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Old 19th March 2008, 10:55 PM   #11
SY is offline SY  United States
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25k is mighty low for an anode resistor. Very high second harmonic.

I went back and looked at the EA230 schematic. A few things struck me. First, the feedback from the output tube plates to the driver cathodes is miniscule. I think it helps bias that stage while keeping the cathode resistor low, but the AC feedback seems almost nonexistent.

The overall feedback of that amp also seems extremely low. I haven't seen one in years, but I'll bet the distortion figures are not impressive.

In any event, you'll need a gain of about 100 from input to the output tube screens just to get to max power without feedback. Your first stage looks like about 35, the second stage about 20. So you've got 10dB or so to burn. Unfortunately, the output tube has a gain of something like 6-8dB! And if you burn that, the driver will have to swing several hundred volts. So you really need to include the driver in that loop.

If it were me, I'd linearize the input stage so it doesn't need feedback (for a 12AT7, figure 2mA current, 150k plate load, and a correspondingly bigger B+ rail), then run symmetric feedback paths from the transformer secondary to the driver tube cathodes.
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Old 20th March 2008, 03:03 PM   #12
GordonW is offline GordonW  United States
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You know, I have been puzzling through the EA230 schematic some more, and I'm more and more thinking that the loop from the output tube anode to the cathode of the driver tube, is actually supplying POSITIVE feedback.

The driver inverts polarity, the cathode follower preserves it, and the output inverts it again. So, since the cathode of the driver tube is in SAME POLARITY as the grid, and with the two inversions, the plate of the output is in the SAME POLARITY as the grid and cathode of the driver tube... putting drive from the output tube anode there should INCREASE the gain, unless I'm missing something. Almost sounds like a Stuart Hegeman type trick... my HK A300 has that sort of thing going on, to increase the gain of the driver stage...

I'm almost tempted to go back to a similar arrangement as the original Berning input stage (47-68K anode resistor, about 2.2-2.7K cathode resistor, or as close as I can get and still make the voltage work to bias the LTP properly) and see what happens... or, maybe even better- if I can linearize the input stage as Sy suggested... even if it means I need to use a negative voltage instead of ground on the bottom (I have a negative rail available, might as well use it if it's beneficial), to get the stage linear on its own... maybe that might be a way to go...

I'm wondering why the 12AT7, with three times the transconductance of the 12AX7 (about 5000, the way I'm using it, as opposed to about 1600 for the 12AX7) and ONE FOURTH the output impedance of the 12AX7 (about 15K as opposed to about 65K or so for the 12AX7, IIRC), would need such a high-resistance anode resistor. 12AX7 circuits work just fine with 220K and even 160K anode resistors... I'm wondering why a proportionally smaller anode resistor (to the transconductance ratio and/or impedance ratio) of something like 39-68K shouldn't work out?

Regards,
Gordon.
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Old 20th March 2008, 04:06 PM   #13
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No, it's negative. It's returned to the cathode of the driver tube, so the signal is not inverted at the driver plate. That means it's not inverted at the output grid, hence applied in a plate-to-grid (inverting) sense. But there's no more than a dB or so at AC.

Set up a 12AT7 in a test jig and play around with plate loads. You'll find pretty quickly that it needs to be much larger than you're proposing. A hint of this can be seen in the datasheet curves for plate resistance versus current- the closer you get to constant current, the more linear. 10x rp is not unreasonable.
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Old 20th March 2008, 07:34 PM   #14
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Ah, yes. I had to start thinking of the Berning voltage amp in terms of how the FEEDBACK loop drives it- it's basically a grounded grid stage, when you think of it that way (driven from the cathode). And grounded grid stages are, naturally, NON-inverting...

I guess it might be time to start thinking about how to apply feedback to the cathodes of the phase splitter... though, I can't get my head around solving the apparent problem of what happens when the feedback interferes with the drive for the negative side of the phase splitter... seems like it would interfere with the balance of the stage. Though, with the adjust-ability I have with the anode resistors... that might not be a problem, maybe?

This stuff is a bit esoteric sometimes... takes more thought than would first seem!!

Regards,
Gordon.
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Old 21st March 2008, 04:24 PM   #15
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Gordon,

Have you thought about using transformers instead of caps to couple the stages.

Figure what the hell. You mentioned transformer farm
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Old 22nd March 2008, 01:05 PM   #16
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Quote:
Originally posted by The Donk!
Gordon,

Have you thought about using transformers instead of caps to couple the stages.

Figure what the hell. You mentioned transformer farm
Well... the intent of this is to INCREASE bandwidth. I don't think interstage transformers really fit into that scheme.

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Gordon.
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Old 22nd March 2008, 01:15 PM   #17
GordonW is offline GordonW  United States
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OK. I think I may have found a bug in TubeCad.

I have been modeling my LTP with tube-CCS, compared to the SAME LTP without CCS... and TubeCad keeps telling me, that with the SAME anode resistors and same current, that the CCS version has almost TWICE the gain of the non-CCS version... 40 vs about 21, with around 36K anode resistors.

Is there anything so "magical" about the CCS that would cause this sort of gain increase? I know it's more linear than a resistor... but if anything, shouldn't an 'infinite resistance' like a current source actually DECREASE gain a bit as a cathode "resistor replacement" (since gain is generally set by the ratio between anode and cathode resistance)?

Sorry if this is a bonehead question, but I'm questioning EVERYTHING at this point!

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Gordon.
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Old 22nd March 2008, 02:06 PM   #18
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Nope, that isn't right. What CCS loading of the LTP cathodes will do is equalize the gains on each side. The side with lower gain will increase slightly, but nothing like 21->40, more like about 20->21.
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Old 22nd March 2008, 10:35 PM   #19
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Quote:
Originally posted by SY
Nope, that isn't right. What CCS loading of the LTP cathodes will do is equalize the gains on each side. The side with lower gain will increase slightly, but nothing like 21->40, more like about 20->21.
That's what I thought... it's just basic common sense, in essence.

Thanks for the confirmation... I was briefly being sold the Brooklyn Bridge by the sim program, I guess.

On that tack- I'm thinking about re-visiting the 6GH8A as an input/CCS tube. Gain on the triode part is 46 (I think I should be able to make that work in place of the 12AT7 section, in the CCS for the phase splitter), and the pentode can probably give me enough gain on the first stage while maintaining sufficient bandwidth to keep good stability. On that subject- anyone got a favorite high-gain (at least 40 or so) grounded-cathode alignment for the pentode part of the 6GH8A? Preferably, with about 200v across the tube at idle, if possible... that would make my phase splitter happy...

I definitely need to get more practice at doing these curve/load-line fittings for pentodes... just haven't gotten it into my brain completely yet, I guess, cause it just seems to take forever, as of now...

Thanks for any help!!

Regards,
Gordon.
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Old 27th March 2008, 07:38 PM   #20
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I'm keeping on trying...

I decided I HAD to do something about the front-end gain. I was having to ask way too much gain from the LTP, to get enough good voltage swing. Also, this was impinging on the bandwidth of the whole amp... the phase splitter was the "bottleneck", there, as well.

So, the question became: HOW could I get a LOT more gain from the first stage, without throwing everything off? I did a lot of modeling of various topologies, with various tubes.

Also, in the phase splitter, I wanted to find more voltage swing... so, I was off looking for any ways to get that, too. After a lot of research, I found a neat, somewhat obscure but easily available, tube, that gave me quite a bit more voltage, with the same B+, without having to run outrageously large or small current levels.

So, here's what I've got, now:

Input stage: 6N30P, in cascode. Almost INFINITE bandwidth (over a megahertz in open-loop!), with a gain of nearly 90!

Phase splitter: the smaller triode section of the 6CS7. 500v plate voltage... not as much gain, but with the input stage as it is, it didn't need much gain here!

Cathode follower/output driver: Larger triode section of the 6CS7. This tube was made for the same duty as the 6BL7 I had spec'd before, and is capable of enough current (60ma continuous, IIRC) to do ANYTHING the output tubes might demand. Gain is pretty good- about .9 (a tad lower than the original .95, but good enough to work fine).

Output tubes: 6JB6- same as before. I was checking curves and the original Berning EA230 design, and it looks like they're getting a gain of around 3 to 4 out of these... which should work fine, with the rest of my gain structure.

With this setup, and mimicking the Berning input requirements (about 1.4v for full output), I get an open-loop gain of between 1900 and 2500, and a needed closed-loop gain of about 283. That gives about a factor of 6.7 to 8.9, for feedback... between 16-19 dB or so. I'd think that should work OK...but I'd like to hear any comments or suggestions, thereof...

I really think this is a much more 'sensible' design than the others... it requires much less power supply complexity (the only non-normal thing is a negative rail of about 200v, which shouldn't have to carry more than a few watts of power consumption) than before... I will probably still use a secondary transformer for the 350V and the -200V, but the other rails will ALL run off of the SAME 550V main B+...

As I said, I'd really appreciate any continuing comments... thanks in advance!!

Regards,
Gordon.
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