[kentuckytukker]

Hi DR103 fans,

We all now that the original old Bias winding voltage of the power transformer was to less. (35VAC) to use to set a proper operatin bias voltage. I do have a Dutch clone which has only 32,5VAC winding for this purpose. So digging through the excelent info, gathered together by Mark Huss at hiwatt.org, I planned to upgrade the amp with his improved Bias schematic. As no actual physical layout was provided on Marks website I have designed my own. It still uses the 23 way turret strip to accomodate most additonal electronic parts. It is recommended to instal the Bias pot or pot's by drilling a hole(holes) in the top plate. This pending you integrate a common bias for a matched quad output tubes or use matched pairs. To my opinion we should go for a matched quad, as when doing a replacement we most of the time will replace all tubes at this stage. I'ts also recomended to add 1 Ohm bias measuring resistors at each output tube. This will eleminate the need for buying a special adapter. Pardon using English in this topic, but although I can read, speak and understand German language very well, it takes me to much time to write this down in proper German

[kentuckytukker]

It is recommended to use the original layout of Mark as a reference too, because not all the wiring details at the 23 way turret strip is shown in my sketches.

[kentuckytukker]

It is recommended to use the original layout of Mark as a reference too, because not all the wiring details at the 23 way turret strip are shown in my sketches.

[kentuckytukker]

Hi DR103 fans,

We all now that the original old Bias winding voltage of the power transformer was to less. (35VAC) to use to set a proper operatin bias voltage. I do have a Dutch clone which has only 32,5VAC winding for this purpose. So digging through the excelent info, gathered together by Mark Huss at hiwatt.org, I planned to upgrade the amp with his improved Bias schematic. As no actual physical layout was provided on Marks website, I have designed some bymy own. It still uses the 23 way turret strip to accomodate most additonal electronic parts. It is recommended to instal the Bias pot or pot's by drilling a hole(holes) in the top plate. This pending you integrate a common bias for a matched quad output tubes or use matched pairs. To my opinion we should go for a matched quad, as when doing a replacement we most of the time will replace all tubes at this stage. I'ts also recomended to add 1 Ohm bias measuring resistors at each output tube. This will eleminate the need for buying a special adapter. You just measure the mV across these resistors and it will give you the actual bias current in mA. Allt htis without any additional calculation. Pardon for using English in this topic, but although I can read, speak and understand the German language very well, it takes me to much time to write this all  down in proper German language.

[kentuckytukker]

Hi DR103 fans,

Unfortunatelly a small mistake has been discovered in both the layout circuits. A jumper was missing between turret 6 and 13. I've updated the sketches now.
See rev.1 of the attached documents

[bea]

Thanks. Very useful information and documentation for people who own DR103s or maintain them.

[kentuckytukker]

Hi DR103 fans,

Unfortunately the jumper wire between turret 6 and 13 was not indicated in the 2 pair EL34 version. The documents has meanwhile  been updated(see the 2 small arrows on rev.2)

[carlitz]

Cool, you are already in the year 2020.........1 year ahead

[kentuckytukker]

Going back to the future. Oops, yes it is still 2019. Noted also a mismatch in document names . Attached both final documents for all of you Hiwatt lovers. For the quad BIAS use only this attachment.
Please delete all previous downloaded doc's. Man, I must had an off day(s).

[kentuckytukker]

Another way to represent the actual arrangement of the components on the turret strips for a single potmeter to adjust the bias for all 4 output EL34 tubes

[kentuckytukker]

Hi HiWatt lovers,

Attached you will find 2 updated physical schematics for improving the BIAS circuit on old DR103 amplifiers. One is for a single fixed BIAS (4 output tubes) the other is for a dual fixed BIAS for 2 pairs of output tubes

[kentuckytukker]

Hello Hiwatt fans,

Unfortunately I discovered another error in the layout. The minus connection of one of the 220mf capacitors was connected to terminal 5. This had to be terminal 6. Attached you will find an updated layout for the single bias circuit in combination with 4x EL34 tubes

mfrgr, René

[kentuckytukker]

Hello Hiwatt fans,

The same error showed up in the dual fixed bias version . So this has been corrected too. See new attachments.

regards, René