Tube Amps / Music Electronics
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|11/18/2003 3:16 AM|
|Major Pain||Re: Thanks Don|
I thought that was what you meant but I just wanted to make sure, Thanks again Don
I guess I shouldn't have slept thru math at hi school or some of the other classes that I needed in life now like spelling too
Too much like Spicolli in those dayz. LOL
|11/18/2003 11:30 AM|
If you go the resistor route I suggest you separate the resistors that feed the output tubes from the preamp tubes. Most of your current draw is from the outputs and if one of these filaments opens all the other tubes will see much higher filament voltages with the reduced current through the resistors. If I use resistors to lower filament I size ones to feed the output tubes and separate ones to feed the preamp tubes - just a little Ohm's law and a bit more protection (then again, I often use NOS/UOS "vintage" tubes and want more filament protection).
|11/18/2003 8:03 PM|
Right now the amp im working on is running the filament @ 7.5vac and that is really making the preamp tubes hot. Seems using the hammond PT is kind of a pain since these are set at the factory for 115vac on the primary and here in the US we have any where from 120 to 124 vac varing over the US.
I would have used a different trannie but this was a spare so I figured time to use it and be done with it. I won't buy any more hammond PTs so I can avoid the filament over voltage problem in the future.
Which method do you use and prefer? resistor or diode to get your filament voltage down to par?
Thanks for the info
|11/18/2003 9:55 PM|
What is your Hammond PT's filament winding rating?
|11/19/2003 3:13 AM|
|Major Pain||Re: Filament rating|
The rating for the filaments are at 6.3vac @ 5amp
The trannie ia a 273BX. I had it laying around and finally said what the heck I'll use it in another build. To bad the Hammonds are 115vac primary instead of 120 like most other power trannies.
I guess Canada has a 115vac household setup there like the USA used to be......
|11/19/2003 3:07 PM|
Actually, we run about 120, same as the States!
Damifino why Hammond is stuck on 115...
|11/19/2003 3:15 PM|
Your heater winding rated @5A is overrated for your amp, which draws 2.7A heater current (2x6L6 + 3x12AX7 = 0.9Ax2 + 0.3Ax3=2.7A). I have found that when using Hammond trannies with 115V primaries AND overrated filament windings, the heaters can easily get overvoltaged. For kicks, try subbing in EL34's which draw 1.5A each and you should see the voltage drop to within the +/-10% margin.
Also, I have found that the back-to-back parallel diode trick works, but it can inject noise into the circuit. The resistors are IMO, a better solution. BTW, you only need the resistors on one leg of the secondaries, I believe. Or just stay with the EL34's
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