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Old 27th July 2008, 08:58 PM   #1
kevinkr is offline kevinkr  United States
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Smile Oddball Console Amplifier

Wandering down the fair streets of Quincy near my house yesterday I stumbled upon a stereo console on the street with the words "free" tacked to the front of it..

Brazenly I dashed home and grabbed some tools to salvage the electronics.. My wife commented that that was probably not quite what they intended. I commented anything to keep the electronics out the hands of the local scrappers who literally will walk off with anything they can turn in for a dime. (long story)

I did not recognize the brand name which was apparently that of a local cabinet maker.

The unit had a stereo TT (ceramic cart) and a monophonic tuner with the Monarchy brand name as well as this amplifier separately mounted.

I did not get the speakers or tuner.. I would have probably gone back for the speakers at least, but the torrential rainfall last night has ruined it.

The amplifier is a complete mystery. It is not a Magnavox or anything I recognize and the engineering is very odd. (It does not look anything like the Monarchy tuner.) The chassis is copper clad steel. It's stereo, has two fixed biased (!) PP output stages using Telefunken 6BQ5 pentode connected.. Each channel uses a single Telefunken 12AX7A as a amplifier and split load (cathodyne) style phase splitter. The unit has one additional 12AX7 shared between both channels, stereo bass and treble controls, and a source selector in addition to the volume control.

Here is where it gets very odd:

Filaments - the 5V and 6.3V filament windings are connected in series, this feeds the filaments with the 6BQ5 being wired in series connected pairs. The 12AX7 are run on 12V.. Filaments are single wire with return through chassis.

Bias- Half wave rectifier with a single cap for filtering.

High voltage supply - Although the transformer has a conventional center tapped HV secondary only half is used and it is silicon half wave rectified at that! The other secondary lead is accessible, but is snipped about 1" from the transformer housing.

Filter Caps - Two are used, both dual section, but one half of one can is not used at all!

The iron looks like it is good quality and has endbells unlike other amps in this price range. What I suspect is the 16 ohm secondary is snipped about 3/8" from each transformer.

The sockets are molded micanol.

No star ground or buss in this design, and the high voltage ground is located many inches from the supply caps so there are both high filament currents and the HV supply rectifier pulses flowing in the chassis.

The tubes are very close to the transformers as well, not something I would recommend. The unit seems to have very low hours as the tubes show no real evidence of usage. (Still to test)

I'm planning to repurpose it after a redesign/rebuild as an amplifier for my computer..

Here's that picture..
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Old 27th July 2008, 09:17 PM   #2
kevinkr is offline kevinkr  United States
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Here's a view of the inside of this amplifier.. As you can see there is not a lot of room..

Lots of crapulous ceramic caps..

I do have the possibility of using something like a 5Y3 rectifier in this thing and might do it or not.

I will retain fixed bias but probably double from the filament supply or find a tiny 100mA 12V transformer for that. (I think I might still have one.)

That fuse is actually not a line fuse, but is in series with the half wave HV rectifier.. It's still good..

Thinking something along the lines of an RH84 but PP..

Those teles would fetch big bucks on eBay, but I think I shall keep them..
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Old 27th July 2008, 09:18 PM   #3
kevinkr is offline kevinkr  United States
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Another pix, maybe it will help with ID..

Unit was originally vertically mounted, knobs facing up into a compartment with a lid over it. In that compartment just behind the amplifier was the Monarch tuner which was a basic Monophonic type which appeared to be Japanese made and if not bottom of the barrel it was certainly relatively close.

The maker's plate was between the tuner and the amplifier panel.

No, I don't remember the maker's name unfortunately, and haven't been back to see whether the unit has been removed. (Still there last night when I drove by it on the way to dinner.)

There is a constant stream of pickers driving through our neighborhood looking for loot.. And on trash day I have to be vigilant that people from the surrounding towns don't dump old car batteries and carpet here - Quincy has an excellent recycling program which costs the city, and us a lot of money. Someone last year dumped nearly a ton of carpet by our house and the city initially refused to take it - I called the DPW and explained how it got there and they sent a crew for it. One other incident was when I put out some windows for someone to pick up a scrapper removed the glass, left it behind and took the metal. The guy I gave them to was a bit disappointed and although the city took the glass I cringe to think what could have happened.

I caught a guy dumping carpet the other day, yelled at him - you never saw anyone move so fast!

Car batteries on my sidewalk.. Nightmare...

End rant..
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Old 27th July 2008, 11:02 PM   #4
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That first post should read "Monarch."

I replaced a few of the electrolytic caps and all of the rectifiers and am reforming the original supply caps as I write this. They WILL be replaced later.

The power transformer has what appears to be ~500VCT secondary and is quite beefy. (One of the first things I will do is convert to FW rectification.) I doubt I will have problems with it regardless of what the final approach I take happens to be.

Death cap which is a 0.01uF ceramic has been snipped out.

Caps are reforming, but leakage is definitely high.. These aren't going to be long for the world. I will check to see if I have something else.

I'm curious to hear what the stock amplifier sounds like before I modify it beyond all recognition..
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Old 27th July 2008, 11:43 PM   #5
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Nice amp, especially for free. At worst, you've got a fine set of parts.
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Old 28th July 2008, 12:27 AM   #6
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I'd gut the thing. The spacing between the tubes is too close, and the chassis doesn't even have a bottom. Iron looks nice though
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Old 28th July 2008, 12:53 AM   #7
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Caps were hopeless. Modding beyond all resemblance now..
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Old 28th July 2008, 01:57 AM   #8
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Didn't Harmon Kardon copper plate their chassis? Nice score on the Telefunken tubes.
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Old 28th July 2008, 04:17 AM   #9
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Well the re-configured unit is working.. It's now got 6.3V filament supplies, a voltage doubler for bias, and full wave rectification using a 5Y3..

The filaments are wired properly now, the HV center tap goes to the recycled dyna replacement quad cap..

I need to figure out what impedance the outputs are configured for, will measure voltage ratio and calculate the tap impedance from that. Could be either 8 or 16 ohms, suspecting it's 16 from the amplifier behavior with 8 ohm speakers connected.

Raw supply is about 360V..

Output hum and noise is just 2mVrms - not bad for the minor mods done so far..

Sounds ok, not great. I'd say it needs more work. Mids nice, highs extended, lows - well bonky describes them..

Surprisingly given the cramped layout it does not run that hot.

Unit dates from 1959 or 1960, pre mpx stereo based on date codes on components.
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Old 28th July 2008, 02:03 PM   #10
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Likes 16 ohm speakers, that's a clue.. It's a bit buzzy and not totally satisfying to listen to. Promising though the original design is not all that great. Must rework..
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