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 Post subject: Zenith 6V27 AC conversion questions
PostPosted: Nov Mon 06, 2017 5:32 pm 
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Joined: Jan Fri 20, 2017 9:36 pm
Posts: 141
Location: Centuria, Wisconsin
I have been evaluating my collection of radios I have found over the years with the plan to start restoring some of them this winter (now that I am retired). One of the radios is a Zenith 6V27 that was originally designed for 6 volt operation, and had been converted to AC operation sometime in the past. Some of the issues I found is that the tube layout is now different. It was 15, 6A7, 15, 75, 76, and 19. It is now VT49, 6A7, VT49, ? (suspect still 75), 76, 6N7, and a 5Y3 added. One issue is that the 15 and 19 tubes have 2 volt filaments, and the VT49 and 6N7 have 6.3 volt filaments, so it must have been rewired. Not only that, but the 19 is a 6 pin tube, but the 6N7 is an octal socket (more rewiring). The cabinet is in decent shape though!

At this point, I have been just gathering information and have not done much tracing out the circuitry. I was wondering if anyone has run into a similar situation and what are your suggestions and tips? At this point I am fairly new at restoring tube radios, but have been an electronic technician all my life, mostly solid state devices. It appears that if I could get this one running, the next one would not be as difficult. I am not overly concerned about restoring to its original state, just getting it functional again.

Thanks, John


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 Post subject: Re: Zenith 6V27 AC conversion questions
PostPosted: Nov Mon 06, 2017 6:01 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 30663
Location: Livermore, CA
Hi

VT49 is a 39/44 tube. Similar to a 15 but has 6.3 volt filament. Reasonable to use a 6N7 in place of 19 tube.

39/44 and 15 pin outs are the same. Best to use a tube manual or check online for 19 and 6N7 differences.

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 Post subject: Re: Zenith 6V27 AC conversion questions
PostPosted: Nov Mon 06, 2017 10:13 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 36177
Location: SoCal, 91387
This model has one of the best looking dials when lit;


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 Post subject: Re: Zenith 6V27 AC conversion questions
PostPosted: Nov Wed 20, 2019 10:48 pm 
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Joined: Jan Fri 20, 2017 9:36 pm
Posts: 141
Location: Centuria, Wisconsin
After sitting on the shelf for a couple years, I decided to take another look at this radio. The 15 tubes have been replaced with 39/44 tubes (6.3 volt filaments). The 75 was replaced with an 85. The 76 (5 pin socket) was broken so I installed an 8 pin octal and planned on using a 6c5. The 19 was replaced with a 6N7. However, I discovered that the audio inter-stage transformer has an open primary, so will have to find a new one or redo the audio section.

All the coils seemed to be ok, so I replaced the paper caps and out of spec resistors. Next I disconnected the power supply that someone had added years ago, and connected it up to an external power source leaving out the output tubes. I kept the voltage about 200 volts (max on tube plates is 250 volts), and the current draw was about 28 ma. This is about right if you add up the max draw of each tube from the tube specs. By monitoring the output at the vol control with the scope, I was able to align the IF and determine that that section was working. Hooking up an antenna showed that it was able to receive stations (trace on scope).

Now for the problem: The transformer that was installed has a secondary voltage of 700 volts (350-0-350), which gives a way too high B+. The way it was installed, the B+ was about 450+ volts, which is way over the tube limits. Is there any practical way to reduce the B+ other than a different transformer? I would have to reduce it by 200 to 250 volts! Just what would be a reasonable secondary voltage for a 200 volt B+? Would 400 CT or 500 CT be about right?

I was doing a search online and run across several suggestions. One was to put a resistor in the primary, but that would also lower the 5 and 6.3 volts. Another was to put resistors between the secondary winding and rectifier plates, but that would require quite a drop and wattage dissipation. The same would be for a resistor before the filter choke. Someone even suggested using zener diodes to regulate the output. I suspect that any series resistors would have to be selected for the proper voltage drop at the operating current? Would the voltage spike when the unit is powered up, or would the delay of the rectifier tube prevent that?

Any suggestions would be appreciated.

John


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 Post subject: Re: Zenith 6V27 AC conversion questions
PostPosted: Nov Thu 21, 2019 1:36 am 
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Joined: Jun Wed 14, 2006 11:49 pm
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Location: Leo, IN or Zellwood, FL
I would find another suitable power transformer. Something that produces around 200 to 250volts DC. Then it is simple enough to create a voltage divider with power resistors for all lower voltages needed.
John k9uwa

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