HH SCOTT 233 rebuild

[billyz]

I just took possession of a very nice Scott 233, completely original as far as I could tell.
I ran it up on a variac to 117vac , did a few voltage checks and set the bias and balance.
Sounded pretty good a bit muddy and blurry and hum. I played it awhile anyway to get a good feel for it.
Next I proceeded to order new can caps from Antique Radio Supply and a few Musicap coupling caps.
After I installed the Electrolytics and the coupling caps to the output tubes. I reset the bias. And Played it again. Much improved and the hum was minimal. I had the preamp cover off and it was better with it on of course. Next I decided to replace a few of the pf value ceramics and some more coupling caps with parts from JustRadios. I used polystyrene for the PF values, much nicer again. More Air and depth. So off we go. I then installed some PIO caps to the line stage and some more Musicaps to the Phase invertor driver. I also replaced the very difficult to get to .047uf caps on the phono output. I use PIO again.
WOW, huge improvement but not at first . At first there was distortion and I thought I had screwed up. But I could hear it getting better by the second so I left it and hoped it was the old caps forming or settling in. It appears it was something breaking in, because it was sounding much better, more depth and air, reverb, details I had missed earlier in the music.
I still have to rebuild the Phono stage. I think I will replace the PEC phono eq with some discreet parts ie. polystyrene and quality resistors. Then the phono coupling caps. I did try the phono stage but it was rather lackluster compared to the line input. I also built a bucking transformer that drops 12vac. It measures around 108-9 vac. Seems Low but the internal voltages now read very close to the schematic. Sounds really good.

The hardest part is getting to the .047 input caps. I let the front panel down by removing the screws on the sides and bending it froward and down. That helps alots.

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[trainbuftony]

Did you get your hum sorted out? Some scott amps have a special can capacitor which is built with the can as common positive which performs double duty as a filament supply for the front end and as a bias supply for the power tubes. If those caps have failed they may be the source of a significant hum. Somewhere there is a thread on how to rebuild them. I'll bet it sounds fantastic. I know all of my Scott amps do.

 

[kvflyer]

... Somewhere there is a thread on how to rebuild them. I'll bet it sounds fantastic. I know all of my Scott amps do.

Is this the one that you remember?

http://www.audiokarma.org/forums/index.php?threads/rebuilding-electrolytic-capacitors.448773/

I do like Scott amplifiers. I just need to get back on the bench and work on mine. (Been on a long and drawn out move...)

 

[juddboz80]

Looks great! This is one I'd love to hear!

 

[billyz]

Did you get your hum sorted out? Some scott amps have a special can capacitor which is built with the can as common positive which performs double duty as a filament supply for the front end and as a bias supply for the power tubes. If those caps have failed they may be the source of a significant hum. Somewhere there is a thread on how to rebuild them. I'll bet it sounds fantastic. I know all of my Scott amps do.

Yes, The Hum is pretty well sorted out. I knew about the reverse polarity Bias/Filament Can. I have not replaced it yet. If i do, I will restuff it with 100uf 100v caps though. I did replace the other two Cans with new ones from CE Distribution.

I measured every Cap and Can I took out. The Main Filter Cap Can was oozing black tar but measured OK on my Heathkit IT28.
All the other caps measured within tolerance except one .1uf Ceracap to the 7591 grids .

 

[billyz]

Looks great! This is one I'd love to hear!

Ill post some more pics when I get it back together, After I rebuild the Phono section. I am very impressed with the PIO caps and the polystyrenes.
I replaced the ceramic pf caps after I listened to it after the Musicaps, and they really took the grainy-ness out and sweetened the presentation.

The oil caps were last and they made a big improvement as well. I purposely mixed the oil caps with the polypropylene Film n foil caps so as to not have too much of a good thing.

Bypassing the cheap .0015 film caps with the polystyrenes made a nice improvement that was audible. I listened before and after.

 

[juddboz80]

I just got done recapping a scott lk-30 and a scott 340b with all sozo caps, and can caps from hayseed hamfest. it took them to a whole different level from the original ceracaps, I've never replaced many of the ceramic caps unless they tested bad....is it worth it?? Now I'm debating changing them out. Not that they tested bad but I've never tried replacing them to see if I could hear the difference

 

[billyz]

I've been rebuilding/repairing/restoring guitar amps for over 30yrs. Sometimes a ceramic gives a nice grit to the signal and for guitar that's fine. But if you want a pure sweetness , clear as light sound the polystyrene are the best. Silver mica fall somewhere between I think.

They made a real improvement in this amp . I'm running some modified klipsch cornwalls with altec mid horns. So you'll hear everything that's wrong or right. But it's like being there.

I just got done recapping a scott lk-30 and a scott 340b with all sozo caps, and can caps from hayseed hamfest. it took them to a whole different level from the original ceracaps, I've never replaced many of the ceramic caps unless they tested bad....is it worth it?? Now I'm debating changing them out. Not that they tested bad but I've never tried replacing them to see if I could hear the difference

 

[billyz]

Just recapped the Phono section . I used a .027 uf PIO cap in place of the .022uf ceracap. and a Musicap .02 for the big black ceramic cap.
Definitely added a bit more life to the phono stage. but the bottom end is rolled off a bit more than the line stage. I using a Shure V15 iv with a jico SAS stylus. Sounds very good thru a CJ PV12. Same loading, 47k.

 

[billyz]

I now believe the poor low end performance of the Phono Stage is due to the 0.02 coupling caps. I believe 0.1uf would be much better. I have tacked on some cheapo caps and now have 0.5uf and the low end is much better but still not up to the line stage. I also noticed the plate resistors are reversed from the schematic, 100k and 270k along with the corresponding cathode resistors.
I have disconnected the 299 PEC module and it has made a huge improvement. but it does need the gain reduced. Apparently the 299 filter throws a lot of gain away. I am calling it the Vestigial Dynaural filter .

 

[Wharfcreek]

Billy, I too agree.......'Great Job'!! Like Don, I also share a love of Scott amps. I'm just curious about your process for setting/adjusting the bias? In reading through your work thus far, I didn't notice if you had installed any current sensing resistors on a 'by tube' basis or if you're just going off what's there? IIRC, Scott had some kind of like 3.3 ohm resistor or something like that in the cathode side on some amps...... but I've found that eliminating that and simply installing 4 individual 10 ohm resistors off each of the cathodes on the 4 output tubes makes the process of bias/balance MUCH easier!! Honestly, that bias/balance setup that Scott has in these amps is one of the more favorable IMHO. BUT......you need to have the individual test points associate with it. Those aren't too hard to install, and places like Antique Electronics Supply sell what they call 'Tip Jacks' that make perfect test points.

Anyway..nice job! I think my first Scott amp was also a 233...which in retrospect, I think I got really lucky to have found at the time. Now I'm a 'Scott-aholic'!!

Tom D.

 

[billyz]

Billy, I too agree.......'Great Job'!! Like Don, I also share a love of Scott amps. I'm just curious about your process for setting/adjusting the bias? In reading through your work thus far, I didn't notice if you had installed any current sensing resistors on a 'by tube' basis or if you're just going off what's there? IIRC, Scott had some kind of like 3.3 ohm resistor or something like that in the cathode side on some amps...... but I've found that eliminating that and simply installing 4 individual 10 ohm resistors off each of the cathodes on the 4 output tubes makes the process of bias/balance MUCH easier!! Honestly, that bias/balance setup that Scott has in these amps is one of the more favorable IMHO. BUT......you need to have the individual test points associate with it. Those aren't too hard to install, and places like Antique Electronics Supply sell what they call 'Tip Jacks' that make perfect test points.

Anyway..nice job! I think my first Scott amp was also a 233...which in retrospect, I think I got really lucky to have found at the time. Now I'm a 'Scott-aholic'!!

Tom D.

Tom, Thanks.
I used the Scott stock 3.3 ohm resistor. I measured them and they were pretty close. Like 3.43 , 3.37. I balanced it by measuring plate to plate voltage.
I may install 1 ohm Dale resistors one day. Right now Im trying to fry bigger fish, like detail, depth, wide range response. I made a huge improvement by disconnecting the PEC 299 filter and reducing the gain with a 27k resistor( i paralleled across the 1meg grid load.) Then increasing the coupling cap feeding the line stage to a .47uf. . I tried unbypassed cathode with 2.7k and it was very clear but ultimately went with the stock bypass cathode for better low end. I might should have gone lower , like 22k or less. I dont have a scope or distortion analyzer, so it's all by ear and some experience.
I used .47uf because thats what I had , probably too big . I ran out of 0.1uf. Maybe after I get it right i'll spring for some good ones.
I have had an account with CE Distribution (AES) for many years, got my can caps and the Musicaps from them.

 

[Wharfcreek]

Billy, I 'get' where you're going. A couple of questions though: 1) Are you at all concerned about 'stability' issues, particularly in the super-sonic area, but also in the LF range? 2) I'm just curious about your musical choices with this amp? I too have some Cornwalls in my shop......and that's where things get 'sorted out.. Once done, the amp moves from the back of the house to the front, where I have some 20' elevated ceilings in my main audio listening area, and I have Forte IIs in there. The open nature of the room provides a pretty nice environment for what I listen to....which is a lot of 'guitar player' type music. I'm a fan of electric guitar....consider myself a 'wash-out' from my old band days when I never was any good!! I still have fun with it, and have gone back to my 'guitar amp' roots of late and built a few guitar amps for myself and some friends. I just did 2 of the 'kit' versions of the Tweed Deluxe as supplied by MojoTone. Each build took about a day...... beginning one late afternoon, and 'completing the following day by about lunch time! Two in a row was kind of fun, and I'd say the 'experience' gained from the first one definitely made for a better 'second' on, though they actually sounded identical!

Anyway...... sounds like you're a bit more advanced than I when it comes to the "....bigger fish....."......lol I've never really gone that deep into a re-build. I guess since I usually start with something that's generally inoperative or 'badly' lacking.....to me, getting them back to seemingly 'fully functional' usually is success enough. And, admittedly, I've never been that discriminating a 'listener', however I do have some pretty good hearing according to some 'testing' I had done on myself a few years back. You may laugh at this, but I was getting complaints from the 'significant other' about how loud I listened to music....and more so the TV and movies. I was accused of hearing loss while arguing that it was simply a preference. It took going to a local Hearing Lab and some testing to reveal that I not only had far better than average sensitivity, but I also still had some frequency range capability that was well above average as well. Sadly, I also suffer from tinnitus, which I blame on a trip to the Drag Strip with a full pass, and spent the night on the starting line without ear protection, then rode my Harley home!! My ears haven't quit ringing since that day!!

Again, my compliments on the job well done. I'll keep following along and won't thread-gank any further!! Best regards, Tom D.

 

[billyz]

Tom,
Gank all you want. I am not an expert or even educated (no degree in electronics).
I am always concerned about anything that does not sound right. And its all a balancing act.
If a truly knowledgeable person would offer some direction Id gladly accept .

Particularly I was concerned about too much low end , maybe subsonic oscillation robbing power. But I am not hearing it. I do think the .47uf may be a bit much. I will probably go to a .22uf . and better quality.

Basically, I am using a few resistor and capacitor substitution boxes to dial it for my taste.

I just ordered some sprague hyrel caps .1 uf for the phono stage. I will do them all even replace the .047uf with .1uf.

I see most scotts have both sides of the V1, V1a ( phono) with 270k and 2.7k. This one has 270k on one side and 100k on the other. So I will try the dual 270k setup and see or hear.

My first tube set was a Harmon Kardon Citation 1 and 2. I guess I cut my teeth on those. I did rebuild the pre and some work on the power amp.
But I after I heard the CJ stuff I knew where I wanted to be.
Ive had others but those are the ones worth remembering. I suspect I'll die with the CJ gear.

I am pretty happy with the line stage and power amp sections as of now. It's that phono section, (but I am comparing it to the CJ) , if I can get it 80% there Ill will be happy.

If I had PSPICE or LTSPICE with the tube data. I would try to play with them. I did check a few different online calculators and Randall Aiken etc.

Oh, I relate to the guitars and tinninitus. My ears really ring after the doc started me on Thyroid medication.

 

[Wharfcreek]

Billy, this is yet another testament to the old adage we have in the automobile business, which is: There's an ass for every seat!! And that's a literal expression, though it's often referred to sarcastically as well!! I've never owned any CJ equipment though I had a chance to play with a nice pre-amp at one point. I was able to do an A/B comparison between it and an ASL unit that I still own. Frankly, I didn't hear much difference. BUT......I'm a 'tone control' guy......and neither the CJ nor my ASL have any kind of compensation built in. The CJ had the advantage of a balance control, which my ASL does not have, but my ASL unit is built with a separate Power Supply..... so, perhaps a bit quieter? I think it would take some pretty sophisticated equipment to actually measure the distinctions between the two. At normal listening levels adjusted so I felt the SPL was equal, I really couldn't tell any difference. And.......I didn't like either of 'em! I go for the far more 'colored' PAS.......OR.....I tend to opt for an SS pre with a tube power amp. I use an old Kenwood with the power amps I build.....and it does the job for me. Built in 5-band EQ...which I like!! BTW, I have a C-5 here along with every one of Upgrade/Mods that are sold by Mr. HK himself, Jim McShane! Unfortunately, that too is a 'project' I've yet to get in to!!

Anyway....pleased to have made your acquaintance...and I'll keep watching as you progress with all this.

I wonder, does BP and Cholesterol medication also affect tinnitus?.....lol

Later, Tom D.

 

[billyz]

Whats the best Capacitor ? NO capacitor! I just figured out I could remove the .0015 coupling cap to the line stage. I had a .47uf in there yesterday.
It is the best yet. I was getting some weird faint distortion on the leading edge of the signal. When I removed the cap it went away. Sounds full and clear.

edit.. When I say removed it, I mean I replaced it with a jumper wire.

I may even remove the .047uf from the phono stage. I measured it and there is 5mv there, not sure that is a problem, might cause some popping when switching.

 

[billyz]

Here is a pic of the current line stage, without the coupling cap. The Best sounding yet.

The White and Yellow wires replaced the cap. Also, see the Polystyrenes I used for the Tone controls.

 

[billyz]

Today I replaced the .047uf coupling cap from the phono stage going to the line stage with a wire. Sounds much better. Much improved low end definition and body. Fuller , firmer sounding, yet more clarity. I measured 5mv dc, no popping when switching . I think this is part culprit in the weak bass performance of the stock phono stage. I have NOS Sprague HYREL Q caps ,0.1uf coming for the phono stage as well.

I imagine HH SCOTT was trying to trim the low end due to expected placement in a stereo cabinet with built in speakers. And the quality of the average turntable in those days. I am using a modified Lenco L75, no problem with rumble.

 

[billyz]

Several folks have a requested a schematic of what I did. I had to use a Scott 299D schematic since it was in a JPG format. Dont have the software to edit PDF.
There may be mistakes but here is what I cobbled together. The Amp is sounding really good after a little break in. The NOS Caps did need a little time to start sounding really good. Most improvements took less than an hour.
I still plan to rebuild the PEC 222er phono eq, as I do hear a little grit/grunge on the left channel phono only. I'll be rolling the phono preamp tubes as well. I have a good stash of tele's, Amperex, RCA, Mullard etc. Wish I had a stash of 7591's ! It has the original Scott labeled Westinghouse 7591's.
Amperex Bugle Boy 6u8's , RCA 12ax7 line stage. Not sure whats in the phono, Slyvania I think.

I critically listened to every change I made before proceeding to the next. So I am confident of what is what. I am sure these changes would be applicable to most of model scott's . I left the vestigial Dynaural filter PEC in place.The gain must be reduced after disconnecting the PEC circuit.
That proves how much it is affecting the circuit. The Tone controls still function well and that is one reason I wanted this amp. Though I hardly use them. All changes can be reversed. I would love to A/B this one with a Bone stock unit.

I used my Conrad Johnson PV12 and Premier 11a ( with Selected NOS tubes ) mostly listened thru a set 83' Modified Klipsch Cornwalls with Altec 511b mid horn and ALK Extreme crossovers. I used my Lenco l75 with shure v15 IV Jico SAS, Modified Marantz CD player, IPOD and even Bluetooth iphone inputs.

I did not bother to change the caps in the loudness circuit as I don't need or use. I did not change the caps on the ceramic phono input either, although I should relook at that to be sure it would have no impact on the MAG LOW input.

I am using the stock Scott 3.3 ohm resistor to bias and measuring the dc from plate to plate of each pair 7591's to set balance. I do have some DALE precision 1 ohm resistors I could replace the cathode ground and use to set bias and balance. I could also use the transformer shunt method to measure ma of each tube as well. But I am pretty comfortable with the 3.3 ohm setup. I did measure them and they are pretty close to 3.3ohms. My amp has a test point on the top of the chassis( factory) to use the 3.3 ohm.

I know some will think it's Blasphemy to do what I did. I say , let's A/B amps and then judge.

 

[billyz]

I apologize for the poor image quality. It looks much better on my photobucket account.
I'll gladly email it to anyone who wants it.