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Old 27th February 2011, 10:24 PM   #1
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Default AA 151 or Scott 222

Hi all,

I am starting out as one of you said "jump in the pool" with the dismantling and eventual building up of a tube amp project. I have two amps to work with a Scott 222, the other Heathkit's AA 151. The Scott works with a hum in one channel and one OT getting warmer than the other. The AA 151 was fired up but one of the out put tubes got red hot so I pulled the plug, literally!
My aim is to have one amp able to play records ( have a pre-amp section but no tone controls) while the other would only need a CD input so it could be stripped of these stages. I think less is better but understand that some things are necessary.
My speakers are horns, mostly self designed/made, but some with 16 ohm drivers.
I have read many a thread regarding these two amp and find the comments funny and insightful. I gleaned much information already but have these preliminary questions.
Which of the two is inherently better designed to have a working/upgraded pre-amp with out the tone controls?
How do I test the out put transformer for damage? With the ohm meter both read the same specs.

Doing the breast stroke!
Robert
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Old 28th February 2011, 12:38 AM   #2
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Robert,

You have (presumably) efficient horn speakers. Therefore, power is not an issue.

Scott sold GOOD stuff, while 1 fellow (Joe Rosen) made the disparaging "griefkit" remark, more than once. If the "iron" is intact, overhauling and refitting the Scott 222 seems best, but do take Mr. Rosen's remark with lots of salt. Which 222 version do you have? All the schematics can be found here.

How far from OEM are you willing to go? FWIW, I always advocate replacing the source selector switch and wiring a single set of I/P jacks directly to the magnetic device section. Permanently configure the mag. preamp section for phono. The fewer mechanical connections in the signal path at mV. levels, the better. If you strip out tone controls, the voltage gain device in the power section can change to a triode. The pentode is needed to compensate for the insertion loss of tone controls.
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Old 28th February 2011, 02:29 AM   #3
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Default AA 151 - scott 222

Thank you Eli,

Yes, power is not an issue. The Scott that I have is the 222, no letters, maybe the first model? The OPTs are smaller than the AA 151's.

About the iron, I just saw, looking through thread pages a way to check the OPT. I don't remember the author right now but it worked great and thank you! The suggestion was to connect 6.3VAC through a 5-10 Ohm 5 Watt resistor to the 16 Ohm secondary tab of the OPT and to then measure the voltage in the primary wires 'plate-plate'.
In my case 6.8VAc (no load) to the tabs yielding 163.2 volts ac plate-plate. The second OPT reads slightly lower 161,8. Also the voltage to each tube tabs is between 46.3 and 46.6 Volts AC in this test set up. So I think the two OPT are fine. I checked the main power transformer and it checks out too. This is all regarding the AA 151. Tomorrow I'll check the scott.
So what I understand is that the scott would be a better amp to make it a dedicated phono amplifier and that taking out the selector switch and other filters, selectors and switches would clean up the signal path as well and in this case, make the power tubes able to 'run' in triode mode. I am not tied to keep the Scott in original state. What would it then take to make that switch as that is off schema?
The AA 151 would then become a dedicated CD player amp and it too can then be stripped of all the 'pre-amp and tone control parts. Is the same true for it that it may be run in triode mode as it uses the same model power tubes.
This is already taking me into adventure! thanks for you reply and opening doors.

Best.
Robert
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Old 28th February 2011, 03:52 AM   #4
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I have a 151 that I was rebuilding, and now it's on hold. I personally didn't really have the skills and confidence to finish it and feel like it had any chance of working. From a practical standpoint, some of the layout is really tight and difficult to work in. However, if you have experience with point-to-point work in tight spaces you probably won't have trouble. I sure did.

The phono section of the 151 is not so great... it's one half of a 6eu7 feeding the other half which is shared with the line-level input. It never had enough gain for my cartridge so I planned to shoehorn the RCA 12ax7 circuit in there. If you choose to skip over the tone and the preamp sections choose the appropriate pot values, since 1Meg is probably not a great match if it's not coming after the tube sections. I also planned on splitting up the bias on the outputs so one resistor wasn't being used for all 4 cathodes.

Here's a glued together and cleaned up 151 schematic, since all the scans out there that I could find were in two or more parts.
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File Type: jpg aa151scematic.jpg (642.4 KB, 187 views)
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Old 28th February 2011, 04:04 AM   #5
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I'm inclined to agree with the thinking that the Scott is an "A".

If you do things "right", a single amp will work for both phono and CD. Do you own 2 sets of speakers? I'm inclined to think Scott used better "iron" and that definitely makes the amp. You want triode wired to go with your horn speakers. You have the makings of an "El Cheapo" (schematic attached) variant. Send planet10 (DD) a PM. He worked on an EC using the similar LK48 as his donor.
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File Type: gif ElCheapo-23jun06-map.gif (36.3 KB, 166 views)
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Last edited by Eli Duttman; 28th February 2011 at 04:07 AM. Reason: completed post
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Old 28th February 2011, 04:20 PM   #6
dtut is offline dtut  United States
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The Scott phono pre is well regarded - with good reason I believe. I use a DIY clone and really like it. It can be tweaked a tad. Check the AX archives for an article by Charles Hanson (sp?) about going through a 222.

The AA 151 I have hums so badly, I've never seriously listened to it, phono or otherwise.
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Old 28th February 2011, 08:44 PM   #7
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Thanks all for your comments and the two schematics.

It sounds like I may have to rethink gutting the 222, anyway before I aquired it many of the caps have been up graded, though two of the electro's are still in use! Both OPTs test, with 6.8Vac applied to the 16 Ohm tab secondaries at 145 VAC 'plate to plate'. I think they are fine. The power T runs a bit high: 348 Vac on the secondaries (this is with 117 VAC through a Varivac) and 390 V goes to the output tubes. On V11, the rectifier, the resistance on P3 is 39.6, P5 42.5 roughly half of what is called for (80 Ohms) on the trany.
Further, one of the AC balance pots R8 reads 356 Volt! That ain't right!
And there's no voltage to the heaters of V7, V2, V6 and V1.

So I am in the thick of it now. I have the AX articles by Chuck Hanson (sp) but was not prepared then to get so deep into it. I may now not have a choice, though!

Any help is appreciated.

Robert
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Old 1st March 2011, 01:34 AM   #8
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A way to bring the B+ rail voltage down is to use a "potato masher" 5R4 as the rectifier, instead of a 5AR4. Planet10 did that in his EC.

Do you want to stay close to OEM or do you want to "get adventurous"? With "clever" use of SS diodes, I think the OEM power trafo can take care of energizing B+, B-, and a 12 VDC phono section heater supply. Your "El Cheapo" would use 4X 6BQ5s, 2X 12AT7s, 2X 12AX7s, and a 6AL5.
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Old 1st March 2011, 06:22 AM   #9
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Quote:
Originally Posted by Eli Duttman View Post
You have the makings of an "El Cheapo" (schematic attached) variant. Send planet10 (DD) a PM. He worked on an EC using the similar LK48 as his donor.
Ours ended up as an incredibly good 3.2 w amp (qw made 2 actually -- one had power transformer failure, and got recycled into a tubelab PP for Chris' wife). Not pretty, we're going to retool it, and have it switchable for more power.

dave
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Old 1st March 2011, 07:50 AM   #10
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The Scott has better resale value. I would use the Heathkit as guinea pig and modify it to your hearts content.

The AA-151's plate voltage can be too high for the plate if your AC is higher than 117. Mine went as high as 420v! With that voltage slamming the plates AND screen grounds in ultra-linear mode, tube life can be shortened and can sound harsh. Check the operation points against the schematic.

The phono stage is very noisy, good luck making it quiet. Many integrated amps have noisy phono stages, albeit sometimes sounding quite good. You can bypass the line stage and tone controls completely and just keep the selector and volume control and route the signal straight to the grid of the 6AN8, using just the amp section since it has enough gain for horn speakers.

If the plate runs red hot, most likely the coupling caps have DC leakage and must be replaced. All the power tubes share the same cathode resistor so I would suggest using two resistors (double the value), one for each channel, plus two bypass caps for better stereo separation. I would increase the value of cathode resistor to lessen the current so the tubes don't have to run so hot. Ultra-linear mode at high voltage can lessen tube life for certain tubes, be careful.

After recapping power supply, the AA151 can be very quiet. All Heathkits in this series use really good output transformers and after cleaning up can sound very transparent and extended. They are underrated. But the sad part about their design, like many integrated amps, is that they try to squeeze more power and end up running the voltage too high, especially on the screen grid. High screen grid voltage is one of the main reason of short tube life span, especially in UL circuits.

Anyway, I highly recommend experimenting with Heathkit amps. They can be highly rewarding. Scott amps are great too if you decide to use that. I just like working with sleepers. And if you mess up, it's not a huge loss financially. Enjoy and happy soldering.

.
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