capacitor that you are talking about has one leg tied to ground, the other let goes to the Yellow-Green wire which is the center tap of the filament supply. The Red-Yellow wire is just going to ground.

Your link did not work for me.

Bryce -- For whatever reason, I can use gyazo to look at pictures that I posted a week or 2 back for the SP12 in a PAT4 Chassis posting, but can't get it to work today for a new picture???


I forgot to tell you that Pavel's Picture, shows the Filament Outputs of the AB-D-Mkiii Board going to solder pads 8 and 9 of the original Dynaco Driver PCB.   Pads 8 & 9 are also connected to V1's Filament Pins (2 & 7).

Stew


https://gyazo.com/399d13b74d3d5ca14be1f7d3768ef197

BryceJ wrote:Hi Stewdan,

I have not installed Pavel's AB board on a Mark III.

I asking Pavel specifically about the Model AB-D-MKIII and Erhard Audio's comment regarding it: "It is important to note that the AB-D-MKIII MUST use an isolation transformer, as the Dynaco MKIII's filament supply is referenced to ground." Again, Pavel's answer was "The original Dynaco do not have a DC filament connection to ground. They use a capacitor, which does not matter when connecting the AB module."

Pavel's answer is exactly the opposite to Erhard Audio's comment. Take your pick.

It can be said that the AB-D-MKIII module operates in Dynaco MKIII without an isolation transformer. However, there have been some problems with Dynaco amplifiers in the past, so it is recommended to use a 1: 1 12VAC isolation transformer for safety.
These transformers are sold by Erhard Audio:

http://www.erhard-audio.com/AutoBias.html

Pavel