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
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
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.
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.
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
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
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.
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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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.
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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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.
The AA 151 I have hums so badly, I've never seriously listened to it, phono or otherwise.
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
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
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.
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.
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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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.
.
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.
.
Hi all,
You guys are very helpful, thank planet10 for the picture. I have ordered electro caps for both amps so the power rails can be more secure, and by pass and coupling caps. What I am moving to is to take one project at a time and directdrive is right to 'practice' on the AA151 first.
My background is that I have build several solid state kits, 4 Borbely 60's and 4 regulated power supplies, so I am not new to tight places or soldering. Still I am a newby and by any standard know parctically nothing. This however does not deter me from venturing into the tube world.
The 'iron' of the AA 151 is in good condition and this seems as good a starting point as any. What I would like to do is to take the selector, source, treble and bass features out and hard wire the mag phono input.
I don't know if that means I can take a AF stage out or not or what the ramifications are in terms of Pentode or Triod mode.
Maybe I should just work with what I have in terms of the power tube setup as I have no clue how to change from one to the other and I have the schema of the first.
The Power Trany of the Scott is a Triad R-20B transformer and it's specs as printed on the case are with 115 Vac primary. My house main is 124 Vac, so that would maybe explain the hotter B+.
At any rate my plan is to start with the AA151 and get my feet wet, and park the Scott. Is there a benefit to changing the dual volume pot to two isolated pots, as I now room on the face plate for this?
I may not have responded to all your advise, sorry for that.
My speaker system is all horns Siare 16 VR drivers for midrange and a self designed/build base horn with a 12" EV, a Jensen imperial I build with a Stephens 15 driver with tweeter in center, and Visaton 16 tweeter horn. That's one set. I also have a 13 foot long sub base horn in the basement that when fired up with a mono tube set taken from a 147 Leslie Cabinet provides fundamentals very well. An other set of horns are a prototype, using a concrete tractrix midrange in front and an Edgar 70 hertz corner horn in back of the driver, currently a 4" Audax, I forget the numbers. Very natural and so simple, no cross over. This second set is in a different room. I am also building midrange horns made out of doorskin, six panels that flare out like a petunia. Much more appealing than concrete to some.
Thanks again, I am waiting for my parts to come in and then I'll report further.
Robert
You guys are very helpful, thank planet10 for the picture. I have ordered electro caps for both amps so the power rails can be more secure, and by pass and coupling caps. What I am moving to is to take one project at a time and directdrive is right to 'practice' on the AA151 first.
My background is that I have build several solid state kits, 4 Borbely 60's and 4 regulated power supplies, so I am not new to tight places or soldering. Still I am a newby and by any standard know parctically nothing. This however does not deter me from venturing into the tube world.
The 'iron' of the AA 151 is in good condition and this seems as good a starting point as any. What I would like to do is to take the selector, source, treble and bass features out and hard wire the mag phono input.
I don't know if that means I can take a AF stage out or not or what the ramifications are in terms of Pentode or Triod mode.
Maybe I should just work with what I have in terms of the power tube setup as I have no clue how to change from one to the other and I have the schema of the first.
The Power Trany of the Scott is a Triad R-20B transformer and it's specs as printed on the case are with 115 Vac primary. My house main is 124 Vac, so that would maybe explain the hotter B+.
At any rate my plan is to start with the AA151 and get my feet wet, and park the Scott. Is there a benefit to changing the dual volume pot to two isolated pots, as I now room on the face plate for this?
I may not have responded to all your advise, sorry for that.
My speaker system is all horns Siare 16 VR drivers for midrange and a self designed/build base horn with a 12" EV, a Jensen imperial I build with a Stephens 15 driver with tweeter in center, and Visaton 16 tweeter horn. That's one set. I also have a 13 foot long sub base horn in the basement that when fired up with a mono tube set taken from a 147 Leslie Cabinet provides fundamentals very well. An other set of horns are a prototype, using a concrete tractrix midrange in front and an Edgar 70 hertz corner horn in back of the driver, currently a 4" Audax, I forget the numbers. Very natural and so simple, no cross over. This second set is in a different room. I am also building midrange horns made out of doorskin, six panels that flare out like a petunia. Much more appealing than concrete to some.
Thanks again, I am waiting for my parts to come in and then I'll report further.
Robert
Robert,
Look at the AA151 schematic spopepro was kind enough to post. You keep the 6AN8/6BQ5 power sections and eliminate the remaining signal circuitry. The least expensive way to control volume and channel to channel balance is with individual mono controls in each channel serving as the 6AN8 pentode control grid to ground resistance. The downside to the setup is channel to channel tracking, but it is quite workable. I suggest inexpensive 50 KOhm Alpha/Taiwan Carbon controls from Mouser.
Heath's phono section leaves (IMO) much to be desired. While 6EU7s help, AC heaters in phono sections is highly problematic. Use the tweaked RCA circuit I've uploaded. A TO92 case LR8 regulator IC in each channel will take good care of the 250 VDC needed. I'll get back to you, after doing some research, on the DC heater supply for the phono section.
Look at the AA151 schematic spopepro was kind enough to post. You keep the 6AN8/6BQ5 power sections and eliminate the remaining signal circuitry. The least expensive way to control volume and channel to channel balance is with individual mono controls in each channel serving as the 6AN8 pentode control grid to ground resistance. The downside to the setup is channel to channel tracking, but it is quite workable. I suggest inexpensive 50 KOhm Alpha/Taiwan Carbon controls from Mouser.
Heath's phono section leaves (IMO) much to be desired. While 6EU7s help, AC heaters in phono sections is highly problematic. Use the tweaked RCA circuit I've uploaded. A TO92 case LR8 regulator IC in each channel will take good care of the 250 VDC needed. I'll get back to you, after doing some research, on the DC heater supply for the phono section.
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Eli, thank you, and hi all,
About the heaters, the wiring to the heaters is consistent, I mean from the hum balance control one pole goes to only the 4 pin and the other to the 5 of the set. Does this have to do with phase?
I have gutted the unit and am now building it back up. One of the hum balance pots is 40 % out of spec of what is called for (200 Ohm).
Is this then feeding higher voltage running the tube hot.........?
I am proceeding making this AA 151 a dedicated phone amp. It's my warm-up to learn from.
So I will take all the front panel wipers out and use two separate volume controls.
Is it advantageous to shield the AC heater wires?
And SY: I do this for my therapy!
Best,
Robert
About the heaters, the wiring to the heaters is consistent, I mean from the hum balance control one pole goes to only the 4 pin and the other to the 5 of the set. Does this have to do with phase?
I have gutted the unit and am now building it back up. One of the hum balance pots is 40 % out of spec of what is called for (200 Ohm).
Is this then feeding higher voltage running the tube hot.........?
I am proceeding making this AA 151 a dedicated phone amp. It's my warm-up to learn from.
So I will take all the front panel wipers out and use two separate volume controls.
Is it advantageous to shield the AC heater wires?
And SY: I do this for my therapy!
Best,
Robert
Hi All,
To update you on my AA 151 project. After gutting and cleaning I am now rewiring the heater supply first using three stands O2free Teflon 24 solid wire in a braid. I had a left over spool of this stuff. A little extra work but very cute. I measured the Hum Balance pots and read 157 and 186 Ohms, quite different from 200. Will this be a problem?
Following Eli Duttman's suggestion and reading Spopepro's schema, when I discard the parts prior to the 6AN8A, may I also discard R64 (1000 Ohm), R63 (22000 Ohm) and C2A (30mfd) and C2B (30mfd) in the power supply? these supply the 275 and 205 Voltages to parts I am not using anymore. And if so do I connect the center tab of the hum pots via the 150 KOhm to the 285 Voltage Source? And do I have to adjust the resister?
About the volume control hook up. It is the wiper or middle pin, of the 50 Ohm pot that I connect to the grid ,pin 8 of 6AN8A, and the first to Input signal and the third to ground, correct?
As to lay out, I moved the two 6AN8A to the V1 and V6 position.
You help Eli and others is appreciated!
Robert
To update you on my AA 151 project. After gutting and cleaning I am now rewiring the heater supply first using three stands O2free Teflon 24 solid wire in a braid. I had a left over spool of this stuff. A little extra work but very cute. I measured the Hum Balance pots and read 157 and 186 Ohms, quite different from 200. Will this be a problem?
Following Eli Duttman's suggestion and reading Spopepro's schema, when I discard the parts prior to the 6AN8A, may I also discard R64 (1000 Ohm), R63 (22000 Ohm) and C2A (30mfd) and C2B (30mfd) in the power supply? these supply the 275 and 205 Voltages to parts I am not using anymore. And if so do I connect the center tab of the hum pots via the 150 KOhm to the 285 Voltage Source? And do I have to adjust the resister?
About the volume control hook up. It is the wiper or middle pin, of the 50 Ohm pot that I connect to the grid ,pin 8 of 6AN8A, and the first to Input signal and the third to ground, correct?
As to lay out, I moved the two 6AN8A to the V1 and V6 position.
You help Eli and others is appreciated!
Robert
Eli,
Sorry, my mis-speak. It's 50kOhm.
Splitting up the cathode resister/cap set, so each EL84 pair has its own set, I'm thinking of increasing the resister value, maybe 200Ohm, making the tube run a little cooler.
About the hum balance wiper through the 150 k resister to 285 vdc....I am alright keeping the values the same?....
plodding along.....
Robert
Sorry, my mis-speak. It's 50kOhm.
Splitting up the cathode resister/cap set, so each EL84 pair has its own set, I'm thinking of increasing the resister value, maybe 200Ohm, making the tube run a little cooler.
About the hum balance wiper through the 150 k resister to 285 vdc....I am alright keeping the values the same?....
plodding along.....
Robert
Hi all,
As the AA151 lay open, I took a break and changed the remaining electro-caps in the Scott and play it now. Wonderful to hear it sing again as I plod along the with the AA 151. Too soon for updates. I did find some other threads showing upgrades of it, beautifully done, colorful and very neat. Encouraging!
Bert
As the AA151 lay open, I took a break and changed the remaining electro-caps in the Scott and play it now. Wonderful to hear it sing again as I plod along the with the AA 151. Too soon for updates. I did find some other threads showing upgrades of it, beautifully done, colorful and very neat. Encouraging!
Bert
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