Unusual Motorola Console

[Trout]

Oh My I love summer outdoor auction season.

Found another unusual Motorola Console this past weekend.

I really dont understand how these babies work though, 3 Output channels.
Left, Right, & Bass.

As It sits, Center(bass) and Right channel are VERY Loud, Left Channel appears dead. I have only just begun trying to figure out the point of failure. I suspect its off chassis on either the preamp chassis, Or the weird speaker test switches. All the tubes check out fine, In fact they are still over 100% on my tester. Nothing looks burnt.

Photos: Chassis label ,Top View , Huge Bottom View

Interesting to me was the nearly exclusive use of ceramic caps. The sound quality from the 2 working channels is outstanding and well defined.

I have never seen a console utilizing ECL82/6BM8's prior to this unit.
Its not uncommon to see motorolas using 3 output stages I guess, This is the second unit I have gotten this summer,
The Other Using 2- 6V6 on bass channel, And 2 EL84 SE for left & right channels.
Gene

 

[Trout]

After Further digging in this console, as it turns out, The preamp powers off of the power amp chassis via a long cord. The Tuner on the other hand actually is a free standing (has its own power supply) Unit.

Photos below are of the preamp chassis. A bit more conventional I might add.
Label Preamp Chassis Large Chassis Underside

Getting close to setting it up minus the speaker test selector switch and extra junk not needed. Then I will try to locate the reason for no output on left channel.

<Crossing Fingers>
Gene

 

[Sherman]

Oh My I love summer outdoor auction season.

...I really dont understand how these babies work though, 3 Output channels.
Left, Right, & Bass.

...Gene

Gene,
When I was a kid we had a console with three channels. IIRC the bass channel worked kind of like a modern subwoofer in that bass from both left and right channels were fed into it. The left and right channels, handled only the higher frequencies (I have no idea what the XO point would be) and could be lower power with less expensive OPTs.

I didn't look closely at the bottom chassis photo but I'm guessing you'll find some sort of XO combining the two main channels.

 

[planet10]

is that a copper chassis?

3x 6BM8 PP time to do up the Yves/gingertube style Frugal-phile(tm) amp

dave

 

[Trout]

is that a copper chassis?

3x 6BM8 PP time to do up the Yves/gingertube style Frugal-phile(tm) amp

dave

haha, When I first opened the unit and saw all that copper, I was Thinking SWEEEEEEEEEEEEEEEEEEEEEEEEEEEET!

But sadly, its some kinda of plated steel. Theres a couple spots where oxidation has revealed the steal underneath.

Im Really liking all the British tubes relabeled Motorola!

Although, The 2-12AX7's in the preamp were I beleive GE.

Gene

 

[Trout]

More Info:

The Speakers are set up , 5 1/4 and 3 1/2 on each channel (L & R)
The Center Has a 12 inch Golden Voice Motorola ( Looks like Alnico)

I have a schematic for a different (working)unit that I bought earlier this summer, AMP PreAmp

Possibly it will give some insight as to how motorola did things back then. That particular unit has ended up in my collection and in fact gets used almost daily. My younger brother(also a musician) Stopped over one evening to hear that unit, 6 Hours of Vinyl later, I told him to go home! lol
He has been hitting garage sales ever since!

I now have a line on 3 more consoles for this weekend. The more interesting one is a very old Dumont with 2 floor speaker units.
I am unsure of the other 2 brands as its at a local auction.

Picture Of Dumont

Gene

 

[planet10]

Im Really liking all the British tubes relabeled Motorola!

Made in England or made in Gt Britain?

dave

 

[Trout]

Made in England or made in Gt Britain?

dave

Great Britain, Very Clean and date coded, Though I find the codes on these things totally confusing.

So far of the 3 motorolas and 2 zeniths I have recovered this summer 97% of the tubes tested over 95%. Most still over 100% and only 2 at about 895. Pretty damn good.

2 Old RCA /Cunningham 6V6's Testing at 103% matched and date coded 1948. Truely a tribute to the craftsmanship of the era.
gene

 

[Sch3mat1c]

Interesting to me was the nearly exclusive use of ceramic caps. The sound quality from the 2 working channels is outstanding and well defined.

Hahahah. Now you know what all the capacitophiles have feared men discovering.

The chassis is in wonderful condition, almost mint! I've never seen one that nice. Even the tone amp merely has a layer of dust. The copper plated chassis I have (from a Westinghouse radio) is all rusted to heck. You should mask that off and give it a spritz of laquer to ensure it stays that way, if it hasn't had it already.
Thing that sucks about those chassis is they are anti-galvanized: the iron underneath actually protects the copper! Unlike zinc plating, which corrodes sacrificially instead of the iron. That's why you'll need a layer of paint to keep it nice.

Tim

 

[Trout]

Re: Re: Unusual Motorola Console

The chassis is in wonderful condition, almost mint! I've never seen one that nice. Even the tone amp merely has a layer of dust. Tim

Hey, The chassis in my other motorola (schematics above)has what appears to be a gold anodized looking finish. It makes this one look bad.

I wonder if some kind of copper cleaner/polish should be used to bring up the appearance a little. Its only has a couple green spots and only on the outside. The inner chassis is still looking factory new.
Gene

 

[planet10]

Great Britain, Very Clean and date coded, Though I find the codes on these things totally confusing.

Got a B in the code? 1st letter of 2nd line. (ie Mullard Blackburn)

http://www.audiotubes.com/mullcode.htm

97% of the tubes tested over 95%. Most still over 100% and only 2 at about 895. Pretty damn good.

My recovery rate isn't quite that high (& dependent on tube type, but i have found many a tube that tests as good or better then kew (particularily EL84s)

dave

 

[Trout]

WOOHOOOOOO

I got the chassis and preamp wired up on the bench and Self cured the dead left channel. As I suspected the speaker test switches were bad.

They had this (what were they thinking) feature where each channel had a speaker kill that defaulted to a load resistor while speaker was in off position. The left channel switch had stuck internally in load position.
So, wiring the speakers direct to connectors on chassis instantly cured the problem.

Now, Should I try to bring full range, or fuller range to left and right channels?

Currently, I have mounted a pair 2 way awia bookshelf speakers on L & R channels, and the original Jensen 12 inch on the bass (center) channel.
The L&R channels produce some of the most detailed Mids & highs I have ever heard. But those channels seem to be limited to just mid & upper freqs. The bass channel is VERY strong, And also limited to just lower freqs.
All in all, as it sits, Even at 100% volume, no signal, this baby is dead silent. The original stereo turntable produces great sound. I trashed the tuner, yucky multiplex mono junk lol.
Gene

 

[Sherman]

...

Now, Should I try to bring full range, or fuller range to left and right channels?

Currently, I have mounted a pair 2 way awia bookshelf speakers on L & R channels, and the original Jensen 12 inch on the bass (center) channel.
The L&R channels produce some of the most detailed Mids & highs ...

The bass channel is VERY strong, And also limited to just lower freqs.
...

The apparent imbalance between the bass channel and the others is probably due to the efficiency of the speakers. The Jensen is probably >92 db/W and I'll wager the Aiwas are more in the 86 dB range.

You could pad the woofer but I think I'd try to go with more efficient mid/tweets or try for full-range. The potential gotchas for going full-range seem to be-

* The L & R OPTs may not be up to the task of handling the low frequencies (hence the bigger OPT for the center channel).

* Finding the XO circuit and removing it shouldn't be too difficult. It is possibly just one cap per channel between the input and the L & R amp circuits but since it seems to be a pretty high quality amp there is probably a low-pass also.

* What do you then do with the center channel? (I'd be tempted to use the tubes and OPT to make a little guitar amp!)

I'm getting inspired to go hunting at flea markets and garage sales again!

 

[EC8010]

That's an unusual amplifier. It just crys out to be hooked up to some "full range" drivers (perhaps Fostex FE103) on its baby channels and to use that 12" woofer in a decent enclosure as a centre bass. I'm considering something similar myself; Jordan JX92S for left and right, and a pair of 15" drivers with their chassis tied together to cancel the reaction force for mono bass below 70Hz. Crush the pre-amplifier.

 

[Sherman]

That's an unusual amplifier. It just crys out to be hooked up to some "full range" drivers (perhaps Fostex FE103) on its baby channels and to use that 12" woofer in a decent enclosure as a centre bass. I'm considering something similar myself; Jordan JX92S for left and right, and a pair of 15" drivers with their chassis tied together to cancel the reaction force for mono bass below 70Hz. Crush the pre-amplifier.

That sounds like a great idea for the high end! The efficiency of the Fostex drivers should be a much closer match to the 12" driver than the Aiwas resulting in a more balanced sound.

I would be very careful about an enclosure for the 12" driver though. Most of these old console speakers were essentially open baffle. The rear of the console was usually covered with thin wood or masonite with lots of ventilation holes.

A nice looking enclosure with an open back might be the best match for that driver.

 

[planet10]

* What do you then do with the center channel? (I'd be tempted to use the tubes and OPT to make a little guitar amp!)

I had a similar RCA amp... i managed to find someone with the same amp and swapped him the single for a power supply.

EC... those kinds of amps may be uncommon in Europe, but seem quite common here. Add the magnavox to Motorolas & RCAs using the same scheme. They even went as far as using a PP OPT with 2 SE OPTs and a pair of 50C5s to do the same scheme on the cheap.

dave

 

[Trout]

I swapped speakers around a few times and even putting old vintage 12" speakers from another old console netted me about the same results. The Bass/Center channel remained just bass, And the left and right remained generally speaking, Just mids and highs.

Im thinking theres 2 reasons, #1 The left & Right OPT's, Just a tad small, And #2 Possibly a cap value that may need changed.

Speakers for these units are piling up here, Back then motorola used Jensen Alnico PM5 concert series for woofs and Mids and Tweets.
One thing I am finding is that these seem like they lived an easy happy life. Very Clean,and apparently never abused.

Darn cabinets(5) are piling up as well as those old made in england turntables!! I swear, My Gal is a pack rat with crystal and depression glass,, And YES Its contagous!!!
Took me 3 weeks to entirely disassemble the 2 wurlitzers I got for $7.00, Now I have 4 of these console cabs to dispose of. Im keeping the original Motorola as I like the tube choices in those better.

I have run this newer unit now for about 3 hours continuous after playing with different speaker arrangments.
This one might just end up on ebay as other projects are piling up as bad as my "honey do" list.

HaHa, Btw I was talking to my Mom on the phone and the motorola subject came up. My Mom actually worked for Motorola in the 50's and well into the 60's here in chicagoland. Yep, You guessed it, Line soldering on chassis.
Gene

 

[EC8010]

Thanks for the information, Dave, that was certainly a better engineering solution than attempting two full-range boxes.

Trout: Don't try to make those baby channels full-range, the output transformers just aren't up to it (that's obvious by their size), but they probably have quite good HF response. Similarly, the bass transformer probably has poor HF response. You see, audio transformers are all about bandwidth in octaves. More bandwidth is more difficult to achieve (and costs more). It's far better to sidestep the problem by using an active crossover and limiting the number of octaves each amplifier has to reproduce (in exactly the same way that we do with multi-way loudspeakers). Work with these amplifiers, not against them; they are the product of an enlightened designer.

 

[Trout]

Thanks for the information, Dave, that was certainly a better engineering solution than attempting two full-range boxes.

Trout: Don't try to make those baby channels full-range, the output transformers just aren't up to it (that's obvious by their size), but they probably have quite good HF response. Similarly, the bass transformer probably has poor HF response. You see, audio transformers are all about bandwidth in octaves. More bandwidth is more difficult to achieve (and costs more). It's far better to sidestep the problem by using an active crossover and limiting the number of octaves each amplifier has to reproduce (in exactly the same way that we do with multi-way loudspeakers). Work with these amplifiers, not against them; they are the product of an enlightened designer.

Honestly, at this point, after listening to this amp&preamp combo alot more, I think they did a fine job of engineering on these units.
Seeing as each of the amps are seeming to do exactly as they were designed to and the perspective freqs are tight and clear in all channels.
Im thinking only a couple safety issues like changing power cords,
adding better speaker jacks, And maybe a mod for aux input. Right now its Stereo Phono & Mono for tuner.

I would really like to hear this baby with a CD player, But Im not sure the line out on my audio card or walkman CD player will match in the phono input. Im not exactly up on that matching stuff as generally I just build or repair based on schematics

Im really surprised these Motorola's arent more sought after than magnavox's as they out of the box clearly sound better.

I know This, I will certainly be adding ECL82/6BM8's to my supply of Tinker Tubes as they are certainly valuable audio tubes for moderate budget amps.
Gene

 

[EC8010]

Drive a CD player directly into the power amplifiers. Crush the pre-amplifier.