Magnavox 8802 amp conversion from 6v6 to 12v6 filament tranny size?

[kirkendoll]

I have a magnavox 8802 amp and was wanting to add a filament transformer so I could run 12v6 tubes (i have close to 30 nos ones) in it and change over the 6eu7 to 12ax7s. On the data sheets it says that a 12v6 uses .225 ma of current and the 12ax7 uses .3 ma of current in series .15 in parallel. That comes out to a maximum of 1.5 amps. I guess I'll be using a 120 primary 12 volt sec tranny. What would be a good size tranny to look for, can I just find a 1.5 amp one or do I need to get one that has power to spare. Will running the 6ca4 alone on the old heater curcuit cause any problems and is there anything else that I have not thought of that might give me problems.

Thanks Luke

 

[whoaru99]

A little bigger is probably good, but a bunch bigger and the voltage will be something higher because the rated voltage is typically with the rated load applied.

Same goes for the 6CA4. With only that tube on the filament circuit, voltage will be something higher than ~6.3V. How much higher is hard to say.

 

[whoaru99]

Maybe it's already part of the plan, but I'd go with a 12.6V center tapped transformer. That way you could run tubes with 12V filaments or 6V filaments (or both) with the proper wiring.

 

[kirkendoll]

I guess on the rectifier fil curcuit the best thing to do is calculate a resistor of appropriate size to add load to the curcuit to get the voltage down to the right value. Maybe? Or would that take a huge resistor to use up 1.5 amps.

Got that in after the last post which seems to be the best answer

 

[cademan]

You could leave the 6EU7 filaments alone and just swap wires to accommodate the 12AX7. Basically the two tubes are the same. You don't need 12.6 volts to run 12AX7 tubes, just your 12V6 tubes so your 6CA4 and 12AX7 tubes can stay with the 6.3 volts, and your 12V6's will have their own separate filament voltage source.

If you wire like I described above, the 6CA4 along with the 2 12AX7 tubes will be a lot easier on the power transformer and will probably run noticeably cooler.

 

[Kidmoe]

Another option would be just to run the 6CA4 and 12AX7 off the 6.3v filament winding and just the output tubes off the 12v transformer.

 

[Tom Bavis]

The 88 series has an unused 6.3V winding for the tuner (brown wires to Molex connector). Connect it in series with the existing winding, and connect ONLY to output tube sockets, leaving remainder on 6.3V. If the output tubes don't light, reverse the brown wires. You could even add a 6V/12V switch.

 

[kirkendoll]

The y1 and y2 extra tap that is 6 volts. Would you need to tie the brown and green together to make the center tap? Disregard the red writing on the pic.

 

[Tom Bavis]

Yes. Though it might be the brown/yellow - in fact that's the one I'd try first. You should then get 12.6V on the brown wire, with respect to the green-yellow.

 

[kirkendoll]

Well that's awesome I don't have to buy anything, thanks a lot guys

 

[century tek]

Wow, good job Tom. I didn't even think about the extra winding because most people here like to use those extra taps for bucking action.

 

[gadget73]

I was thinking exactly the same thing when I first saw this. If you get 0 volts, reverse the brown and brown/yellow leads.

 

[GordonW]

The cool thing, is that if you do what Tom said with the heater windings- with a DPDT switch, you can make the amp switchable for 12v or 6v heaters for the output tubes.



Obviously, you'd want to put the switch somewhere it couldn't get accidentally switched randomly (like underneath the amp, on a bracket, or something)...

Regards,
Gordon.

 

[gadget73]

heh, yeah that would be a good way to roast a set of 6V6's. probably have screaming output for a short bit before they died though.

 

[kirkendoll]

Ok I checked and it is green/yellow wire connected to the brown wire to get 12 volts across on the brown/yellow and green wire to the filaments of the 12v6 tubes. For the 6ca4 I just run one side off of the brown/yellow or the green and the other to the center tapped green/yellow and brown to get 6 volts on the filament. Now the question is what do I do with the hum balance? Do I leave it in place with 12 volts on it. I know I'll have to disconnect the 6ca4 off of it and wire it separately.

Thanks Luke

 

[gadget73]

leave it with 12v, or ground your new "center tap" where the two windings connect together. Either will work fine.

 

[kirkendoll]

And on the center tap for the 12 volt filaments, should I float it or tie it to the chassis?

 

[gadget73]

It has to be ground referenced somehow. You can tie it to the cathodes of the output tubes to get a bit of DC on the heater and reduce any heater-cathode voltage differences.