New 300B PP amp completed, phase shift questions

[zigzagflux]

Hi Bud:

Very thankful you have elected to chime in. Two hours and two beers later, I have some more data that should help the process.

I've held off on the phase shift issues for now, and just investigated the peaking over about 5K. The gain across each differential stage is fairly constant, so that doesn't seem to be the problem. It is each IT that is contributing to the rise in response.

I've experimented with resistor loading each IT, and around 30K for the first stage and 22K for the second stage are fairly happy values that tame the peak. These values could probably be increased a little, and a series cap used to bring it in near 5K, for a nice smooth response.

However, based on Lynn's issues with a current source loaded differential stage, this peaking may all be moot anyway. My prior experience with the Lundahl's was similar to your statement; measurements don't always make for good sound. It was Kevin Carter's recommendation to NOT add zobels to transformers (or very rarely), as they don't improve the sound. At least in my amorphous transformer coupled preamp, he was right. So before I go throwing zobels everywhere, I'll have to address Lynn's recommendations in the next post. I suspect (hope) everything will fix itself.

Happy to report the output transformer, as you stated, does not exhibit peaking. This is only affecting the IT's. Why the output stage is distorting, I guess that's a third issue, but one step at a time.

[zigzagflux]

Hello Lynn:

Good to hear from the amp's creator. It's truly honorable to have you spend the effort posting. As I know you understand, building one of these isn't exactly cheap, so your help is greatly appreciated.

Originally posted by Lynn Olson
Some quick questions - what is the Rp of the triode-connected 46? The 45 PP driver was specifically chosen for low distortion

Rp is about 2300K, compared to 1700K of the 45. I chose to use it because it costs less than half the 45, and a number of people have had very good things to say about it as a driver. Supposed to sound very similar to the 45. Hope I'm not shooting myself in the foot?

Another quick question - what is the purpose of the 33 nF input cap on the primary of the input transformer?

It is serving as my high pass filter, around 115Hz. I admit this is a questionable implementation, but I couldn't come up with a better method for a fully balanced system. With test equipment, it actually works pretty well; sonically, we'll see.

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......I've tried differential, and the performance is substantially poorer in every respect - more distortion, less available current, higher Zout, and sounds worse, too.)

Well (*(insert profanity here)*) !! And here I thought I was improving the situation. Just to explain myself, I got this idea from Gary Pimm's site, where he was using the resistor in the cathode, and by replacing it with a CCS, was able to remove the capacitor with no sonic loss. Getting rid of a capacitor is always a good thing. Wrong was I. I got a bunch of oil cans and soviet teflons, I'll give this a try.

If you are right, it is very possible all other problems will be solved (crosses fingers). One question though; at around 20kHz 5W output the output stage is clearly distorted, while the signal at the grids is nice and clean. This would indicate my driver is capable, but the output stage itself is distorting, wouldn't it?

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.

Now you tell me !

On direct A/B comparison, the differential circuit sounded thin, scrawny, and with "pinched" and closed-in HF, with poor dynamic reserves.

Yep, that would be a very good description. Closed in. Frustrating, in that the midrange really is beautiful. Highs is where everything starts to suffer. What is more interesting is that even though there is peaking of the HF range, the shimmer and glitter of cymbals has just disappeared, and actually sounds quieter in a relative sense.

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.

I guess I deserve this. Thank you for the marching orders. I'll pursue all the other 'stuff' (peaking, phase shift, distortion) once I make those changes.

[ErikdeBest]

Very interesting thread and a revelation for me as well. Just like Zigzagflux I was also thinking that CCS's in the tails could only improve the situation...

Thanks for sharing this with us, Lynn!

[Lynn Olson]

Gary Pimm actually built the Karna, and I took them back to Gary for a second go-round to further optimize the circuit. But the design is mine, with Gary acting as mentor for the instrumentation grounding techniques. If the grounding is carried out as drawn, the Karna should be entirely free of buzz and hash. With DC supplies for the DHT's (not tried yet), the output noise should down in the microvolt range. Building a good-sounding DC supply is not trivial, though - Gary Pimm's DC supply for his 300B SET was elaborate as any I've ever seen (very high speed shunt and series regulation), and he STILL found that battery-powered filaments outperformed them.

Gary Pimm's amp is headed in the same direction as mine, but he is using a completely different approach. In fact, the two don't commingle that well, although I have certainly contemplated using his front end instead of mine. We found that the most of the distortion in the Karna isn't in the 300B's, nor the 45's, but the input tube!!! To say the least, this is a comment on the linearity of the driver and output stage.

Gary gets rid of the Miller capacitance by driving pentodes, which have very low capacitance. I use a less elegant brute-force approach, with lots of linear current from transformer-coupled Class A PP DHT drivers. One of the biggest differences between this amp and a conventional RC-coupled PP amplifier is that the current from both sides of the driver is available when one of the 300B grids demands grid-current (they take turns going into grid-current, of course). So the transition into Class A into Class A2 happens with no visible "bump" on the scope at all - in fact, Gary found the 300B's operate quite happily with the grid voltage going 30 volts positive on the peaks!

With a conventional RC-coupled PP amplifier, as soon as the grid draws current, it discharges the coupling cap, and the amplifier is seriously misbiased for the length of time it takes to recharge the coupling cap - this commonly takes a half-second or longer, stretching out what would have been a millisecond overload into a lengthy fraction of second. This is a good reason to keep the RC time constant as short as possible consistent with acceptable bass response. By contrast, the transformer-coupled amplifier enters into Class A2 with no visible transition, and recovers instantly from overload, with no coupling caps to discharge.

That is why the modest-looking 16-watt amplifier (30 watts in Class A2) sounds as loud as a 60-watt Citation II (with much harder clipping thanks to feedback), or a 100-watt transistor amplifier (even harder clipping and a Class AB transition at a few watts).

It is true that there are still time constants in there - a cap-bypassed cathode is not the same as dedicated negative DC supplies for the grids. But then there is still a coupling cap lurking in the grid circuit (connecting the two halves of the secondaries together), although this can be a lot smaller than a cathode-bypass cap, and doesn't carry as much current. But this approach is certainly more complicated, since there then has to individual bias adjustments (and switchable metering) for each DHT to prevent it from running away.

If you really want to spend a lot of time twiddling with the circuit, a good-sounding DC supply for the filaments should meet the bill. You can spend many months or even years on that. But for now, get some clip-leads, a collection of GE or similar oil caps, and bypass those cathodes (I'd suggest to B+, it's a shorter current loop).

Here's the presentation on current loops.

[JoshK]

Great thread! Thanks Lynn for the additional rationale on differential versus parrellel PP.

I too am a Karna builder. I am much earlier on in my build...finishing up the remote power supplies. I took a couple years to collect parts, build a couple of teething projects (which I am finishing final touches now) and read/learn all I could. I spent so much on this project (all amorphous Lundahl iron) I wanted to bring my understanding to a level that would exact the potential.

I finally finished Morgan Jones Valve Amplifiers this weekend. I am more than halfway done with Building Valve Amplifiers.

[zigzagflux]

Good for you, at least you can learn from my mistakes. I also found this project to take somewhere around 3 years, and you and I didn't design a thing !! Happy there are people like Lynn who take the time to document and share their designs. No doubt he has spent 3 times as much as us in order to reach the final state.

You also using AC heating? I'm blown away as to how silent it is, though I admit this is my first DHT amp.

Let us know how the Lundahls work out for you. I should have my repairs completed sometime before the weekend, and I'll report back. Got the GE 97F caps, just waiting for the right size WW's to come in.

[zigzagflux]

I think I have some progress on at least one of the root causes.

I changed all the stages out per Lynn's request: 40uF GE 97F oilers plus 0.1uF teflon bypass from B+ to the cathode. Bias for the 12A4 stage is about 18V grid with 18mA per triode. Bias for the 46 driver is 28V grid at 28mA per triode.

Listening session was improved; midrange was definitely better, lows need help due to my selection of the HPF; I'll tweak with that in a few days. But the highs still were not satisfying, regardless of volume level.

Repeated the measurements of gain/phase from 20 to 20K. Response was essentially identical to that done before the modifications. I would say the phase shift and peaking must be the IT bandwidth, which may or may not be audible.

Bigger revelation was when I looked at the output near 20KHz. Blue is input, red is grid-grid at the 300B, and yellow is the speaker output:



Distortion at the output is clearly visible. This is running almost 20W at 19kHz. Grids don't look too bad, so I figured this was the output stage distorting. Then, I took a look at the individual grids of the 46's:



O great. Correct me if I'm wrong, but I'm operating my 46 at 0V grid, and the driver ultimately distorts, as the first stage is unable to supply the required grid current to the 46. The differential grid-grid looks okay, but this is deceiving, as each grid distorts somewhat evenly.

I was considering the risks of using the 46 when dealing with the idle conditions. The 45 looks to bias over 50V, while the 46 less than 30V. So I get less allowable signal swing at the input.

So am I back to having to consider using the 45 instead of the 46? In the interim, I would suspect I get no more than about 15W before distortion really sets in.

This still, however, does not explain to me why the highs sound washed out, regardless of volume. I mostly run the amp at less than half the signal strength of these tests, and still am unsatisfied with the highs. Could the 33nF K40Y PIO be the reason for this?

[zigzagflux]

One other thought, upon further reflection.

I did some calculations on the phase shift issue. From about 500Hz and up, the phase shift from input to output is a CONSTANT TIME DELAY, around 15 microseconds of lag. It is constant as frequency increases. This leads me to think maybe the phase shift is not much of an issue in the audible realm, as what you hear from the speaker, though shifted in time, is relatively constant.

Sound reasonable?

BTW, took out that 33nF PIO capacitor and ran the speakers full range (not my long term intention), and the highs are back, sounding very nice and detailed. Frustrating when I can't measure what I hear. Now I'll have to experiment with teflon or other exotics to get the sound I know this amp is capable of.

[45]

Originally posted by zigzagflux


BTW, took out that 33nF PIO capacitor and ran the speakers full range (not my long term intention), and the highs are back, sounding very nice and detailed. Frustrating when I can't measure what I hear. Now I'll have to experiment with teflon or other exotics to get the sound I know this amp is capable of.

The 33nF capacitor together with the inductance of the input transformer is a resonant circuit.
If you want to make it work sufficiently well (but never as good as without the capacitor, IMO) the resonance should be far out of the audio band.

Cheers,
45

[45]

Originally posted by zigzagflux


...cut...

Distortion at the output is clearly visible. This is running almost 20W at 19kHz. Grids don't look too bad, so I figured this was the output stage distorting. Then, I took a look at the individual grids of the 46's:



..cut....

This still, however, does not explain to me why the highs sound washed out, regardless of volume. I mostly run the amp at less than half the signal strength of these tests, and still am unsatisfied with the highs. Could the 33nF K40Y PIO be the reason for this?

Looking at your frequency response I would say yes.

Huge amplification around 20K due to the resonance at the input.

IMO, to be sure about 46s performance you should make the same measurements at 500Hz in the same condition and/or without the 33nF cap.

Cheers,
45