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 Post subject: Type 42 push pull amplifier help
PostPosted: Aug Fri 02, 2019 10:41 pm 
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Joined: May Sat 25, 2019 9:42 pm
Posts: 201
Location: Royal oak MI
Im looking for any good schematics to make a "junk drawer" amplifier. I just kind of want to do this for fun because I have a radio chassis I'm not really going to be able to restore so I figured I'd reuse to old parts and turn it into a cool project at the same time. I have 2 type 42 tubes, 2 76 tubes 2 6a7s a 6c6 a 6d6 I have a power transformer, an output transformer with a 6-6.6k winding all the sockets I would need I have a choke if I need one, a chassis ect. I just kind of want to do this for fun to help with my building skills, as well as have a nice little amp for my phone to use in the garage.


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 Post subject: Re: Type 42 push pull amplifier help
PostPosted: Aug Fri 02, 2019 10:44 pm 
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Joined: May Sat 25, 2019 9:42 pm
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Location: Royal oak MI
Also I forgot to mention if anyone has radio schematics that would work I'd be happy to look at some of those as well! I was looking for some, but I found a lot with Interstage transformers, which is kind of like to avoid. And I thought I should add too, Im not looking to make a high end amp or anything like that!


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 Post subject: Re: Type 42 push pull amplifier help
PostPosted: Aug Sat 03, 2019 3:46 am 
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Location: Tucson, Arizona U.S.A.
If you find a schematic that uses 6F6s you can use that. A 6F6 is a 42 with an octal base. Just make the appropriate pin number changes for the 6 pin socket.

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 Post subject: Re: Type 42 push pull amplifier help
PostPosted: Aug Sat 03, 2019 4:57 am 
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Location: Livermore, CA
Look at schematics on nostalgiaair.

http://www.nostalgiaair.org/Resources/

RCA K80 seems to match what you want?

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 Post subject: Re: Type 42 push pull amplifier help
PostPosted: Aug Sun 04, 2019 9:56 pm 
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Joined: May Sat 25, 2019 9:42 pm
Posts: 201
Location: Royal oak MI
Thank you! That should be perfect! I can sub the 6f6s for the 42s the 6sq7 for a 76 tube and I should be able to use a 76 instead of the 6sf5 too.
would these changes work?? Sorry for all the questions I just wanna make sure I have the right values before I start buying caps and resistors...


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 Post subject: Re: Type 42 push pull amplifier help
PostPosted: Aug Mon 05, 2019 1:11 am 
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A 76 is very different from a 8SQ7 or 6SF5 so different resistor values and possibly circuit changes would be required to make it work right.

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 Post subject: Re: Type 42 push pull amplifier help
PostPosted: Aug Thu 08, 2019 5:08 am 
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Joined: Dec Sat 24, 2011 9:17 pm
Posts: 6216
Location: Vancouver Island, Canada
Looked at Norm's suggested RCA K80, and yes, it would make a fun radio, but needs the IF transformers, tuning cap and other complexities.
Since you just want an amp, I searched for 6F6 push pull amp and found a lone schematic but the phase inverter seems awkward.

Since the 6V6 is the successor to the 6F6, how about a simple schematic for 6V6, then convert it to 42's? There are plenty of examples.
I like this one, but there are many variations. You will need to adjust resistor values a bit. You can use your triode tubes for V1a, 1b and V2.


Attachments:
6V6 cathodyne inverter.jpg
6V6 cathodyne inverter.jpg [ 274.62 KiB | Viewed 1344 times ]

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 Post subject: Re: Type 42 push pull amplifier help
PostPosted: Aug Thu 08, 2019 9:28 am 
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Joined: Sep Thu 23, 2010 6:37 am
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Location: Powell River BC Canada
In the suggested schematic, there should be a coupling capacitor before the grid of V1.

Otherwise a signal source with a DC component will shift the operating point.

What could do that ? An old tuner with a leaky output capacitor, or a hi fi nut
insisting on cathode follower output.

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VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
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 Post subject: Re: Type 42 push pull amplifier help
PostPosted: Aug Thu 08, 2019 4:50 pm 
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OK, good tip, and on that subject, what is the ideal coupling cap value for an audio amp of this type?

Also we know the plate voltages will be lower for the 42's, so voltages on the schematic are just a guide.

Here's the Fender Deluxe, another simple 6V6 design, but with much larger coupling caps (and too many input jacks.)

So again, you would sub the 3 single triodes in ahead of the output tubes, and for cathode resistor values, I would study the data sheets and old schematics using those tubes you have selected.


Attachments:
deluxe_5e3.jpg
deluxe_5e3.jpg [ 199.56 KiB | Viewed 1320 times ]

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 Post subject: Re: Type 42 push pull amplifier help
PostPosted: Aug Thu 08, 2019 5:53 pm 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 12437
Location: Powell River BC Canada
On the grid side of the input tube the designer may want a bit of rolloff to
prevent hum from getting through, especially if the output transformer
never deals with anything below 100 Hz.

With the Fender using 0.1 uf throughout, and that being a not very common
value to locate, they can concentrate of finding a particularly good brand/maker
and (when the amps were current), provide packs of four with overhaul
recommendation to change them all on routine service cycles.

And where these days do you find good 12AY7 tubes ?

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VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
ve7aso@rac.ca


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 Post subject: Re: Type 42 push pull amplifier help
PostPosted: Aug Thu 08, 2019 7:56 pm 
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Joined: Dec Sat 24, 2011 9:17 pm
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Location: Vancouver Island, Canada
Right, a 12AY7 tube can still be bought, NOS or Russian, for $40-50 US. But for the current challenge, how about we use a 76? :lol:

The 76 has a relatively low gain of 13 or so.
For the 2nd preamp, could we utilize the 6D6? The gain is variable, can be adjusted, and I would crank it up to max for starters, and then the other 76 simple triode can be the phase inverter.

Someone with the templates could draw up the schematic with all the correct pinouts and then resistor values can be inserted.
Bingo, there's your schematic.

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Burl Ives, RIP, oldtimer.
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