Split load LTPI tail resistor? Can someone explain?

[Gainzilla]

Hi all,

I've been really studying the LTPI this week, looking at how different topologies utilize different sets of values throughout, but paying especially close attention to the variances in EL84 based amps. During my travels, I uncovered what looks like a split-load tail resistor. What does this thing do exactly? I've noticed it specifically in designs that appear optimized for high gain over at the 18 watt forums.

Does it provide the appearance of a smaller value tail resistor for the second input/NFB, but an overall larger tail for the LPTI (and thereby allowing a bit of fine-tuning NFB/PA distortion balance)? Apologies if this is ubiquitous knowledge.

Cheers!

[tubeswell]

You mean a tail resistor and a bias resistor (the latter being between the junction of the grid leak resistors and the joined cathodes on a cathode-biased* LTP).

The tail resistor functions as a constant current source. The bias resistor sets the grid-to-cathode voltage.

* You can bias a LTP differently, i.e. use fixed bias, in which case then you only need a tail resistor.

[Malcolm Irving]

Merlin discuses this particular circuit on pages 164 to 167 of the second edition of his pre-amp book:

http://www.valvewizard.co.uk/

It's a bit too involved to try to summarise here. But that book is highly recommended, if you don't already have it.

[jazbo8]

And please do not refer to the circuit as a split load, it only causes more confusion. It's just a LTP.

[pdf64]

Also see Aiken's pages http://www.aikenamps.com/index.php/the-long-tail-pair and http://www.aikenamps.com/index.php/desi ... e-feedback

[Gainzilla]

Thanks for the replies, and apologies if my naïveté around the correct nomenclature caused any confusion.

I have the first edition of Merlin's preamp book and have been wondering if it's worth it to get the second... I suppose the answer is yes.

To clarify a bit, the exact resistor I'm talking about is the 4.7k resistor just below the tail. I realize I didn't articulate that very well in the original post. Does that clarification change the way you might answer the original question? How does that little guy impact the circuit beyond what the tail resistor does? I've read both Aiken's LTPI and designing LTPI pages as well as Merlin's circuit explanation. I feel like I have a handle on the tail resistors function ("pseudo constant current source" providing local negative feedback, setting the overall headroom of the PI...). I'll look into the second edition of Merlin's book, but until then, any info is most appreciated.

cheers!

[brewdude]

Thanks for the replies, and apologies if my naïveté around the correct nomenclature caused any confusion.

I have the first edition of Merlin's preamp book and have been wondering if it's worth it to get the second... I suppose the answer is yes.

To clarify a bit, the exact resistor I'm talking about is the 4.7k resistor just below the tail. I realize I didn't articulate that very well in the original post. Does that clarification change the way you might answer the original question? How does that little guy impact the circuit beyond what the tail resistor does? I've read both Aiken's LTPI and designing LTPI pages as well as Merlin's circuit explanation. I feel like I have a handle on the tail resistors function ("pseudo constant current source" providing local negative feedback, setting the overall headroom of the PI...). I'll look into the second edition of Merlin's book, but until then, any info is most appreciated.

cheers!

I'm no expert, and it's hard to tell from the portion of the schematic posted, but could that be part of the nfb loop/presence circuit?

[Colossal]

I'm no expert, and it's hard to tell from the portion of the schematic posted, but could that be part of the nfb loop/presence circuit?

Yeah, that's the later Marshall presence circuit. 4k7 in parallel with 25kB-0.1uF. The original 5kB-0.1uF (DC on the pot) sounds better, IMO.

[jazbo8]

To clarify a bit, the exact resistor I'm talking about is the 4.7k resistor just below the tail. How does that little guy impact the circuit beyond what the tail resistor does?

You can think of the tail as being 14.7k but with the lower 4.7k also serving as a part of the negative feedback network that's connected to the output of the amplifier. Hope this helps...

[Gainzilla]

Aaaaaah, ok I see it now. I just got done doing a ton of googling after reading your responses and feel like I understand what's going on. Colossal touched on it being the newer style Marshall Circuit, which if I understand correctly, the main differences are that it always keeps a certain amount of NFB in the circuit, and provides a bit more balance to the PI.

Is the general consensus that the older style sounds better? In what way does it sound better to you, Colossal?

Cheers, and thanks again!

[Colossal]

Aaaaaah, ok I see it now. I just got done doing a ton of googling after reading your responses and feel like I understand what's going on. Colossal touched on it being the newer style Marshall Circuit, which if I understand correctly, the main differences are that it always keeps a certain amount of NFB in the circuit, and provides a bit more balance to the PI.

Is the general consensus that the older style sounds better? In what way does it sound better to you, Colossal?

Cheers, and thanks again!

The "newer" Marshall presence arrangement (as shown on your schematic) keeps DC off the pot since it is blocked by the 0.1uF. The older version can get some hiss/scuffing DC-on-the-pot noise when you turn the knob, but it's really minor. I find that the older version has much more range and usefulness in the rotation where the newer version kind of comes on towards the end of the rotation.

[Gainzilla]

Cool, thanks man! Of course, now this means I'm gonna to have to try it. Lol