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Old 20th April 2004, 01:25 PM   #21
Mike C is offline Mike C  United Kingdom
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Location: South east Essex, UK
Default Suggestions

Hi 's2kov',

I think your design would work and sound reasonably good, but I have
a few suggestions that I think might improve it and I'd like others
here to comment.

First, in my experience the 12AU7 is a poor sounding valve and this
will severely limit your results.
You could consider E80CC or 6SN7 for somewhat similar valves (no,
they are not the same!); for the E80CC you need a higher heater
current than the 12AU7, and the 6SN7 is of course an octal valve.
But see later for another much better suggestion ...

Next, the overall design.
It's simple, and I like that. So there are very few design parameters.
These are the load resistance (in particular, the ratio of this to
the valve internal resistance). And the output impedance.
A good rule for low distortion is to make the load 5x the valve Ri.
Taking 7kohm Ri (for 12AU7, E80CC or 6SN7), parallel that's 3.5k.
Five times that is roughly 17kohm; 15kohm would be fine.

However, 15kohm and 3.5kohm (parallel) will give about 3kohm output
impedance, which really isn't bad for such a simple cct. Even so,
I'd personally prefer it to be lower. I like to have a ratio between
driver and driven impedances of 20:1 or more, and you don't quite get
this if you're driving a 47kohm input power amp.
Which I guess is why your design uses a lower anode load resistance;
but this may slightly increase the distortion, I think.

Finally, the use of paralleled sections of the valve.
I can see why it's done here, and it'll work and give you the right
output impedance, so it's not a bad design.
But I'd prefer to keep it simple.
If you need twice the Gm and half the Ri, why not just use a valve
with twice the Gm?

My suggestion would be the 5687 twin triode.
See what this does to the design!
Typically, Ri is roughly 2kohms instead of the 7kohms of the 12AU7.
This gives several benefits.
First, you don't need to parallel the sections; Gm of *one* section
is higher than the paralleled 12AU7; Ri of *one* section is less than
Ri of the paralleled 12AU7.

Next, you don't need to compromise on the load resistor. Use the rule
of thumb of 5 x Ri, giving 10kohms. Nice and easy.

Finally, with the 10kohms load, the output impedance will be roughly
1800 ohms.
If driving a 50kohm load, the impedance ratio is about 28; really
nice.

I guess at an operating point of about 120V anode, 4V bias, 20mA but
that's a guess as I've never used this valve.

I think this would be usefully better than the 12AU7 design, but
perhaps others will comment.
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Old 20th April 2004, 03:15 PM   #22
s2kov is offline s2kov  Canada
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Location: AB
Default Re: Suggestions

Quote:
Originally posted by fscarpa58
Hi s2kov



have you downloaded it?
if not try at


http://www.spectrum-soft.com/demodown.shtm

bye

I have it already unfortunately cannot utilize full functionalities....


Quote:
Originally posted by Mike C
Hi 's2kov',

I think your design would work and sound reasonably good, but I have
a few suggestions that I think might improve it and I'd like others
here to comment.

First, in my experience the 12AU7 is a poor sounding valve and this
will severely limit your results.
You could consider E80CC or 6SN7 for somewhat similar valves (no,
they are not the same!); for the E80CC you need a higher heater
current than the 12AU7, and the 6SN7 is of course an octal valve.
But see later for another much better suggestion ...

Next, the overall design.
It's simple, and I like that. So there are very few design parameters.
These are the load resistance (in particular, the ratio of this to
the valve internal resistance). And the output impedance.
A good rule for low distortion is to make the load 5x the valve Ri.
Taking 7kohm Ri (for 12AU7, E80CC or 6SN7), parallel that's 3.5k.
Five times that is roughly 17kohm; 15kohm would be fine.

However, 15kohm and 3.5kohm (parallel) will give about 3kohm output
impedance, which really isn't bad for such a simple cct. Even so,
I'd personally prefer it to be lower. I like to have a ratio between
driver and driven impedances of 20:1 or more, and you don't quite get
this if you're driving a 47kohm input power amp.
Which I guess is why your design uses a lower anode load resistance;
but this may slightly increase the distortion, I think.

Finally, the use of paralleled sections of the valve.
I can see why it's done here, and it'll work and give you the right
output impedance, so it's not a bad design.
But I'd prefer to keep it simple.
If you need twice the Gm and half the Ri, why not just use a valve
with twice the Gm?

My suggestion would be the 5687 twin triode.
See what this does to the design!
Typically, Ri is roughly 2kohms instead of the 7kohms of the 12AU7.
This gives several benefits.
First, you don't need to parallel the sections; Gm of *one* section
is higher than the paralleled 12AU7; Ri of *one* section is less than
Ri of the paralleled 12AU7.

Next, you don't need to compromise on the load resistor. Use the rule
of thumb of 5 x Ri, giving 10kohms. Nice and easy.

Finally, with the 10kohms load, the output impedance will be roughly
1800 ohms.
If driving a 50kohm load, the impedance ratio is about 28; really
nice.

I guess at an operating point of about 120V anode, 4V bias, 20mA but
that's a guess as I've never used this valve.

I think this would be usefully better than the 12AU7 design, but
perhaps others will comment.

Hi Mike C,

Thanks for the comments.... I just chose 12AU7 for my first attempt to tube design. I just want to know and welcome others opinion regarding this.... Just like what you said instead of using 12AU7 in parallel to give me twice the Gm with other valve that has a high Gm. This will added note on my head that will consider every time the design takes place.

Regards....
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Old 21st April 2004, 12:20 AM   #23
arnoldc is offline arnoldc  Philippines
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s2kov,

i simulated your circuit on TubeCAD and it will have a voltage gain of 13.13 (22.4 dB), input impedance of 168K, output impedance of 2.97K, PSRR of -14.1 dB

also, the Rk of 200R gives more balanced maximum output voltage of +- 100V

i hope this helps

jojo,

you can borrow my foreplay anytime. but i will get it back from you when the owner of the Wright WPL11 pre i'm currently using demands i return it to him.

frank,

intriguing, that foreplay is. i am considering other pre-amp options though i currently use one. i think it does have this "mass following sonic appeal", if that even makes sense. i think what bottlehead offered is something you can start with and tinker later on. i'm a lazy guy, i will not go and add CCS, make a change to 6SN7, etc. i'd rather build your ultimate pre once i get hold of elusive 12BH7s!

- sorry for a bit of OT -
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Old 21st April 2004, 01:22 AM   #24
billr is offline billr  New Zealand
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Join Date: Dec 2003
Location: new zealand
Quote:
Originally posted by tubetvr


Also I dont think anyone can stop you for copying a patented or copyrighted circuit for DIY purposes, what you are not allowed to do with a patented circuit is to build it and sell it . For copyrighted material you are not allowed to publish it without owners consent.

For DIY, use whatever patented or copyrighted circuit you want if you can find it!

Regards Hans

Hans.

you can only copyright original work, not just a tube circuit with different value tubes, resistors or caps, especially for a circuit that has been around for 70 odd years. so basically unless the author can prove that the circuit is unique, not just the values, then no copyright exists.

Patents are a different kettle of fish, if a patent exists, you have to pay for a licence to use the patented material.

kind regards

bill ramsay
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Old 21st April 2004, 03:33 AM   #25
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tubetvr, billr,

thanks for the enlightment about the patent stuff. You're both right, this is what I want and I will build it.

arnoldc,

thanks for the offer to lend me the foreplay you have. I didn't saw your post because I was so busy researching about that preamp.

anyway guys, here is what I have already prepared for my foreplay. I even have the power trafo and filter chokes ready. I hope to gather more info soon. wish me luck.

JojoD

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Old 21st April 2004, 04:00 AM   #26
arnoldc is offline arnoldc  Philippines
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jojo, souped up FP with chokes!
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Old 21st April 2004, 04:32 AM   #27
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Quote:
Originally posted by arnoldc
jojo, souped up FP with chokes!

Yep, this is a heaily modified version. I thought that if I build something based on the original but with a bit of difference then no one will come after me. It was then that some of our friends here at diyAudio said that the technology existed a lot of years ago and not to worry about patents. Whew!

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Old 21st April 2004, 04:35 AM   #28
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A fellow diy friend said that the psu is too serious... But I already have the parts then why not use them right? The heaters are regulated with slow turn-on too. Both HT and heater rectifiers have their own snubbers.

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Old 21st April 2004, 05:51 AM   #29
s2kov is offline s2kov  Canada
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Location: AB
ArnoldC,

Thanks much for the help. I'm beginning to construct this once my power trafo and choke completed.

JojoD,

Foreplay I'm also interested with that, let me know when you've done....

Regards to you guys!!!

Andy



Quote:
Originally posted by arnoldc
s2kov,

i simulated your circuit on TubeCAD and it will have a voltage gain of 13.13 (22.4 dB), input impedance of 168K, output impedance of 2.97K, PSRR of -14.1 dB

also, the Rk of 200R gives more balanced maximum output voltage of +- 100V

i hope this helps

jojo,

you can borrow my foreplay anytime. but i will get it back from you when the owner of the Wright WPL11 pre i'm currently using demands i return it to him.

frank,

intriguing, that foreplay is. i am considering other pre-amp options though i currently use one. i think it does have this "mass following sonic appeal", if that even makes sense. i think what bottlehead offered is something you can start with and tinker later on. i'm a lazy guy, i will not go and add CCS, make a change to 6SN7, etc. i'd rather build your ultimate pre once i get hold of elusive 12BH7s!

- sorry for a bit of OT -
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Old 21st April 2004, 07:21 AM   #30
arnoldc is offline arnoldc  Philippines
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s2kov, good luck bro... let us know

jojo, let's AB with my stock FP when you're done
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