Advice on this linestage build
 

I've put this together from different sources.

I basically want a buffered line stage with minimal gain since most digital sources today are able to drive a power amp fine, but may still require a little gain for the phono, I will have an RIAA stage fed into this for the phono input, but may need some gain depending on how much gain I get from the 6922/E88CC that it uses.

I have built on the Baxandall design found here:
https://www.angelfire.com/electronic...mp-Tone-A.html
https://www.angelfire.com/electronic...e-fig-06-1.gif

It uses a 12AU7 as a cathode follower for the input, I've modified it by adding an EF86 in triode mode at the end, which should give a modest gain, but make up for the losses in the tone stack.
The EF86 configuration is essentially based on the Svetlana preamp schematic.

I have also added balance controls and a tone bypass. I'm not 100% sure how this will behave.
Im also interested if there is a way to add NFB to the EF86 to reduce gain and improve linearity like ive read a bit about, I might be able to incorporate a switch into this, or utilize the tone bypass switch to do this, as its possible the volume/gain may increase when bypassed?

Re: Advice on this linestage build
 

The second valve misses a grid resistor.

I am afraid that your tone control is not going to work very effectively because the gain of the second valve is only about 20. That gain is used (by feedback) for the tone control. A gain of only 20 is too low to cover the range of the tone controls from the lowest to the highest settings (about - 15 dB to + 15 dB). In the schematic you linked to, the gain of the second valve is about 70.

Re: Advice on this linestage build
 

I wasnt sure if a grid resistor was needed or not on the second tube, the first one didnt have one.

Anyway, as far as gain goes, I thought 25 was going to be too much, If i configure this as a pentode will I be any better off?

Re: Advice on this linestage build
 

The first valve does have a grid resistor (470K).

With the EF86 in pentode mode the gain will be about 110.

To be honest: I am not sure about the effect of the high output impedance of the second valve in your circuit (which will be even higher with the EF86 in pentode mode) on the way the controls will work (will the frequency curves be the same?). In the schematic you linked to, the second valve is followed by a cathode-follower, which has low output impedance (and a 'gain' of a little under 1).

Re: Advice on this linestage build
 

Sorry, I was referring to the first schematic, neither of the last two stages have any grid resistors, on either the 12au7 or 12ax7.

I was hoping I could have reduced the tube count by using an EF86 in this way, plus i have a ton of the things so thought it would be good to make use of them.

I have been reading this here and it shows it in a pentode configuration.
It says the EF86 has alot of gain so has used an EF184. I have a few of these too.
https://www.sciencedirect.com/topics...ring/baxandall

Would I be better off keeping the EF86 in triode mode and adding a 12AU7 cathode follower at the end?
From what i understand, a 12AU7 can take a high impedance load and give a low impedance output which is useful in a line stage.

Re: Advice on this linestage build
 

I am not sure what you mean by the first schematic. If it is the schematic of the tweaked Brimar tone control, than the 12AX7 section has its grid returned to ground through the 470K and 2.2 M resistors. The grid of the 12AU7 section following the 12AX7 is directly coupled to the anode of the 12AX7 and therefore does not need a grid resistor.

Like I wrote, I think that the gain of an EF86 in triode mode is not high enough to cover the range from - 15 dB to + 15 dB. An EF184 in triode mode (mu is about 60 so the gain can be about 40) would be somewhat better. I have never used the EF184 before but I have read that the EF184 can be microphonic.

I hope some other forum members join in because the precise workings of tone controls is a little over my head (except in guitar amplifiers, I never build tone controls in my projects).

Re: Advice on this linestage build
 

It was this one here.
https://www.angelfire.com/electronic...e-fig-06-1.gif
I think you are looking at a different schematic.
There is a 1M from the grid to ground and perhaps thats what im missing?

I hear alot say the EF86 is microphonic too, im not sure if there is much difference between the two in regards to microphony.

Re: Advice on this linestage build
 

The EF86 was designed for low level audio applications. It has a bifilar filament (like the EF40 and the CF50 before it) to minimize heater hum. With respect to noise and microphonics I think they are rather good. The EF86's I use in the first stage of two of my guitar amplifiers are not prone to microphonics (but they are stand-alone amplifiers and the filaments are being fed dc voltage).

The EF184 was designed for the IF stages of televisions, so also mostly for low level signals. But they were not designed with audio in mind (as far as I know, they do not have bifilar filaments). I know that Radford used them in their STA-100 amplifier as phase splitter but there they operate with high input signals.

The forerunner of the EF184 was the EF80. Some two years ago there was a discussion on this forum about its usability for audio: https://www.vintage-radio.net/forum/...d.php?t=157707

Re: Advice on this linestage build
 

What you perhaps could do is use the schematic of the passive tone control on the site you linked to, preceded by an EF86 in triode mode + a 12AU7 cathode-follower (dc-coupled). The EF86 gives you a gain of about 20, so of about 26 dB, while the tone control takes 20 dB off of that. You than have about 6 dB headroom.

Re: Advice on this linestage build
 

Interesting what you say about the EF184, I have no idea if it has a spiral heater or not.
I guess going to DC heaters could solve this?
I know both the EF80 and EF184 are popular tubes with builders.

I was actually wondering about that myself.
I had also considered using it as the first gain stage, but I think impedance matching would be an issue?

From what I understand, with a line stage, particularly if you are using it as a buffer with little gain, is you want to keep output impedance as low as possible, but able to match various loads on the input?

Re: Advice on this linestage build
 

Mainly cos the EF80 is cheap and the EF184 cos it's mentioned quite often in thread's. The 184 comes up a lot I think cos it was used in a lot of scope front ends, it's also developed a self perpetuating self affirmation cult due to all the threads on it, not to say it's a "bad" valve.

That tone control circuit you attached is touchy, as regards gain as Robert says, I tried modding it using an ECF82 after the tone control to get extra gain in one valve base, no go. As per the author's schematic it works, mess with the gain and it's essentially an attenuator. In my case I added an ECC82 gain stage, 47k Ra, 1m grid leak. 1k ish Rk after the published circuit. You could use a CCDA line stage though.

Andy.

Re: Advice on this linestage build
 

OK, well I may just end up going with the schematic as it is.
Will adding balance controls or a tone bypass affect anything?

I was hoping to make use of the EF86 in my build just for the sake I have so many of the things.

If anyone has any similar schematics to share, that would be great.

Re: Advice on this linestage build
 

EF86 or better still EF184 in pentode form giving around x 400 gain followed by a cathode follower then wrap the feedback around the Baxandall network.

There are many ways of making a pre amp and hundreds of schematics on the web.

There will be plenty of designs from back in the day which you could just remove the phono section from. Beware of some old designs being intended to drive >100K load though! Keep the cathode follower.

If you didn't want tone controls I'd have linked to a design of my own intended for easy DIY build.

Re: Advice on this linestage build
 

I put this together (my suggestion in post #9 but with a cathode-follower after the tone controls).

Re: Advice on this linestage build
 

FWIW after seeing reports of super linearity from EF86 when triode strapped I tried it....

It's not true! I tried several samples, tried different methods of triode strapping and experimented with load line etc and distortion was disappointing.

Re: Advice on this linestage build
 

That sort of sounds similar to what Robert has suggested in post #14 except his is not a baxandall feedback loop, I may be tempted to go with something like that and source some EF184 tubes perhaps.
How easy is it to implement a bypass switch on a baxandall design? Ignoring the rest of my schematic on the first post, would that way of inserting a switch work? Will the volume level change significantly when switchong on or off the tone stack?

Thanks for that, this looks similar to what I was looking at to begin with, but then I got onto the baxandall unity gain design and thought that may have been a better option, but if i can make this work with a baxandall layout, ill probably go with this.


Thats interesting to hear, I have read from others experiences that if you feed negative feedback into a triode strapped EF86, that it improves linearity immensely.
Does that mean that an ef86 as a pentode has poor linearity?

Re: Advice on this linestage build
 

Feeding an EF86with an active load improves linearity, just a MJE350 CCS. That said I don't mind a bit of distortion in my amps, else why use valves?

Andy.

Re: Advice on this linestage build
 

Hello Jez,

Could you give values for Vb, Ra, Rg2 and Rk to achieve a gain of 400 with an EF86 and an EF184?

Can you share some of your test results with us? Did your results differ from the distortion figures in this datasheet (see page 3), and if so, in what way? https://frank.pocnet.net/sheets/030/e/EF86.pdf

Re: Advice on this linestage build
 

There seems to be some sort of cultural imperative driving people towards using triodes everywhere, but as Robert has pointed out, triodes don't have the necessary gain to make the active Baxendall tone control circuit work. Towards max boost it will run out of loop gain.

It's a feedback circuit and all the usual equations apply.

You could try a 2-stage amplifier, but then that doesn't invert so the feedback is the wrong sense. Alternatively a cascode would boost the gain.

Even with a pentode amplifying its little heart out, the gain is somewhat limited by the impedance of the feedback network loading the anode resistor. It's still not exactly wonderful, but nevertheless, a lot better than a triode alone.

What the circuit really needs is an NE5534. Don't worry, it won't pollute your sound. It's likely that some of what you listen to has already been through a load of 'em in the studio.

David

Re: Advice on this linestage build
 

I'm afraid I have better things to do than act as a free R & D service! There will be many designs already around. With active load you can get gains of thousands!

The distortion figures in that datasheet look about right yes and similar to what I obtained. There is a Svetlana datasheet suggesting 0.1% for something like 10V output!! No way. Suggestions of triode strapped EF86 being super linear abound on the internet also....