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Lafayette Two Tube Preamp

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Posted on December 8, 2014 at 06:50:09
jec@afo.net
Audiophile

Posts: 100
Location: Philadelphia area
Joined: January 15, 2005



Good morning everyone,

I picked up a Lafayette two tube preamp, #99-0139. I want to use a Shure M3D with it. It comes with a 6K input resistor. Can I just replace this with a 47K resistor and that's it? Also, it requires an amplifier with a 100K or higher input impedance. I am going to be using a Magnavox #9302-00 push-pull 6BQ5 amp. I am clueless when it comes to cartridge loading, etc. so any help is greatly appreciated. I have included the schematic.

All the best,
Jim

 

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RE: Lafayette Two Tube Preamp, posted on December 8, 2014 at 08:11:44
Eli Duttman
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Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 That seems to be a VERY early design that was used with GE variable reluctance cartridges, along with mag. tape heads. Among other things, notice the (yuck!) 1/2 wave rectified B+ supply.

Those early setups used the inductance of the coils in the moving iron cart. as part of the equalization network. Therefore, simply changing the 6 Kohm parts to 47 Kohm parts, while necessary, is not sufficient.

IIRC, a cap. in parallel with each of the 100 Kohm resistors near the O/Ps is necessary. Somebody else will confirm that and provide an appropriate value.

In any event, the almost certain selenium B+ rectifier MUST be replaced. Use a bridge of 4X UF4007s. All of the decrepit electrolytic caps. require replacement too.

The design uses AC heating and the ONLY tube readily available that will have even the slightest chance of yielding an acceptable hum level is the Sovtek 12AX7LPS. The 'LPS is a genuine 7025 equivalent that contains a spiral wound, hum bucking, heater.

That looks like a diode connecting the filament winding's CT to ground. Get that out of there and bias the CT off B+ with a resistive voltage divider set for approx. 60 V. The CT's connection to ground is made with an electrolytic capacitor.

You should be OK as far as loading goes, provided you use short low capacitance cables between the phono preamp and the "Maggot Box".


Eli D.

 

RE: Lafayette Two Tube Preamp, posted on December 8, 2014 at 09:48:12
jec@afo.net
Audiophile

Posts: 100
Location: Philadelphia area
Joined: January 15, 2005
 Thank you Eli. It looks like I have a bunch of work to do. I am certain I will need additional help.

All the best,
Jim

 

RE: Lafayette Two Tube Preamp, posted on December 8, 2014 at 10:30:14
mobiasloop
Audiophile

Posts: 100
Location: SW Missouri
Joined: August 14, 2014
 Sell it! Start over, or virtually replace every part in it. The bias is grid leaked because each cathode is grounded. So it is necessary to have those caps on the input grids (.05 and .005 uf shunted with 3M resistors) They are probably ceramic (boo-hiss). We are listening to bad DF capacitors. The power supply is laughable (half wave-only one diode and it is probably selenium which drifts with age. Replace with 4 UF4007 diodes. Using a Shure M3D might be problematic because this was designed for the GE variable reluctance cartridge and the EQ may be off. I could recommend other designs that use the same tube. Dr. Torsten Lousch(AKA Que Yang Wang) has a design on his THUNDERSTONE AUDIO website that I would investigate if I were you. YOU MUST use DC on the filaments if you don't want boo-koo oodles of HUM and the versatility of differing brands of tubes and the Lafayette is using AC filaments (another BOO-HISS). There are many DC filament designs on the web that are simple. One big capacitor (4000uf, 25v) and four UF4007 diodes is all it would take to convert to DC filaments. M.B.
"...the fool doth think he is wise but the wise man knows himself to be nothing but a fool." Will Shakespeare

 

RE: Lafayette Two Tube Preamp, posted on December 8, 2014 at 10:33:02
Caucasian Blackplate
Industry Professional

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 Do you really have to use this phono preamp? I'm not a huge fain of grid leak bias if you don't need the massive input impedance, but that's just me. I'm also not a huge fan of the EQ network implementation, and the low impedance loads used.

Eli pointed out plenty of other issues as well!

 

RE: Lafayette Two Tube Preamp, posted on December 8, 2014 at 11:58:31
Eli Duttman
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Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 FWIW, it has been said that the "acid test" of a phono preamp builder is achieving sufficiently low hum levels with AC heated filaments. Frankly, I'll never "sit the exam". However, it's not accidental that the "classic" passive EQ setup found in RCA tube manuals calls for 7025s. Hum bucking heaters give AC heating some chance of being satisfactory. Back in the day, DC heater supplies were not the simple thing they currently are. Look at the numerous "integrated" amp and receiver designs that used the "Cheap Charley" method of DC heating, in which the phono section filaments were used as part of the O/P tube bias network. I LOATHE "Cheap Charley". :>(( Consider that phono section heater current can be modulated by the power O/P signal.

I would not attempt to rectify the filament winding of the OEM power trafo in question. It takes approx. 2X the AC RMS current compared with the DC draw, when building a DC supply. Given that unit's "Bargain Basement" nature, there is no reason to believe the current handling capability necessary is present.


Eli D.

 

Similar Issues Old Thread, posted on December 8, 2014 at 13:08:52
Eli Duttman
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Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 A link is provided below.


Eli D.

 

He should sell it., posted on December 8, 2014 at 13:10:30
mobiasloop
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Location: SW Missouri
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 Also the power transformer is probably too close to the 12ax7s if it is what I think it is so it's inductive coupling (Lenz lines) is gona cause hum no matter what he does even if the tubes have shields over them. He would be better off to power it from the Magnavox power amp power supply.
"...the fool doth think he is wise but the wise man knows himself to be nothing but a fool." Will Shakespeare

 

RE: He should sell it., posted on December 8, 2014 at 14:08:38
jec@afo.net
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 Looks like the "sell it" idea might be the way to go......unfortunately. I want to thank you all very much for your input. It has been greatly appreciated.

All the best,
Jim

 

Why do these fetch so much on Ebay?, posted on December 9, 2014 at 12:42:04
gusser
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Location: So. California
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 I have seen these Lafayette tube preamps go for over $100. Yet in their day they were junk at around $10. I have some old Lafayette catalogs that show them. As discussed the circuit design is dirt cheap and a poor performer.

I don't get it?

 

RE: Why do these fetch so much on Ebay?, posted on December 9, 2014 at 12:43:24
Eli Duttman
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 Wishful thinking? A fool and his money ...


Eli D.

 

Photos?, posted on December 9, 2014 at 13:06:29
Eli Duttman
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 Will an "inmate" please post photos of the unit? I want to see what the chassis looks like, both inside and outside.

There is an outside chance of converting these crappy things into RCA circuit units, without spending much money. If the Allied 6K27VF power trafo fits mechanically, a voltage doubler B+ supply would feed LR8 3 terminal regulators (1 IC/channel). As AC heating would be retained, use of the Sovtek 12AX7LPS is mandatory.


Eli D.

 

RE: Photos?, posted on December 9, 2014 at 14:23:37
jec@afo.net
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Location: Philadelphia area
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 I will get some photos up soon. One good thing is that it does have a NOS pair of Mullard tubes anyway.

Thank you,
Jim

 

Have you ever even heard these Preamps ?, posted on December 9, 2014 at 14:28:05
Interstage Tranny
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 Eli constantly finds faults but might not have even heard the potential within. AC filaments can sound fine in these chrome preamps. Half wave rectifiers can be found in more than a few classic sounders.
Marantz 7, Lafayette KT-600, Shure M65 are great sounding examples. Have you heard these ?

Power supply, input R and EQ upgrades can make these cute chrome preamps sound incredibly sweet...

 

Great Potential Lies Within these Cute, Chrome Preamps..., posted on December 9, 2014 at 14:32:58
Interstage Tranny
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Posts: 3063
Location: Eastern
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Anyone who denounces these has simply not heard them, nor have they attempted to refine them...

 

These fetch so much on Ebay for their looks and sonic potential...plus..., posted on December 9, 2014 at 14:38:20
Interstage Tranny
Audiophile

Posts: 3063
Location: Eastern
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 They can easily be modded to accommodate two turntables. With power supply augmentation, proper input Rs and proper EQ networks, these can sound awesome !
Unchecked with decent cosmetics, they can fetch over $150 on the bay. Upgraded properly and with nice cosmetics, they easily bring $300. They are cute, good lookers and can easily sound very endearing. That is why they bring the $$$...

 

More Threads in Vintage Forum re:"cute, chrome preamps"..., posted on December 9, 2014 at 14:40:23
Interstage Tranny
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Posts: 3063
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 These can easily sound very enjoyable...

 

Upgrade and LISTEN...then u too will ENJOY !... (nt), posted on December 9, 2014 at 14:42:34
Interstage Tranny
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Posts: 3063
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 NT

 

RE: Great Potential Lies Within these Cute, Chrome Preamps..., posted on December 9, 2014 at 14:51:39
sony6060
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Location: USA
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 I see what I believe to be those great Russian green color caps. So, why the at best midgrade Orange Drops under the chassis?

Orange Drops are a bargain and I use Orange Drop capacitors in shortwave receiver rebuilds all the time.

 

Upgrade it...Listen to it...then decide..., posted on December 9, 2014 at 14:52:03
Interstage Tranny
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Posts: 3063
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These sound awesome when upgraded. The power tranny is fine. Grid Leak bias is found on many classic phono stages which also sound nice. The AC filaments can sound hum free; or, at least the hum is non-obtrusive. These preamps simply need a little TLC...

 

RE: Great Potential Lies Within these Cute, Chrome Preamps..., posted on December 9, 2014 at 16:52:40
Eli Duttman
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 There are "Orange Drops" and there are "Orange Drops". The 716P series are discrete aluminum foil and polypropylene film. The leads are copper, not steel. 716P series parts make fine coupling caps., especially inside NFB loops.


Eli D.

 

RE: Photos?, posted on December 9, 2014 at 17:42:40
Eli Duttman
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Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000



The photos Interstage Tranny provided contain a pleasant surprise. The power transformer seems fairly substantial. Somebody allowed for the "standing" DC 1/2 wave rectification places on a power trafo and that DC causes considerable stress in the part.

It seems that buying a new trafo might not be necessary. A voltage doubler B+ supply energized by the trafo in situ looks possible.

I've uploaded the venerable RCA circuit. Other than changing R1 to 24,000 Ω (a value not available back in the day), you could use that circuit as shown. If you want to feed a recording device or long cables, a buffer will be needed.

Save the genuine Mullard tubes for another job. Notice that RCA called for the 7025, which is a 12AX7 that contains a spiral wound, hum bucking, heater. The Sovtek 12AX7LPS is a genuine 7025 equivalent. LPS stands for long plate spiral. ;>)


Eli D.

 

Here are some pictures, posted on December 11, 2014 at 05:33:13
jec@afo.net
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Posts: 100
Location: Philadelphia area
Joined: January 15, 2005












Here are a few pictures and again, thank everyone for their help. I am still undecided what to do. I wouldn't mind a little project over the Holidays.

All the best,
Jim

 

RE: Here are some pictures, posted on December 11, 2014 at 09:17:04
mobiasloop
Audiophile

Posts: 100
Location: SW Missouri
Joined: August 14, 2014
 Actually, you're better off than I originally thought. I expected to see a sea of ceramic capacitors. I see only one ceramic, that buried orange flat thing near the tape-phono switch. Replace it with a polystyrene or SBC716P. Also the aluminum polarized power supply caps are the same color gray as the dielectric mylar films. The aluminum electrolytic cans HAVE to be replaced because of age. Panasonic or Nichicon radials or axials are your best bet from Digikey or Mouser. Their di-electic is caustic and destroy themselves after about 15-20 years of age. SBC716P capacitors are your best bet for the small value caps like .1uf. Antique Radio Supply in Tempe, AZ is another good sourse. M.B.
"...the fool doth think he is wise but the wise man knows himself to be nothing but a fool." Will Shakespeare

 

RE: Here are some pictures, posted on December 11, 2014 at 11:39:46
Eli Duttman
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Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 "I wouldn't mind a little project over the Holidays."

How far do you want to go? :>D ;>)

At a minimum, you have sheet metal, a power transformer, and shock mounted/shielded tube sockets that will be reused. The clamp holding a multi-section 'lytic down is good news too. Some sort of new part will go into that space. Unless they are physically breaking down, wirewound resistors are recyclable. OTOH, carbon composition resistors drift in value and go noisy, over time.

Sadly, those Japanese oil caps. are known to go leaky and they are likely to need replacement. Where oil caps. are of benefit, Soviet surplus K40s are the thing to use.

I was SERIOUS about using the RCA setup. I will post a revised PSU schematic, after I get home from work. In the meantime, peruse the LR8 3 terminal regulator data sheet. A LR8 + ancillary parts will be used to "nail" the filtered DC at the 250 V. the RCA setup requires.

I dislike switches in mV. level signal lines. FWIW, I would rework the front panel to hold a proper grounding post, along with a pair of I/P jacks and a pair of O/P jacks. The rear panel openings would be used for the power cord, a fuse holder, and an on/off toggle switch.

It was originally stated this preamp will be used in combination with a "Maggot Box" amp. Please upload the schematic for that unit.


Eli D.

 

Eli is correct, posted on December 11, 2014 at 14:05:36
Michael Samra
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 The Japanese oil caps have to go and I would wire this for the RCA and you have to pay close attention to the AC heaters as they are an issue.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

New PSU Schematic, posted on December 11, 2014 at 17:35:44
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000



My "hen scratches" are uploaded for review and comment.


Eli D.

 

Incredible, posted on December 11, 2014 at 19:12:39
jec@afo.net
Audiophile

Posts: 100
Location: Philadelphia area
Joined: January 15, 2005
 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

 

RE: Incredible, posted on December 11, 2014 at 19:55:14
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 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.

 

Magnavox 9302-00 Schematic, posted on December 11, 2014 at 20:09:14
jec@afo.net
Audiophile

Posts: 100
Location: Philadelphia area
Joined: January 15, 2005



Hello Eli,

Here is the amp schematic. Again, thank you very much.

Jim

 

RE: Magnavox 9302-00 Schematic, posted on December 11, 2014 at 21:13:49
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 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.

 

RE: Magnavox 9302-00 Schematic, posted on December 12, 2014 at 04:32:46
mobiasloop
Audiophile

Posts: 100
Location: SW Missouri
Joined: August 14, 2014
 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

 

RE: More, posted on December 12, 2014 at 04:54:41
mobiasloop
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Posts: 100
Location: SW Missouri
Joined: August 14, 2014
 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

 

RE: Magnavox 9302-00 Schematic, posted on December 12, 2014 at 08:07:24
Eli Duttman
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Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 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.

 

RE: Magnavox 9302-00 Schematic, posted on December 12, 2014 at 08:45:10
Tom Bavis
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Posts: 961
Location: Upstate NY
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 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.

 

Yes, but the rectification is much gentler than SS., posted on December 12, 2014 at 09:01:52
mobiasloop
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 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

 

Eli, if your integrated uses..., posted on December 12, 2014 at 10:34:10
gkargreen
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Posts: 1562
Location: DC
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 the cheap charlie method, can you go further to explain the deficiences in that arrangement, and how one could go about making changes to the cheap charlie that would make for a better phono stage... Thanks!

 

what are those green square caps?, posted on December 12, 2014 at 10:35:38
gkargreen
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Posts: 1562
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 not familiar with them although I do know the green cylindrical K42y caps...

 

RE: Magnavox 9302-00 Schematic, posted on December 12, 2014 at 11:40:48
Eli Duttman
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Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 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.

 

RE: Eli, if your integrated uses..., posted on December 12, 2014 at 13:56:04
Eli Duttman
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Location: Monroe Township, NJ
Joined: March 31, 2000
 Sorry, I missed this post until now.

IMO, 2 obvious defects of "Cheap Charlie" DC heaters are the possible modulation of phono stage heater current by the power O/P signal and the use of a single bias network for all 4 O/P tubes.

The solution to the problems is a separate, true, RC bias network in each channel's O/P stage and a regulated 12 VDC phono section heater supply.

A good 12 VDC heater supply for 2X 12AX7s can be made from an AnTek AN-0112 toroidal transformer, a MBR20100CT common cathode twin Schottky diode, a large valued 25 WVDC 'lytic, and a 7812 3 terminal regulator IC. Wire the trafo's secondaries in series, FWCT rectify, and cap. filter. Refinements include a 1 muF. Panasonic ECQ-V cap. physically adjacent to/electrically across the regulator IC I/P and a 15 muF. 'lytic physically adjacent to/electrically across the regulator IC O/P. Don't forget a clip on heatsink for the regulator IC. OTOH, the (sic) 10 A. diodes in a MBR20100CT will be loafing.


Eli D.

 

RE: Great Potential Lies Within these Cute, Chrome Preamps..., posted on December 12, 2014 at 14:35:30
Jim McShane
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Posts: 5910
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 What Eli said!! Thanks for posting this Eli - it saves me from having to post it!

 

DC Heater PSU Schematic, etc., posted on December 12, 2014 at 19:13:45
Eli Duttman
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Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000






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.

 

RE: DC Heater PSU Schematic, etc., posted on December 12, 2014 at 19:47:24
jyourison
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Posts: 279
Location: Wisconsin
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 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.

 

RE: what are those green square caps?, posted on December 12, 2014 at 19:51:32
jyourison
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Posts: 279
Location: Wisconsin
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 I think these are polystyrene, and if so, they offer a neutral, uncolored sound much like mica caps. Like Mike Samra, I like to suggest a mix of dielectrics. My preference is mica, 'styrene, and film and foil. I used these green caps in my build of the RCA Hybrid MOSFET phonostage.

 

Magnavox Pictures, posted on December 13, 2014 at 05:07:36
jec@afo.net
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Posts: 100
Location: Philadelphia area
Joined: January 15, 2005


















Here are a few pictures. As you can see, it is a project waiting to be done. Resistors hanging loose, etc.

Thank you,
Jim

 

Good idea: 5V4 nt, posted on December 13, 2014 at 07:34:56
mobiasloop
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Posts: 100
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 nt
"...the fool doth think he is wise but the wise man knows himself to be nothing but a fool." Will Shakespeare

 

RE: Magnavox Pictures, posted on December 13, 2014 at 12:02:09
Eli Duttman
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Location: Monroe Township, NJ
Joined: March 31, 2000
 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.

 

Some ideas....., posted on December 14, 2014 at 05:04:27
mobiasloop
Audiophile

Posts: 100
Location: SW Missouri
Joined: August 14, 2014
 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

 

RE: Some ideas....., posted on December 14, 2014 at 09:28:19
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 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.

 

RE: Good idea: 5V4 nt, posted on December 14, 2014 at 13:07:10
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 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.

 

RE: Some ideas....., posted on December 14, 2014 at 13:12:05
jec@afo.net
Audiophile

Posts: 100
Location: Philadelphia area
Joined: January 15, 2005
 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

 

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