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SCOTT 222D is a Keeper!


AndyKubicki

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Well, finally, after touring the USA to visit the 2 Scott Doctors, my Scottie came home. In case you missed my previous post, I purchased her via eBay and sent her off to Craig to get recapped and checked out. But after recapping, Craig said she didn't sound right, and after consulting Ryan, decided the best bet would be to send the amp to Ryan. He indeed found wiring errors which happened AT THE FACTORY! This was his email to me:

I found many problems inside the 222D, but only one was causing the crosstalk distortion, and this problem can be traced back to the amplifier's degenerative feedback. Each channel of the 222D uses a feedback loop, which connects to the output transformer's 16 Ohm impedance tap, and is fed back to the phase splitter. The purpose of these feedback loops is to mask crossover distortion. Crossover distortion occurs in any amplifier that is not biased for class A operation. The 222D, along with every other Scott integrated, operates in class AB1. At low volumes, the amplifier operates in class A, as the volume is pushed, the amplifier starts to operate in class B. This allows the amplifier to have a larger power output. When the amplifier switches from class A to B, crossover distortion occurs, since each output tube is now conducting for only 180 degrees of the sinusodial waveform, instead of 360 degrees for class A operation. The degenerative feedback loop compensates for this crossove. Crossover distortion can be reduced to almost nothing by using a phase shift network.

There is a feedback loop for each channel, and the problem was that the factory tied both feedback loops together, and connected them to ground. This bypassed the RC network, and the amplifier was running without feedback. Cross Modulation also occured between the two phase splitters, so both of the amplifier's channels were shared, this amplifier was not capable of producing a stereo output, and the sad thing is, it left the factory in this condition.

This amplifier ran nearly 40 years incorrectly, until now.

Well, I brought her home while on lunch, with only enough time to unpack her. Damn she looked good! Smelled great too! Got done with work around 4-ish, but had to go to Home Depot and get some plastic to cover the roof of my patio which was leaking (rain forcasted, and showed up later indeed). Then I had to get some banana plug jacks so I can hook up some wires with spades to the speaker terminals. I was even somewhat reluctant after all this time to plug her in. What if I don't like it? But I plugged her in. Sounded weak and tinny at first. What's wrong? I checked all the switches, the tape switch was on. Soon as I moved it over, boom! Now that's more like it!

Before I ever heard this amp, I was worried about bass response and the ability for this 20 wpc amp to handle transients. I have only listened for about 3-4 hours tonight and I can tell you for sure, no problem in either case! Now believe it or not, I don't really think that's a lot of listening yet. Not enough to notice everything (new) I have heard (on my old CDs). But here are some initial observations...

Since wifey came home about the time I got things ready, I wanted to see what she thought of her vocal-based music. We put on Phantom of the Opera. The organ opening was awsome! The bass was not only stronger, but you can hear the note played, rather than just a deep sound. The next song was vocal and it sounded different, but we weren't sure how. We hooked up the Adcom again, and wifey said the vocals were more "balanced with the music" on the Adcom. They were right out in front with the Scott. She said she liked the balanced sound better. I said give it some time. We got used to the brighness for all these years, now lets just listen.

I went to my (Stravinsky's) Firebird CD. It starts out with the basses playing. Again, what struck me was the ability to hear the note instead of a deep sound close to a note. Stravinsky's melody builds and after a few minutes, mellows out to a false ending which lulls the unsuspecting listener into a state of relaxation. My cat sat next to me on the couch getting his chin scratched when all of a sudden, BOOM! Transient #1 hits full force. Bonkers (my cat) jumps to his paws! Even after hearing this piece many times, I was stunned for a few moments at the easy with which this amp handled this powerful passage spanning the audio frequency spectrum from low (bass drums) to high (flutes, horns, etc). But wait, the melody goes to the violins and after some time I am hearing some dissonant notes being played that have previously escaped my ears! There's a new world in the mid range!

On Bach's Toccata and Fugue the organ sounded magnificent! I couldn't listen to this CD before thinking it was recorded poorly. (Incidentally, I wanted to have this played at my wedding...for some strange reason was talked out of it!)

King Crimson's DeJaVROOM DVD sounded great! It was recorded bright, but tamed down a bit thanks to tubes.

Anyway, these were just initial observations...and this is all with an inexpensive CD player (the Pioneer). The overall biggest improvement is tonal...that mid range (which I thought I would never miss!).

BTW, everything was played with the tone controls at 0 and loudness off, so as to get as close to a setting as my tone controlless preamp. Now I need to get some sleep so I can listen more tomorrow!1.gif

A great big thanks to Craig and Ryan for your great work!

Also, a great big thanks to all who have responded to my tube questions in the past few months!

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Congratulations on your venture back into the world of tubes. I expect your entire cd, and record? collection will take on added value.

Are you using the 8 Ohm output on the Scott? You might try the 4 Ohm taps. You have power to spare with those KHorns and the detail may get even better.

I think your noticing low level detail is consistent with tubes' tendency to distort less at lower levels unlike the ss tendency to get worse. How are the "s" sounds on vocals. I would expect the ss "s" sounds to be more intense and all sound the same. With the Scott, I would expect each "s" to sound different, often more like an "sh" sound. If you listen to people (real people, not recordings) talk, you'll notice the "s" sounds are in fact softer and more varied than we are used to hearing through ss distortion.

leok

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Thanks Leo. My record collection is merely calling out my name from the garage as my TT is missing a needle and a plug. I also need to take it apart and clean the tracking mechanism (it's a Beogram 4002 with tangential (sp?) tracking. I powered her up and everything seems to work, but the movement of the tonearm assembly is a bit slow and erratic. Still, I wonder if it's worth it because I remember another problem I've had since it was new, and that was that once in a while, it would just out of nowhere slow down slighlty for a brief moment and then return to normal speed. I took it back to the dealer who sent it to B&O, and it didn't do it for them, so it never got fixed. I was looking at some Marantz TTs on eBay and maybe it's wiser to go that route.

Thanks for the tip on the impeadance taps. I didn't know that increases detail...I'll check it out. Any additional danger in doing that?

You are correct about the "s" sounds. Matter of fact, when we put on the vocal for my wife last night we went from the Scott back to the Adcom and I was kidding her about the SSSSSS!

I was really surprized at the detail in the bass.

Craig, I think I might need to get some different software, not yet sure.

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Nothing "dangerous" about going to the 4 ohm taps unless you decide to grab a power supply cap with a hand while your doing it. Even an unplugged tube amp can kill you. Keep one hand in your pocket while working.

Going to the 4 ohm taps will cut your output in half, but the amp will be better able to handle swings in impedance. Makes sense with a SET amp, but I don't really understand it in this instance. Maybe Leo can enlighten us.

Seems to me it's a double edged sword. You get more stability from the amp with the 4 ohm taps, but since the power is cut in half -- you'll go into class B much sooner. Of course, with K-horns -- you'll be at earblistering levels at four watts.

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I always thought the first 1/3 of the rated output in a Class AB amp was Class A. So, with a 20 watt amp on the 8 ohm taps -- I figure about 7 wpc of Class A. If going to the 4 ohm taps, that's 10 wpc -- with a little more than 3 watts of Class A.

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I didn't know this. Thanks Ryan.

I don't remember where I heard this exactly. Just something to the effect that tube amps in general carry lethal voltages even if unplugged, and one should work with "one hand in the pocket" whenever possible.

Is the center channel on all the Scotts "powered"? I was under the impression that the center channel output was a low level output?

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deang,

That's mostly the part I don't know. The Heath W5M was over 50% class A. That's why it was so easy to adapt it to no feedback, triode operation .. the KT66s were already biased about where I wanted them.

The reason for trying lower Ohm taps is: the Ohms seen by output tubes (tube side of the output xfmr) is often a compromise between highest power and lowest distortion. For the KT66, 5KOhms gives max power while 8KOhms gives lowest distortion. I am using an 8K Ohm transformer (normally used with EL84 tubes). When my RF-7 is 4 Ohms, the KT66s see 8KOhms. I am not using the stock Peerless transformers that came with the Heaths.

The Scott, with a stock transformer, may present a more friendly load to its output tubes (twice nominal) if the 8 Ohm KHorns connect to the 4 Ohm tap. The power will be lower (with KHorns this is probably not an issue), but distortion may be lower also, resulting in finer detail and also lower noise due to lower gain. Damping will be greater which may smooth the freq. response.

It's not a big deal, but it's easy.

leok

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Deang,

The only Scotts that have powered center channel ouput are the 222D,299D,LK-48B and LK-72B all others are designed with a low level center channel output and in my opinion a better setup. The powered center channel is a compromise at best. The good part is these amp still have a low level center output also !

Craig

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Hey Dean, I've heard of what you mentioned, especially about working with one hand in your pocket. Stay out of puddles too! I believe other older tube sets may not have had the bleeder resistors Ryan mentioned are in the Scotts. I remember as a kid watching the TV repair dude taking a wire (after the TV has been off) and touching it to ground to discharge the cap. It made a cool spark!

Well, I did a bit more listening today and dicovered another deficiency...in my Adcom/Audire. I have a DVD of Bruford's Earthworks in Footloose in NYC. I played it and noticed that the acoustic bass can be heard when playing lower notes, but the upper notes were lost. You could see the bass player plucking them, but the notes were lost under the rest of the music. Thought we could blame this one on the recording, or I thought maybe the acoustic bass just doesn't sound as loud in the upper bass range. Well, those note WERE there when played through the Scott! They were still not as loud as the deep notes, which on the Scott WERE DEEP as well, but you could hear them!

I have yet to hook up a speaker to that center chanell. Since my Khorns are 21' apart, it may fill in that hole in the middle. I'm just having too much fun playing music to get into experimenting right now! Besides, I do want to let my ears "burn in" to this new sound!

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Uh oh... I didn't hear this before, but tonight I am getting some line noise in the form of the 60 cycle buzz. It's at a low level and independent of volume setting. It's especially obvious when listening through headphones. I did not hear this yesterday. When I reverse the plug it gets worse. I can pull out all interconnects and it's still there. Am I going to have to do something with my outlets or get a line conditioner?

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Andy,

The hum/buz could be a weak electrolytic in the power supply or simply excessive gain. Since you probably don't use much of the amp's power, you might be hearing it amplify stray electronic noise which is mostly 60Hz power based.

Is the noise the same in both channels? Is it the same for all source selection (aux, phono, tuner, tape, etc)?

If the noise is simply a normal characteristic of the amp, there are things that can be done.

You could try the 4 Ohm taps, they will have reduced gain, and maybe a few other advantages.

Other things involve power supply upgrades.

If this is normal, and knowing that is all you need to know, then probably, if both channels have the same noise level, you don't have a problem. Does anyone know if this amp has seperate filters for left and right power?

I don't think a line filter will make any difference.

leok

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Yes, Leo, it's fairly equal on both sides regardless of source. I think you're right in that this may be a normal thing for this amp and if I'm reading the schematic correctly, left and right are not separate filters. They are separated into the filament, B+ and pre-amp B+. I think I need to find the offending piece of electronics outside my amp that made for the excess noise last night. It's not bad tonight, so whatever was causing the interference is not on tonight. And I do crank 'er up occassionally.

I can't even listen to my shortwave because of all the rf noise in this house...

Perhaps this is an area where the Adcom wins...0 noise from the Adcom at any volume level.

Craig, it's because of the Khorns that I need headphones for that late night listening!

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