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Old 20th December 2008, 11:46 AM   #21
45 is offline 45  Italy
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Some toughts.

You told your shift measurements are differential so your real zero phase could be in the high frequency range.

I don't know the specifications of your transformers,
however (from your first measurements) I would say you have a (parallel) resonance at (or just above) 20KHz.

A parallel resonance means you have the equivalent capacitance and inductance in parallel. It gives rise to a 180 degree shift (going from far below the resonance to far above it).
The phase is conventionally 0 degree at the resonance, so you go from +90 to -90, passing through the resonance.

Such parallel resonance is typical of any transformer: it is the first of a series.
The first resonance is usually due to the coupling of the stray capacitances (all in parallel among them) and leakage inductance of the transformer.
Note that the typical stray capacitances of audio transformers are in the range of nF (can be a fraction of nF or several nF's, depending on their inner coupling and the transformer design itself). So I am not suprised about the bad effect of the 33 nF cap!

If you say there is no problem with high frequencies when listening without the 33 nF, I think you have a resonance given by the parallel coupling of such 33 nF cap with the leakage inductance of the input transformer.
Lets assume the leakage inductance is 1 mH (typical), than you get a resonance at 28Khz!
Very probably it is not well damped beacuse your source has a low impedance.

Maybe if you tell the specification of your input transformer I could be more precise.



Cheers,
45
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Old 20th December 2008, 01:43 PM   #22
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Default Re: New 300B PP amp completed, phase shift questions

45:

Thanks for the thoughts. You seem to be really zeroing on the input cap, but I think that capacitor is only contributing to a washed out tweeter, nothing else. In the first post of the thread I noted:

Quote:
Originally posted by zigzagflux
For the purposes of eliminating a possible issue, the 33nF HPF cap is BYPASSED for these measurements.
All the rising response and phase shift issues are independent of this cap. In fact, when I had experimented with possible values for this capacitor in choosing the HPF frequency, I had swept the system over the audible range looking for resonance. I did find some, with smaller values of capacitance. The higher I went in capacitance, the less prominent the peaking. The values chosen worked good with swept sine waves, without adversely loading the source.

Further, I noted there was zero phase shift and flat response 20 to 20K from source to 12B4 grids. While the capacitor may not SOUND good, it is not contributing any resonance that I can see.

I guess where I'm heading is asking the question if other people's non-GNFB IT coupled designs exhibit the phase shift (constant time delay) I am experiencing. It no longer sounds bad, with the input cap removed, but the shift is still there.
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Old 20th December 2008, 02:01 PM   #23
45 is offline 45  Italy
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Default Re: Re: New 300B PP amp completed, phase shift questions

Quote:
Originally posted by zigzagflux
45:

Thanks for the thoughts. You seem to be really zeroing on the input cap, but I think that capacitor is only contributing to a washed out tweeter, nothing else. In the first post of the thread I noted:



All the rising response and phase shift issues are independent of this cap. In fact, when I had experimented with possible values for this capacitor in choosing the HPF frequency, I had swept the system over the audible range looking for resonance. I did find some, with smaller values of capacitance. The higher I went in capacitance, the less prominent the peaking. The values chosen worked good with swept sine waves, without adversely loading the source.

Further, I noted there was zero phase shift and flat response 20 to 20K from source to 12B4 grids. While the capacitor may not SOUND good, it is not contributing any resonance that I can see.

I guess where I'm heading is asking the question if other people's non-GNFB IT coupled designs exhibit the phase shift (constant time delay) I am experiencing. It no longer sounds bad, with the input cap removed, but the shift is still there.

In fact, in the last post, I was speaking in general about transformers as well.

The phase shift could be due to a transformer alone or in combination with the tubes (i.e. one or more differential pairs do have quite high and/or different capacitances within the pair).

The 33pF cap brings an additional and similar effect being in the same (or close) range of stray capacitances.

Cheers,
45
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Old 20th December 2008, 07:40 PM   #24
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If the tubes are distorting due to excessive capacitance in the loads, this defeats the whole purpose of the Amity/Karna family of amplifiers. The goal is to operate the most linear devices with most linear loading system - but - this is critically dependent on the right choice of tubes, transformers, and operating points.

This is most awkward in the input stage, which has the least ability to drive the stray capacitances of the first interstage. (The 5687's and 7119's I am using now are barely adequate for the job.) Rather going to the additional complexity of a cathode-follower, you might revert to the previous differential input stage that is then simply RC-coupled to the grids of the driver stage. This eliminates the input and first interstage transformers, allows you to select the RC coupling value that gives you the highpass characteristic you are looking for, while retaining the Class A PP driver that is transformer-coupled to the output stage.

You still need a very low-distortion input stage, but the standing current and Zout no longer matter that much with RC coupling. A nice vintage 6SN7 will probably do just fine - and you can keep the current source you already have. Since the ideal coupling caps will be small at these impedances, you have the option of Teflon or other exotics, and don't have to chase after an exotic high-Z interstage transformer for the first interstage. High source impedances are the enemy of good transformer performance, and that's unfortunately what you tend to see in input stages.

To recap, the suggested topology is:

1st Stage: Input is direct-coupled to one side of a differential 6SN7 (or similar) dual triode with a 15~40 mA current-source on the cathodes. The plates are then conventionally RC-coupled to the following driver stage.

2nd Stage: The driver stage is the same as the previous recommendations - NO current source, merely a common resistor to ground, and the previously suggested bypass cap between B+ and common (virtual) cathodes. The grid resistors should be 100K or less.

3rd Stage: As before. No current sources here either.

This gives you an amplifier which is an Acrosound for the first stage, and an Amity/Karna for the driver and output stages.

Sidelight/distraction: If this works for you, an interesting alternative would be a Mullard first stage, with the PP/differential part of the Mullard using transformer coupling at the output (a Mullard with a twist). Brief walkthrough of a Mullard: single-ended first tube is plate-resistor loaded and directly-coupled to the grid of a second stage, which sits at an elevated voltage and uses a long-tail resistor to create a differential pair. The grid of the "other" half of the diff pair goes to two places; a 1 megohm resistor connected to the plate of the first tube, and a large-value bypass cap that goes to ground. If the coupling is RC to the following stage, it is a traditional Mullard, and the kinky variation is transformer coupling instead. But this nutty digression aside, I'd try the straight differential RC-coupled Acrosound version first.

By eliminating two sets of low-level transformers, you should see a lot less phase shift in the whole amplifier, while retaining their advantages in the stages that operate at higher power and at lower impedances.
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Old 20th December 2008, 11:35 PM   #25
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More thoughts - the data you're showing indicates that the tubes involved cannot handle the capacitances that they're seeing on their plates. Distortion in vacuum tubes isn't just matter of the real (resistive) term of the load, but the reactance as well - and elliptical load-lines have far more more distortion that straight load-lines.

That's the reason I've suggested trying a simple RC-coupled first stage to supply the voltage swing for the driver stage, so it no longer has to drive the stray capacitance of the first interstage transformer. It will have to drive the plate-load resistor, so there is merit in using a high value of B+ and a high value of plate resistor.

A mildly exotic variant of the RC-coupled input stage would use separate inductors between the B+ supply and the plate-load resistors, which would act to increase the effective value of the resistor in the midband (where it counts), while the existing plate-load resistor would buffer the stray capacitance of the inductor from direct exposure to the plate. This means that at DC and at extreme HF, the minimum load would be the plate-load resistor, while in the midband, it would be the much higher value (100K) of the grid resistor of the following stage.

The combination of the inductor, plate-load resistor, coupling cap, and grid-resistor would be a much easier load to drive than the existing transformer, and less reactive as well, since the plate and grid resistors would limit the phase angles and resulting elliptical load-lines to real values at the frequency extremes.

I agree the bias condition of the 46's does not look good. There should be lots of headroom at the 46 grids, and the input shouldn't be working that hard. You need to measure the frequency response of each stage, using direct injection on the primary side of the relevant transformer.
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Old 21st December 2008, 01:37 AM   #26
45 is offline 45  Italy
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Quote:
Originally posted by Lynn Olson


I agree the bias condition of the 46's does not look good. There should be lots of headroom at the 46 grids, and the input shouldn't be working that hard. You need to measure the frequency response of each stage, using direct injection on the primary side of the relevant transformer.
Lynn, I have no experience with PP transformer coupling driver, however I do have experience in driving a 300B SE with an LC coupled 46 (triode connected).

This amp was a variant (with the cheaper 46 instead of a 71a) of this amplifier:

http://www.audiodesignguide.com/se/katelelo.html

My amp was able of 13W at the (extremely soft) clipping and the driver was still far from going into positive grid field. The power bandwidth was 30Hz-50KHz.

The operative conditions were:
1) driver : 250V / 22 mA (as from data sheet), L = 60H, C = 4,7uF and 10K for the 300B grid resitor.
2) 420V / 70 mA / 4 Kohm for the 300B

The 12B4 is certainly "stronger" than the PT8 (that is, more or less, like a 26).

It seems very strange to me here we have only 20W and the 46 is swinging into positive grid.

Cheers,
45.
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Old 21st December 2008, 01:46 AM   #27
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Lynn:

Thanks for not giving up on me. I'll give some further opinions on what I hear (though I suspect nothing will surprise you) then I'll have a few questions in hopes of saving the current build.

Had two of my friends over to listen and compare this amp to my existing amp. Interestingly enough, your suggestions are very similar to the existing design; it is basically a Mullard 5-20 design with ST70 iron. 12AX7 gainstage, DC coupled to a 12AU7 LTP with CCS tail, cap coupled to triode connected EL34's for the output.

The basic summary was the 5-20 blows away the new amp. Not by design, of course, but my choices. Even with the input capacitor removed, BTW. Open and detailed soundstage versus a flat presentation, sounding compressed would be the best way to describe it.

1. One preliminary question for you, Lynn. Knowing you use similar iron, do you have the same type of phase shift on your amp? If you do, but your sound is still top notch, then I'll forgo stressing out about it any further. Part of me thinks the missing presence and imaging is in part due to this phase shift throwing off the recombination of sound, but of course I have no idea.

2. I will certainly consider your suggestions above, but if at all possible, I want to try and salvage what I have without replacing the entire front end. Since it works for you, it has to work for me. I placed the 12B4 at the front end because it has a very low Rp and just the right gain needed for my system. I am surprised to hear it may be a limiting factor in the amp. I DO have a population of 7119 tubes. What about paralleling two 7119 triodes for a really beefy driver for the first interstage? Would this not be more than adequate for the first interstage then?

3. If I need to bite the bullet and throw in 45's, I will. Again, if it works for you, it must work for me? However, would it not make sense that the 46 would do just fine at low output, before zero grid voltage is even approached? Because at present, the amp isn't right regardless of volume.

Not trying to ignore your advice, just trying to make right what I've done wrong. The 12B4 being a wrong choice really surprises me. Rp is 1K, how can that be a bad thing?

Thanks for your patience. And thanks, Bud, for some nice trannies, guess I just can't drive them properly.
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Old 21st December 2008, 10:45 AM   #28
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Great thread. lots of important information.

I built a little 5687 6080 amp with interstage phase splitting and it sounded good but had roll off issues. The pointers in this thread have clarified my thinking on a rebuild, keeping the two stages - but splitting at the front with a transformer and then running PP all the way to the 6080 finals. The 6080 works very well at 100V 100mA so the 5687 has plenty of gain and grunt to keep them happy. I was going to build it with differential stages - but your comments have given me pause. I have built a version of Gary Pimms Tabor amp which is differential front to back, so I was totally sold on the idea - but experience is a great educator.

Shoog
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Old 21st December 2008, 02:48 PM   #29
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Experience IS a great educator; it's the lesson that can be most painful No problem being the guinea pig, as long as I find my way out of the cage.

I will confirm the prediction that removing the lower CCS and adding the B+ to cathode capacitor improved the sound. Funny thing is, I've found many designs of IT applications with anemic highish Rp drivers, often running in the sub-10mA range. If I'm hearing Lynn correctly, the idea is to run a ballsy driver at every stage of the amp, not just before the output stage. Distortion is important, obviously, but it seems to me if I can produce sweet sound with the 12AU7 in a LTP, which has a poor reputation for linearity, application is critical.

Not to bleaguer, but it is surprising to me a 12B4 doesn't have enough. It is no slouch in linearity or Rp, as is the 46. Most surprising.

If I migrate to the 7119, what about a 2:1 IT ??? Don't need additional gain... Hmmmm....
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Old 21st December 2008, 03:20 PM   #30
45 is offline 45  Italy
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Quote:
Originally posted by zigzagflux

Not to bleaguer, but it is surprising to me a 12B4 doesn't have enough. It is no slouch in linearity or Rp, as is the 46. Most surprising.

About the 46 (as for many 2A3's) you should need to have a good number and make a selection.....then you can get almost as good linearity as the 45.

With the 45 linearity is never an issue unless it has been heavily used.
Many 45's around look new at a first sight however then you find they have less emission tha expected....

45 is now hard to find in really new condition and is expensive, so ask for serious testing before buying.
If you don't find what you are looking for could also consider something like Emission Lab 45 (solid plate) which can take up to 13W plate dissipation.

Quote:
Originally posted by zigzagflux

If I migrate to the 7119, what about a 2:1 IT ??? Don't need additional gain... Hmmmm.... [/B]
IMO, you should go for those IT's designed for maximun overall bandwidth, minimal insertion loss and that can take a little unbalancing with minimal loss in performance.
Typically IT's (and OPT's) designed for minimal insertion loss and extended high frequency response give up something in low frequencies however, in my experience, I cannot hear a difference between a transformer that can go down to 1Hz and another that starts to roll off at 20Hz.
This is practically the philosophy of the best japanese transformer companies.

Cheers,
45
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