Hey all !
I've just started the testing phase of my transformer coupled 76 preamp (http://members.myactv.net/~je2a3/JELsc-TClinepre.jpg) (nothing caught fire, which was awesome !).
Instead of a gridstopper, I'll be using a TVC (currently there is no gridstopper, or TVC installed, the grid is unconnected), and for the cathode resistor, I'm using 2.7K (with 5ma from the tube, should give 13.5 V ...)
The votlages seem a fair way off though. I've attached an image of the psu (note that the 10ma load represents to 5ma loads in parallel, being the 76 tube, and the 1.6k resistor is 2 3.2K resistors, diverging to the tubes seperately).
Anyway. With tubes installed, I get the following voltages :
315VAC before the rectifier
263VDC at C1
200VDC at C2
188VDC at the 76 tube
44V at the cathode (should be 13.5 ...)
I'm really at a loss as to how I got these values. 44V at the cathode, with a 2.7K resistor suggest the tube is drawing 16ma, rather than 5ma, which is way too much.
The goal was to get 263 (ish) VDC at the tube, so I may run it at 250V.
Unloaded (no tubes or resistor, I get about 330V at all points).
The transformer I'm using is a hammond 372DX, using 240V primary, 600V CT secondary.
If there's any more info I could provide, let me know, and I'll see what I can do
I've just started the testing phase of my transformer coupled 76 preamp (http://members.myactv.net/~je2a3/JELsc-TClinepre.jpg) (nothing caught fire, which was awesome !).
Instead of a gridstopper, I'll be using a TVC (currently there is no gridstopper, or TVC installed, the grid is unconnected), and for the cathode resistor, I'm using 2.7K (with 5ma from the tube, should give 13.5 V ...)
The votlages seem a fair way off though. I've attached an image of the psu (note that the 10ma load represents to 5ma loads in parallel, being the 76 tube, and the 1.6k resistor is 2 3.2K resistors, diverging to the tubes seperately).
An externally hosted image should be here but it was not working when we last tested it.
Anyway. With tubes installed, I get the following voltages :
315VAC before the rectifier
263VDC at C1
200VDC at C2
188VDC at the 76 tube
44V at the cathode (should be 13.5 ...)
I'm really at a loss as to how I got these values. 44V at the cathode, with a 2.7K resistor suggest the tube is drawing 16ma, rather than 5ma, which is way too much.
The goal was to get 263 (ish) VDC at the tube, so I may run it at 250V.
Unloaded (no tubes or resistor, I get about 330V at all points).
The transformer I'm using is a hammond 372DX, using 240V primary, 600V CT secondary.
If there's any more info I could provide, let me know, and I'll see what I can do
Does that mean there is literally nothing connected to the grid pin (it is left floating) ? Some bias will develop this way but usually it is not enough, except with some tubes designed for such operation and in specific circuits.
Put a 470K-1M resistor from grid to ground and measure again.
If that is not the case please post actual schematic.
Put a 470K-1M resistor from grid to ground and measure again.
If that is not the case please post actual schematic.
Anyhow, if you're concerned you can simply change filter resistor (R1 in your PSU schematic) for slightly larger value. The larger the value, the larger the drop across it and consequently the lower the anode voltage.
Remember that all high impedance input devices need to have their inputs referenced to some sane voltage level, otherwise large current may flow and device may selfdestruct itself (tubes, FETs, integrated circuits built with MOS process ...).
Remember that all high impedance input devices need to have their inputs referenced to some sane voltage level, otherwise large current may flow and device may selfdestruct itself (tubes, FETs, integrated circuits built with MOS process ...).
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Because it is cheap, readily available, and reliable?
It'll probably live forever in this application, and looks a lot cooler than some smaller rectifiers..
(I use a real Mullard 5AR4 in my 26 dht pre which in total draws about 15mA.)
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I'll concede that the 5u4g rectifier DOES look cool, it sits quite nicely next to the smaller 76 tubes ... and they really are everywhere
(let's face it, small rectifiers ... I dunno )
I'll get some more testing done over the weekend, and hopefully I'll have the sucker sealed up and singing away in less than a week
(let's face it, small rectifiers ... I dunno
)I'll get some more testing done over the weekend, and hopefully I'll have the sucker sealed up and singing away in less than a week
Hey all !
Bit of an update ...
The amp is "complete", and sorta working. However, it seems to have revealed a noise problem with my phono stage that didn't happen with my previous preamp. It produces a loud buzz when the turntable is connected, and a hum when its disconnected.
Other sources don't produce any such noise.
The input TVC primary is floating (not connected to the ground bus), and the output Tx secondary is tied to the ground bus.
Not sure why this preamp will reveal a noise problem that the previous one didn't though ...
All RCA's are isolated, and the 0V lines aren't connected to safety earth, no chance of a ground loop.
Brain hurts ... Maybe I need to sleep ...
Bit of an update ...
The amp is "complete", and sorta working. However, it seems to have revealed a noise problem with my phono stage that didn't happen with my previous preamp. It produces a loud buzz when the turntable is connected, and a hum when its disconnected.
Other sources don't produce any such noise.
The input TVC primary is floating (not connected to the ground bus), and the output Tx secondary is tied to the ground bus.
Not sure why this preamp will reveal a noise problem that the previous one didn't though ...
All RCA's are isolated, and the 0V lines aren't connected to safety earth, no chance of a ground loop.
Brain hurts ... Maybe I need to sleep ...
?
It's almost surely a ground loop, or at least a grounding problem. And not connecting anything to safety earth is probably not a good idea.
You may recall from earlier that leaving things floating is often a recipe for problems.
At any rate, a picture and a schematic would be invaluable at this point.
I'll try to get a picture tonight, but the schematic is the one on my first post, except rather than an input pot, there's a TVC, wired as described.
Currently safety earth is only tied to the chassis (which is what I've been doing for my other amps ... except the power am, that has a thermistor between 0V and earth), but I'll try tieing the input primary to the groundbus, and connecting the groundbus to safety earth, either via a thermistor, or a loop breaker.
Currently safety earth is only tied to the chassis (which is what I've been doing for my other amps ... except the power am, that has a thermistor between 0V and earth), but I'll try tieing the input primary to the groundbus, and connecting the groundbus to safety earth, either via a thermistor, or a loop breaker.
Here are some snaps of the guts
I've since tied the TVC primary to ground, and no noise whatsoever
http://img194.imageshack.us/img194/2159/p1000989v.jpg
I'll take some "glamour" shots over the weekend, and post in the pics thread
I've since tied the TVC primary to ground, and no noise whatsoever
http://img194.imageshack.us/img194/2159/p1000989v.jpg
I'll take some "glamour" shots over the weekend, and post in the pics thread
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