Motorola SK44BW - 1961 3 ch phono

[Motorola minion]

I have an interesting Motorola to post. HS-838 amp with 6CA4 and (6) 6BM8

[Motorola minion]

I have been impressed so far by the lack of wax-paper caps and way-off-tolerance resistors so common to RCA, Mag and to a lesser extent, Zenith. This is more of a repair and routine service than restoration, mainly due to its clean, low-hours condition.

The two 12AX7 on the preamp were Motorola branded but sourced from EIA 280 Raytheon, the 6BM8/ECL82 and 6CA4 were all branded originals as well, made in Great Britain. Thankfully all tested good, even the Mallory multi-section passes tests, reforming quickly at 300 volts. I cant wait to play this in the garage tonight

On the flip side, this preamp has the same MONO tape input limitation as two of my own models . This really bites, like Motorola couldn't put one more contact on a two-position selector switch.

[Firedome]

That's a beauty Dave. Like the really clean copper chassis ala early H-K and Westy, and it has a bunch of Mullards, even the cabinet looks pristine too!
6 6BM8s? does it have 3 amp sections? The Moto in my avatar ---> did, but they were all 6V6GTs iirc and it had the big 15" speaker center mounted with mid+highs mounted on the sides. I was very impressed with it's build quality and sound as well.
Good luck with it, minimal repair needed is always good!

[Motorola minion]

This woofer is a 12" flanked by 3" cone tweets and 6" round midranges, just like the SK112 in my home office and that uses 6 x 6V6 like the 1964 SKR151 in your avatar. After quickly hooking everything up in the cabinet, I dropped a changer in this console this afternoon and played a Chet Atkins stereo on an RCA pressing. It was balking at playing all three channels but that was due to those very dirty and worn channel verification switches that allow you to switch off any or all of the 5 speakers to confirm balance, etc.

I confirmed all three do play well, just not together our garage has been a good test range due to mild temperatures along with all the clouds and rain, leftovers from the midwest ice storm I suppose. I would be happy for some sub-30s just for an excuse to fire my outdoor wood boiler up.

Once I get out there later this evening, I need to pull the channel verification switch enclosure apart, clean the three slider switches. Its on top of the record compartment lamp, which is a nice large diffuse lens that miraculously has not yellowed

Fortunately this job does not involve a record changer, so I can give it back real soon along with the little blond SK12

[electra225]

Dave, I have never really worked with 3-channel stereo much. What do you think the advantage to a 3-channel setup would be? Motorola used it a lot. They made good stuff, so there must have been a good reason. What are the pitfalls, or the challenges of 3-channel stereos?

[TC Chris]

I was wondering the same thing. Since bass frequencies aren't directional as higher frequencies are, it saves one speaker's cost. It also allows Motorola to advertise "3 channel," possibly fooling people into thinking they were hearing three separate channels of audio information as opposed to the two in standard stereo. Thus, it had some advertising advantages.

My question is how it sounds--whether the bass is nondirectional enough that you don't miss the separate-channel woofers. So, how does it sound?

Chris Campbell

[Firedome]

Mine at least sounded great, with the non-directional quality of low bass frequencies the fact that it had only a single bass 15" speaker wasn't evident at all in the delivery of the over sound stage, and concomitantly gave the kind of advantages found from bi-amping that some super hi-end no-expense limit audiophiles seem to embrace with multiple mono amps for bass, mid, and treble frequencies, 3 amps per channel in the extreme case. By limiting the frequencies that each amp section has to reproduce each one has less demanded of it, reducing clipping, saturation &c in theory, and measurable probably.

[Motorola minion]

The sound of this console is not exactly the same as if it used 6 x 6V6 or even 4 x 6BQ5 for three channel operation. But it is very clear and bright, even more so with certain material. The Admiral test record is very entertaining and showed how good the stereo separation is.

The 3 x PP 6V6 Motorolas from 1962-64 were rated the highest power at 50 watts, followed by 1960-61 models using two SE 6BQ5 and PP 6BQ5 bass channel are rated at 40 watts, with this 1961 coming in a bit lower per ECL82/6BM8 ratings in RCA RC30 tube manual.

The odd thing about the 6BM8 is that data sheets list single-ended rating of 7 watts of the pentode section, no ratings given for push-pull AB1.
For comparison, two 6V6 in a push-pull circuit, can drive 10-14 watts. one 6BQ5 can drive 5 watts, two 6BQ5 can drive up to 20 watts.

Going by the volume control setting and using the same VM1200 player with a Sonotone 9t cartridge, the 6V6 amps get very lively at a setting of "2", the 6BQ5 amps usually sound similar at "4" and this 6BM8 model needs to be at "6" for that level of loudness.

[Motorola minion]

Dave, I have never really worked with 3-channel stereo much. What do you think the advantage to a 3-channel setup would be? Motorola used it a lot. They made good stuff, so there must have been a good reason. What are the pitfalls, or the challenges of 3-channel stereos?

I think Roger touched on the fact that more power is possible when you limit/optimize the frequency spectrum to be amplified, which explains the advantage. Separation of bass from left/right is accomplished using a low-pass filter at the input to the bass amp with a high-pass filter at the right-left inputs. Magnavox was the only "bi-amped" model I am aware of, which took advantage of this principle. Magnavox also used complex crossover networks in comparison, because of the horns maybe ?

RCA, Philco and a FEW others offered three-channel stereo while Magnavox, Zenith and MOST others did not. This can be explained by comparing consoles to component systems. Increased separation of right and left speakers can make a huge difference in perceived sound staging, which a console with fixed-locations cannot accomplish without some additional gimmick. Enter 3-channel amps and Zenith's Extended stereo consoles. Bass is non-directional, like a subwoofer, so there is little advantage to splitting bass into two channels.