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Cathode Bias questions

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Don't 7788 vary quite a lot, like 6S45S, 417A and 5842?

I'll change it to 50 and see what I get...

a little later...

OK, changed mu of 7788-triode to 50.

Due to the unbypassed cathode resistor, that changed the Zout of that stage to 2760 (up by a couple hundred Ohms).

This makes very little difference, but there is a difference.

If the following 5687 stage's Miller capacitance is 80pF, then it makes no difference.

If the following stage's Miller capacitance is more like 72 or 76, then it makes a little difference and the C1 should be a little larger value (maybe by 10pF or so).

This is a great exercise, but I'm wondering... Am I close enough?

I need to get some values in there and take the preamp to a friend who has an MJ (Japanese) inverse RIAA he'll let me use. That'll be the start of a whole new thread, I'm sure.

Anyway, I think I'm pretty close. Not perfect, that's for sure. But close.

Thanks for all this. I'm learning a lot, and really looking forward to the book coming (where is it??).

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Well Philips manufactured E810Fs are reputedly quite uniform. Whose 7788s are you using? Most of my E810Fs are labelled Mullard and "manufactured in Holland", but some are "made in USA". They're all pretty much the same on my AVO tester, so I imagine Philips' figures are accurate. If you weren't using an unbypassed resistor, it wouldn't matter terribly. I see you've changed to an LED for biasing on the second stage, why not do the same on the first stage (I do)? You'd have to recalculate the capacitor, but you'd have a bit more actual gain.
 
Well, to add even more confusion... It's a Russian "E810F". It's not a real Euro E810F or a USA 7788. So who knows?

Only reason I'm not using an LED in the first stage cathode is that I can't find one that will drop only 1V. A -1.8V bias leaves the tube running too cool, and doesn't sound as good. Seems the tube likes to see at minimum 15mA at these voltages.

Also, I don't need max gain. It's at a good place for my needs now. Matches the output of my SACD player pretty closely, so I'm happy with that.

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sorry to jump in on the thread but while you're on the subject of leds i'd like to know how to work out the current flowing through a yellow led?

Just swapped the cathode resistors for yellow leds so I've got a voltage drop of 2 Volts.

Of course I used to calculate the current with Voltage/resistance.

But now? Maybe a dumb question.

:xeye:
 
The LED is a constant-voltage device whose current has to be set by external components (like a tube, a CCS, or a resistor).

If it can be used as a direct replacement for a cathode resistor (that's what I understood), in my case I have 2 volts sitting on the cathode, without any additional resistors, then how do I know what current is flowing through the plate?
eg. If I want 9mA as an optimum value?
 
rongon said:
This is a great exercise, but I'm wondering... Am I close enough?

I need to get some values in there and take the preamp to a friend who has an MJ (Japanese) inverse RIAA he'll let me use. That'll be the start of a whole new thread, I'm sure.

Anyway, I think I'm pretty close. Not perfect, that's for sure. But close.


Headed to the OBX of North Carolina, so didn't carefully check calcs., but I think you've got the idea down. It's a little complicated, but really not that hard with the simulators that are available. Nice thing is that you can do a lot of iterations quickly. That begins to give a good sense of the relative effect of the various variables, and builds confidence in the design.

Yes, close enough for sure to start testing and listening. You may find some physical issues to track down, given the high amplification. Time to get to it.

The reverse RIAA is a good check. But I doubt that small deviations from ideal will be all that audible. More important probably, is getting good channel to channel matching. You may have to try a few tubes to get the best matches.


Brit01 said:


If it can be used as a direct replacement for a cathode resistor (that's what I understood), in my case I have 2 volts sitting on the cathode, without any additional resistors, then how do I know what current is flowing through the plate?
eg. If I want 9mA as an optimum value?

Measure the voltage across the plate resistor. And remember what our good friend Ohm taught us: I=V/R.

Sheldon
 
Measure the voltage across the plate resistor. And remember what our good friend Ohm taught us: I=V/R.

i was always used to measuring the current using the value on the cathode and the cathode resistor.

So I have 130 Volts on the plate and a 15K resistor.

So 7.53 mA is good for me.


Look at it from the other direction. Take the tube curves, look at the grid bias line corresponding to the LED voltage, then see where that falls on the load line.
Thks SY.
I'll have a look at this once I get a few minutes free at my office:D
 
Yes, indeed. The learning continues...

Thank you Sheldon, SY, barretter for all your input, help, guidance and prodding.

Sheldon, you pulled a Karnak -- I'm having channel balance problems. I have a cheap LCR meter, so I was able to match components. I'm not confident in the accuracy of the actual values it's reading out, but if they match, they match. It must be the tubes. But that's OK, I've got more...

Did some listening last night with this latest set of RC's. The tonal balance sounds more "normal" compared to CD. Maybe the vinyl sounds a little darker than the CD, but one, I can tweak that and two, I might like it that way. I think I'll change the 840pF in the first stage (too high by a smidge) to 800pF (750pF // 49pF). That'll be enough for now.

My initial problem was that the preamp sounded too harsh 'n bright. I thought the RIAA values were totally off. Hence this "quest."

Now that I'm nearing the end of this phase, what's funny is that the C values ended up exactly where we started! It was the shunt resistor in series with the .033uF (the shelving resistor?) that was wrong. It was 10k and should have been approx. 9.4k. Paralleling 10k with 150k gives 9.375k, which is close enough, right? Just changing that made a huge difference.

The other possibility is that a friend had given me some F-Dyne capacitors to use for this, labeled "PE". I think that's "polyethylene" (as in Mylar). I've switched them to polystyrene film/foil and I like the sound much better. Less "shout-y", definitely more relaxed. I prefer that.

Any opinions re: silver mica vs. polystyrene for the 3180uS filter? I have both available.

And... Just now... "Valve Amplifiers" has arrived! OK, now my free time is toast.

Thanks again, y'all, this has been a lot of fun, and a successful exercise to boot.
--

PS -- thanks for the beta on the IR LEDs for 1.2V.
 
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