Help ID this 12AX7/EL84 Console(?) Amp

[duganj]

I just picked this up at my local surplus shop and I'm having a hell of a time trying to track down any info on it.

Surplus Mystery Amp on Flickr - Photo Sharing!

The labeled tube sockets are from when somebody recently pulled the tubes out of it. I believe the 6BQ5 and 12AX7, but that octal is not a 6SN7, its a rectifier tube for sure.

There are some numbers on the transformers and the unit itself all of which have lead me nowhere on the Google.

Chassis:
8466942
062903

Power Xfmr:
95 1743
524022

Outputs:
95-1790-AJ
172 602

95-1780-B
172 6022

There are 3 pots on the front, as well as a 2 or 3 position lever switch.

There is a sticker labeled "Speaker Terminals" that lists "TV Audio In" as well as a red, black, and white quick connect.

Looks like it had a 120V cord thats been snipped as well as an aux 120V plug.

Thanks for any help.

 

[Tom Bavis]

Zenith chassis 4F20. Schematic is in Sams folder 558-15. With some Zeniths, two different output transformers give a "sum" output for a mono extension speaker.

 

[duganj]

Fantastic, thanks. Of course its the one number on the thing I didn't search for.

I found a nice looking example currently on eBay:

VINTAGE ZENITH 6BQ5/EL84 TUBE AMP STEROPHONIC 12AX7 - eBay (item 400103615640 end time Mar-16-10 20:26:24 PDT)

A little steep if you ask me.

Know I don't know if I want to bring this guy back to life or just rob it for the power transformer and chassis for my Blue Velvet preamp build.

 

[zelgall]

It would seem apparent that it's not a stereo amp as advertised due to only one out put tube.

 

[duganj]

Are you sure about only 1 output tube? I count 2 6BQ5's.

 

[Eli Duttman]

Fantastic, thanks. Of course its the one number on the thing I didn't search for.

I found a nice looking example currently on eBay:

VINTAGE ZENITH 6BQ5/EL84 TUBE AMP STEROPHONIC 12AX7 - eBay (item 400103615640 end time Mar-16-10 20:26:24 PDT)

A little steep if you ask me.

Know I don't know if I want to bring this guy back to life or just rob it for the power transformer and chassis for my Blue Velvet preamp build.

I think Tom Bavis has provided an important clue. Those O/P trafos appear asymmetrical.

FWIW, my inclination is to rebuild the bugger into a nice little SE amp, as that power trafo energized both the amp and a tuner, which makes it way oversized for a preamp. Consider a variation on the RH84 (schematic uploaded) theme. Install a pair of $37.01 Edcor GXPP15-6-8K O/P trafos. The heft associated with a 15 W. rating should give you extension into the 1st audio octave, at the 5 W. a pentode mode EL84 yields. An additional tweak is to regulate each EL84's g2 B+ with an individual LR8 IC.

It's likely the OEM rectifier was a 5Y3. You can increase the B+ rail voltage, without overstressing the 5 VAC winding, by installing a 5V4.

 

[duganj]

Agreed that the OPTs appear asymmetric. Still, I would be interested in using them if possible, looking at the faceplate of the amp in the eBay auction, it does show a 'stereo' mode as well as mono and ext stereo. So I would think there must be a way to use them as relatively symmetric.

I'll need to head to the library and dig up the Sams schematic.

 

[Eli Duttman]

Agreed that the OPTs appear asymmetric. Still, I would be interested in using them if possible, looking at the faceplate of the amp in the eBay auction, it does show a 'stereo' mode as well as mono and ext stereo. So I would think there must be a way to use them as relatively symmetric.

I'll need to head to the library and dig up the Sams schematic.

Zenith was both clever and CHEAP. The 2 sections operated out of phase. They functioned to produce a mono center bass channel and 2X stereo HF channels. Unless you want to construct a speaker system for that sort of setup, replace the O/P "iron". A pair of 95+ dB. efficient speakers will mate nicely with 5 WPC. In a small space, a little less efficient will work.

 

[duganj]

That sounds like just the kind of odd ball setup that intrigues me.

I might have to send these OPTs over to my DC Double Darling and free up the 125ESE's on it for this SE EL84 duty.

Assuming I can get the primaries to work out OK.

 

[Eli Duttman]

That sounds like just the kind of odd ball setup that intrigues me.

I might have to send these OPTs over to my DC Double Darling and free up the 125ESE's on it for this SE EL84 duty.

Assuming I can get the primaries to work out OK.

A look at Hammond's connection diagram suggests using the green and black wires, as the speaker connection.

 

[duganj]

There's enough hookup combos on the Hammonds that they will just about work with anything. It's more the Zenith's I'm wondering about. A glance at a few SE EL84 designs show pri impedances of 2.5-5k, so I should be able to make that work on the Double Darling.

 

[duganj]

Unfortinately the power xfmr was fried, so I yanked the 270FX out of my DC Darling along with it's 125ESE's and I'm in the middle of rebuilding this thing into a SE EL84 amp to listen to. Clearing out all the tone controls and recapping here and there.

The original ideal was to snag this power tranny for a preamp project of mine that needs a beefy power supply, but since a wrench was thrown in those works, I was able to dig up a Hammond (Organ not Canadian Transformer) M-3 amp who's power supply will work great.