Such a common misunderstanding...
White Cathode Follower sound
- Thread starter MaxDM
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Winston OBoogie
Well-known member
I like anode chokes myself too. But one thing to think about if you're then coupling to a White follower - or even a humble cathode follower - is the voltage swing capabilities of each stage. Your choke loaded anode will be able to swing almost double the H.T. supply voltage it's supplied with.
In order for your follower to swing the same amount of signal and preserve that headroom, it'll need twice the H.T. supply voltage that the anode choke has.
You could cap couple the White and then establish it's own middle DC bias point, but if that is run from the same H.T. voltage as the proceeding stage, you just lost half your voltage swing on the output stage and it has 6dB less headroom.
Yes. Always use a high Gm tube. And the E88CC is a great one for that.
My understanding is that the designer actually didn't "design". He took parts of existing, tried and true schematics and put them together. I'm not dissing, I think he was concerned about making this product within time and money constraints.
Winston OBoogie
Well-known member
Sure, why not. There are gazillions of solutions and ways to do what that Motown amp did. Just pick any decent valve mic amp circuit that tickles yer fancy, rip out it's input transformer, install a suitable 1st grid leak of 2.2M (that's my usual 'go to' but season to taste if you need), wire in a 1/4" input jack... And "Robert's Your Live In Lover", "Bob's Your Uncle", "Etcetera" "Et Viola!".
Plate inductors are about increasing output voltage, when cath-followers are about increasing current output. One does not necessarily need both. It's usually one or the other.
Winston OBoogie
Well-known member
Sure, the choke increases the valve's output voltage swing cabability, but It's not always the only reason I'd go for it though. The loadline becomes more horizontal and the valve linearity goes up too.
And if we've just increased headroom and linearity by spending good money on a choke, then why not keep that benefit in hand by making sure our follower is able to swing the same amount of voltage while supplying the current we need for the outside world. Yes? No?
Just my own thoughts on what Max is proposing.
Any one of us on here could come up with really good implementations and, left to our own devices, they'd probably all be quite different but equally as good at the job.
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jacomart
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jacomart
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Winston OBoogie
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ruffrecords
Well-known member
This has always puzzled me. Making a decent anode choke that will carry dc and have a decent frequency response is pretty tough but quite possible. Why add a follower to lower the output impedance. Why not just add a secondary?
Cheers
Ian
Winston OBoogie
Well-known member
Fashion? Current trends?
Seriously though, I suppose you could look at the effect of the DC current through a coil which has a secondary carrying your signal.
Some folks might prefer to not have those non linearities.
A follower into a cap that goes to the outside world would certainly measure better in that regard, loading considerations etc notwithstanding.
If using an output transformer:
A choke fed parallel feed into a suitable step-down would be better than a transformer loaded stage if the effects of DC is your concern.
And a directly coupled (to the preceding stage) White follower into a smaller step-down would be even better if that's your goal.
I don't know that I'm advocating any particular way to do it, I'm still trying to figure this stuff out myself.
My comments re. the choke and the follower were only pertaining to what it seemed Max wanted to do.
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ruffrecords
Well-known member
I was wondering if there was any particular engineering reason for doing it? The combination of plate choke followed by a capacitor coupled gapless transformer is common enough but pretty much all the designs like that I have come across are using pentodes. Maybe it is a pentode specific thing.
Neve's early class A transistor designs use a transformer to obtain both the double Vcc output swing plus some gain; but there is an option to have just an inductor if a balanced out is not required.
Going back further, Rupert used an EF86 with a plate transformer in one of his early tube mixers.
You know me, just trying to understand these things better.
Cheers
Ian
Neve's early class A transistor designs use a transformer to obtain both the double Vcc output swing plus some gain; but there is an option to have just an inductor if a balanced out is not required.
Going back further, Rupert used an EF86 with a plate transformer in one of his early tube mixers.
You know me, just trying to understand these things better.
Cheers
Ian
jacomart
Well-known member
Winston OBoogie
Well-known member
Yep, me too Ian.
You know as much and probably more than I do so I'm probably not much help in that regard!
However, if I ever got to the point where I felt I understood most things, it'd probably take a good bit of the fun out of it all.
.
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ruffrecords
Well-known member
Absolutely. The fun is in the discovery alright.Yep, me too Ian.
You know as much and probably more than I do so I'm probably not much help in that regard!
However, if I ever got to the point where I felt I understood most things, it'd probably take a good bit of the fun out of it all.
Cheers
Ian
I've seen chokes used on triodes as well, by Neumann, for example.
having a transformer would load the stage differently than an anode choke. I would rather keep the voltage amplification separated from the output load, hence the WCF.
As soon as you start to put a complex load in the system, it's bound to be affected by it. The WCF would ideally mirror the signal, without loading the voltage amp in any significant way.
Generally speaking, you can use a choke to load anything, including transistors, and it behaves sort of like a current source, in terms of resolving the signal, sound-wise. An active current source is technically better, but chokes do have something about them, which I find makes things sound punchy, and not too 'electronic', which sometimes happens, with active loads.
I did a bit of research, and it would appear that for an ECC88, the anode resistor should be closer to 140 ohms, than the 809 ohm resistor I saw in the Mastering Lab pre.. Either I misread the color codes, or the PC board, or, they did it on purpose?
ruffrecords
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Winston OBoogie
Well-known member
Your question earlier regarding "engineering reasons"for doing something or not got me thinking.
It's an interesting question. And one I don't think I have an all encompassing answer for.
I was pondering why, at times, I've gone one way rather than another...
So I started thinking about the reasons I could give, if I were to go with a purely engineering approach, as to why I would even be interested in building stuff with tubes.
From a purely engineering point of view, at the signal levels we're dealing with on here, surely an IC with magnitudes less distortion would be a more sound engineering choice yes?
Same with a transformer on a mic input. We don't generally need galvanic isolation there, so why spend upwards of £100 for a nice big expensive bulky thing when I can just parallel a few $0.25 pairs of transistors, or get it All in one easy IC package.
Compared to a couple of caps into a low noise BJT diff pair, that transformer coupled tube thing measures worse in the usual ways this industry classifies things, uses big parts. And big nasty voltages to boot.
Yet, I still would rather go with the tubes.
I just like it I guess.
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Stevie342000
Active member
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- Jan 4, 2014
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It might show my lack of understanding on how the WCF or a SRPP works but do you not run into Heater Cathode Insulation Voltage Breakdown voltage issue with either circuit if an ECC88 valve/tube is to be used?
Winston OBoogie
Well-known member
Yes, good point. If you had a heater supply referenced to ground as is norm, then you very well could/will have those issues.
What I've done, is elevate the heater to between +40 - +60 volts above ground or somewhere thereabouts. I guess it depends on what your main H.T. supply is, and which valves we're using, but just elevate it so that both top and bottom are within the specified range.
You could use two heater supplies and just elevate for the top valve (if it's a separate bottle) but it's extra stuff to buy and do.
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