Anode choke/Bypass cap/Fixed bias

[Frankkk]

Hey everyone,
I am new here and I am already on a phono preamp project
In fact it's the really beginning and I am doing my first stage with E180CC then 7119
The problem is that I have many choices :
Direct coupling between the two first tubes and use cathode caps.
Capacitor coupling and I can choose best operating point for both tube, little Rk so I can remove cathode capacitor.
I saw we can use CCS too etc..
And is it better to use anode choke or resistor ?
I search a lot in the forum but I can't find a lot informations... Everything seems good and bad..
Any advice ?

Thank you

 

[jazbo8]

If you need a lot of anode swing, then use a choke, whereas anode CCS can really lighten the anode load and reduce the distortion (if you don't need the large swing).

 

[rayma]

I am already on a phono preamp project

Which cartridge will you use, and how much gain do you want?

 

[kaputt]

It is "better" to use anode choke than resistor because your load gets more linear. Ri of the E180CC is low enough to work with a choke as load. A CCS might work even better as the load impedance is higher and that makes it easier to linearise the equalisation circuit (what do want to use? LCR, LR or lame RC ) needed in a phono stage. The E180CC draws enough current to easily work with LED bias to get rid of the cathode resistor and needed bypass cap.

 

[Frankkk]

So many reply, thank you !!

I use MC cartridge, I already have step up transformer who give me 24dB.
A preamp with 45dB of gain will be really confortable for me.
I will go for LRC, the Silk 10k is interesting to use 10k:10k transformer after the 7119 tube.
The silvercore too : riaa filter | silvercore.de

So in fact I already have the good load for the 7119. The question is for the E180CC.



So, the best choice to load the E180CC seems the anode choke and CCS.
I am afraid with CCS because CCS don't fix the cathode tension so I can have so noise. With Rk, at least, I fix the tension Uk.


For the coupling, I also have the choice between direct or cap coupling.
The advantage of cap coupling is that I can choice the best operating point to have little Rk and remove bypass caps.

 

[john_tracy]

One thing to keep in mind when using a CCS load: the bias current will be fixed and with either LED bias or a cathode resistor the grid bias voltage will be fixed. That means the plate voltage will go where it needs to go as the tube ages or different tubes are substituted in. With a direct connection the voltage on the grid of the second stage will not be fixed and this fact must be accounted for in the design. You could I guess use a CCS on the cathode of the 2nd stage bypassed with a cap and it would set the bias and allow the voltage on the grid to change.

With a plate choke and a regulated B+ you can more or less fix the plate voltage and in the case of LED bias the plate current will vary as tube parameters change and with a cathode resistor there will be some "compensation" as the bias current and grid voltage adjust to changes.

 

[Frankkk]

So, the match is between anode choke and CCS. I am not fond of LED bias :/

Can you describe me the subjective pro and cons for the sound of removing the bypass capacitor ? And between DC or cap coupling ?

I remember someone saying that remove the bypass capacitor make the sound "slow" and thin.

If I do direct coupling, I am obligated to bypass the cathode because Rk will be high.

 

[kaputt]

There are different ways of dc coupling and there are other means of getting cathode potential higher than grid potential than just a huge Rk.
Even for the input stage you will need to bypass the cathode resistor if you need the full mu of the tube. When you use an anode choke on the input tube, why not reconsider and use an interstage transformer? Something nice with an 80% nickel core goes a long way in a phono stage; at least for me.

 

[Frankkk]

I already have a interstage transformer before the filter RIAA and one output transformer.

Add a third transformer will add a low-cut frequency too much maybe

But it's true, with my three tubes E180CC, 7119, 7119 I need almost full mu to have my 45dB of gain.

 

[6A3sUMMER]

The moving coil step up transformer does not have to deal with DC through the windings, which would saturate the core, so the core is not normally air gaped.

Air gapped transformers and chokes on the other hand, are more sensitive to hum that comes from any stray AC magnetic fields, such as a power transformer, or a turntable motor.

And a plate load choke has DC through it, so it is air gapped.
Air gapped chokes and transformers need more turns to get the same inductance (and low frequency response), as non-air-gapped chokes and transformers.
More turns may mean more distributed capacitance, and high frequency roll-off.

Phono signals are low level, so a little hum pickup 'goes a long way'.

Just be sure of the hum sensitivity of the choke you pick, and also the distance and strength of magnetic fields that are near to that choke.