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Old 13th December 2008, 06:20 PM   #1
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Default New 300B PP amp completed, phase shift questions

Just completed the following amp build, basically a Lynn Olson Karna amp with a few differences. This is my first non-GNFB build, so I have some questions about performance.

http://www.just4sheep.com/public/schematic.pdf

Things appear to operate well statically; hum is between 400uV and 1.5mV with AC heating throughout; pretty happy about that. All DC idle conditions are appropriate, and the amp puts out close to 30W at 1kHz at the onset of visual distortion.

Sound-wise, haven't been all that happy. Midrange is truly excellent, no complaints there. Hearing a lot of details I haven't noticed in the past. Midbass is poorer than my ST70, although I am still experimenting with the transformer coupled HPF at the input.

Biggest concern is in the high frequencies. Washed out, and audibly quieter than expected. Cymbals are just not there.

Did some measurements to see what was going on. For the purposes of eliminating a possible issue, the 33nF HPF cap is BYPASSED for these measurements. Following is a simplistic gain/phase plot:

http://www.just4sheep.com/public/response.pdf

So not only do I have a rising response with frequency, but the phase shift is waaaay out there (output lagging input). Did some further testing to see where the shift occurs.

20kHz input, loaded with 4 ohm noninductive resistor (all phase shifts relative to input, measurements are differential):
at grids of 12B4: 0 degrees
at plates of 12B4: could not obtain safely
at grids of 46: -39 degrees
at plates of 46: -73.4 degrees
at grids of 300B: -78.5 degrees
at plates of 300B: -90.7 degrees (measurements taken through decoupling 68nF orange drops)
at speaker terminals: -115 degrees

I am surmising most of the phase shift is occurring at the differential stage, and not across the interstage transformers. Output transformer does have around 25 degrees of lag.

One other observation: With a 20kHz input, the voltage at the grid of the 300B's is very clean. However, starting at about 5W output, the 300B stage begins to visibly distort (seen at the plates). Grid stays clean, tubes are distorting. With only 5W output? Clean output at 1kHz, distorted at 20kHz.

ideas ???

Power transformers are Electraprint, audio/filament transformers are O-Netics, input transformer is Tribute. 300B's are JJ.
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Old 13th December 2008, 07:57 PM   #2
DougL is offline DougL  United States
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Just a quick question?
What are you using to drive the amp for these measurements?
A sound card may not be able to drive the input transformer capacitance.

Just a thought.

Doug
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Old 13th December 2008, 09:14 PM   #3
kevinkr is offline kevinkr  United States
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The leakage inductance (or stray capacitance) in each of the interstage transformers is what is causing the HF phase shift you are seeing in each of your amplifier stages, Fisher and others installed caps across the primary to secondary windings (plate to grid) to reduce the effect of leakage inductance on the HF phase and frequency response. (You could try something like 0.022uF or larger since you are using 1+1:1+1 interstages - this might work reasonably well.)

Sometimes a little resistive loading on the secondary of the IT will help with the HF response issues.

It is highly likely that capacitance in the output transformer is responsible for the bad waveforms you are seeing, practically speaking you do not need much power response at 20kHz, try checking it at 15kHz where hopefully it is at least 6 dB greater than at 20kHz.

The design I published in VTV 10 yrs ago had a usable bandwidth of 30kHz and was mostly limited by the effects of miller capacitance in the output stage. Leakage inductance in the output transformer also contributed.

Generally I think more than one interstage in the signal path is design overkill and now you probably know why...

Somewhere in there there is also some ultrasonic peaking going on - you should try to establish what stage(s) are responsible.
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Old 13th December 2008, 09:15 PM   #4
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Good point. I am using a sound card to inject.

However, as shown, there is zero phase shift at the grids of the 12B4, so I would say the source is irrelevant. The shift is occurring internal to the amp, no ?
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Old 13th December 2008, 09:29 PM   #5
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by zigzagflux
Good point. I am using a sound card to inject.

However, as shown, there is zero phase shift at the grids of the 12B4, so I would say the source is irrelevant. The shift is occurring internal to the amp, no ?
I agree with your assessment.

What is the actual idle current in the output stage? I use JJ300B in both my 300B SE amp and the PP 300B mentioned in a previous post- they perform well. Take a look at the ac signal voltage appearing across the cathode bypass capacitor, shouldn't be much, but if there is try some additional bypassing.
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Old 13th December 2008, 10:48 PM   #6
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Thanks Kevin:

Won't be the first time you come to the rescue.

For starters, I appreciate your feelings about too much iron; you're not the first one to give that advice. Maybe I'll a little thick headed, but I still believe this can work. I've heard the same number of people swear by xfmr coupling as those that avoid it. I must be somewhere in the middle

I'll do some measurements in the next few hours in accordance with your previous post. The statements about leakage inductance got me thinking, and I went back to Morgan Jones on transformers. He says to expect an 18dB/octave LPF based on the characteristics of a transformer, the frequency dependent on transformer design. Also has some suggestions regarding loading and/or zobels to improve performance.

I think the O-Netics are a good tranny, so I'll continue to burn in the amp (currently have about 10 hours on them) and see what happens in about 20 hours. I will swear by a break-in period with some Lundahl's I have (love them). Who knows, these may require the same.

Quickly checked the AC voltage across the cathode cap. It's essentially immeasurable from 1kHz to 20kHz; no change that I can see. The search continues.....
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Old 13th December 2008, 10:52 PM   #7
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Hmm, those are not very happy measurements. Some quick questions - what is the Rp of the triode-connected 46? The 45 PP driver was specifically chosen for low distortion and low output impedance, as was the previous version using 5687, 7044, and 7119 drivers.

I was not entirely happy with the bandwidth of the O-Netics interstages. The Lundahls were going out to 80 khz without any peaking, while the O-Netics were making it out to 25 kHz with about 3 dB of peaking. I like the sound of the O-Netics better, but I've never tried the amorphous-core Lundahls, which many people say are very good.

The amplifier must be flat to 20 kHz at a minimum, with 50 khz a desirable target. RC loading of the secondaries on the interstages does remove the overshoot and peaking, but it also loads the tubes much more heavily at the highest frequencies. I found it sounded worse, but then my amplifiers didn't have the severe overshoots that your amplifier is showing.

The best way to optimize the impulse response is with a scope and a fast-risetime square wave. This can be injected at any stage of the amplifier, and the bad actors found. I'd start with the grids of the output tubes, work your way backward, and find out where the overshoot is coming from.

Since there is no global feedback and no shared grounds, it is easy to isolate and debug the amplifier stage by stage. If any given stage is the cause of the trouble, there isn't much to change except the tubes and the transformers.

Another quick question - what is the purpose of the 33 nF input cap on the primary of the input transformer? Do you have sources with several milliamps of DC offset coming from them? If so, you'll be hearing crackles as the volume control is rotated.

The reason I mention this is that small-value caps resonate with the transformer inductance and you can end up with very large LF peaking in the 10~50 Hz region. One of the annoying things about parallel-feed transformer coupling is that the required coupling cap almost always has to be 5 uF or larger to avoid the subsonic peaking - and these caps don't usually sound very good.

I would also remove the 0.068 uF caps shunting the VR tubes, at least during the debugging phase.

I'd also add 20~40 uF of capacitance between the virtual cathodes of the driver stage and the B+ of the driver stage and replace the negative-supply current source with a simple resistor to ground; what you have now is differential drive, and this has substantially higher 3rd-order harmonic content (2 to 3 times) and far less peak current drive capability than Class A PP (which is what the bypass cap gives you).

This one change alone might improve the frequency response and distortion substantially. (I'd also do it for the input stage as well; the improvement might actually be greater, but the driver stage MUST be Class A PP, not differential. I've tried differential, and the performance is substantially poorer in every respect - more distortion, less available current, higher Zout, and sounds worse, too.)

You can do this quickly with clip-leads; once you audition and measure Class A PP versus differential drive, I don't think you'll be going back.
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Old 13th December 2008, 11:54 PM   #8
BudP is offline BudP  United States
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zigzagflux

I too would add a 10 k termination to both IT's for measurement purposes. Listening might be another story, as the terminating resistor will eat up depth of field info badly. These IT's simply do not have enough distributed capacitance, or leakage inductance, to provide flat performance much beyond 20 K. In their defense, they were designed for audible characteristics and not test equipment characteristics.

Your findings on the OPT do surprise me. Typical phase performance has been flat to about 35 k with a gradual roll into lag from there. They do require about 40 hours to charge the dielectric circuit in the coil however and the audible changes from that charging are not subtle at all.

Might be a good idea to get Gary Pimm involved here too as he has a wealth of detail experience making the Karna amps sing sweetly.

Bud
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Old 14th December 2008, 12:26 AM   #9
kevinkr is offline kevinkr  United States
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Hi Lynn, (and Zigzag)
Took me a moment to realize what you were saying in regards to differential vs class A pp, because obviously the differential stage is operating in class A as well. If I understand you correctly you are talking about eliminating the differential local feedback in the cathode circuit of each of the preceding stages by getting rid of the CCS and independently biasing each tube in the pair. (Or bypassing the common bias point so there is no common ac between the two devices.) Clearly the mechanism here is that the less perfect balance results in some residual 2nd harmonic and perhaps slightly less 3rd. It seems like a good suggestion and I would further suggest the use of the existing negative supplies to provide fixed bias to these stages in lieu of cathode bias to avoid the need for very good cathode bypass caps.

You have another degree of freedom in that if the problem is truly caused primarily by the transformer's leakage inductance you may be able to improve the overshoot performance considerably by raising the effective rp of the tubes driving the transformer - local cathode feedback (just an unbypassed cathode resistor) will do this.. Note as well you can do this with your current ccs just by adding resistors between the cathodes and ccs. (Technically with a really good ccs only one resistor is needed, but this bothers me from a semantic if not technical perspective.)

IIRC the 46 in triode connection has roughly an rp of 2 - 2.2K , but I could be substantially off.

Stray winding capacitance is less of an issue with low rp triodes than leakage inductance in many instances IMO, but I am unfamiliar with the specific transformers.

Zigzag keep us posted, and good luck!
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Old 14th December 2008, 01:46 AM   #10
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To clarify the discussion of differential vs Class A PP: Differential is series operation; if one tube is pulled out, clips, or runs out of current, the whole stage shuts down, just like old-fashioned Christmas tree lights. By contrast, what I'm calling "Class A PP" operates in parallel; if one tube is pulled out, clips, or runs out of current, the other tube takes over, and keeps on going. The stage slides into Class AB if it needs to, while the differential stage just flat-out clips.

This may seem like semantics, but the transition into Class AB with vacuum tubes is actually surprisingly broad, not the 0.7V diode drop we see in transistor output stages, which hard-switch on and off. I was still seeing residual transition effects with many tens of volts of negative bias on the DHT grids, which is why the output-tube distortion is sensitive to drive impedance.

The impedance between the center-tap of the interstage (or output) transformer and the virtual cathodes of the pair of tubes is what controls the ratio between the two modes of operation. John Atwood made a detailed series of measurements by varying this impedance and did find with medium-distortion tubes the best place of operation was fairly close (but not the same as) as the traditional Class A PP mode - that is, parallel operation.

He also measured it again with lower distortion tubes in the DHT family and the optimum, lowest-distortion mode was the Class A PP flavor. When the potentiometer was moved away from the lowest-distortion point, what was significant was the higher-order distortion came up first - 3rd, 5th, 7th, all the ugly ones. In addition, and probably closely related to the prior finding, was the linear current delivery dropped rapidly as well. The ability to deliver linear current is closely related to the absence of high-order distortion content, and this is where the differential circuit falls down the worst.

Returning the previous post, the decision to switch the input and driver stage to pure differential operation is almost certainly the reason the amplifier is grossly underperforming compared to the Karna. Although I didn't mention this on the Web-page, the decision to avoid differential operation was based on a series of measurements and audition.

On direct A/B comparison, the differential circuit sounded thin, scrawny, and with "pinched" and closed-in HF, with poor dynamic reserves. Measurement confirmed this: distortion was several times higher, the proportion of upper harmonic to lower harmonics was unfavorable, and most important, peak current delivery was curtailed by severalfold. This was the real source of the problem: the driver stage was unable to provide peak current to the 300B grids when the output tubes were getting near Class A2 conditions or Miller capacitance + interstage stray capacitance were demanding more and more current from the driver at high frequencies. When it comes to driver stage design, it's all about linear current delivery, particularly at high frequencies.

This is not the same as just throwing in a cathode-follower; that lowers Zsource, but has NO effect on distortion or the ability to deliver peak current into a load with complex (and nonlinear) characteristics. In effect, the driver really needs to be a small power amplifier in its own right, or at least able to drive headphones.

One of the most revealing tests of the Karna was a test I did a while ago to see if I could slew the amplifier; although the HF rolls off above 40~50 kHz, there's nothing to stop me from increasing the input level. So I fed in a sinewave, increased the gain to just below clipping (16 watts output), and kept raising the frequency, and then level, keeping the amplifier just below clipping the whole time.

I finally lost my nerve at 500 kHz, just below the AM radio band - the sine wave was still looking very close to a sine-wave, no sign of flat-topping or starting to turn into triangles, and the amplifier was acting like a 16-watt radio transmitter putting out a carrier wave. I never did see slewing, although I shudder to think how much current was going into the plates of the driver stage (the driver was looking into a nearly pure reactance, so all the power was getting bounced right back into the driver-stage plates).

A test like this would destroy most transistor amplifiers, and I very much doubt a differential driver would have cheerfully slid into Class AB to deliver the amount of current the 300B grids were demanding.

So ditch the negative-voltage current sources for the input and drive stages, replace them with wirewound resistors, and shunt the cathodes to the B+ supply with a good-quality 40 uF oil cap. One thing to be careful of: the VR tubes do NOT like to see capacitive loads (they will oscillate), so either put the bypass caps between the cathodes and ground, or cathodes and B+, but not both.
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