chrisdrop
1000+ Head-Fier
You are going to need a bigger, cozier chair if you have any hope of sitting there for long!
You are going to need a bigger, cozier chair if you have any hope of sitting there for long!
You are going to need a bigger, cozier chair if you have any hope of sitting there for long!
Hi LG,
Re your mentioning of various voltages coming from the transformer windings, I am under the impression that in the tube era a 10% variation was considered within specs. Is it really that important to have the exact voltage available, especially since the mains voltage may fluctuate a little as well?
Hi LG,
Re your mentioning of various voltages coming from the transformer windings, I am under the impression that in the tube era a 10% variation was considered within specs. Is it really that important to have the exact voltage available, especially since the mains voltage may fluctuate a little as well?
Is it possible to hear a difference if the voltage changes somewhat more than the 10%?
From personal experience I have found that late at night or on weekends my equipment may sound better, possibly because of less electric interference from all kinds of devices and machinery.
@A2029 rocksContinued working on the tube heater dropping resistors today.
I wired a pair of 4V driver tubes with the heaters in parallel, MOV MHL4. The heaters are center tapped and referenced to ground with a pair of 100ohm resistors, so this was a ton of fun with a thousand alligator clips. Thankfully, the voltage is right where I want it ~3.9V with 124VAC into the mains transformer.
Next, I checked the voltage of the 4V rectifiers. First, a Marconi rebranded Mullard FW4/500 which draws 3A:
Then a GEC U18/20, which draws 2.8A:
As expected, their filament voltages are slightly off due to the difference in drawn current, but they are right where I want them to be, looks great.
Last thing I did was put together one of the CCS kits. I hooked it up to a 30VDC power supply and adjusted the trim pot until I got 6mA, the plate current for the driver tubes. This corresponded to 395ohm on the trim pot, so I'll likely get a 400ohm set resistor. Thanks for the idea, @A2029
So looks like my math was correct on the dropping resistors, now I can confidently drill holes in the chassis when it arrives. Supposed to be finished re-plating Tuesday, hopefully will get it by Friday and the real building will begin next weekend.
Continued working on the tube heater dropping resistors today.
I wired a pair of 4V driver tubes with the heaters in parallel, MOV MHL4. The heaters are center tapped and referenced to ground with a pair of 100ohm resistors, so this was a ton of fun with a thousand alligator clips. Thankfully, the voltage is right where I want it ~3.9V with 124VAC into the mains transformer.
Next, I checked the voltage of the 4V rectifiers. First, a Marconi rebranded Mullard FW4/500 which draws 3A:
Then a GEC U18/20, which draws 2.8A:
As expected, their filament voltages are slightly off due to the difference in drawn current, but they are right where I want them to be, looks great.
Last thing I did was put together one of the CCS kits. I hooked it up to a 30VDC power supply and adjusted the trim pot until I got 6mA, the plate current for the driver tubes. This corresponded to 395ohm on the trim pot, so I'll likely get a 400ohm set resistor. Thanks for the idea, @A2029
So looks like my math was correct on the dropping resistors, now I can confidently drill holes in the chassis when it arrives. Supposed to be finished re-plating Tuesday, hopefully will get it by Friday and the real building will begin next weekend.
IXTP08N50D2 is an interesting choice for top device in the CCS cascode. I believe K&K used the IXTP01N100D in the past, but it's more expensive and not as hefty of a chip (but has lower capacitance values). What MOSFET is used as the bottom device on the other side of that heatsink?
Someday in the future I'll have to build a bunch of different CCS circuits and do an in depth shootout. I'll maybe build a blinding box to make it easy to switch and so that I can't see which CCS I'm listening to.
IXTP08N50D2 is an interesting choice for top device in the CCS cascode. I believe K&K used the IXTP01N100D in the past, but it's more expensive and not as hefty of a chip (but has lower capacitance values). What MOSFET is used as the bottom device on the other side of that heatsink?
Someday in the future I'll have to build a bunch of different CCS circuits and do an in depth shootout. I'll maybe build a blinding box to make it easy to switch and so that I can't see which CCS I'm listening to.
Hey Mischa - the top device is the chip on the opposite side of the heatsink, and it is the IXTP08N100D2. The IXTP08N50D2 in the photo is the bottom device. For some reason, Kevin has the bottom device marked Q1 on the board and the top device Q2.
That would be very interesting, would be very curious to hear the results! maybe I would make a change based on your test.
There is also the issue of thermals. The IXTP08N50D2 will only rise 2 degrees C per watt where as the IXTP01N100D will rise 5 degrees C per watt. Some tubes like the 5687 can create a few watts of dissipation across the CCS. Just 3 watts across the CCS is the difference between 6 degrees vs 15 degrees and may be the difference between bolting the mosfet to the top plate vs buying a dedicated heat sink.