Modifications to Magnavox 8301

[SpareRibs]

Hello,
       I would like to convert the stereo amp into mono, does this diagram look reasonable ? I was going to convert  to 12AX7 but there would be a problem with heater voltage. Thank you in advance for any replies.

[sluckey]

Looks workable to me. You may want to decrease the output tubes cathode resistor to 130Ω or 150Ω.

There's no problem running a 12AX7 as you've shown in your modified drawing.

Let us know how it turns out.

[SpareRibs]

Hello,
        I am hesitant to try the 12AX7 because of the heater voltage. In the modified schematic I think I will use the 6EU7 instead of converting to the
12AX7 as that would require less rewiring, thereby leaving less room for mistakes.
     

[sluckey]

I have added another schematic which I think will be helpful as a guideline for the transition.

This schematic is for a push/pull amp. It's quite different from the first schematic you posted. I think it will just add confusion.

[SpareRibs]

Hello,
        OK, Thank you I will delete that one and take it out of the equation.

[PRR]

Don't feed your preamp from the same B+ node as your power tube screens. (This gave big problems on my first amp.)

An amp this scale, you need 3 B+ nodes. Final plates, final screens, and then both preamp stages can share the 3rd B+ node.

I understand sticking with 6EU7 for less work (less mistakes). However the supply of fresh 6EU7 is dry, fresh 12AX7 are everywhere. And preamp tube "tone" is something you may want to play-with in voicing. (Actually it would be cute to make a 12AX7/6EU7 adaptor.)

You have not changed the number of tubes on that cathode resistor, so it is reasonably the same size as Maggie had it. Since you have upped the Screen voltage, it does need to be a bit bigger, 130-150 is probably right-size.

A 4,200 pixel wide image is MUCH too big for a forum. Monitors are 1,200-1,600 pixels wide; some folks use cell-fones which are even smaller. Grey-on-grey is hard to read. JPG makes a mess of line-drawings. (It is however a *lovely* bit of Drafting!)

Topic: Image size-How to reduce it

The attached RE-Revised image reads well and does not fall off the end of my monitor to scare the dogs.

[SpareRibs]

Hello,
        PRR, I am trying to emulate this amp using the Magnavox as a platform. I am not as electronically proficient as most of you folks here. So any
suggestions would be welcome. Thank you for any help in advance.
        Hope this diagram came in at the right size.

[SpareRibs]

Hello,
        Does anyone know how I can use the Magnavox platform and pull the 12 volts needed to supply  a 12AX7 ?

[sluckey]

Does anyone know how I can use the Magnavox platform and pull the 12 volts needed to supply  a 12AX7 ?

You don't need 12 volts to heat a 12AX7. It's very common to use 6.3VAC. Just jumper pins 4 and 5 together. Connect the 6.3 to pin 9 and pin 4/5.

[SpareRibs]

Hello,
        Thanks a lot sluckey, That has been the holdup. Will the 12AX7 fit into the 6EU7 socket

[HotBluePlates]

Yes, they use the same 9-pin socket. However the socket wiring is entirely different. You will need to look at how 12AX7's are normally wired.

[SpareRibs]

Hello,
        Thank you. I thought it was possible. I will look for it on line.

[AZJimC]

I rigged up a DPDT switch with a wire jumping diagnally opposite pins, with the intent of wiring it for a 6v to 12v wiring for the ability to swap out tubes like those. I forget the actual hookup but the intent is to electrically move the wire on pin 9 to pin pin 4 while breaking the pin 4-5 connection. I will be trying out the very cheap russian tubes that are like a 6ax7. Those can be had for pennies compared to 12a*7, and some folks say they're great.