Pioneer SM-B201 Vintage Tube Amp

StarMover

AlienToAllThingsAudio
I ran across an old vintage Pioneer SM-B201 tube amp today at the thrift. :yes: It has a great late 50s deco look. Must be very early Pioneer. I'm not sure if it works or not. I don't know anything about it. I can't wait to get it home and test it out. The power cord is going to need replacing, but otherwise the unit appears to be very very clean, so i'm hopefully optimistic. I ripped off a picture of one like it I found on ebay so check it out.

I hope this baby works, I have a pair of late model pioneer speakers I'd like to put with it.
 

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love the symetric front i bet it sounds marveles i had the model after yours
but sold it this baby looks very nice inside maybe a picture?
 
The eye tubes are good for about 1000 hours... so this probably doesn't have a lot of hours on it. I would LOVE one of these old monsters... The word "Stereophonic" may be AM-FM simulcast - AM on one channel, FM on the other. Pioneer built various versions for the US, Japanese, and Aussie markets. In the US, it was AM-FM stereo, for Japan it had dual FM tuners (stereo on separate FM channels), and in Australia, it was AM-AM stereo (plus mono FM). And throw in shortwave, while yer at it... Ain't history fun?
 
Good news! It WORKS!

I got a chance to look this baby over last night. The cord has a crack in the insulation, but is intact, so I taped it up and plugged her in. The lights came on, the eyes lit up, an I eventually managed to get music to come out. :banana: Yes the 'eyes' are actually tubes, and they both still work. As I said before, this unit is remarkable clean, almost mint.

Here's the freaky thing. It has TWO separate tuners. Apparently each is configured to pick up AM, FM. The bottom register on both dials has an (SW) reference, which sounds like Short Wave, but I'm only getting AM on it. There are two rows of screws that make up the speaker connections on the back. Apparently one tuner is connected to the top row, the other to the bottom row. I think this may have been produced back when FM was still MONO. Perhaps the idea was to simulate stero by tuning both tuners to the same station? :scratch2: Also it looks like it was built to accomodate large console type speakers that were popular at the time. Very curious. I didn't really have time to mess with it much. I don't totally understand some of the functionality of the front control knobs. Tonight I'm gonna remove the cover, dust it off and fix that cord.

BTW, the cord is original. It has the little round "Pioneer" plug on the end, so I can tell it is original. The problem is that the insulation around the cord is very stiff and brittle from age. There is no way I could wind it up. Should I replace the cord? Is there a way to soften it up again? I hate to ditch the original cord.

I'm dying to get my hands on some sort of manual. I'll have some pictures perhaps tomorrow.
 
More pics

Again, I ripped these pics off an ebay auction, but they show the inside and back quite clearly....
 

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That one is mine!

I am the lucky person who purchased the pictured unit off of Ebay. This is my first piece of tube gear so hopefully it will teach me a lot. I would also love to find a manual/schematic or any additional info. Please keep your findings coming as I need all the help that I can get.
-osh.
 
I noticed the guy put a BIN of $99 and you snatched it up pretty fast. Congrats! I haven't seen many of these laying around.
 
StarMover said:
Here's the freaky thing. It has TWO separate tuners. Apparently each is configured to pick up AM, FM.

This was a very short lived early stereo scheme. To broadcast in stereo, two towers were needed. One channel was broadcast on AM, the other on FM. You would need to tune both dials to the right frequencies to get a stereo broadcast. FM stereo saved everyone from the madness before AM-FM stereo got very far. Very few stations could to afford to have 2 transmitting towers. There are some other notable AM-FM stereo receivers from the same time frame, notably the Fisher 800.

Mark
 
I suppose I am also a member of the "club". I bought a Pioneer SM-B201 about a month ago. Mine is in excellent condition, shows very little wear, chassis is super-clean, all tubes are original except for the 5AR4 rectifier and one of the 12AX7's. I also have the separate Pioneer MXA-1 Multiplex Adapter which was an original match to the SM-B201, which allows it to receive FM stereo broadcast. In addition, I have an original operating manual and schematic. I would be glad to furnish copies of these to other SM-B201 owners if you need them, just email me.

Poston
 
Wow, I have never seen a SM-B201 before.
Congrats. to you guys that have found them. Looking at the receiver, I think that at $100.00 or so that would be a steal. What type of output tubes do they use?

RC
 
They use a quad of 6bm8's and a 5ar4 rectifier. Posten Drake, how do you like the sound and what type of speakers are you using? Would love to know more as mine still has not arrived.
 
The SM-B201 has 19 tubes, which includes the two 6E5 eye tubes. Each channel uses a 12AX7, driving a pair of 6BM8's in push-pull output. 5AR4 rectifier. The filaments of the 12AX7's are supplied via DC from a separate power transformer winding, rectified by a germanium diode full-wave bridge rectifier with a R-C filter network, which also supplies the fixed-bias for the output tubes. The separate MXA-1 multiplex adapter, which I mentioned above, uses a matching cabinet and has three tubes. Pioneer later issued a model MXA-3 multiplex adapter, but I am unsure of its tube lineup and whether or not the cabinet matched the SM-B201. Of course, similar early 1960's external multiplex adapters, such as manufactured by H.H.Scott, Fisher, Marantz, Pilot, etc., would also work fine with this Pioneer.

VANADAR, mine sounds OK but needs a little work. Cosmetically it is excellent and super-clean, but in a brief bench test audio is a little muddy and somewhat less gain than it should have. I tested it hooked up to a pair of H.H.Scott S-10B speakers (c. 1969), but the speakers are not the problem. I suspect there is some leakage of the audio coupling caps which is upsetting bias on the output tubes. The Toshiba paper-oil coupling caps which Pioneer and other Japanese manufacturers used in their early sets are known to be notorious for leakage, and I plan to replace them in the near future. Also, I have not tested the 6BM8 output tubes yet, or the 12AX7 drivers, so that could also be a potential source of trouble. Although the power supply seems OK, I also plan to replace the electrolytic filter capacitors as "preventive maintenance" to safeguard the power transformer and rectifier tube in case these 45-year old filter caps should short. I am certain that once this Pioneer is recapped and the tubes checked, it will be a great performer.

Poston
 
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Sm B201

glad to have found this post. i, too have a SM B201. the AM /FM circuit is broken and needs a lot of work. sound was distorted and hum was audible. replaced all 6bm8, the lone 5AR4 and all the PS caps. i replaced the volume pot and rewired it direct to the grid of the 6bm8 triode input. sound is clean and balanced.

Poston Drake, can you post schematics?

thanks.
Jeff
 
Hi Jeff,

My scanner is "down", and I won't be getting a new one for a couple of weeks. But if you will email me with your regular mailing address, I will be glad to send you a xerox copy of schematic via US mail.

Did you also replace the four .05 coupling caps to grids of 6BM8's? These are leaky on my SM-B201, upsetting bias on output tubes. Hard on the tubes and output transformers, not to mention audio quality.

Poston
 
Hi Poston,

yes, i replaced all coupling caps. don't worry about the schematics, i can wait.
i was just wondering about the 10k feedback resistor, does it have a bypass cap across it? and also, the value of the cap in series with a resistor connected to the plate of the 6bm8 input triode. i can only read PF.

thanks.

Jeff
 
Jeff,

On the schematic, the 10K feedback resistor is paralleled with a 20 pF ceramic cap.

If in your second question you are referring to the 22K resistor connected from B+ to the plate of the input 6BM8 triode, it is in series with a 50 pF ceramic cap. This 22K resistor/50 pF cap series is also paralleled with a 330K resistor.

Schematic does not list voltage rating of any caps, except for electrolytics.

Poston
 
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Poston,

it seems the previous owner messed up with the circuit. no bypass cap actually across the FB resistor. and the phono input cables snake all the way across the middle, in between the power tubes, across the AC filament wires, through the AM/FM stages to "land" at the input grids of the 12ax7s.

one other thing, the Svetlana circuit has the cap (in series w/ 22k resistor) connected to the B+, while in our case, it is the reverse (22k resistor to B+, cap to plate).

Svetlana 6bm8

i understand that this RC network is a bandpass limiter to safeguard against oscillation and to reduce the high gain of this triode half of the 6bm8. other than that, can the RC connections be reversed?

thanks,
Jeff
 
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