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Old 15th January 2003, 09:28 AM   #1
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Default ECL86 preamp?

Hi!

After some succesfull solid state projects some years ago, I thought it would be fun to build something with tubes.
I really like my Zen amp and don't have the money yet to buy some good output trannies (i think), so i'll start with a tube preamp first.
I have some ECL86 from an old radio, so I'd like to make a preamp with these. The Pass Zen amp has quite low input impedance but since the ECL86 has half a EL84 (if I am correct) i thought it would have enough power to drive it.

I found a schematic for a ECL86 preamp here:

ECL86 preamp (in french)

Anyone any suggestions? Or could anyone point me to a thread or link with a list to think about when building your first tube amp.

Thanks, Hans-Willem
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Old 15th January 2003, 12:12 PM   #2
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Hi,

That preamp will have a mu of 100 in mu-follower configuration which is way too much for a CDP.

The ECL86/6GW8 comprises 1/2 ECC83/12AX7A and a EL84/6BQ5.

While the output impedance will be lower then a classic anode follower it will be far from ideal to drive the Zen + I/C.

Personally I'd rather build a small SE amp with a pair of those tubes,it would drive the most difficult headphone with aplomb.

Cheers,
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Old 15th January 2003, 01:56 PM   #3
Elkaid is offline Elkaid  Canada
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I built exactly the same one and I don't regret it at all.
This schematic comes from a French magazine called "LED" which provide generally more high-end (and costly) projects than any magazine I'm aware of.

I haven't built many preamp in the past, however I can assure you that I was quite amazed by it. No "hum", no background noise, well, no noise at all even at full power.

If you're beginning like me, this is an ideal project
1. The PCB is well designed
2. No many components
3. Cheap and easy to find power tranformer (mine is Hammond 269GX)
4. Results garanteed

However, high voltage capacitors are quite expensive and use only good resistors.

Here's mine. The only thing left is to build a nice enclosure as nice as Peter Daniel's one with his Acrylic GainClone

The fdegrove's comments seems pertinent but I haven't tried my Preamp with Zen.... However mine's connected to a TDA7294 based amp and it seems to work fine. Also, I had to add a resistor along my volume potentiometer because the volume was way too loud past 1/16 turn
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Old 15th January 2003, 02:13 PM   #4
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How about if i put a resistor in series with the output to virtually make the imput impedance of the zen higher?
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Old 15th January 2003, 02:18 PM   #5
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Default Zin.

Hi,

Quote:
How about if i put a resistor in series with the output to virtually make the imput impedance of the zen higher?
That would only act as a voltage divider.
It would by no means make the input impedance of the amp higher.

Cheers,
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Old 17th January 2003, 05:48 AM   #6
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Quote:
Originally posted by Elkaid
Schematic for the "famous", "strange", "odd" and "useless" ECL86 preamp
Not useless at all... totally funky. An SRPP with a pentode on the top. You should try disconnecting C10/C11 and see if things get better. Should lower the gain, give you a little regenerative feedback and eliminate a cap from the circuit.

dave
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Old 17th January 2003, 06:18 AM   #7
halojoy is offline halojoy  Sweden
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Default Re: ECL86 preamp?

Quote:
Originally posted by Hanzwillem
Hi!
I found a schematic for a ECL86 preamp here:

ECL86 preamp (in french)

Anyone any suggestions? Or could anyone point me to a thread or link with a list to think about when building your first tube amp.
Thanks, Hans-Willem
Is this the Preamp you are discussing.

Is this a "normal" and good application,
if I want to try to build my own ECL86 preamp?

Any alternatives?

halojoy - likes to SEE the circuits
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Old 17th January 2003, 06:26 AM   #8
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Default Re: Re: ECL86 preamp?

Quote:
Originally posted by halojoy
Is this a "normal" and good application,
if I want to try to build my own ECL86 preamp?
Creative is probably a better description. Good, well build it and see... we have one thumbs up in this thread and it was good enuff that the French magazine published it.

Pretty simple...

dave
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Old 17th January 2003, 08:11 AM   #9
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Quote:
Originally posted by planet10
An SRPP with a pentode on the top.
Now that i went back to actually attempt to understand what it says, they are calling it a Mu-follower.

dave
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Old 17th January 2003, 11:49 AM   #10
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Default LE MAXIMUM.

Hi,

They probably named it that way because of the maximum gain one has using the mu-follower.

Some remarks:

If you decide to get rid of the cathode bypass caps you need also to consider a considerable rise in output impedance (Zo).

On the circuit the gain is already reduced by the voltage divider at the input.
You can change gain setting there if you like.

Also,at switch on the output cap will charge to ~ 6VDC before settling to 0.

So to prevent this I would recommend using a mute switch at the output and have it engaged for at least one minute prior to operation.

All coupling caps should be of premium Q and it would not hurt to make the HT rails independent from each other to avoid IMD and crosstalk.
It will take a second xformer and a handful of components.

Alternatively one could use a pair of chokes to split the B+ into L + R channels.

Cheers,
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