Hello everyone, starting a new old build here - I built a D'lite about 7 or 8 months ago, and while i thought it sounded good, i really wanted to build the #102.
Long story short, i gutted my D'lite down to the bare bones (big thanks to members on the forum for buying my boards to fund my conversion ) and built up a set of 102 boards.
My initial question is this: the D'lite uses a diode rectified circuit like the blackface fender amps. The 102 uses a full wave bridge rectifier without a center tap. I am using fender bassman iron in this build - my PT has a center tap. Could i disconnect this center tap from ground and redo my rectifier into FWB configuration and achieve the proper setup for #102?
"It's like what Lenin said... you look for the person who will benefit, and, uh, uh..."
beasleybodyshop wrote:
My initial question is this: the D'lite uses a diode rectified circuit like the blackface fender amps. The 102 uses a full wave bridge rectifier without a center tap. I am using fender bassman iron in this build - my PT has a center tap. Could i disconnect this center tap from ground and redo my rectifier into FWB configuration and achieve the proper setup for #102?
No. It is possible to use a center tapped PT in FWB mode by disconnecting the center tap, but you are going to get a B+ voltage that is basically twice what you want it to be. See http://www.hammondmfg.com/pdf/5c007.pdf for a rundown of rectifier configuration options. Sticking to regular full wave rectification is a pretty minor deviation from the original design.
The rectifier circuit you will use will depend on the power transformer. You should copy the basic topology used in the original amp your PT came from, if it is an old Bassman the rectifer will most likely be identical to the one in this D'Lite schematic:
You don't want to use a bridge rectifier with a grounded center tap transformer as your DC from the bridge will be 2X higher than the other. I don't think the rectifier type will have any effect on the amp's tone at all, extremely little if any.
Most 50W Fenders have the grounded center tap style of full wave rectifier, while many 100W amps have bridge rectifiers.
Actually I have wondered for years why this was so; basically the secondary voltage for a grounded center tap rectifier has to be 2X what is needed for a bridge rectifier circuit to get the same rectified DC voltage output. The only reason I can think of is the grounded center tap style is useful with tube rectifiers, it is expensive and difficult to do a bridge rectifier with tube rectifiers (although it can be done, my home stereo amps are designed this way).
I see. I dont understand why Dumble wired the 102 with a full wave bridge rectifier but used fender iron - which uses a center tapped PT/Filament setup.
"It's like what Lenin said... you look for the person who will benefit, and, uh, uh..."
I posted some unclear information in my above post; the grounded center tap rectifier I referred to is better called a full wave rectifier. Just calling a rectifier a "center tap rectifier" is a bit misleading as some amp makers (Marshall in particular) use center tapped transformers with bridge rectifiers in some 100W amps. They then connect two filter caps in series to the output of the bridge, and connect the center tap of the transformer to the junction of the two filter caps. This ensures the two caps have 1/2 voltage divided across them without using dropping resistors.
To get an idea what rectifier you need with a particular PT, do the following:
1) Does the HV secondary winding have a center tap? If not, multiply the voltage by 1.414, and see if that is in the ballpark for the DC B+ you need. If so, use a bridge rectifier. You cannot usually use a full wave rectifier without a center tap.
2) If the HV secondary has a center tap, multiply the voltage by 1.414, and see if that is in the ballpark for the DC B+ you need. If so, use a bridge rectifier. If this voltage is way higher than what you need, multiply the voltage by 0.707; if that is closer, use a full wave rectifier.
Yes, I was looking at the layout for 102 as inspiration and saw what appeared to be a rectifier bridge setup, however power tube plate voltages were similar to a twin reverb, which led me to speculate that it wasn't being used in a voltage doubler configuration with a typical twin power transformer.
"It's like what Lenin said... you look for the person who will benefit, and, uh, uh..."
) and built up a set of 102 boards.
