|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
69.141.96.215
In Reply to: RE: Here are some pictures posted by Eli Duttman on December 11, 2014 at 11:39:46
My "hen scratches" are uploaded for review and comment.
Eli D.
Follow Ups:
I am dumbfounded by the knowledge that you gentlemen possess! My hat is off to you. Bottom line....is it worth working with or should I start from scratch?
All the best,
Jim
If you are up to working from scratch, contact "inmate" Jeff Yourison. He can provide feedback on his successful build of my tweaked version of the RCA setup. The tweaked setup can drive a downstream load, unlike RCA's original.
What I'm trying to do with the Lafayette unit, within its limitations, is give you something that will work reasonably well with a Shure MM cartridge.
You should end up better than OK, in combination with the "Maggot Box" amp. I want to see the power amp schematic in order to tweak it into a synergistic whole, when combined with the reworked Lafayette phono preamp.
Eli D.
Hello Eli,
Here is the amp schematic. Again, thank you very much.
Jim
The point "mobiasloop" made about being able to power the phono section from the power amp is important.
Forget yesterday's doubler B+ supply. All that needs to placed in the phono preamp chassis for B+ is regulator circuitry. The inter-chassis umbilical cable will feed the regulator circuitry.
The umbilical will also carry the "extra" AC filament current. We will use that current to energize a proper DC heater supply. Those Mullard tubes you already have can stay put. ;> )
I will upload a schematic for the heater supply, this evening.
BTW, the power trafo on the phono preamp chassis will removed. Save it, as a future use will be found.
Don't forget uploading photos of the "Maggot Box" amp.
Eli D.
Here are a few pictures. As you can see, it is a project waiting to be done. Resistors hanging loose, etc.Thank you,
Jim
Edits: 12/13/14 12/13/14 12/13/14 12/13/14 12/13/14
First it is obvious that someone tried to upgrade it albeit rather sloppily. The first thing I would do is convert the grounding system to a star ground system using a solid, tinned copper buss bar wire of about 12 awg. This buss bar would be connected to the chassis at only one point (preferably the large can filter cap where you see the splash of solder to the chassis). This is the star ground. The 110V AC green ground lead, the CT of the power transformer, the speaker common of the output transformer all connect to this point. The buss bar would travel from the input jack to the large can filter capacitor which would be the only ONE ground point on the chassis. All grounded resistors and capacitors would connect to this buss wire throughout the amp instead of the chassis here and there. I would insulate the input jacks using Switchcraft 3505F or similar input jacks and a nylon shoulder washer on one side and a flat washer on the other side of the input jacks. The ground stud would connect to the buss bar wire. This would make the chassis a SHIELD not a conducting ground. I would install a Cinch 141Y bakelite connector as speaker output connector. M.B.
"...the fool doth think he is wise but the wise man knows himself to be nothing but a fool." Will Shakespeare
Hello all,
I am trying to get a feel for all the great tips you gentlemen have provided and determine how to go about implementing them. Seems like it will be a challenge but I hope to learn a lot in the process. Thank you.
Jim
The remarks M/B made about grounding and shielding are spot on. "Single" point grounding (SPG) goes a long way in reducing residual hum. Two proven methods of achieving SPG are "star" and buss. Either way, the idea is to keep the large currents close to the unitary point of chassis contact. Sensitive, "low" level, circuitry is kept away from the hum sources.
Eli D.
Jim,
That is a ROYAL mess! It is painfully obvious that the unit has been gone through, at least once, and that the workmanship is/was poor. ;> (
The need to (IMO/IME) completely rebuild is an opportunity. ;> ) We can employ better circuitry than that used by Magnavox. Several possibilities come to mind. The bulk of my thinking revolves around SS rectified B+ and combination biased O/P tubes. Combination bias derives the total grid to cathode negative potential from a RC bias network and a C- supply. SS rectifying the B+ frees up the 5 VAC filament winding, which will be voltage multiplied to generate the C- rail.
Are you willing to work with me and other "veteran inmates"?
BTW, the O/P transformers are on the small side, which hampers obtaining full bass extension. That can be dealt with, by high pass filtering at the inputs to the circuitry. I don't think you will have problems listening to a double bass. OTOH, don't try recordings of 32 foot organ stops.
Eli D.
I've uploaded the phono section DC heater supply. Yes, it's another voltage doubler setup. :> D
As powering the phono preamp from the "Maggot Box" disposes of concerns about sufficient B+ current, it now becomes possible to use the tweaked version of the RCA setup. I've uploaded that schematic too.
As stated previously, the tweaked setup can drive downstream loads, courtesy of the MOSFET source follower. This project is growing into something quite interesting. The front panel switch will be retained to provide a choice between phono and 1 "line" level source, like a CDP. While (IMO) switches in mV. level signal lines are bad, a switch in "line" level circuitry is fine. The O/P of the phono preamp is what will connect to the switch. The phono I/P jacks will be hard wired to the phono preamp.
Eli D.
This doubler should work well to supply the regulator. If I recall, the similar setup I used in the RCA hybrid MOSFET follow circuit provides about 14VDC and regulates to 12VDC well. The 12AX7s don't pull much current for their heaters.
Use a heat sink on the 7812 -- it gets warm.
IMHO, you would be better off to power the Lafayette from the Magnavox power supply which is much better designed (slow turn-on tube rectified) via a 3-5 ft shielded cord and dispense with the Lafayette power supply. This would get inductive coupling and hum out of the picture. The connections on the Mag power supply transformer marked Y1 and Y2 is obviously to power a preamp (probably DC filaments) that is not shown on your Mag schematic. Most magnavox units had a seperate control box with volume, bass and treble controls (it probably had tubes). You will have to add a volume control to the input of the mag power amp if you don't have this preamp unit.M.B.
"...the fool doth think he is wise but the wise man knows himself to be nothing but a fool." Will Shakespeare
The 5U4 is directly heated and turns on almost as quickly as SS diodes do.
You are correct about being able to power additional equipment. IIRC the "extra" filament winding is 6.3 VAC for the tuner/preamp. Turning that into a proper DC filament supply for the phono preamp is easy.
Magnavox ran "117" VAC through the Molex connector in order to put the on/off switch in the tuner preamp. I strongly prefer keeping the AC mains line out of the umbilical between chassis.
For a console amp, this "puppy" is top of the line. The power trafo is "meaty". There is lots to work with.
Eli D.
I know I'm correct. We need to keep it simple for this beginner. M.B.
"...the fool doth think he is wise but the wise man knows himself to be nothing but a fool." Will Shakespeare
Measure the voltage at Y1 and Y2 either at the molex connector or at the power transformer of the Maggie. I bet it's 12.6v or 6.3v and ready to power a preamp. Also the maggie power supply is well designed and choke loaded and you can't do better for a power supply. All you would need to do is add another electrolytic cap at the end of the tri-cap filter and one voltage divider resistor to give you B+ for the Lafayette. M.B.
"...the fool doth think he is wise but the wise man knows himself to be nothing but a fool." Will Shakespeare
Some fast remarks. Details tomorrow. Photos of the amp will be helpful.
Do you have the tuner/preamp that was associated with the power amp? If you don't, we have to build something. Notice that no listening level controls, whatsoever, are to be found in the schematic. What's labeled Bal. is a trim pot., not a control.
BTW, a "quick and dirty" solution to the volume control issue is to replace the R107/R207 fixed resistors with 500 Kohm log. taper controls and connect the pot. wipers to the grid circuitry.
Eli D.
Yes, dual log-taper pot will be needed - 500K is probably best, as the phono stage has a high output impedance. If the 750 Ohm balance control R117 is installed, leave it as-is - if not, you need to add two 390 ohm resistors in its place to use the amp stand-alone. If you have the Molex harness from the console, it can supply the power to the preamp as Eli described - if not, ask around the forums - most haven't used it, and eliminated the connector on the amp. A 5V4 can be used in place of the 5U4, giving slow warm-up and less load on the transformer heater winding.
Edits: 12/12/14
nt
"...the fool doth think he is wise but the wise man knows himself to be nothing but a fool." Will Shakespeare
I'm not sanguine about using a 5V4. Check the data sheet out. Current capability is 175 mA., which should be enough for the power amp, but may be problematic if ancillary equipment's B+ is to be provided. Another thing to watch out for is the small 1st filter cap. value limit.
A 5AR4, like the 5V4, draws only 2 A. of heater current, but like the 5U4, the type is good for 250 mA. Also, the 1st filter cap. limit is greater than that of a 5U4. Therefore, the 5AR4 is the vacuum rectifier type I'd look at, IF a change was to be made.
If either rectifier with a cathode sleeve is installed, B+ has to be taken from pin 8, not pin 2. Both the 5V4 and the 5AR4 will raise the B+ rail voltage, with the 5AR4 raising it more.
Eli D.
Post a Followup:
FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
This post is made possible by the generous support of people like you and our sponsors: