Please advise RE Console Pull

[Hi-Fi Low-Life]

Hi Guys… I recently pulled an amp out of a working Zenith console. It looks like a PP EL84 design with a 5U4 rectifier & 6FQ7 input tubes. This thing has a big honkin’ PT which I suspect powered the entire console. The OPTs don’t look like anything special & I got this info about them off the net: “EIA code "1386131" says these were made by Stancor in 1961. Zenith part number 95-1913.”

I figured I could rebuild this amp, but the thing is, there’s 44 volts AC between the chassis and house ground! Is this one of those “Hot Chassis” designs I read about somewhere? Or does this indicate a bad cap somewhere? I measured power cap impedances and they were around 26 million ohms. The speaker outs measure zero volts, both AC & DC so it looks like the amp is dead quiet.

If it is a hot chassis by design, I don’t think I’m qualified to safely use this amp. I could sell it on eBay, but I’m concerned somebody could get shocked by it! Should I just Sh**-Can this amp? Or part it out & sell the parts on eBay? It’s got old stock tubes in it (Zenith brand, made in Holland EL84s for example) and half a dozen Good All caps plus a couple “Bee” caps.

TIA for any advice you can offer!

[erichayes]

Hi All,

There are several things that can cause the phenomenon you're seeing, HFLL, some of them intentional. If you have a model or chassis number, I'll see if I have a PhotoFact for it and give you some ideas on where to go with it. The fact that it has a power transformer rules out an AC/DC design--as does the fact that there's only a 44 volt differential. Check for a .05 µF paper cap going from one side of the line to chassis and remove it. Also try reversing the line plug in the outlet and measuring again. Sometimes there's also a 2.2 meg resistor going from the line to chassis that can create a phantom voltage reading that a DVM or VTVM will see because of their input sensitivities, but is inconsequential otherwise.

P.S. Zenith never made their own tubes. Your Dutch EL84s are rebranded Amperexes that are almost as highly sought after as Mullards.

[EWBrown]

I would assume that this has a two line power cord, due to its age.

Sometimes they used a cap from one AC line, or both lines, to ground for an RF ground. If there are no caps, you can still get some capacitave leakage from the primary winding through the trannie core to the chassis.

The voltage is there, but the leakage current is microamps, or even nanoamps. If there is no DC resistance between the primary leads and the chassis (best to unsolder the primary leads from the AC input circuitry to measure this)., then the trannie should be good to go.

/ed B in NH

[mesherm]

Adding to what Ed and Eric have said, if you don't find a cap connecting the chassis to one side of the power line then the voltage probably is just leakage from the trannie and probably very very low current. install a 3 wire plug and connect a 0.05 to 0.10 uf or so ceramic cap from the chassis to the third (ground) wire...that should drain the small leakage current and make your mystery voltage dissapear.

[Hi-Fi Low-Life]

Thanks for the info guys...

The number on the chassis is: 0913 01 and there's a paper sticker on the chassis with the number: 7H30

Are the Good All 600 volt caps that are in this amp considered desirable or should I replace those when I replace all the carbon resistors?

Thanks again!

[erichayes]

Good All caps were almost always paper, and if they're rated at 400WVDC should definitely be replaced. I'd replace 'em even if they're rated at 600 volts.

I'll go see if I can find that schematic . . .

[erichayes]

Hi All,

OK, found it. This is an interesting system in several respects. First, it's one of the first (if not THE first) tuners to incorporate the GE-Zenith FM stereo multiplexing system that the FCC approved the year it was made (1961). Unless it was from the greater Atlantic region, it probably didn't receive an FM stereo broadcast for a couple of years. The decoders (or "adapters", as they were called then) had marginal performance at best--if you got 10 dB of separation, you were lucky.

Another interesting feature that guitar players might get off on is a reverb unit that can be operated dry (bypassed), with monaural injection (L+R sum signal) into the left and right channels, or in-phase mono injection to the left channel and out-of-phase injection to the right. This was called expanded stereo that enhanced the sound of mono signals, and was pretty heady stuff for 1961.

The power amp is a pretty straight forward push-pull 6BQ5, Cathodyne- split, cathode biased, non-UL amp with 430 volts on the plates and 15 volts on the cathodes. What's novel is what they're doing with the NFB: they route it to the preamp chassis where it goes through a variable RC network that acts as a "presence" control. I might just cobble this up to see what it sounds like.

The power transformer runs the power amp and preamp only; the tuner has its own power supply, as it apparently was available as a stand-alone unit. And, yes, the PT is pretty impressive: 680 VCT @ 240 mA, 6.3 VCT @ 6 A and 5 V @ 3 A.

I'll see if I can get the schematics posted over the weekend.

[mesherm]

That NFB circuit sounds like a rudimentary parametric equilizer. Is the variable RC circuit changing the center frequency point or is it changing the amount of NFB also? and is it controlled by the user or factory set?

[TerrySmith]

I also have a Zenith amp from a console, it has a 12AX7, 6CG7, EL84 (2), and a 5Y3. It's a SE with slightly larger than normal outputs.

I recapped it to get rid of the snap crackle pops and it sounded very good! It has a wierd switch that almost sounds like surround sound when you turn it on.

Then one day the PT went open while in use, it smelled horrible. Yellow_Light_Colorz_PDT_31

[Hi-Fi Low-Life]

A schematic would be SWEET!

Since part of the feedback circuit resided in the pre-amp (which is now residing in our local land-fill) what should I do to close the feedback circuit of the power amp? Would it be harmful to the amp to run it with its feedback circuit basically disconnected (sans the pre-amp portion)?

Thanks for your input guys. I really don't know a whole lot about electronics & am just a hi-fi nut with a soldering gun in one hand and a beer in the other! Yellow_Light_Colorz_PDT_07

[SteveH]

I HIGHLY recommend this site:

http://www.nostalgiaair.org

Go under 'Resources', and you will find your schematic. Also, the guys that hang out in the forums are awesome.

Hope that helps some,
Steve

[Hi-Fi Low-Life]

A fine gentleman provided me with a schematic for this amp & pre-amp :)

If anyone is interested in the feedback circuit, just PM me your email addy & I'll send it your way.