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 The Fender 5C5 Pro mod project,
Or how to get more power out of your Tweed Fender

 

 A client came to me with his Fender clone of the 5C5 circuit complete with the 50's Jensen P15N speaker freshly reconed by Austin Speaker Works.

If you don't know, Ron Condon and his crew do great work on vintage speakers.

          

 

 After discussing the many options, Jeff opted for some Big Iron; in the form of the Mercury Magnetics Bassman Output Transformer, with all the bells and whistles. This will gain you some bottom end and better transfer of the available power.

     

             here you can see the original sized OT next to the Fat Boy.

 

As the New OT had extra wiring for Ultra Linear connections and every possible secondary impedance.

I choose to tuck them away inside the end covers after insulating the ends of course. I needed to drill a few extra holes in the chassis to accommodate the four mounting feet and another hole for the secondary wiring.

 

 

This really does not gain us more power so I installed a set of Silicon Diodes across the Rectifier tube pins.

This was the easiest and least expensive way to do it. The Rectifier is now just for looks. I recommended not using it at all to reduce heat.  

 

  Before                               and After, notice the bias tap.

 

We are now getting 480+ VDC well beyond the stock voltage of 395 or so.

What more can we do, hmm, how about fixed bias to really goose things. Jeff wants a tighter more focused low end, more cut.  Personally, I prefer cathode bias for that compressed singing quality. I let Jeff try out various models with all the different options, he wants fixed bias.  It is pretty easy to go from a Fixed bias to cathode bias in most amps, The other way round is not so easy. And because of the Paraphase Inverter on this amp it required a little thinking. We need to isolate the negative voltage from the PI.

Ok, let's install a couple of signal coupling capacitors to block the voltage to the PI. At the same time we wanted to clean up the low end a bit, so I used .02uf instead of the .05uf already present.  Then we need a source for the bias voltage. As the Power transformer did not come with the secondaries usually used for this, I tapped one side of the High Voltage taps before the B+ diodes. A 220K 2 watt resistor worked well into a standard 1n4007 diode reversed to develop the negative voltage.  Jeff wanted to have an adjustable supply too.

Let's build a circuit.

 

A little schematic I drew up to start with. I used Whatchyagot engineering and ended up with a 50kl pot and a 220K 2W resistor.

 

Layout with isolation caps and bias feed resistors. I ended up with slightly different values.  

 Tube rectifier to Silicon diode layout.  

 I opted for series diodes to spread the load out and build more reliability into the circuit.

I used two diodes per side instead of the single ones shown.

It will work this way though.

I could have bypassed the diodes with small ceramic caps

for lower switching noise, but I don't hear it.

 

Before                                              After

Oh, and don't forget to remove the Cathode Bias resistor and Bypass Cap.

 

 Pretty simple, just remove the resistor and cap and solder in a wire to reconnect the cathodes directly to ground  

 Here is what I settled on voltage and bias current wise.

 

I choose to bias the standard 70% of Max plate dissipation, which for a 6l6gc is 25 watts (30w).

we used 5881 so I used 25w for calcs.

 The formula is: 25w/486v = 51ma current. Taking 70% of that we get 36ma current.

Actually, the voltage will drop when you increase the bias current. I think I started at 489 VDC.

 How does it sound? It is a lot louder and tighter, Jeff jumped back when he first heard it, he is going to gig it and get back to me. Then I am going to suggest we parallel the inputs, lowers the impedance to drive the next stage. I find it really opens up the dynamics and note articulation, better detail and I like the tone. Some say it increases the 3rd harmonic over the second, but it is very subjective. I think it really fattens up the tone.

Z