Minor mods to DR103 - what do they do?

[therealjoeblow]

Hi,

I found these pics of a vintage DR103, it has 3 subtle mods that I can see in the photos, and as indicated on the schematic:

1) There is a capacitor installed across the V1A plate load resistor (value unknown)
2) The V1A coupling capacitor appears to have been changed, but the value is not known
3) There is a capacitor installed across the V4A and V4B (phase inverter) plates (again, value unknown)

(It also has the Canadian Importer "1-wire" cascaded gain mod, I do know what that is and what it does).

Code: Select all

Schematic:
http://postimg.org/image/hozcta2xt/

Photo1:
http://postimg.org/image/cbojq9wmv/

Photo2:
http://postimg.org/image/flyvnvicl/

Without knowing the exact values, can an expert please help me to understand in theory what these capacitor alterations would do?

Many thanks
The REAL Joe

[mhuss]

Both the cap additions will decrease the high frequency response, but I'm not sure what the 'cloud' will do.

--mark

[therealjoeblow]

Thanks Mark. The "cloud" just indicates that the coupling cap was changed, but I don't know if it was raised or lowered in value since I only saw these 2 photos.

I also note that all of the 60's and up to the mid-70's 4 input preamps had 68k grid resistors on V1, but then in the late 70's and 80's (Biacrown) and the 90's (AudioBrothers), V1 did NOT have any grid resistors at all.

Do you know why they omitted them? Most references I see say that a 68k grid stopper on the first valve is very important to prevent oscillations.

For example:

http://www.aikenamps.com/index.php/grid ... -they-used

(read 2nd last line... "It is not usually a good idea to eliminate this resistor.")

thanks
TRJB

[mhuss]

Not true, all the Hylight and Biacrown amps used 68k input resistors; these were always mounted on the input jacks, like Fender did back then.

Note that these input resistors work best when mounted with a short lead directly to the tube socket, but almost no one does this.

--mark

[therealjoeblow]

Strange, because the 68k resistors are NOT shown on the schematics on your page for the late 70's 2-input models, or on the schematics Biacrown 80's models, and also on the AudioBrothers schematics (both the 19/05/95 and the c.2001 versions). And also none shown on the schematic for your own 504OL on your web page.

I'm looking at mine now (1979 hylight 2-input) and there are no 68k resistors on the jacks (just the 2 x 1M brown-black-green), and the coax leads go directly from the input jack to thhe V1 pins; in fact there are no blue-grey-orange resistors in this amp at all anywhere.

Also, I have high-res photos of 2 different DR504OL's that I saved from ebay over the years, and they are exactly the same, just the 2x1M on the input jacks with the coax going directly from the input jack to V1 with nothing between.

And another 2-input 103 that the photo on your site shows the same thing.

Plus, finally, the one with the extra capacitors that I initially posted the pic of above, exactly the same, there are no 68k grid resistors on any of these.

TRJB

[mhuss]

You're right! You'd think I would know better since I have one, and I drew the schematics. I was thinking strictly of the four-input amps.

Sorry about that.

On the input, they don't prevent oscillation but do cut down on the "amp as an AM radio" phenomenon.