Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Ed_Chambley on February 24, 2014, 07:10:38 am

Title: Zenith Stereophonic Schematic Hunt
Post by: Ed_Chambley on February 24, 2014, 07:10:38 am

I harvested this amp from a 1963 Zenith Console Phonograph and I am trying to find the schematic for it.

Tubes, 1 x 12Ax7, 2 EL84, 1 5Y3. 

Chassis has sticker 4K21, Model SK2502MT, Manual states 8.5 watts.

Looks like some older Zenith Hi-Fi tube with added OT for Stereophonic.  Stancor Trannies and appears to be nice stuff.  It is a tad newer Zenith than I am familiar with.

SN stamped in chassis S378782.  Just wondering if anyone knows of a corner of the internet where someone may be keeping Zenith schematics that are not radio.

Title: Re: Zenith Stereophonic Schematic Hunt
Post by: sluckey on February 24, 2014, 07:41:20 am

Google for 'zenith chassis 4K21' will probably turn up a Sam's Photofact. That looks like the Zenith amp that I built for my mother-in-law back in the '60s. Nice sounding amp for a low end system. Must use a ceramic cartridge pickup.

Should be very easy to draw a schematic. It's basically a two channel amp driving two speakers. Each channel uses 1/2 12AX7 and SE 6BQ5. Does the dual volume control have an extra lug on each pot? IE, four lugs per pot? Also, IIRC, the OT had an extra winding (or tap) but I don't remember how it was used.

Title: Re: Zenith Stereophonic Schematic Hunt
Post by: Ed_Chambley on February 24, 2014, 08:23:42 am

Quote from: sluckey on February 24, 2014, 07:41:20 am

Google for 'zenith chassis 4K21' will probably turn up a Sam's Photofact. That looks like the Zenith amp that I built for my mother-in-law back in the '60s. Nice sounding amp for a low end system. Must use a ceramic cartridge pickup.

Should be very easy to draw a schematic. It's basically a two channel amp driving two speakers. Each channel uses 1/2 12AX7 and SE 6BQ5. Does the dual volume control have an extra lug on each pot? IE, four lugs per pot? Also, IIRC, the OT had an extra winding (or tap) but I don't remember how it was used.

Yes the dual vol pot does have an extra lug.  The extra OT winding goes to a bass enhancement switch, but I am not sure what it does yet..  I have the whole console and it was working well, but the cabinet was Grandmothers finest.  Early american whitewash finish.  The turntable works great and will go on ebay.  If it were 50's moderne I would keep it together.  It is too new for my collection.

Actually it was $269.00 new.  Still has all tags and manuals with instructions on how to replace the album holder with a newly FCC approved a FM Stereophonic tuner.  Even though it is sort of homely, I still am having a hard time parting it out.  The sound quality is surprising.

I am thinking easy guitar amp here.  A quick look it seems like a dual SE.  Should be nothing to it, I just wanted to see what the voltages were on the schematic.  I will get a read on them later.  The Transformers appear to be very overbuilt.

Couldn't find by 4K21. :sad2:  You are correct in the notion it will be easy to draw it.

The caps are old Bumblebees.  If they are good and any have Gibson Guitar values they will ebay nicely.  I still cannot believe what people pay for these things.  Their ears are much better than mine.

Title: Re: Zenith Stereophonic Schematic Hunt
Post by: scrimpus on February 24, 2014, 06:08:22 pm

I have that same amp but without the bass contour switch and I love it. I have it connected to my PC and I sounds great, plenty loud too. I can email you the schematic if you still want it.

Title: Re: Zenith Stereophonic Schematic Hunt
Post by: Ed_Chambley on February 24, 2014, 07:57:44 pm

I want it.  Thank you.  I will pm my email address to you.

Title: Re: Zenith Stereophonic Schematic Hunt
Post by: RWood on March 03, 2014, 07:26:46 am

Ed,

Here is the 4L21 schematic - it might be very similar (same model, a year apart).

These Beitman's Schematics are available for download here:
http://makearadio.com/beitmans/ (http://makearadio.com/beitmans/) It requires the DJVu plugin, which makes for very high resolution viewing.

RWood

Title: Re: Zenith Stereophonic Schematic Hunt
Post by: Ed_Chambley on March 03, 2014, 07:46:59 am

Thanks RWood.  You have a very familiar name, you ever play with the Faces?

Title: Re: Zenith Stereophonic Schematic Hunt
Post by: RWood on March 03, 2014, 10:26:03 am

That was a long time ago, but yeah. I played along with The Faces while they spun on one of these: Ooh La La!
(http://i181.photobucket.com/albums/x304/vortigerncrancocc/motorolaSH12N.jpg)

Title: Re: Zenith Stereophonic Schematic Hunt
Post by: Ed_Chambley on March 03, 2014, 10:41:21 am

Quote from: RWood on March 03, 2014, 10:26:03 am

That was a long time ago, but yeah. I played along with The Faces while they spun on one of these: Ooh La La!
(http://i181.photobucket.com/albums/x304/vortigerncrancocc/motorolaSH12N.jpg)

You know a nod is as good as a wink to a blind horse.  Has to be one of my favorite bands.  Cool photo, love those old phonographs.  I guess you do too.