A Bogen line transformer amplifier with 13FD7 tubes
|The demise of the SPECO T7010 transformer presented a challenge and the next best line transformer was the Bogen T72510. But it has half the primary inductance of the original SPECO so what to do? The answer was fixed bias triodes using the 13FD7. Triodes provide low impedance for driving the transformer and fixed bias prevents bias shift when working hard to do bass while still allowing fine balance of idle current.|
Working from right to left on the schematic we have the speaker connection through a 1/4" phone jack (speaker external). The Bogen is driven by the power section of the 13FD7s which are biased and balanced through the R20/R15 network, the negative supply for which comes from the -180 V through separate 1 Meg resistors. This network also does double duty as a semi-floating paraphase input to V2AA via the 18.7 K resistor, which provides just enough offset to match the gain of the V2AA. The semi-floating nature means it's self correcting. If the output of V2AA is larger than V1AA it will lower the signal and, so, tend to pull V2AA back down. Phase balance is further enhanced by the long tail on V1AA and V2AA, which tends to act as a constant current source forcing a see-saw action. When one goes up the other must go down the same amount and, so, remain in balance. Finally, the 12DT8 input triode has a portion of the output feed back to it's cathode linearizing the whole amplifier.
Volume is a dual pot with the input coming from 1/8" jacks. Two are simply daisy chained so a PC Speaker can be left plugged in and used for non-musical computer bleeps and blips, saving the Paraphaser for musical use. Lastly, a switched jack is provided to over-ride the input with an external source, such as an iPod or cell phone.
Bias is sampled by the 10 Ohm resistors and brought out on the 1/4" jacks so the Paraphaser doesn't have to be opened up to bias/balance the tubes.
Working from the bottom up, the heaters are powered by a Radio Shack 273-1511 12.6 V, 3 Amp, transformer. Since the current draw is only 1.95 A it's output is 13 V, correct for the 13FD7s. It's then lowered to 12.6 V for the 12DT8 through the double, to preserve balance, 1.33 Ohm resistors. The 'LED' indicator is fed through a 10 K and reverse diode (not shown).
The upper transformer is a N-68X 115/230 VAC to 115 VAC transformer. When run in reverse, like this, the regulation factor works backwards and the N-68X output is only 181 VAC, which is needed to keep power under the 50 VA rating.
The negative rail is cap isolated, full wave rectified, and fed to a primary filter cap and two RC filters. Finally, a 5 K pot is for 'fine tune' of - V (since it's in the bias section).
B+ is full wave rectified, fed to a primary filter cap, and then, to cut cost and lower hum to inaudible levels, a Q1 capacitance multiplier is employed. A further RC filter feeds the preamp section.
|Templates were made of the layout and holes drilled following them. This was a test fit to make sure everything was copacetic.|
|And the sides. Note, the sloping sides and rounded corners of the loaf pan meant the templates couldn't be reliably registered and had to be visually oriented.|
|The power supply and heater wiring. Note how the hetasink was center mounted to clear the Bogens and the side vents. Two top vents were added between the transformers. I also added some 'stiffeners' in the socket area. Perhaps unnecessary but near impossible to add later should it be found they were needed.|
|The completed Paraphaser|
|And the final touch, a base to cover the insides.|
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