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Old 12th February 2019, 07:19 PM   #11
GoatGuy is offline GoatGuy  United States
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I'm a little confused by this comment…
Are you saying I shouldn't be using any local feedback around the triode?
Can you explain further?
Local negative feedback is a fine thing; it is quite commonly used to "tame" the nonlinearities of the triode (and to some degree, pentodes, too.)

BUT … if you were asking my opinion as to the better of many possible local negative feedback topologies, I prefer the non-bypassed cathode resistor method, and balancing the VA operating point to somewhere between ½ and ⅔ of B+ nominally.

The unbypassed cathode resistor triode stage has a significant drop in gain (which sounds per your discussion as beneficial!) compared to either bypassed resistor biasing or LED (or similar) biasing. Because it is also negative feedback on the most local level, it also works well to linearize the triode gain stage. Try it: you need only temporarily disconnect the CK cathode bypass cap from the 560 Ω or 600 Ω cathode resistor. Remember: output will be lower. But listen for the "nicer" acoustic signature of the triode.
Initially I ran the triode open loop (and so the pentode…), after degenerating the pentode because gain was rather high, then I went back to the triode and degenerated that too.
See above.
IIRC at either bias point, the triode still has headroom, Well after the pentode grid is driven far enough to clip the output.
One thing to consider is … that you really don't want the output of the “front end” to be higher than the negative bias of the output pentode. If VK is running 30 volts above ground (say), then you definitely don't want the output of the prior stage to have more than a ±15 volt swing. Above that, and you get into source material driving the pentode grid into positive-relative-to-cathode territory, which of course causes it to draw real grid current, … which in turn seriously changes the load to the prior driver stage. All of which tends to create both linear and nonlinear distortion.

Hope that helps.
GoatGuy
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Old 12th February 2019, 08:38 PM   #12
mondogenerator is offline mondogenerator  England
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GoatGuy,

I can't disagree with any of the facts, my opinion isn't formed yet, regarding sound, but yes I probably could use some gain reduction in gain.
The comment about stage loading and interaction is spot on the mark. The triode stage is loaded down to a degree, i cant recall the exact figures but i think gain was somewhat less than what i had expected.

A couple of days back, I'd tried 6C6B triode, with a mu of 20-25. My thinking was there wasn't enough drive current from the higher mu triodes.

As it happens it's not the case at all haha!

What I learned after a few days pottering about, considering if I should try to use a intermediate follower stage or not, was that the pentode stage was happier with higher gain.

The unbypassed cathode on the triode stage is an easy one to try

Which makes me wonder....

Zener regulated screen voltage might be something I try next, I think I have some 47V zener diodes....May be A string of red LEDs for fine adjustment?

Hmm...food for thought. Thanks.
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Last edited by mondogenerator; 12th February 2019 at 08:44 PM.
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Old 12th February 2019, 09:34 PM   #13
GoatGuy is offline GoatGuy  United States
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PS: when you "try" the unbiassed cathode resistor, remember also to unhook the local negative feedback. Its gain reducing won't be needed (or desired).

PS: In general I tend toward 3-stage small amplifiers such as you are building. One of the more significant reasons (rarely cited!) is that it allows the tube curve-shape asymmetries to be somewhat balanced between signal phases. Better, not best.

The "preamp" stage should be spec'd with "one of the most linear, least power-law" triodes. The next stage should be low-mu, and only "barely amplify at all" (maybe 3× or so) This sets things up for the final stage, well.

GoatGuy

(PS: a zener regulated screen isn't a bad idea, all in all. I've used it quite successfully. Tho design usually calls for a fairly high screen, I tend toward 150 V… or 3 of your Zeners in a string. Remember to feed with a "low enough" resistor.

If your B+ is 300 V, and we're shooting for a 150 V drop, and recognizing that the screen can easily sink 5 ma, then …
E = I R
R = E / I
R = 150 ÷ 0.005
R = 30 kΩ (nearest is 27 kΩ)
Also… tho it is a bit of a stretch, you could use the same trick for the VK of the final valve. Purchase a bunch of 5 volt Zeners, string in series, making your VK = 30 volts. No capacitor and resistor needed for biasing.

Just saying,
GoatGuy
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Old 13th February 2019, 12:42 AM   #14
kevinkr is offline kevinkr  United States
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The degeneration provided by that 1M resistor around the driver stage is negligible which is a good thing otherwise the input impedance would be pretty close to Rin which currently it isn't. The open loop gain of that triode is far less than the ratio of Rin/Rfb so it's not doing much of anything except perhaps providing a path for bias current.

What drove the choice of the 5744? I've not found it to be terribly linear tbh..
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Old 13th February 2019, 02:10 AM   #15
GoatGuy is offline GoatGuy  United States
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What drove the choice of the 5744? I've not found it to be terribly linear tbh..
LOL… there's no accounting for why designers (you and I and many of the readers) do what we do. Opportunity is part of it. I would be surprised if you actually went to the trouble of sourcing the almost rare 5744 … it was a WW2 / military tube that was supposed to be miniature, tough-as-nails, direct-circuit-solderable, and almost infinitely EMP/radiation proof. Pretty good characteristics when germanium transistors would go bad just by glancing at them the wrong way. In the height of the megaton cold war era. Just saying…

OK, so the 1 megger does near nil.

Try out the no-cap RK bias on stage 1.
Tho' most commenters might demur, instead of LNFB
(local negative feedback)
to tame the output gain, use the LNFB degenerate cathode bias instead.

Yes, you lose some power.
Yes, there are more cool ways of doing it.

Still … it works pretty well.

For “trial 2”, instead, drop in an appropriate chain-of-Zeners for the RK of the final stage; I've personally found that the sound is somehow "sweeter" for most music program material. Can't quite put my finger on why, but it is nice.

Best of luck to you.
I'm on a flight in the AM…
To someplace 155° West of where I am today.
So… replying isn't much of an option.

Looking forward to recover this thread in 3 weeks!!!

GoatGuy
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Old 13th February 2019, 06:44 AM   #16
mondogenerator is offline mondogenerator  England
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Kevinkr,
thanks for you reply. I am not happy with the feedback arrangement as it stands, and considered that as it stands, perhaps it isn't doing much of anything (I.e. i thought 1Meg....not much is getting through that)

GoatGuy,

Firstly, enjoy your holiday!

I may try the zener string for screen voltage, but presently with around 100uF on it, it seems fairly happy.

Since I have B+ of 300V and Vg2 of 250V and Rg2 is 47k, I figure:

(300-250)/47k = ~1.1mA Screen current

(I'm not sure this is the right way to calculate screen current, but its the only wat i can think of at 6:30am.

It sounds like I need to try without cathode bypass firstly.


As for the choice of 5744 tubes...well...I happened across a boatload for a good price and bought them. I'll probably die with the tubes still in the attic, but I've wasted larger sums before and lived...
(I have 6021, 6111, 6112, 6C6B, 6C7B, 6N16B, 6N17B, 6N21B and some rod pentodes, but the 5744 are the vast majority if my tube stock - better tubes such as 6021 or 6N17B may end up being preferred choice for Hifi. For guitar the 5744 might find a good home)
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Last edited by mondogenerator; 13th February 2019 at 06:47 AM.
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Old 14th February 2019, 07:58 PM   #17
mondogenerator is offline mondogenerator  England
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Hi,

I haven't had much time since last post, but I tried unbypassed cathode, and the reduction in gain is clear. Rk at 820R and I just about make 1.7V. Gain is probably 15 V/V.

Time for the LED. Better performance immediately, but back to a slight excess gain. Catch 22

Then I read the thread through again and This popped out and really made sense.

Quote:
Originally Posted by GoatGuy;5695740

PS: In general I tend toward 3-stage small amplifiers such as you are building. One of the more significant reasons (rarely cited!) is that it allows the tube curve-shape asymmetries to be somewhat balanced between signal phases.

The "preamp" stage should be spec'd with "one of the most linear, least power-law" triodes. The next stage should be low-mu, and only "barely amplify at all" (maybe 3× or so) This sets things up for the final stage, well.

Just saying,
[b
Goat[/b]Guy
Suffice to say I think I will abandon the first stage, keep the pentode stage and work backwards.

Now that, (surprisingly) I seem to have the pentode working nicely, and I know I need 30V pk-pk to drive it to full output, I'll leave that well alone.
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