Author Topic: 1960 Magnavox Imperial restoration  (Read 5092 times)

electra225

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1960 Magnavox Imperial restoration
« on: September 08, 2014, 02:13:29 PM »
Here are some pictures of my Imperial while I had it torn apart.

The first picture if of the amp chassis.  81-02-00 dual frequency allegedly 30 watts per channel.  This particular amplifier sounded like a transistor radio when I got it, and had a NASTY hum out of the left channel.  The second picture shows the bottom of the chassis.  The third picture shows why I had both poor performance and the nasty hum.  That is the 130 ohm, 10 watt cathode bypass resistor for the bass amp.  A 6V6 ouptut tube had developed a heater to cathode short and took out this resistor.  Very fortunate it did not take an output transformer with it.  I replaced every tube in this instrument, all 22 0f them.  Since this picture was taken, I have replaced all the 6V6's, 5U4's and 12AT7's with JJ's.  The 12AX7 in the amp is a Telefunken I had in stock.  I replaced 80% of the resistors with metal film resistors with 1% tolerance.  I replaced any resistor that was more than 5% off value.  I changed all the coupling caps and increased the cathode by-pass caps in the bass amp from 20uf at 25 volt to 50 uf at 60 volts.  I changed the input filter cap from40uf at 450 volts to 5o uf at 800 volts by connecting two 100uf at 400 volt Nichicon caps in series.  I changed the filter cap following the choke to the same value, done the same way.  I left the 10uf at 450volt cap the original value, as it powers the tuner.  I changed the two filter caps in the tuner from 30uf at 450 volts to 34uf at 800 volts by connecting in series two 68 uf at 400 volt Nichicons.  These two caps in the tuner provide power for the pre-amp tubes.  This all done to enhance bass response, and to provide a silent power supply with reserve power.  Most of the tuner is blessed with couplets, so there are really very few components to change.  There is a double 25 uf at 25 volt filter cap for the tuner and a 4uf at 50 volt electrolytic that works in the tuning eye circuit.  The last one is on the IF strip PC board, so it is kinda tricky to replace.  My eye tube operation was wonky until this cap was replaced.  I also replaced the 6EA8 FM RF amp with a 6GH8A in an attempt to control the drifting during warmup that these old Magnavox mono tuners were famous for.  It seems to have helped.  Magnavox dual frequency stereo amplifiers are frequently criticized for poor bass response.  My experience has been that aging, drifted value components have a greater effect on bass response than perhaps would be experienced in other amplifiers.

« Last Edit: September 08, 2014, 02:44:04 PM by electra225 »
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Pat L

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Re: 1960 Magnavox Imperial restoration
« Reply #1 on: September 08, 2014, 06:02:10 PM »
Greg, that is fantastic information on the upgrades you performed to the amplifier and tuner. I like the hi-res pictures a lot of the as found condition. That is the first one of those 10 watt resistors I have seen fail like that, interesting. Let me ask you, why did you change all the tubes? Were they weak or damaged? I usually don't, but I sure do like the Russian tubes, I often use the 6BQ5 and 6L6 equivalents, and I've had great luck with both. I have not tried the JJ's or the Russian 6V6's yet, but I think I will if you recommend them.

Also, Im glad that you are dispelling the myth of weak Magnavox bass response. In my experience the worst resistors I have run across are in RCA amplifiers. That is not a knock on RCA consoles. I find them to be very stylish, quite attractive if you will. Just don't expect the quality of build, power, or great speakers that you would get in a high end Magnavox.Pat

electra225

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Re: 1960 Magnavox Imperial restoration
« Reply #2 on: September 08, 2014, 07:16:31 PM »
Pat, I commonly find the 220K ohm grid resistors in Magnavox amplifiers that have drifted high.  One in that Imperial chassis measured 494+K!  I alsways replace every 220K ohm resistor I find.  I always use metal film 1% resistors for replacement.  I replaced all the tubes in this instrument because it was a "high mileage" instrument, and I had that short in the output.  I wanted to take NO chances of a recurrance, hence all the tubes went.  The 6E5 eye tube was dead.  JJ's were recommended to me when I did my Symphony, with the 9302 chassis that uses 6BQ5's.  I was so pleased with its performance that I did the same with the Imperial.  Now the Concert Grand may be another story.  A quad of JJ 6V6's is $64.00!  I experimented a bit with that 6EA8.  My wife is always complaining about having "to tune that thing three times to get it to stay", so I tried 6GH8A's to see if that tube would stabilize the RF circuit a bit.  So far, so good.  It is an experiment at this time, not necessarily a recommendation.  A 6EA8 is listed as a sub for a 6GH8, but not the other way around.  The "A" was developed to stabilize the chroma circuits in color tv's.  The "A" is a souped up 6GH8, and the GH8 is a souped up 6EA8.  I will let you know how this works out.  I did not mention this, but I also use 300K ohm resistors paralleled with the enhanced filter caps that I used.  A friend recommended this setup, so I cannot take credit for it.  This Imperial is a sweetheart.  The wife turns the bass down, because it rattles the dishes in her hutch.  I really think, at least until I go thru it, that this Imperial sounds as good as my Concert Grand.  I expect huge improvements in the CG using these mods.  Best..........GREG

The following pics are of when I got it home, complete with a Philco part on the changer.  The last picture is of my hotrodded power supply filter caps.  That is the one following the choke with a by-pass cap as part of the assembly.
« Last Edit: September 08, 2014, 07:35:00 PM by electra225 »
I'm great at multi-tasking.  I can listen, ignore, and forget all at the same time.

Pat L

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Re: 1960 Magnavox Imperial restoration
« Reply #3 on: September 08, 2014, 07:32:02 PM »
Interesting that you think it sounds as good as the Concert Grand, I do too. In my estimation the sound quality and realism is incredible, the only real difference between the two is how far you turn up the volume to get the same result. Thanks for the information on the resistors, in truth I have seldom checked or changed them. I usually do the caps only, but I will start checking them per your recommendation. Pat

electra225

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Re: 1960 Magnavox Imperial restoration
« Reply #4 on: September 08, 2014, 07:38:44 PM »
Those grid resistors drifting so high mess up the bias on the stage.  Any degradation in performance, multiplied several times, adds up to one sick amplifier.  I have gotten where I don't even bother to check them.  I know they are bad, so out they go.  Magnavox quality control and build quality were second to none.  In fairness, electronic components have gotten better, smaller, and more reliable since those old babies were assembled.  They benefit greatly from improved technology, once you learn where to apply it.  Best..........GREG
I'm great at multi-tasking.  I can listen, ignore, and forget all at the same time.

electra225

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Re: 1960 Magnavox Imperial restoration
« Reply #5 on: September 08, 2014, 09:50:31 PM »
And another point that I have figued out messing with Magnavox amplifiers.  Bass performance and volume are a result of carefully adjusting the various controls provided on one of these chassis.  Timbre, treble, bass, hum balance, balance, phono balance, and treble amp outputs need to be carefully adjusted for maximum performance.  This is not a radio with just a catch-all tone control.  These are instruments designed and built to accurately reproduce recorded music as it was recorded, not necessarily like you want to hear it.  If you want boomy bass that will damage the drywall, get a Pioneer SX-1050, driving a pair of Peavey 18" studio speakers.  If you want accurately reproduced bass, consistant with the rest of the recording, then you will be satisfied with a Magnavox dual frequency stereo amplifier.  Kinda like driving a Pinto or a Packard.  The Pinto has four wheels and sounds like marbles in a tin can going down the road at 30 mpg.  The Packard is dead silent, silky smooth and moves with precision and grace, fuel mileage be damned.  It is the same with a Magnificent Magnavox..............GREG
I'm great at multi-tasking.  I can listen, ignore, and forget all at the same time.

electra225

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Re: 1960 Magnavox Imperial restoration
« Reply #6 on: September 09, 2014, 09:14:31 AM »
Another little tidbit of information.  Having a good FM antenna seems to be an important consideration when expecting optimum performance from the 6000 series tuners.  Just using the antenna on the back of the cabinet seems to make the drifting worse.  The signal using the built-in antenna may cause the eye to close, but the signal is still not strong enough, given the digital stereo composite that is currently broadcast.  Alignment could perhaps be touched up to compensate somewhat for this condition, but the problem is not severe enough for me to be tempted to possibly introduce another problem tinkering with alignment.............GREG
I'm great at multi-tasking.  I can listen, ignore, and forget all at the same time.

Consoleman

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Re: 1960 Magnavox Imperial restoration
« Reply #7 on: September 28, 2014, 08:23:30 AM »
Another little tidbit of information.  Having a good FM antenna seems to be an important consideration when expecting optimum performance from the 6000 series tuners.  Just using the antenna on the back of the cabinet seems to make the drifting worse.  The signal using the built-in antenna may cause the eye to close, but the signal is still not strong enough, given the digital stereo composite that is currently broadcast.  Alignment could perhaps be touched up to compensate somewhat for this condition, but the problem is not severe enough for me to be tempted to possibly introduce another problem tinkering with alignment.............GREG

Which FM antenna do you use Greg? I've been looking at the omnidirectional ones on Amazon.

Awesome job on that amp. I found one of the power supply resistors to be way out of spec on my Amp-150 so I replaced almost all of them throughout the amp.
Mark

electra225

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Re: 1960 Magnavox Imperial restoration
« Reply #8 on: September 28, 2014, 09:12:07 AM »
My wife welcomes the various items I drag home being brought into the house, but goes bonkers over dangling wires.  So I use a common old di-pole antenna, laying on the floor below the stereo.  Fortunately, the cabinet on my Imperial sits low to the floor, so she can't see the antenna under there.  I have tried TV antennas and they have not worked as well for me as a di-pole.  Just twin lead in a "T".  I tried tying it to the back of the cabinet, but it was too directional like that.  The antenna is now just wadded up under the cabinet and seems to work better.  Our HOA will not allow an antenna on a tower.  Another important point I should mention is to by-pass the internal FM antenna when you connect another antenna..........GREG
I'm great at multi-tasking.  I can listen, ignore, and forget all at the same time.

AMP82-01-00

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Re: 1960 Magnavox Imperial restoration
« Reply #9 on: October 01, 2014, 09:13:09 AM »
on russian tubes i ran a set for several hours and they did sound great until one shorted out and luckilly i added fuses to the power transformer so it just popped my center tapped high voltage fuse 1/2 amp. this was in a amp 148.

i threw all 16 tubes in the trash after that. i will not run cheap crap tubes again!

but it screwed something else up in the amp! im still trouble shooting the issue. these were russian military OTK labeled. i will never use another russian tube again!!

now ive heard nothing but good things about running JJ's. That's next on my list but its expensive to buy for all my stereos.
David        "If it ain't interesting, its really just boring"

Pat L

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Re: 1960 Magnavox Imperial restoration
« Reply #10 on: October 01, 2014, 09:46:17 AM »
I'm old school on tubes, I prefer the vintage originals in my own consoles. I know a lot of people prefer to put in new tubes, but I only do so in units I'm selling or gifting. For my personal use, it's vintage.

As far as the Russian tubes go I've had pretty good luck. Specifically the EL84 equivalents. I put them in a dozen or more units and never had a complaint. I've also used the Russian 6L6 equivalents with good success. I have never tried the 6V6 equivalents so I can't speak from experience on those.

electra225

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Re: 1960 Magnavox Imperial restoration
« Reply #11 on: October 01, 2014, 10:39:10 AM »
My experience with NOS tubes has been the opposite of yours, Pat.  I put JJ's in mine and NOS in ones I work on for a VERY limited number of people.  I have had issues with using 50 year old tubes in Magnavox amplifiers.  I personally treat this issue like the old "polyurethane vs. lacquer" that gets talked about so much.  Whatever works best for you is what you should continue doing.  NOS 6V6's are pricy and my supply is not the best.  6EU7's are even worse, both for supply and price.  I may start converting my instruments that use 6EU7's to 12AT7's.  The supply is better and they are not so pricy.  And they are better tubes.  I use NOS in the tuners, but I routinely replace all the amp and power tubes with JJ's.  My Concert Grand may be a different story.  A quad of matched 6V6 JJ's is $64.00.  Since my CG appears to be a low-hour instrument, and since it will see very limited use, I may just retain the originals and go from there.
I'm great at multi-tasking.  I can listen, ignore, and forget all at the same time.

electra225

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Re: 1960 Magnavox Imperial restoration
« Reply #12 on: October 01, 2014, 10:51:26 AM »
I don't get all hung up on audiophile terminology, like "fuzzy crunch" and that stuff.  All I can tell you about JJ's are that my amplifier works, and the magic smoke stays in.  I have three sets of JJ's currently in use.  A 5U4, and a quad of 6BQ5's in my Symphony.  A quad of 6V6's in my Concerto.  Two 5U4's, two 12AT7's, and a quad and a half of 6V6's in my Imperial.  So far, so good.  The JJ's in the Symphony have been there four years.  We have a 300 disc CD changer connected to that instrument, and it sees lots of hours of use.  The Imperial is going to AZ and it will have its own 300 CD changer.  I hope to use it a lot more as well.  Would I recommend JJ's?  Probably.  I really hesitate in making recommendations, since expectations and the difference in "ears" return different results.  I will say that I have been well satisfied, and leave it there.
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Consoleman

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Re: 1960 Magnavox Imperial restoration
« Reply #13 on: October 01, 2014, 11:21:24 AM »
I recently acquired a Mighty Mite tuber checker and use it on all NOS tubes. Anything that tests OK stays, and I'd say 95% of the original Magnavox branded tubes in my consoles have been fine. The Berkshire has all original tubes except for the dual rectifiers.

When I do need a replacement, and NOS is too pricey e.g. 6V6s, I order Electro Harmonix from Tube Depot. I believe these are Russian but I've had a pretty good experience with them. They come with a warranty and the service is great (no affiliation). They also match them for free.
Mark

electra225

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Re: 1960 Magnavox Imperial restoration
« Reply #14 on: October 03, 2014, 10:04:26 AM »
Tube Depot, huh?  I may give them a try.  I have been using AES, but they charge for matching, and that gets expensive when you buy a couple quads at a time.  Their customer service is not very good anymore.  They do not take walk in trade.  I would welcome an opportunity to give my money to anyone but those people.  I appreciate your input.
I'm great at multi-tasking.  I can listen, ignore, and forget all at the same time.