Maggie 175-67 restore....

analog addict · Feb 20, 2011

Gonna try and detail this closely, since there seems to be a bunch of differing views on how best to do this restoration. Therefore, I'm gonna do it as best I can and leave some bits alone and evaluate after the initial job to see what might be an improvement.

BTW, if anyone has a good copy of the 175-67 schematic, I'd appreciate a copy, since I'm working off a mish mosh of partials and blurry copies.

Raw material....

One of the primaries cut out of the molex plug. Looks like the other is a red-black rather than another black wire, since the black/red-black measure about 1.5 ohms DCR....

Cutting out the rest of the Molex....

All that's left, one primary, and a couple of filament supplies....

next order of business....The power supply. Found a 2 x 50uF/500V cap in the stash, Drilled a couple of holes in the side of the chassis, and zip tied the big sucker into place. tied the two positive terminals thogether to make 100uF, and soldered it in as a substitute for the 30uF C1b.

Next are the 40uF first stage for the 5U4GB and the 10 uF 3rd stage. I'm gonna put these on a terminal strip and then attach the terminal strip to one of the lugs of the now defunct can cap. Also gonna add the cap for the hum balance pot on this strip for convenience sake...

Here we are all soldered into place....

Next are the coupling caps and any associated resistors as I go. Gonna use these big ol double 0.1uF oil caps for the stock 0.047 uf caps, and see how they sound. Also replacing the 0.0033uF caps with 0.0075uF orange drops.

Also needed to replace the 20uF/25V caps, but I only had short lead 22uF radials, so needed to do a little creative soldering.

Note the small drill hole on the left side of the chassis. That's where the other oil cap will go, but I'm waiting until I've done all the work in the area, since it sucks up a lot of space. Also removed the first of the input caps off the RCA plug, and dropped a 470 Kohm resistor to ground. I'll start off with these input resistances/impedances, but may drop down to 220K/22k from 470K/47K over time.

Wrapped up at 0130 this AM, but hope to get some more done tonight...:thmbsp:

MWalt · Feb 20, 2011

Is that the 175 that I sold you? If so, I'm glad it went to someone that would do it justice. I'll be following your build up closely. :thmbsp:

bobabode · Feb 20, 2011

love these amps. I've a schematic ... somewhere around here.......pm me for higher rez.

bobabode · Feb 22, 2011

One that I left out...

.

analog addict · Feb 22, 2011

Balance control circuit equalized...

Most of the work is done I think.....

Here's how it looks so far....

Need to clip the speaker selector molex out and hardwire the speakers, add the requisite fuse, switch, safety cap, indicator light and binding posts.

analog addict · Feb 22, 2011

bobabode said:
love these amps. I've a schematic ... somewhere around here.......pm me for higher rez.

Thanks for the high rez documents Bob!...:thmbsp:

bobabode · Feb 22, 2011

You are welcome! Following with interest, man . I surely wish there was a thread like this when I started down this road. Didn't know anything regarding tube amps or any other amp for that matter. Took alot of skull sweat to figure it out.......off topic, well a little maybe :smoke:

There are very high voltages in these and other audio equipment-- just in case your eyes started glazing over--THERE ARE LETHAL LEVELS OF DC CURRENT IN THIS AMP-300 VOLTS AND HIGHER. THIS IS NOT YOUR TYPICAL 120 V AC. IT'LL KILL YOU! So don't fart around with it until you have an understanding of how to safely do it. You won't get a second chance!!!! Think electric chair..........
now back to our regular programming
Cheers and all the best, please be safe.
Bob

bobabode · Feb 22, 2011

Sorry for the rant, gals and guys and thanks Analog Addict for letting me voice my concerns. I can imagine some putz reading these (kind and polite threads) in the future and opening up a tube amp and promptly gettin' their heart stopped dead. (Insert Sam Kennison voiceover)We have another winner of the Darwin award!!!!

kpaxfaq · Feb 22, 2011

Thanks for the nice pics Allan. Looking forward to following the progress.

Kory

airdronian · Feb 22, 2011

good rant !

bobabode said:
Sorry for the rant, gals and guys and thanks Analog Addict for letting me voice my concerns. I can imagine some putz reading these (kind and polite threads) in the future and opening up a tube amp and promptly gettin' their heart stopped dead. (Insert Sam Kennison voiceover)We have another winner of the Darwin award!!!!

You can call it a rant, but I think it is well intended. I have a console pull to work with but no recent electrical experience. So, there is much reading to do on safe work practices in store for me.

And I appreciate this project being posted, as I want to soak up other's experiences as they go thru these projects.

Thanks to all !

bobabode · Feb 23, 2011

Thank you and keep one hand in yer pocket and make sure your feet are dry!Then, It'll only burn for a week or two, depending on where it arcs back out.

analog addict · Feb 25, 2011

Almost there...

Top side of the work in progress with switch, neon light, and fuse holder added.....:thmbsp:

FoolForARadio · Feb 25, 2011

Nice work! I am way behind you on mine, so far I am just moving it from place to place in the garage so I can get at what is underneath it.

analog addict · Feb 27, 2011

electricals are finished...

...at least the first go -round.

Here's the finished view from underneath....

Power supply. I may split the bias circuit 2 or 4 ways in the future...

Coupling caps and 12AX7 sockets.....

Binding posts and feedback circuit balanced out....

And the switching circuit. I did drop the B+ voltage by wiring in the extra filament section so that the voltage at the rectifier is 560 VAC at 120 VAC from my variac. With my wall voltage at about 123 VAC, that should leave a nice safety margin.

Details are here....

And the "finished product"

Note that everything added topside except for a recycled vintage fuse holder is Rat Shack. Yeah I know it's not top quality, but I don't mind patronizing them, just so that I do my part to keep a retail electronics store around in my local area

I also tend to keep things with their original patina and generally as undisturbed as possible, but over time I may strip this one and paint it, and put a wood frame on it too.

Fired it up with some weak tubes and promptly got a light show when one of the 6V6's arced, but since I got a second of nice music, I dropped a quad of frosted Maggie labeled Sylvania black plates in it, and fired it up again. Absolutely no problems with good output tubes in it. There is a tiny hum that can be heard with your ear up against the speaker, but I'm wondering if that's AC filament hum.

Anyway, I'm going slap a few Mullard 12AX7's in it and hook it up to the K-horns tomorrow and see how it really sounds.... :music:

dcgillespie · Feb 27, 2011

Analog --Nice project!

A couple of things you might consider as you finish your project out over time:

1. The 47K series input resistors are part of the feedback stability circuit. I would leave them them same, although the 470K input resistors can be of whatever value you wish as appropriate for the driving device you are using.

2. The circuit provided shows the coupling caps to the output stage as both being .047 mfd, yet your unit originally, and as restored shows these to be wildly different in value. Sometimes that is done for stability reasons, but typically they should be the same if possible.

3. Bathtub capacitors typically make for lousy coupling caps, since the can (when grounded) adds significant capacitance to ground in the circuit being coupled. These were originally designed for bypass use in power supply installations. If used in coupling circuits, they should at least not be mounted to ground, and even then, care needs to be used to make sure the can does not act as an antenna to broadcast or receive signals as the case may be to other nearby circuits. At the very least, using only one in a push-pull signal path will cause unbalanced coupling in the circuit.

4. It appears that one channel uses a 4.7K cathode resistor in the first AF amplifier stage, while the other uses a 3.3K as shown on the schematic. The schematic shows both channels to use the same 3.3K value in this location, so that again is another discrepancy between the schematic and your original unit. In any event, these should be identical for balanced operation between the channels. You have made the feedback networks balanced between the channels, but it appears that the cathode circuits still need to be balanced out.

No doubt your amp is sounding quite nice. I hope these comments help as you finish out your project!

Dave

analog addict · Feb 27, 2011

dcgillespie said:
Analog --Nice project!

A couple of things you might consider as you finish your project out over time:

1. The 47K series input resistors are part of the feedback stability circuit. I would leave them them same, although the 470K input resistors can be of whatever value you wish as appropriate for the driving device you are using.

I picked up some 47K resistors at Rat Shack the other day. I can swap them in without too much difficulty.

dcgillespie said:
2. The circuit provided shows the coupling caps to the output stage as both being .047 mfd, yet your unit originally, and as restored shows these to be wildly different in value. Sometimes that is done for stability reasons, but typically they should be the same if possible.

I only doubled the values of these caps as the prevailing line of thought around here is that this mod helps increase bass response. Given the drivers used in these old consoles, I wouldn't be surprised if the circuits were designed to limit the bass extension. I ran the amp for about 30 minutes last night without any signs of oscillation.

Anyone else care to comment?

dcgillespie said:
3. Bathtub capacitors typically make for lousy coupling caps, since the can (when grounded) adds significant capacitance to ground in the circuit being coupled. These were originally designed for bypass use in power supply installations. If used in coupling circuits, they should at least not be mounted to ground, and even then, care needs to be used to make sure the can does not act as an antenna to broadcast or receive signals as the case may be to other nearby circuits. At the very least, using only one in a push-pull signal path will cause unbalanced coupling in the circuit.

Hmmm... Interesting point. I will measure the capacitance and ESR from the + terminals to the chassis/ground and see what that shows.

Can you suggest any way to determine if these effects are occurring?

Anyone else?

dcgillespie said:
4. It appears that one channel uses a 4.7K cathode resistor in the first AF amplifier stage, while the other uses a 3.3K as shown on the schematic. The schematic shows both channels to use the same 3.3K value in this location, so that again is another discrepancy between the schematic and your original unit. In any event, these should be identical for balanced operation between the channels. You have made the feedback networks balanced between the channels, but it appears that the cathode circuits still need to be balanced out.

Hmmmm....I'll have to go back and look at this. I missed it first time through.

dcgillespie said:
No doubt your amp is sounding quite nice. I hope these comments help as you finish out your project!

Dave

Thanks for the critique Dave.

As in most things, I'm no expert, just a journeyman with respect to tube amps. Being able to continue to learn and improve my knowledge and skills is what life is all about.....:thmbsp:

analog addict · Feb 27, 2011

Ok....

Changed out the 43K resistors to 47K. Also changed the 4.7K resistor to 3.3K.

Measured the ESR of the oil caps w.r.t ground, and got open circuit. Also measured the capacitance of the + terminals to ground, and they don't measure significantly different than the orange drops, maybe a couple of nF.

Also measured the voltages, and everything is running 10-15% under spec, which works for me. The plate voltage on the 12AX7's is a little different, 79 V vs 83V. Stuck some Mullards in since everything looks good. It will come up a little with actual wall current, and the amp sounds just fine.

Lastly, I twisted the filament wires since they weren't really twisted previously. Don't know how much that will really do, since they're also AC, but figgered it couldn't hurt. Amp's cooling off and will be hooked up to the K-horns next... :music:

ducati_EL34 · Feb 27, 2011

analog addict said:
I only doubled the values of these caps as the prevailing line of thought around here is that this mod helps increase bass response. Given the drivers used in these old consoles, I wouldn't be surprised if the circuits were designed to limit the bass extension. I ran the amp for about 30 minutes last night without any signs of oscillation.

Anyone else care to comment?

+1 I would never run that small a value for a coupling cap, in any amp.

analog addict · Feb 28, 2011

Im not sure how much more I'm going to mess with this amp. It really sounds good on the K-horns, with much better bass than the 45 SET amp it replaced. Played a little Blue Man Group and litterally shook the walls. My wife sent my daughter down from upstairs to tell me to turn it down....

(That's a good sign! )

Makes me wonder if the SET amp needs to be massaged...:scratch2:

_d5_ · Sep 12, 2011

I think I missed something... what happened to C6/C12 10000mmf, looks like they are taken out and simply not replaced with anything?

Thanks,
John

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