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Old 3rd December 2009, 09:23 AM   #1
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Thumbs down Please help with my first valve preamp ?

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).

Click the image to open in full size.

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
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Old 3rd December 2009, 09:54 AM   #2
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Quote:
(currently there is no gridstopper, or TVC installed, the grid is unconnected)
Tie that grid down with a 100K to ground,and see if that helps.
leaving the grid floating isn't a good idea.
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Old 3rd December 2009, 10:05 AM   #3
Arnulf is offline Arnulf  Europe
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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.
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Old 3rd December 2009, 12:04 PM   #4
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Yeah ... so let's count this one as a rookie mistake ?

I added a 100K resistor to the grid, and viola ! 270V at the plate, 14.5 at the cathode, so the tube is operating a tad outside its range (will 250V vs. 255V be much difference at the plate ? what about bias at 14.5V vs. 13.5 ?).
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Old 3rd December 2009, 12:28 PM   #5
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Don't worry about the 5V difference in plate voltage. Variations in mains voltage can easily be +-5% anyway.

If you really want to get closer to the desired bias point, you can try slightly lower values of the cathode resistor.

Enjoy your new amp.
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Old 3rd December 2009, 12:36 PM   #6
Arnulf is offline Arnulf  Europe
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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 ...).

Last edited by Arnulf; 3rd December 2009 at 12:39 PM. Reason: misread the voltage values
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Old 3rd December 2009, 02:08 PM   #7
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I am wondering, why use such a "heavy duty" 5U4G rectifier tube for a circuit pulling so low of a current?

Sal
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Old 3rd December 2009, 04:25 PM   #8
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by Sal Brisindi View Post
I am wondering, why use such a "heavy duty" 5U4G rectifier tube for a circuit pulling so low of a current?

Sal
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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Last edited by kevinkr; 3rd December 2009 at 04:27 PM.
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Old 3rd December 2009, 09:39 PM   #9
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My 76 pre uses a pair of 816s. Sometimes it's all about the cool factor.

Scott
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Old 3rd December 2009, 10:03 PM   #10
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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 :P )

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
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