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Zenith 6bq5 PP!

Thespeakerdude8

Thespeakerdude8

Super Member
So I gutted a beat up console I got for $5 (with matching stereo speaker)

It has a niceee ~10-15 wpch 6BQ5 PP amp and I need to find a schematic for it.

The tube lineup is:
6BQ5 x4
12AX7 x2
5U4 x1

Stancor power and output x-frmrs. Original zenith labled AMPEREX!! 6bq5's tested ~90%! :banana:
I thnk the AX7's are made by GE, all tubes in the console were labled zenith and have hardly any use on them :D I was the first one in the cabinet in 50+ years...:yes:

I am probably going to gut the amp and replace the OPT's with Hammond 1650F's, but I would like to look at the original schematic for it too. I was able to trace the front end of the amp section and it seems to be a Paraphase inverter rather than the cathodyne used in similar chassis.

The tuner is cool and pulls in stations as well as a modern servo-locked tuner, and I'd like to know what the Zenith "extended stereo" is all about. Pretty deluxe for its day!
I also saved the VOM built cobra-matic from it and built a cabinet for it. Plays very well!

Anyway I would like to find a schematic for the amp so I can sort some stuff out and figure out how the record player is wired to the power supply so I can convert it to a stand alone unit.
I belive the chassis is 7B31 or 7D31, I don't remember what the tag said before it fell off :no:
 
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Here's the 7D31 - the 7B31 has 12AU7s, this has 12AX7s.
 

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Excellent! Thanks this is just what I needed.
Now to figure out the weird windings on that one OPT....


Is the 9B26 tuner chassis the same as the 9D25/9B24? This is what was listed on the back of the cabinet. Model of the console was SFD2883R.
 
thanks, yeah I found it on a zenith site but that is not the one that I need. I was just asking if it was the same as the 9B25/9D24 or not. The tuner looks physically the same, but I believe the 9B26 has a mono amp so the feedback controls might be different for the "zenith extended stereo."

I am still at a loss as to how the left channel hooks up! Why does it tie into the secondary of the right OPT? Does it sum the outputs so that you get around 25 watts for the woofers, and the difference is played through the midranges? I think that the extended stereo is similar to what a lot of computer speakers use now days: LF stays mono/in phase, and when the "3d sound" effect is turned on the HF is played out of phase to create a wider sound stage. Am I right?

One way I can think that they might have done this is wire the channels out of phase but wire one of the woofers in phase so that the LF is unaffected, and a phase inverter on the tuner inverts one channel above a cutoff frequency. The balance control could controll weather or not the phase is inverted or not, lending to a mono signal or "standard stereo" or out of phase HF for extended.

Puzzling, but sounds easily plauseable even in the 50's. What do you guys think?
 
I think it's just to give a mono output if you didn't spring for the remote speaker... or to give a center channel if you connect two remote speakers.

Zenith DID use the weird "matrix" circuit with at least one single-ended stereo amp - channels were drive out of phase and combined as sum and difference.
 
I noticed that in some of the schematics on the zenith site. I ought to try that sometime with my maggie for grins. :scratch2:

I'm already running a hafler circuit with my four nova 6's and it works pretty well for ambience.
 
:headscratThespeakerdude8 I have been trying to figure out that circuit for 2 years now. If you ever get to understand it let me know. I have hooked it to speakers just the way it shows in the schematic and it does sound as if its in stereo except it seems the vocals always seem to come from the same speaker.:smoke:
 
95-1691-B
172934

is on the one without the terminal stip,

95-1692-A
172934

is on the one with the terminal strip and other wire taps. For some reason I thought I saw 188 on these, I guess not. Power transformer is however a stancor, 138939.

I looked up the EIA code on the outputs and found "Ensign Coil" was the manufacturer. No idea who they are...
 
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