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Old 21st February 2009, 06:11 PM   #101
kevinkr is offline kevinkr  United States
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Hi Zigzagflux,
Have a great vacation.. Awaiting further reports upon your return.

Given that all of my amplifier designs except one in recent years have been fixed bias designs I have to strongly agree with 45's suggestion to try fixed bias - if nothing else you will eliminate the odd interactions between the cathode bypass cap and the opt. Depending on the cap you are using the HF performance of that cap may be nowhere as good as you think as well. In fact in some instances a no cap approach is better than any cap, regardless of how good, and IMO this is one such instance..

I had forgotten that built my ST-70 variant, thanks for the pleasant reminder!
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Old 23rd February 2009, 10:56 AM   #102
Ceglar is offline Ceglar  Australia
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Hello,

I've been running a PP 300B IT coupled amp with DHT input and driver with Buds excellent OPTs (level 1) for a few years now.

This last weekend I substituted the output stage common bias resistor and associated bypass C with the CCS found at Gary Pimm's web-site - heatsink Q2.

Probably the single most positive change that I've heard in a while, very obvious and for the better... you might try it.

Shane
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Old 23rd February 2009, 04:59 PM   #103
JoshK is offline JoshK  Canada
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Your experiences underscore the need for me to breadboard my amp before committing to chassis layout.

Another thing I'd like to try is a comparison between the 5687 > 45 & triode strapped D3A (PP still into the IT). The D3a nets you roughly the same gain, similar rp and 2/3rds the standing current. The miller penalty is mild. D3A is said to be quite linear.

The truth is in the testing. Would the D3A be able to compete with the two stage input, both sonically and objectively? It has the advantage of simplicity and lack of interactions of two stages and additionally is one less interstage.

None of that is related to your problems, so I apologize for the tangent, but it is related to this design.

I'd also be really curious as to how the fixed biased compares to the original.
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Old 3rd March 2009, 12:46 AM   #104
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Back from Mexico. Wished I could stay. Water was pleasantly warm. We spent a day at both Grand Cayman and also Cozumel. Very easy for a old whimp like me to go all the way into the Gulf, instead of the typical up-to-the-knees. Weather perfect every day, upper 70's and plenty of sun. Someone remind me why I live in Wisconsin again ?

Wired up the LL1620 and repeated the measurements. Definitely found some improvement, but as others had suggested, this behavior at high frequencies may be somewhat normal. I do confess I have not messed around with winding configurations this time; I simply followed the manufacturer's recommended wiring being overly anxious to see how performance looked. Maybe I can improve on this further with some experimentation, not sure.

Shot of 300b plates at 4W output (original ONetics xfmr):
Click the image to open in full size.

Shot of same with LL1620:
Click the image to open in full size.
Now it does look like the output is less than 4W with this capture, but due to the lower distortion, less drive was needed to obtain 4W output.

300b plates at 4W output (original ONetics):
Click the image to open in full size.

Shot of same with Lundahl:
Click the image to open in full size.

Then an FFT of the plate voltage (ONetics):
Click the image to open in full size.

Same with Lundahl:
Click the image to open in full size.

Finally a shot of 8W output (ONetics):
Click the image to open in full size.

And Lundahl:
Click the image to open in full size.

So it appears there is a degree of distortion with both output transformers, but the Lundahl is clearly less. While some of these measurements were being made, I would check the voltage across the cathode bypass cap, to see if there was any signal voltage being dropped across it, which would indicate to me some interaction or rising impedance with frequency. I could not measure any increased voltage, so it would appear the affect of the cap is minimal in this case?

There are a number of suggestions I have heard and would certainly like to consider (fixed bias, differential, etc) but for various reasons I am somewhat limited to the design at hand. Foremost, my power supply for the 300b is fixed at 500V, and I would have to make undesirable changes to the power supply chassis to address this. So for now, I will have to remain in cathode bias, sorry.

One other observation I found was that the hum nulling circuit causes some odd wiggle of the grid voltage to the 46 tube. But only with the Lundahl IT; no obvious wiggle with the ONetics. So for now, I have the hum nulling out of the circuit; hum is 15mV at the speaker terminals without it, which is tolerable for me; my speakers don't put out a ton of bass anyway.

I will wait to hear some comments before I finish up with some of my thoughts and conclusions, and possibly some direction for me in the future.

What do y'all think? Soundwise, again in mono, it sounds good, but I wouldn't say it blows me away. Just a good amp, that's how I would summarize it. There's got to be something I'm missing that Lynn's amp can do well.

Thanks !!
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Old 5th March 2009, 08:03 PM   #105
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No thoughts?

Maybe a simpler question; since there are hundreds of 300B push pull output stages out there, may I ask "does your amp exhibit the same behavior?"

It's a pretty easy test to perform; just throw some probes ac coupled on each plate. Takers?
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Old 5th March 2009, 08:49 PM   #106
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Glad you're back! Here's a quick thought - what is the quiescent current going through each 300B? I found that I was still getting some Class AB behavior at 60 mA per device - but that 80 mA got rid of it. Yes, I know that's a lot of current, but the distortion data was clear. The 3rd and higher harmonics dropped really fast in the 60-85 mA region, to my surprise. It changed much less in a SET amplifier with the same output stage and tubes.

Now, I don't know how pentodes vs triodes handle the AB transition, and what bias currents they like to see, but typical pentode Class AB amplifiers are run around 35-40 mA per tube. I was expecting the generic 300B SET figure of 60 mA would be adequate to remove Class AB artifacts, but in fact it wasn't enough. In practice, this meant derating the B+ voltage, or using super-300B type tubes with high-wattage plate ratings. I got good results from beating up the Chinese tubes, though - they seemed to take the abuse just fine - something I'd never do to a NOS tube.

As for sonic preferences, what can I say. I've gotten to a point where listening to Class AB pentode amplifiers with 6~20 dB of feedback isn't something I enjoy that much. I prefer either all-triode DHT SET or Class A DHT PP. But that's just me. Other people like other topologies - OTL with huge amounts of local feedback, FET/MOSFET/tube hybrids, and the classic (Class AB pentodes with feedback) amplifiers from the Fifties.

There are certainly some speakers that seem to demand high-powered Class AB pentode with feedback or 60~200 watt Class AB transistor amplifiers - they just don't work very well with SETs or low-powered Class A PP amplifiers. So it's not entirely a listener thing - speakers play a role as well.
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Old 5th March 2009, 09:32 PM   #107
Tweeker is offline Tweeker  United States
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Hi Lynn,

Quote:
The 3rd and higher harmonics dropped really fast in the 60-85 mA region, to my surprise.
This follows what the WE datasheet claims, nice to see reality backing up the paper. Load is also a variable here.
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Old 6th March 2009, 12:08 AM   #108
45 is offline 45  Italy
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Quote:
Originally posted by zigzagflux
No thoughts?

Maybe a simpler question; since there are hundreds of 300B push pull output stages out there, may I ask "does your amp exhibit the same behavior?"

It's a pretty easy test to perform; just throw some probes ac coupled on each plate. Takers?
As is, your amp looks unbalanced to me. Too high 2nd harmonic for a class A push-pull at a fraction of its rated power. It is true you are monitoring a quite demanding point (a rather high frequency). To be sure about that you should check for the same power level (namely, for the same input signal) at 1KHz and 100Hz.

The distortion of your amp does not surprise me as you did not consider any possibility for dynamic balancing. The only possible calibration I can see in your design is the cathode of the input stage. Remove the current source and use a classical cathode resistor. The total resistance will be a separate (bypassed) resistor for each side and a common pot (approx. 15-20% of the total value) with central pin grounded. I know it is local Fb but I cannot see another way and you will have to find a compromise anyway.

Choosing tubes with very high degree of matching is very difficult and expensive and above all is no guarantee of permanent balance because each tube will grow old its own way....


45
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Old 6th March 2009, 08:43 AM   #109
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Quote:
Originally posted by Tweeker
Hi Lynn,

This follows what the WE datasheet claims, nice to see reality backing up the paper. Load is also a variable here.
Higher standing current (in a Class A PP amplifier) also has a really nice payoff - much less sensitivity to load. As a loudspeaker guy, this is a win-win-win for me. Who cares if it puts out 20~30% less power (thanks to lower B+) if you can use a broader range of speakers?

That's the real reason I like transformer primaries on the highish side (6~8 K) and high operating currents (70~85 mA) - the amp as a whole is much less sensitive to the crazy gyrations of speaker impedance. And the best speakers have a bad habit of the most zany impedance curves.

The opposite would be a SET running at moderate currents and lowish primary values - with the right speaker, magic, with the wrong speaker, not so nice.

Feedback squashes out all of these differences, but the distortion spectra changes in character, so the amp as a whole sounds different. But if the starting point is pentodes, well, feedback makes a lot of sense for many reasons. Horses for courses, as the Brits say.
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Old 30th April 2009, 10:34 PM   #110
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Quote:
Lynn Olson
The fact that the new amplifier is sounding worse than a Mullard 5-20 with Stereo 70 output transformers means that none of the benefits of the Amity/Karna architecture are being realized - it's like a BMW M3 accelerating slower and handling worse than a delivery truck. Something is very wrong with this picture.
How true it is.

So after completing this project, I should probably close out the thread and update everyone on the end result. Nice to know, after so many posts and reads, that there was resolution. Indeed, in this case there was definitive resolution.

A lot of very good input by all, which is much appreciated. There are not many avenues to which one can go to get solid advice on a challenging project such as this. DIYAudio has been a life-saver for me more than once! I would like to summarize by reflecting on some pertinent posts, and provide confirmation of the advice. There were, at the end of the day, two fundamental issues that had produced the poor performance of this amplifier, and one misconception that had to be purged from my mind.

First issue was the interstage transformers. I had tried many different things to attempt to both eliminate some rising response, as well as correct some very serious phase shift issues at higher frequencies. To little avail:

Quote:
Lynn Olson
If there are any questions at all about the current delivery in the first stage, consider paralleled 7119's, a monster tube like the 6H30, or a quick conversion to RC-coupling.

When the capacitance issue goes away, you will hear it in the first few seconds of listening. It isn't subtle. Remember, by doubling the current and halving the Rp, the trouble frequencies are now twice as high. That can make all the difference.
I had tried tubes with Rpís ranging from 1K to 2K, single / parallel, various winding configurations, with really very little difference in performance across the IT. Maybe it's just me, but I could not for the life of me get improvement in the IT's, especially regarding high frequency phase shift. I basically gave up.

Quote:
Lynn Olson
If I were you, I'd buy a pair of Lundahl LL1635's interstages as a cross-check. They are good for 80 kHz (that's what I used in the first version of the Amity), and sound pretty good as well. Replace either the first or second interstage and see what difference this makes to the measured peaking and HF rolloff beyond that. In the Karna I have now, there is NO peaking at 20 kHz, and I would not use any interstages that have peaking in the audible range, no matter how good they sound subjectively.
Per K&K Audioís suggestion, I resorted to the LL1692A, designed specifically for Rpís in the range I was dealing with. Measured performance was extremely encouraging, but since I was purchasing these 4 units one at a time (cautious upgrades and monetary considerations), I never got a good chance to hear a completed version in stereo until only last night. Alas, these are not amorphous core !

Second issue is the measured distortion on the output stage at lower power levels. Grid drive to the output stage looked very good, plate voltages did not. Purchased a LL1620 output transformer to compare to the existing O-Netics. While I was able to get more power out of the Lundahl, the basic behavior was similar, just less pronounced.

Quote:
Lynn Olson
I found that I was still getting some Class AB behavior at 60 mA per device - but that 80 mA got rid of it. Yes, I know that's a lot of current, but the distortion data was clear. The 3rd and higher harmonics dropped really fast in the 60-85 mA region, to my surprise.
Decreased the cathode resistor in the self-bias output stage, and that significantly improved performance at the higher power levels. Running about 71mA per 300B at 410V plate. A very welcome improvement, thanks !

Last item was a misconception on my part. Was seeing decreasing performance over 10k when running in excess of 4-5 watts.

Quote:
kevinkr
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.

kevinkr
I am now wondering whether or not your expectations of the opt performance at 18kHz are realistic or not. I think the only way to know this for sure would be to do the same experiment on another reputedly "unimpeachable" opt.
This was right on the money. It is not practical to expect a 16W amp to produce 5W at 20 kHz. I lessened my expectations in this area, and put down the scope.

With both amps upgraded to Lundahl ITís (kept the O-Netics OPTís) and no zobels anywhere, I had a listening session last night. If I may quote one more time:

Quote:
you will hear it in the first few seconds of listening. It isn't subtle.
Yep. There we go. I didnít need to swap in the ST70 to do A/B testing anymore. There was no question I was getting what I had expected. Very glorious sound, amazing detail and soundstage (insert audiophile superlatives here). Couldnít stop listening; I have never quite heard cymbals sound so real before !

To provide two tangible examples, there is a divergence in the Beatlesí Octopusí Garden where they are simulating being underwater. You can hear a manís voice gurgling; he moves from stage left to stage right. Very distinct, but I never heard it before.

Another example is Fleetwood Mac, Dreams. When Stevie sings her own background vocal, she is in a different location than her lead vocal. Listened to this song hundreds of times, never caught that.

Far as Iím concerned, this project is done. Maybe in a few years I might toss in some amorphous core, but for now, itís time to sit back and enjoy. Been 3-4 years in the making. Thank you everyone for your help and advice. What a wealth of information here ! If anyone is interested in measurements or futher info, feel free to let me know.
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