Hi all.
I am part way through a build of Kevin's Custom Special amplifier from the TUT3 book. Great circuit.
Checking my current needs for the 6.3VAC secondary, I calculate 4x 1.5 A for the 6CA7 valves and 3x .3 A for the 12AX7 valves.
Which comes to 6.9 Amperes.
The rating for the specified Hammond 278CX transformer is 6 Amperes on the 6.3 volt secondary.
15% over the rated capacity seems a lot to me. Am I being overly conservative?
I'm considering a seperate transformer to supply just the 3x 12AX7 heaters and perhaps a cooling fan.
Possibly a Hammond 266M6 transformer - 6.3V @ 3A.
I would rectify and filter to DC and measure under load to see if I need to drop the voltage to the heaters a little.
Any thoughts?
Hi Guys
The heater load in the TUT3 Custom Special is fine for the specified PT. No worries.
The standard Hammond PT and OT are the same as the custom-labelled units used in the Yorkville Sound amps.
As far as DC-ing the preamp tube heaters, you really want more than a 6Vac winding to produce 6Vdc. And actually, if you are intent on DC-ing, use the 12V wiring and then you can use a 12Vac PT, bridge and simple discrete regulator. The 12V wiring (AC or DC) allows all of the audio wiring to never have to cross the heater wiring, provided the sockets are turned the correct way.
Since you have TUT3, note that the DC-stand-off provides the same hum-reduction from heater-line-bourne noise as does a regulator.
Have fun
Thanks Kevin. All great advice.
I wasn't looking forward to adding another transformer with the weight, all the fuses etc.
I should mention that living in England so I'm actually using the Hammond 378CX transformer at 50 Hz.
Does that make any difference to the available current rating?
I know Hammond usually varies the design slightly on their multiple primary transformers to compensate for the mains frequency difference, is that the case here?
I remembered after I posted that you had written previously on the merits of 12V heater voltage, when available. Good tip on the tube socket wiring.
I thought the heater elevation in the circuit was excellent.
I'll handle the cooling with a 120/240 VAC case fan. Sort of like what Pete Traynor did.
Thanks!
Hi Guys
The universal mains PT should be equivalent to the original 117V PT as far as the secondary windings go.
It is a wonder that a fan was used in the 80W and 160W bass amps since neither really warranted it.
If you want to add a fan, do it the modern way. Get the largest 12Vdc fan you can fit into the space, positioning it so that it pushes air past the output tubes and towards the rear. Use a 4-diode bridge running from the 6Vac to generate about 8Vdc with a 10,000uF filter cap. This operates the fan at slow speed and it will be inaudible in front of the amp.
In tube amps (other than Vox or VHT) one does not need massive air flow; instead, just to bias the airflow in the right direction.
Have fun