Fewest gain stages.....

[mooreamps]

At the time, I wasn't trying to assert it was a "new idea".. But, at the time, I wasn't seeing anyone else doing it and I was not getting the impression some of the others were being "lighthearted" about it... But then again, I'm not seeing anyone doing the P/P - S.E. trick with their P/I's either... also made possible using fixed bias inside the P/I...

I don't even think KOC or Mr. Conner knows how I do this one yet ... :}


-g

[Steve Conner]

This is my favourite LED biasing example.
http://syclotron.com/rld_signal_circuit.jpg

[imaradiostar]

This is my favourite LED biasing example.
http://syclotron.com/rld_signal_circuit.jpg

whoa. That is a lot of LED's. He could have just used a big zener or even a light bulb but that is pretty entertaining.

OK, now we're off topic.

Back on topic- fewest gain stages. high gain pentode into triode cathodyne splitter into highish gain output tubes- technically one signal gain stage, one unity gain stage (to each output grid) and power gain at the output. About as simple as can be while still being a usable push pull amp.

jamie

[redelephant]

whoa. That is a lot of LED's. He could have just used a big zener or even a light bulb but that is pretty entertaining.

going off topic again The LED biasing scheme is popular among some HiFi amp tinkerers and they have found out that red LEDs provide the best sonics A light bulb wouldn't work since it effectively works more like a constant current source and has way too high imedance. A zener supposedly sounds bad and probably drifts too much...

[imaradiostar]

redelephant, you're right, I should have said what I meant. Use a light bulb to eat up most of the current and use a parallel zener or LED string to provide a fixed DC bias without the zeners/LED's having to do all of the heavy lifting. For that matter- let a mosfet do the work. And why do any of the above when a fixed bias supply is so easy to build and provides the same effect?

Still, this is off topic and we're adding complexity, not simplifying.

jamie

[mooreamps]

Alright.. Use a 6U8 strapped as an SRPP driving a KT-88..

-g

[Ken Moon]

So, everyone is doing the fixed bias thing on their preamps these days ? ? I thought the thing over there was their ability to run a "green" led in parallel with a "red" led... :|

-g

No, actually "the thing" at AX84 is open-minded inquiry, and discussions of electronics, amp design and amp building, without a devotion to a particular vintage design, by a bunch of friendly and knowledgeable folks, many of whom also come here.

AX84 did start with a simple one-12AX7-and-one-EL84 5-watt SE amp (hence the name AX84), but that was almost 10 years ago. Many amp-building newbies go there to learn, but there are also many respected experts who like to hang out there and help the new guys.

btw, Alex Kenis is one of the sharpest minds out there in the amp design world. I know he has built hundreds of amps, and done more experimenting and critical listening than you would believe possible at his age - as well as being a great guitarist and studio engineer, with 4 studio albums under his belt so far (with Aletheian and Becoming the Archetype).

The circuit he posted at AX84, and I re-posted here, is a very useful circuit - why does the very act of posting it make you react so negatively and in such a snarky way?

What confuses me, Gary, is why you feel such a strong need to put other sites and people down, when your statements are based on nothing but ignorance.

Can you show me a single reference on AX84 where someone bragged about "their ability to run a "green" led in parallel with a "red" led" - I doubt you'll find one, but you will find plenty of discussions about using LEDs to bias tubes, and why it may or may not be a good idea in different situations, going back at least 6 years.

It's really too bad, because you seem to have an interesting electronics background, and a quirky way of looking at things that can add a little "outside the box" thinking to discussions...

[kg]

But mosfets don't count! :wink: In all seriousness- if they're used in places where they don't really contribute tonal change through distortion then they're not part of the overall tone.

if that's the case i'd say they're a part that can (and should) be removed from the design.

put another way, if you can't hear whether a part is there or not, does it need to be in the circuit?

[Russ]

Thanks for the post!

Wow, that seems so simple...so except for the different bias point, can an LND150 be substituted directly for a 12ax7?
Any cases where it can't be substituted?

russ

[Steve Conner]

Well, except for the different bias point and the different gain.

MOSFETs behave more like pentodes than triodes. Even though they only have three electrodes, there is an internal "screen grid" made of quantum semiconductor weirdness.

But bearing this in mind, the LND150 is close enough to being some sort of tube that you could make an entire preamp out of them. They clip differently to tubes, but there are circuits out there for dealing with that. Alex's diode in the source is one of these tricks, it helps to soften the cutoff.

For many positions in a guitar amp circuit (cathodyne PI, cathode follower, trem oscillator) the differences are irrelevant and you can use the LND150 directly.

This is very much my own area of research right now. OK, so I got some LND150 samples and am looking for something to make with them.

[mooreamps]

btw, Alex Kenis is one of the sharpest minds out there in the amp design world. I know he has built hundreds of amps, and done more experimenting and critical listening than you would believe possible at his age - as well as being a great guitarist and studio engineer, with 4 studio albums under his belt so far (with Aletheian and Becoming the Archetype).

.

Fine... then have him show me the P/P - S.E. P/I trick..

-g

[Ken Moon]

MOSFETs behave more like pentodes than triodes. Even though they only have three electrodes, there is an internal "screen grid" made of quantum semiconductor weirdness.

According to Mr. Dumble, that's the problem with semiconductors - the free range electrons get trapped in the crispy lettuce and have to be vacuumed out

[Russ]

For many positions in a guitar amp circuit (cathodyne PI, cathode follower, trem oscillator) the differences are irrelevant and you can use the LND150 directly.

Yeah, like fx loop send/return stages, or the input stage, places where you wouldn't expect or want clipping. Seems like a great way to make a simpler, cheaper, smaller, more realiable amp. By the way, can two LND150s be paralleled like a pair of 12ax7? And if so, would you get a similar 3db of noise reduction?

thanks,
russ

[imaradiostar]

if that's the case i'd say they're a part that can (and should) be removed from the design.

put another way, if you can't hear whether a part is there or not, does it need to be in the circuit?

That's an excellent point. I suppose if you're using an LND150 to replace an EF86 at the front of an AC15 type circuit you ARE hearing it. You're just avoiding a mode of operation that would show a limitation of the solid state part.

I don't think I'd want to parallel LND150's for lower noise because it wouldn't need it. You could parallel them with different source bypass caps like a plexi but I think the LND150 is better as a pentode replacement.

jamie

[Russ]

I don't think I'd want to parallel LND150's for lower noise because it wouldn't need it. You could parallel them with different source bypass caps like a plexi but I think the LND150 is better as a pentode replacement.
jamie

Would paralleling get you lower output impedance?
Sorry, but I'm not familiar with using pentodes in preamps. How would the output impedance of a single LND150 stage compare with a 12ax7 stage in the first place?

russ