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Old 9th February 2005, 03:06 PM   #1
Sheldon is offline Sheldon  United States
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Default 6ck4 pp amp

Hi,,

I'm a tube newbee, but have built a couple of chip amps with discrete regulated supplies, buffers and other goodies. This looked like a nice project, by a respected designer, and I was able to get the tubes readily, some locally. Other stuff I had some of too.

http://tinpan.fortunecity.com/saints...er/6ck4-pp.gif

The tube operating points seem ok. I calculate about 80v at the plates of the BZ7's. With a plate-cathode voltage of about 250 on the CK4's, I get about 105v across the cathode resistor, giving a bias voltage of about 25v (105-80), and a current of about 50ma/tube. That all seems to add up.

The power supply has me concerned though. It's simple enough, but my sims in PSUD don't give me confidence that it will work. The schematic seems to suggest that it's for a stereo amp, but there is no way I can get a B+ of 365V with a current draw of 200ma. In fact, with just a basic C filter (no neg supply, no driver section), I get about 350V at 100ma. So it seems marginal even for a monoblock. I'm thinking I need to up the secondary voltage and add another RC section (to reduce the input C) or use an input L. Am I on the right track?

Sheldon
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Old 9th February 2005, 05:15 PM   #2
SY is offline SY  United States
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You didn't ask about this, but I'll tell you anyway: the 12BZ7 has a LOT of distortion. I spent a couple of weeks trying to get a pp diff amp to work right and just gave up. You'll do better with paralleled 12AX7s, similar specs but better linearity.
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Old 9th February 2005, 06:01 PM   #3
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BZ has curves as linear as an AX, it's exactly twice in fact..(half Rp, double Gm and perveance).. I don't know what your distortion is coming from. I'll admit I haven't used one yet.

At any rate, first we have to fix your circuit...

No-no number one: wayy too much capacitance on the 5U4. Drop to 40uF *MAX*.

Instead of the two stage filtering, you can save some phase shift nightmares and use one 56k resistor for each channel. Which isn't necessary anyway since you have the output stages on the same rail (B+).

No-no number two: direct coupling. Oh the horrors! You MUST use NFB to maintain balance, otherwise you'll never get equal bias current in the output. That's a hassle, so coupling caps are just fine. In the same area, you've got positive grid bias, but the 2k cathode resistors might be large enough for that; I see no indication either way. (It would be helpful if voltages were marked on the schematic.)

No-no number three: your negative voltage supply isn't. It shows a coupling cap (from a filtered DC rail) to a filtered resistor divider. If you had included a diode, it *might* develop some voltage from the residual ripple, but nowhere near 30V. This needs to be a diode in series with a resistor to one 5U4 plate, to a filter cap and so forth, for instance as I used in Frankenhouse.

Click the image to open in full size.

Finally; you noted regulation is half what it needs to be -- use solid state diodes instead. Tubes are just plain horrible at voltage drop. FWIW, at least using a smaller capacitor may give the 5U4 an easier job and increase output a little.

Tim
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Old 9th February 2005, 06:09 PM   #4
SY is offline SY  United States
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Yeah, from the data sheet, the BZ looks like a doubled-up AX, but in practice, the distortion is three or four times higher at the swing you need here (my app was a driver for a pp EL84, so the swing requirement is similar).
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Old 9th February 2005, 11:02 PM   #5
Sheldon is offline Sheldon  United States
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Quote:
Originally posted by SY
You didn't ask about this, but I'll tell you anyway: the 12BZ7 has a LOT of distortion. I spent a couple of weeks trying to get a pp diff amp to work right and just gave up. You'll do better with paralleled 12AX7s, similar specs but better linearity.
No I didn't ask that, but there are at least two reasons that shouldn't stop you. First, I may not have known to ask. Second, and more important, once we post a question here, it's community property and someone else might benefit even if I choose not to pay attention.

That aside, thanks. But do I need to parallel an AX7? Would the current from a single triode be insufficient? I've got the parts so I may as well breadboard up the circuit and learn something.


Quote:
Originally posted by Sch3mat1c
At any rate, first we have to fix your circuit...

No-no number one: wayy too much capacitance on the 5U4. Drop to 40uF *MAX*.

Instead of the two stage filtering, you can save some phase shift nightmares and use one 56k resistor for each channel. Which isn't necessary anyway since you have the output stages on the same rail (B+).

No-no number two: direct coupling. Oh the horrors! You MUST use NFB to maintain balance, otherwise you'll never get equal bias current in the output. That's a hassle, so coupling caps are just fine. In the same area, you've got positive grid bias, but the 2k cathode resistors might be large enough for that; I see no indication either way. (It would be helpful if voltages were marked on the schematic.)

No-no number three: your negative voltage supply isn't. It shows a coupling cap (from a filtered DC rail) to a filtered resistor divider. If you had included a diode, it *might* develop some voltage from the residual ripple, but nowhere near 30V. This needs to be a diode in series with a resistor to one 5U4 plate, to a filter cap and so forth, for instance as I used in Frankenhouse.

Finally; you noted regulation is half what it needs to be -- use solid state diodes instead. Tubes are just plain horrible at voltage drop. FWIW, at least using a smaller capacitor may give the 5U4 an easier job and increase output a little.

Tim

Thanks Tim. I didn't make that schematic, I just downloaded it and pasted, so I can't take any credit, blame, or ownership. I guess, for attributions sake I should have explicity noted that.

I'll try the direct coupling just to see what happens. But I'll make sure I have some good caps on hand too. Yeah, I wondered about that negative supply. I didn't give it too much thought, as might be apparent (course, the other possible explanation is that I'm way behind the curve here - well, the truth will be revealed as we go) I now realize, as you point out, that even with a diode there can't be enough ripple to get the voltage that high. If I model a simple C filter with just a 100uf C and a 100ma current source, I get about 7V p/p ripple at the point where the "negative supply" would be. Partly I didn't think about it much, cause I figured I can always just make a separate negative supply. Your approach is even easier. Mainly, what I'm trying to nail down first is what I'd need in the way of a power transformer, cause the rest of it is easier to change. Since I've got a 300-0-300 tranny on the way, I'll give it a try and see what happens

Sheldon
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Old 9th February 2005, 11:11 PM   #6
SY is offline SY  United States
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With some redesign, a single AX could work. But paralleled AX will drop right in with no other changes. I sort of like the lazy approach.
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Old 9th February 2005, 11:56 PM   #7
Sheldon is offline Sheldon  United States
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Quote:
Originally posted by Sch3mat1c

No-no number two: direct coupling. Oh the horrors! You MUST use NFB to maintain balance, otherwise you'll never get equal bias current in the output. That's a hassle, so coupling caps are just fine. In the same area, you've got positive grid bias, but the 2k cathode resistors might be large enough for that; I see no indication either way. (It would be helpful if voltages were marked on the schematic.)
Tim
Tim, I don't absorb this stuff all at once yet - sort of in little quantum packets at this point. So I hope you don't mind a few more questions that came to me after the previous reply.

Are you saying to add both caps and negative feedback? As I read the circuit, I would think that the differential input stage would have reasonably balanced voltages on the outputs, so I'll ignore that as an issue for now. But would that be an issue I need to worry about?

As regards feedback, the output tubes have separate, fairly large, cathode resistors. Doesn't this provide feedback at the cathode, or is it not enough? As I read the circuit (remember my level now), I see the first stage output (calculated as riding on 80V DC) as an element in the bias level. If I get around 50-55 ma per tube, then the cathode would be at about 100-110V (for a bias voltage around -25). I can't just throw in caps, as then we're talking a whole different deal, aren't we?

As far as the values on the cathode resistors being ok to get to the correct bias point, I'm not sure. But at least it all seems in the right ballpark. I'm basing that only on inference and sorta reconciling the currents and voltages by looking at the circuit values given and at the tube data sheets.

All theory at this point, and that's why I want to do this little project - it's simple, but not too simple. And I can try some different things with it.

thanks,
Sheldon
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Old 10th February 2005, 02:55 AM   #8
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Quote:
Originally posted by Sheldon
Are you saying to add both caps and negative feedback? As I read the circuit, I would think that the differential input stage would have reasonably balanced voltages on the outputs, so I'll ignore that as an issue for now. But would that be an issue I need to worry about?
Getting the DC conditions right, and stable, is always a problem with DC coupled amps. The differential/long-tailed pair is very likely to have a slight DC offset at the outputs (i.e. between the anodes) since the valves are never perfectly matched - leading them to draw slightly different currents. Ohm's law dictates that different currents through the same value resistance (anode load resistors) will create a different voltage.

The PP output stage is in essence a differential amplifier of sorts, and without the coupling caps, the DC offset will cause the operating point to be different for each valve in the output stage, causing DC across the output transformer - which is BAD.

Quote:
Originally posted by Sheldon
As regards feedback, the output tubes have separate, fairly large, cathode resistors. Doesn't this provide feedback at the cathode, or is it not enough? As I read the circuit (remember my level now), I see the first stage output (calculated as riding on 80V DC) as an element in the bias level. If I get around 50-55 ma per tube, then the cathode would be at about 100-110V (for a bias voltage around -25). I can't just throw in caps, as then we're talking a whole different deal, aren't we?
The cathode resistors are bypassed by fairly large capacitors so they're essentially invisible to AC. There shouldn't be any feedback happening there at typical audio frequencies. I'm not entirely sure what you mean by just throwing in caps - the cathode bypass caps are already there. If you mean interstage coupling caps: yes, you'll have to recalculate the cathode resistor values.
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Old 10th February 2005, 03:25 AM   #9
Sheldon is offline Sheldon  United States
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Quote:
Originally posted by audiousername


Getting the DC conditions right, and stable, is always a problem with DC coupled amps. The differential/long-tailed pair is very likely to have a slight DC offset at the outputs (i.e. between the anodes) since the valves are never perfectly matched - leading them to draw slightly different currents. Ohm's law dictates that different currents through the same value resistance (anode load resistors) will create a different voltage.

The PP output stage is in essence a differential amplifier of sorts, and without the coupling caps, the DC offset will cause the operating point to be different for each valve in the output stage, causing DC across the output transformer - which is BAD.
Ok, I think I get that one. Doesn't seem any trivial way around it. I guess it's a matter of degree.

Quote:
Originally posted by audiousername
The cathode resistors are bypassed by fairly large capacitors so they're essentially invisible to AC. There shouldn't be any feedback happening there at typical audio frequencies. I'm not entirely sure what you mean by just throwing in caps - the cathode bypass caps are already there. If you mean interstage coupling caps: yes, you'll have to recalculate the cathode resistor values.
Yes, I was referring to DC. I intrepreted Tim's advice on caps to mean coupling caps. By feedback (and I'm probably using the term improperly), I meant DC feedback, i.e., the tubes should find reasonably close current values, assuming the grids are at the same voltage. Or is this not true?

Sheldon
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Old 10th February 2005, 03:30 AM   #10
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Yes there is stabilisation of the operating point when using cathode bias (which is why you're generally allowed to use a larger grid leak resistor). It works because the bias voltage is a function of mean cathode current, such that if the mean current increases, more voltage is developed across the cathode resistor and the valve responds by decreasing conduction. The two effects oppose each other, which is desirable for a stable equilibrium to be reached. A similar argument can be made for a decrease in mean current.

I'm not sure if this answers your question, though. Maybe Tim should answer it, but he might be regenerating in his alcove right now
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