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 Post subject: Zenith console mod question
PostPosted: Jan Mon 05, 2015 6:33 pm 
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Location: Warner Robins, GA
I have a mid 50's Zenith console which I decided to mod for more output power.

Bigbandsman had helped me design the amp and power supply.

I have yet to completely finish it.

My question is can I substitute the 10H choke for a resistor and increase the value of the filter cap after the choke at least until I have the money to buy the proper choke? Also I don't know what resistance the choke needs to be or how many milliamps it needs to be able to handle. I have Bigbandsman's PMs about the amp, but I downloaded them to my computer to free up my mailbox here and I'd have to search through several of the downloaded PM files in order to find them.

I really want to get this finished soon as I can so I can enjoy the console.

I have a Browning RV-31 FM tuner I plan on using with the console and it will serve as the volume control given it has three external inputs and the Zenith was modded so the tuner's audio goes through one of the Browning's external inputs leaving one input for my mono AKAI Terecorder and the other input free.

The biggest headache will be the alignment given with all my searching over the last few years I've found absolutely nothing on the console, but figure it has to be an export model as it has a 115/230 volt switch and no FM. Also at the time it was made the Zenith consoles I've seen online all had an AM/FM chassis without shortwave.

Here's the schematic of the amp and power supply . The unmarked resistor connected to the 100 ohm resistor near the 6N7 is 820 ohms. The unmarked resistor in the power supply I am not sure of until I know what the main B+ will actually be unless I could substitute it with Leigh's solid state choke given that B+ line only feeds B+ to the tuner circuitry. The voltages listed for that part of the circuit are what I measured with the console as it was with the original push pull 6V6 amplifier.

Image


Last edited by Tube Radio on Jun Mon 20, 2022 10:22 pm, edited 1 time in total.

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 Post subject: Re: Zenith console mod question
PostPosted: Jun Mon 20, 2022 2:01 am 
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Have yet to do anything with this amp.

I'm seriously considering just building the amp on a separate chassis that has room for the power transformer, choke and output transformer.

It will need 4 octal sockets and I'd prefer an octal equivalent of the 6C4 which will make it 5 octal sockets.

The 6AT6 will not be part of the audio amp at all.

Will there be enough gain from the 6C4 equivalent wired like the 6C4 is and will Miller capacitance be an issue?

Here's the tuner's audio output section.

Attachment:
1 tuner output.png
1 tuner output.png [ 18.66 KiB | Viewed 701 times ]


Given the output circuit of the tuner what is the lowest value grid resistor I can use on the 6C4 equivalent?

That said given who designed the amp should I even use any tube other than the 6C4?

The audio from the AM/SW tuner in the Zenith will go to an input on the Browning tuner.


Last edited by Tube Radio on Jun Mon 20, 2022 10:30 pm, edited 2 times in total.

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 Post subject: Re: Zenith console mod question
PostPosted: Jun Mon 20, 2022 5:46 pm 
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Location: Detroit, Michigan, USA
A 6J5, or half of a 6SN7, ought to be the best octal equivalent of the 6C4 and should not require changing any component values. The only significant difference is higher filament current. There were a few Zenith radios in the late 40's which used 6C4/6J5 in different production runs, probably based on what they had on hand.

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 Post subject: Re: Zenith console mod question
PostPosted: Jun Mon 20, 2022 5:57 pm 
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Joined: Jan Thu 01, 1970 12:00 am
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Location: Warner Robins, GA
Ok thanks.

I might use a 6SN7.

That way I have an extra triode section if I need it.

Only problem is no matter how hard I've looked I cannot find any service info on the Zenith chassis so I can't do a proper alignment.

I already know how good the speaker sounds as I've used it with the amp in the state it is in.

Also I need two more IF transformers for the Browning tuner.

So I'll gather a list of what all I need and as funds become available I'll buy the parts.

Only thing is the Zenith volume and tone controls will not be used.

Im not too fond of that though, but I don't see a way to actually use them properly. Tone controls I don't really need and the volume control is too high in value unless I use a pentode cathode follower or as a gain stage.

EDIT:

Here's the Zenith chassis.

Attachment:
1 Zenith chassis.jpg
1 Zenith chassis.jpg [ 490.76 KiB | Viewed 701 times ]


Due to the size I had to use the tone control, socket for the output transformer connections so it could sit beside the chassis and for the choke I would have had to use the phono power socket.

By moving the amp and power supply to another chassis I free up four octal sockets. Not sure what I will do with them just yet.

If not for who designed the amp I'd stick in the typical phase splitter circuit I like to use

Attachment:
Phase inverter 1.PNG
Phase inverter 1.PNG [ 6.52 KiB | Viewed 701 times ]


and keep it all on the radio chassis, however I'd run into an issue of Miller capacitance for the 6AT6 unless I am able to use a lower value of plate resistance around 100K. That said I'd hate to throw away a design by someone who obviously knew what he was doing given his professional use of some of the designs such as the phase splitter.


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 Post subject: Re: Zenith console mod question
PostPosted: Jul Mon 25, 2022 2:41 pm 
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Location: Warner Robins, GA
Thanks to two forum members I now have two chassis on which to build the amp.

Now I need to decide which one to use.

I'll measure the dimensions of the power transformer and use those dimensions for the output transformer and choke.

That will determine which chassis I use as I don't want the amp to be crammed in the chassis.

For this amp would it be better to use an octal socket on the amp with an octal tube base for the power cable going to the RF chassis with the male octal plug on the RF chassis or is it better to run the wires through the amp chassis with some sort of strain relief?

Whatever chassis I don't use will get used in the future for another amp. Might build one with a pair of 6DE7 or 10DE7 tubes in push pull. Would only need three tubes given the 6DE7 has a power triode and regular triode in the same envelope.

I have a nice V-M Tri-O-Matic record changer from a cheap 1950 Silvertone console. I cannot decide whether to use it with the Zenith or the Magnavox CR-198C console I have. What I might do is look for the appropriate Zenith Cobra changer and adapt the tone arm to work with a stereo ceramic cartridge if that can be done within reason. I could keep the stock cartridge and restore the phono circuitry in the receiver, but then I'd be limited to mono records only.


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 Post subject: Re: Zenith console mod question
PostPosted: Aug Fri 12, 2022 4:24 pm 
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Location: Warner Robins, GA
I don't have any photo of the chassis handy at work, but I decided to go with the larger chassis given it has enough room for two transformers and a choke.

It has a bit of rust on it. Nothing too bad but, I do want to clean the rust completely off the chassis.

What is the easiest way to do that?

I know that I can use elbow grease, but if I can avoid that I'd prefer not to go with that method.


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 Post subject: Re: Zenith console mod question
PostPosted: Aug Fri 12, 2022 11:44 pm 
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Location: Simcoe, ON, Canada
Depending on just where the rust is you could dab some Naval Jelly on it with a q tip.

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 Post subject: Re: Zenith console mod question
PostPosted: Aug Fri 12, 2022 11:48 pm 
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Joined: Jan Thu 01, 1970 12:00 am
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Location: Warner Robins, GA
There's a bit of light rust on a large part of the chassis and some parts have a little more rust.

Nothing major though.

If it was just a few spots I'd try the naval jelly.


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