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Long-Tailed Pair, pse. share your views...

Hi,

I'm in the process of building a Stereo EL34PP amp.
Actually the very first tube-amp I'm putting together and if it sounds well enough, it will also be the last :)
That's why I try to do 'do everything right' which of course won't be possible.
There seems to be as many 'right' solutions as there are constructors :D
Anyway, I've started on firm ground buying Lundahl Transformers and Winged =C= EL34's. That should set me off in the right direction I suppose.
I've also invested in a couple of Tungsol EF806SG which may turn-out more dubious. At least they SHOULD be less noisy that regular EF86's and of course especially run as triodes but I really can't say yet.
What I'm here to ask is more your opinions on the Long-Tailed Pair. I've come across a few NOS ECC808's that I'll try to use here. Since I understand a high AC-impedance of the Tail-resistor is required to balance the output reasonably I've decided to go for a Constant-Current-Source, see the attached pdf.
BUT, IF it's such a great idea to use a CCS, why isn't it done more widely?
Theoretically the AC-impedance of the CCS will be something close to:
R-Emitter x hFE x 2 -> 1000 x 160 x 2 = 320K-Ohm.
First of all; is this equation correct?
If it is is running the ECC808's at 1.5mA CCS reasonable?
What's your opinion on the construction in general?

Thanks in advance,

rgds,

/tri-comp
 

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Last edited:

SY

diyAudio Moderator Emeritus
2002-10-24 10:19 pm
Chicagoland
www.SYclotron.com
Since I understand a high AC-impedance of the Tail-resistor is required to balance the output reasonably I've decided to go for a Constant-Current-Source, see the attached pdf.
BUT, IF it's such a great idea to use a CCS, why isn't it done more widely?

Inertia. Incompetence. Superstition. A CCS is absolutely the right way to go for an LTP.

Noise is generally not an issue for power amp input tubes.
 
Thanks, it's what I feared ;)
Don't rock the boat, stay on the straight and narrow... etc.
I really couldn't see any reason why NOT to use the CCS.
Probably there are more views on the matter.
What about 'musicality', 'fidelity', 'tube-sound' and the rest of the boss-words?

/tri-comp
 
A CCS is absolutely the right way to go for an LTP.

I built my Baby Huey with a resistor in the tail as I didn't have all the parts for Gingertubes ccs. When the ccs was finally added it made imho a noticeable increase in the background detail. Of course it may only have straightened out some non-linear aspect of my personal frequency range - but what do I care!

:)
 
My opinion on LTP biasing is ccs usually is the best solution. On some circuits you don't want fully symmetric output, and in those cases it may be better with a simple resistor. For instance on totempole output following a PI you want one phase a little smaller than the other, and a resistor will be better than a CCS.
As it turns out, when the resistance of the tail goes over a certain value, a CCS really doesn't provide much benefit. I aint smart enough to figure it out in my head right now without looking up in the books first, but with a 6922 LTP a tail over 10kohm, or for petimeters say 20kohms, a CCS won't do much other than complicating your effort.
Of course, if you have limited voltage for the tail, a CCS is always the best way.
Just wanted to put my pennies in because it's not always lazyness or incompetence that lays behind the reasons for not using CCS.
 
http://www.diyaudio.com/forums/tube...-modern-mullard.html?highlight=modern+mullard

Would not change the LTP/CCS for anything in that design. It is a very musical/dynamic amp (Are those the words you are looking for?)


WoW :hypno2: , it took a while to read through all those postings.
VERY nice work. It shows how new parts enhance the vintage ones nicely.
Perhaps the EL34PP won't be my last amp after all... :D
Another aspect of building a nice amplifier is of course the mechanical details.
In your case they're superb! Nice and classy!
My EL will go on a laser cut steel-chassis painted into very deep hi-gloss red, almost black with hi-gloss black wooden side panels. Cover-shield for the transformers in steel painted into the same hi-gloss black. Tubes standing free.
Well, at least it's my INTENTION to have it like that and I know a lot of people in the right places to make it happen :D
It will also be nice to be able to document the working of the amplifier, but I just have basic tools like a scope, a generator and a dummy-load. So far no distortion-analyzer as they don't come cheap around here; working or not.

Rgds,

/tri-comp
 
Not to offend LTP fans but, sound quality wise how does it compare to another popular splitter the (concertina, cathodyne, split phase)? Sometimes called a Simplified Williamson. One member said he prefered it for more it's detailed sound. Maybe there are disadvantages that could be fixed with some sort of CCS?

A third type that seems to be liked is the Floating Paraphase. How too does it compare?

Both seem to be popular on low power amps.

Randy
 
A CCS is ideal, but no real CCS is actually ideal. A resistor is simpler and cheaper. Whether it is good enough depends on its value, and which valve you use. The aim is to get the common-mode gain as low as possible, when compared with the differential-mode gain. Diff-mode gain is a bit less than mu. Common-mode gain is about RL/(2Rk), and is likely to be somewhere in the region of 1 if a tail resistor is used, or 0.1(?) with a CCS. If you are happy with a few percent error (should be good enough - some people seem to deliberately design-in up to 10% error using other phase splitters), then you need mu bigger than around 30-40 for a resistor or 3-4 for CCS.

So if you want to use a resistor, use 12AT7 or 12AX7 as LTP. If you want to use 6SN7 or other low-mu triodes then you need a CCS, unless you like second-order distortion. 6DJ8 is right on the boundary: resistor OK, CCS slightly better.