6GM8/ECC86 Tube preamp for 30VDC supply

[lineup]

I have decided to build my first TUBE amplifier. Using ECC86 triodes from Telefunken.

It is a preamp with a little voltage gain. I hope getting good tube sound.

There are 3 different ways to go. The first is simply using 1/2 ECC86 per each channel. This only requires one tube for stereo.

The second way is to parallel triodes in one tube. This needs 2 tubes for stereo Gives twice the current and lowers the output impedance.

The third option is to make a differential tube circuit. This also needs 2 tubes for stereo. This option will preserve the correct phase. While option one and two gives inverted output.

I post here 3 images with the 3 choices. Which one should I go for

 

[kylej1050]

What's it driving? That'll ultimately decide what circuit you need.

 

[artosalo]

Do you need gain? If not, a cathode follower has low output impedance and does not invert the signal.

 

[popilin]

I post here 3 images with the 3 choices. Which one should I go for

None of the above, an SRPP stage (+B=60V) will have lower THD, mostly second harmonic.

 

[Lingwendil]

What sort of load is the project meant to drive?

 

[lineup]

None of the above, an SRPP stage (+B=60V) will have lower THD, mostly second harmonic.

Thanks for that option.

Now, I want a lot of harmonic distortion.
This is why I not use solid state, transistor/opamp, for this project.
I want to get 'tube sound'.

What's it driving? That'll ultimately decide what circuit you need.

What sort of load is the project meant to drive?

It will drive my Gainclone input.
Which need +/- 0.400 Volt peak for full output.
And the input impedance of this power amp is 47kOhm.

 

[PRR]

T...I want a lot of harmonic distortion....I want to get 'tube sound'.

...need +/- 0.400 Volt peak for full output.... 47kOhm.

You can do that with one of about any triode, 10K load, just a few volts of supply. THD may approach 5% not-nasty.

Given 30V supply you are likely to be nearer 0.3% THD, not "a lot" but perhaps euphonic.

If you truly want "a lot of harmonic distortion" then you want to dial-down the B+ from 30V down to a couple of Volts. At this power level a 10K pot can turn-down the power until the harmonic distortion reaches "a lot".

 

[artosalo]

Bias the stage 1. to high plate voltage (with cathode resistor) and you get high 2nd harmonic distortion.

Below circuit generates 4.6 % 2nd harmonic distortion at 2 Vpp output level.

 

[popilin]

Now, I want a lot of harmonic distortion.
This is why I not use solid state, transistor/opamp, for this project.
I want to get 'tube sound'.

Some of us do use valves because they are way more linear than SS, so "tube sound" is not a "sound" at all.

Even at high THD, the harmonic distribution can make the distortion inaudible, because human ear is deaf to a certain pattern of THD.

Warn you that valves are mostly a no return road.

 

[lineup]

Bias the stage 1. to high plate voltage (with cathode resistor) and you get high 2nd harmonic distortion.

Below circuit generates 4.6 % 2nd harmonic distortion at 2 Vpp output level.

Thanks for your circuit.

What is the cathode capacitor for?

You can do that with one of about any triode, 10K load, just a few volts of supply. THD may approach 5% not-nasty.

Given 30V supply you are likely to be nearer 0.3% THD, not "a lot" but perhaps euphonic.

If you truly want "a lot of harmonic distortion" then you want to dial-down the B+ from 30V down to a couple of Volts. At this power level a 10K pot can turn-down the power until the harmonic distortion reaches "a lot".

Well, the goal is not to produce a lot of THD.

The main goal is to learn using tubes.
Remember this is my first try. I have been into transistors all my life.

As for now I find my option number 2 above most interesting.
To parallel the two halves for an output current about 2.5mA at 30Volt supply.

It may not produce much THD - and this is also fine by me.

Another question.

I have bought 5 ECC86 tubes.
Is there any good way to match the triodes?
How would you setup a test circuit?
How to measure.

 

[artosalo]

The main goal is to learn using tubes.

What is the cathode capacitor for?

Download this book. It is useful to study.

http://www.tubebooks.org/Books/York_Amplifiers.pdf

 

[lineup]

Thanks @artosalo.
I will now download that book.

 

[stocktrader200]

This consistent rant tubes are more linear the SS is pure bunk. I can get 0.01% distortion at 3v p-p from a npn single transistor amp gain of 10 B+ of 25v. A 6922 produces way more distortion but tube distortion is very musical and sounds fantastic if at the proper 1- 3 % second harmonic. To much tube sounds mushy and lifeless. Tubes do not exibit any kind of vbe 0.7v turn on /off knuckle or latchup like bjt transistors. They are most closely like a Jfet transistor.

 

[lineup]

Yes, they are like JFET transistors.

I have changed the 2nd diagram in first post. Now it will give 2x3.0mA = 6mA through each channel. And the cathode voltage is now 1.0 VDC.

I will get the tubes today or maybe at wednesday. I bought 5 tubes ECC86. So I can select a matching pair.

 

[artosalo]

I have never seen necessary to have matched pre amplifier tubes.

 

[316a]

ECC86 is not a particularly good sounding tube , but I suppose it does have novelty value due to the low voltage operating conditions .

There are much better sounding tubes out there...

316a

 

[artosalo]

If you make a listening test for a group of people, how big percentage agree with you?

 

[kylej1050]

If you're using an LM317 at the plate of the tube anyways why not run it as a CCS? Looks like the tube is fairly linear at 1mA between -0.25 and -0.75V

 

[lineup]

Heater for one ECC86

Here is the heater circuit I will use.

It uses LM317T as a constant current source. The current will be 327mA.

Of course I will use 2 such ciruits. For using 2 tubes in stereo. But only one transformer 30VA 2x9VAC.

 

[artosalo]

According to Philips data of ECC86, pin 9 is not connected to the heater at all. So one heater pin should also be grounded.