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Old 16th August 2003, 03:35 PM   #1
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Default Correct values for this schematic?

Hi all,

After I build my vext amp I plan on building a new pre to go with it. I thought about a mu stage using a 6SN7 since I have a few of these on hand. I'm also considering using the design below. Looking at the schematic and being familiar with the 5687 it looks like the values are wrong if these tubes are supposed to be running at "one third of their potential". These are capable of 30 mA (if you would want to push them that hard). If you drop 150 volts across a tube and run it at 10 mA then a 750 ohm cathode resistor is what you would want to use. In this design I would want to drop 165 volts across each tube (including the drop across the cathode resistor). The value of 1.4K (that is definately not a standard value) seems way too high. Also what would I gain by using that much capacitance in the coupling cap position if the input resistor of the next stage is 100K? The spec sheet for the 5687 can be found here:

http://www.diyparadise.com/5687.pdf


Maybe I'm missing something (not that that has ever happened before )
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Old 16th August 2003, 11:07 PM   #2
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Hi,

That circuit will work but don't forget it was developped to work into a 300 R load.

If it's a linestage your looking for using the 5687 using the SRPP
a number of circuits are out on the net already.

I recommend a well regulated B+ for any of them.

Cheers,
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Old 17th August 2003, 03:15 AM   #3
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Hi Frank,

I was planning on using two 20 henry chokes in the PSU and a regulated DC filament supply. Hopefully that will suffice.
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Old 17th August 2003, 04:15 AM   #4
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Default Regulator

Hi Gavin,
The PSRR for a SRPP is pretty poor . You could also use a ss voltage regulator which works very well. Even a capacitance multiplier will probably work very well. But if you really want to use a choke -------.
I have seen many people say that a choke filter 'sounds' better than a ss regulator . I really do not know as Ihave not made any comparison. I will do that one of these days.
Cheers.
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Old 17th August 2003, 07:55 PM   #5
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Also what would I gain by using that much capacitance in the coupling cap position if the input resistor of the next stage is 100K?

Nothing. If the next stage is 100k then 1uF is fine
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Old 17th August 2003, 11:58 PM   #6
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Hi,

Quote:
Also what would I gain by using that much capacitance in the coupling cap position if the input resistor of the next stage is 100K?
A 100K gridleak resistors does NOT necessarily mean a 100K Zin.

Caveat emptor,
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Old 19th August 2003, 04:29 PM   #7
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Default Capacitance

Hi G.
As mentioned above, for 100k input impedance, this value may be much smaller for flat response. The value will depend somewhat on the "sound' of the power supply cap and the brand coupling cap you wish to use.
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Old 19th August 2003, 05:43 PM   #8
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Hi Steve. Since the input resistor of the next stage is 100K I would say that the load on the circuit is going to be somewhere around 90K so obviously I can use a much lower value on the coupling cap. My real intention for posting the design is that the values seem all wrong to me and I wanted a second opinion. I think that the two cathode resistors should be around 750 - 820 ohms instead of the 1.4K shown. It really doesn't matter though. I like simple designs and since this circuit has a bad PSRR factor I'm not going to pursue it. If I can't make it sound good using a couple of healthy chokes in the PSU then I will look elsewhere. Hell I have a PCB layed out for a Mosfet power follower using a cascaded CCS. I may end up using that. It's meant to drive headphones but it would serve my purpose as a buffered volume control also.
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Old 19th August 2003, 09:51 PM   #9
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Hi,

Quote:
My real intention for posting the design is that the values seem all wrong to me and I wanted a second opinion.
Gavin,

They may seem unconventional but if you look at the 5687's curves you'll notice this will work too.

That's the beauty about valves, they may have a sweet spot that suits your application ( a preamp I gather) and than someone else optimises for a different service ( headphones) and decides for what values are best suited.

Loadlines do offer a lot of different dots...
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Old 19th August 2003, 10:01 PM   #10
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"Loadlines do offer a lot of different dots"

You can say that again Frank. I know that tubes are supposed to be simple to use if you have a little book lernin and some imagination but sometimes they git downrite onery. Specially if yer samplin the corn squeezins.
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