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I have used the PowerDrive circuit before in a SV811-10 amplifier and it worked really well, until I had a failure around the circuit. Likely bad execution on my part. But by that time, I was working on the GM70 monoblocks, and needed the output transformers, and also wanted to see what could be done with a reliable interstage transformer coupled approach. I have a pair of Dave Slagle 5K nickel core ITs in circuit. But I may rework the circuit using an IT between a voltage amp stage and a driver tube and then go from there on how to drive GM70. A PowerDrive circuit would be pretty easy to put together for the task.

I heard the expression "Slow boat from China" but perhaps now change that to "Slow boat from New Jersey"! The Gulf has seen alot of hurricanes this season, I have some remote co-workers in New Orleans who have been "battening down the hatches" a number of times this season. Fortunately (for them, not others) the storms have passed west of NO and all they get is rain and not much wind.

David

Well, wait a moment. First of all, as to matched tubes and biasing, the tubes should be *well-matched* using good matching techniques, which some of the better tube dealers employ. This involves more than just throwing them on a stock tester. That said, setting the cathode idle current for quality-matched tubes is perfectly adequate. I mean, let's take a pair of SE amps with poorly-matched output tubes. They're not going to perform optimally either!

As for crossover distortion, this is, I think, an overrated problem. It occurs primarily in deep Class AB or Class B amps. A classic Williamson amp runs almost totally in Class A. Furthermore, negative feedback reduces crossover distortion considerably. This is a old bugaboo that should not be a concern in a well-designed, modestly-powered PP amplifier.

As for PP vs. SE, I well understand the allure of SE DHT amps. It's a very attractive sound in the right system. It also comes with significant costs and limitations, in terms of power and bandwidth, not to mention the output tubes and transformers necessary to get anything approaching decent performance. My Williamsons, by contrast, employ $25 tubes and afford solid bandwidth of 20Hz to 40kHz. (They are capable of 200kHz on the high end, but then you get into stability issues so it's better to limit the HF response.) It would e a rare SE amp that offers that kind of response. As for "realism," it depends on your definition of "realistic." I find the better bandwidth and power make recordings sound more "realistic" but again, it depends on your system, your needs and your listening habits.

..and did I mention harmonic and intermodulation distortion? Magnitudes lower in a PP amp.

I am just looking for the best possible setup I can with PP KT88's. None of which were purchased as matched pairs.
I just have a standard emission tube tester so matching pairs will be minimal.

I have four monoblocks and one stereo amp that allows for setting cathode bias. I usually bias KT88 at 35 ma's. Walt taught me that the lower Bias the midrange just blooms and I agree.

I mean, let's take a pair of SE amps with poorly-matched output tubes. They're not going to perform optimally either!

How is that possible? They're separate single-ended amps not tied together push-pull pairs???

As for crossover distortion, this is, I think, an overrated problem. It occurs primarily in deep Class AB or Class B amps. A classic Williamson amp runs almost totally in Class A. Furthermore, negative feedback reduces crossover distortion considerably. This is a old bugaboo that should not be a concern in a well-designed, modestly-powered PP amplifier.

Whether Class A or AB or B each tube in the push-pull pair hands-off to the other -- it is a differential amplifier after all. What with the manufacturing tolerances that are common to the industry it is impossible to have perfect hand-off.

As for PP vs. SE, I well understand the allure of SE DHT amps. It's a very attractive sound in the right system. It also comes with significant costs and limitations, in terms of power and bandwidth, not to mention the output tubes and transformers necessary to get anything approaching decent performance. My Williamsons, by contrast, employ $25 tubes and afford solid bandwidth of 20Hz to 40kHz. (They are capable of 200kHz on the high end, but then you get into stability issues so it's better to limit the HF response.) It would e a rare SE amp that offers that kind of response. As for "realism," it depends on your definition of "realistic." I find the better bandwidth and power make recordings sound more "realistic" but again, it depends on your system, your needs and your listening habits.

Well the Carys (my Carys as one example) are good for 20-Hz. to 20,000-Hz. +/- .5-db and are capable of bringing a drum set (as one example) into the room with slam that you feel in your chest. That said I have yet to hear in my system any Williamson or Ultra-linear amp. (most recently an Audio Research amplifier that I had here for evaluation) that provides the natural see-through transparency and musicality the rest of my system is capable of.

Granted when trying to drive inefficient speakers even the most power SET's can run out of steam up to a point. Yet there really are not that many PP amps out there that can provide more than 150-watts/channel (re: the 150-watt Cary 211's) that do not cost less than a nice car.

BTW -- the Audio Research amp. was not a dog. Actually -- I liked it -- I liked it a lot. Yet -- it just didn't do what the SET amp. was capable of.

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Walt

In that case, matching the tube current is about a good as you can do. At lower power output it will perform just fine. At power extremes it may not be optimal, but I wouldn't worry about it too much!

How is it possible for poor tube matching to affect SE amps? Simple. They aren't going to perform the same, and depending on the demands placed on them, you're bound to get more distortion, channel imbalance and other issues.

The Cary 805's are well-regarded, and you have every reason to like what they do in your system. But the measurements are nowhere near what a similarly-powered Williamson can offer, even with feedback applied:

https://www.stereophile.com/content/car ... asurements

For contrast, look at my "Update on Williamson Amplifier" thread to compare the square wave responses:

viewtopic.php?f=4&t=1404&hilit=williamson

The Heyboer versions of those amps price in at about $1200-1500 the pair--a very good price-to-performance ratio, IMO.

Now, most of us here agree that measurements aren't everything, so we don't need to get into that. But if you want to talk about distortion and bandwidth, SE amps simply can't compete with a good PP design. Having built many different SE amps over the years, I have found much more satisfaction with the Williamsons than any SE amp I've built. To me they have a very, very "natural" sound due to the low distortion and excellent bandwidth, with added depth and realism afforded by the PP topology.

Regarding the Cary 211s, well, the price of retubing alone is, what, $800 *minimum* and probably double that for quality tubes? The cost/benefit ratio is a little OOC there. And I can't find any measurements but I'll bet they're not spectacular. But again, the sound of a PP 845 amp can be pretty amazing, as memebers of this group well know.

I don't know which ARC amp you auditioned, but the ones I'm familiar with are neither Williamson-style not ultralinear. It's been a long time since I've heard one but my recollection is that I wasn't fond of it. I think that beyond a certain power level the "magic" of tubes startes to diminish significantly.

_________________
Walt

With an SE amp, definitely.

Our Movecube weighed 1600 kg. I think the cost of sending it by air would be the same as buying a used A380 these days.

Deciding what to take is complicated. To be honest, I did not have to take anything and I could have bought new stuff here. I see that I can still order from Mouser and shipping is free when the order is over $60. However, I do have some stuff I don't want to give up and my wife has 40 scrapbooks that she spent years creating. It depends on what is important to you. Just like in amplifier building, your design depends on what is important to you personally and nobody else. The comments I have read about the sound of the powerdrive circuit is that it is sonically benign. Years ago when I built my 6V6 amp with powerdrive, it appeared to have much more grunt than other 6V6 amps we played the same day.

ray