I have a tube question. The radio is a mid-60's AGS, Japanese AM/FM, similar to a Rincan, that has water damage, I wonder if it floated, but is complete. It is a transformerless design that uses 2 12DT8 9-pin tubes in the RF amp stages. One tube is cracked, so I need to substitute, at least for now until I know it's worth continuing. :>)

From my Marconi Tube Characteristics Manual, the pinouts for a 12AX7 are the same as the 12DT8, except for the filament center tap, which I won't need. However, there are differences as follows:
Max plate Watts Capacitance in pfd
Input Output grid-plate
12DT8 2.7 2.7 1.6 1.6
12AX7 1.0 1.8 1.9 1.7
12AT7 2.5 2.2 1.2 1.5

As you can see, a 12AT7 would be a closer match, but I am not sure if I have one handy.
1) Is the plate watts rating a serious factor? Would it be foolish to try a 12AX7?
It looks like one side of the tube will be AM and the other side FM.
2) Should the substitute go in the first socket, at the antenna input, or the second, after the 1st IF transformer?

:I have a tube question. The radio is a mid-60's AGS, Japanese AM/FM, similar to a Rincan, that has water damage, I wonder if it floated, but is complete. It is a transformerless design that uses 2 12DT8 9-pin tubes in the RF amp stages. One tube is cracked, so I need to substitute, at least for now until I know it's worth continuing. :>)
:
:From my Marconi Tube Characteristics Manual, the pinouts for a 12AX7 are the same as the 12DT8, except for the filament center tap, which I won't need. However, there are differences as follows:
:
:12DT8 Max plate Watts 2.7
:12AX7 1.0 W
:12AT7 2.5 W
:As you can see, a 12AT7 would be a closer match, but I am not sure if I have one handy.
:1) Is the plate watts rating a serious factor? Would it be foolish to try a 12AX7?
:It looks like one side of the tube will be AM and the other side FM.
:2) Should the substitute go in the first socket, at the antenna input, or the second, after the 1st IF transformer?

Forget the 12AX7. And I don't have a 12AT7.
What I found in my boxes is 12AU7's and a couple of 5963's, I think they're a milspec twin triode. Here are the specs:

Max plate Watts
12DT8 2.7
12AU7 2.75
5963 2.5

Capacitance in pfd
Input, Output, grid-plate 12DT8 2.7 1.6 1.6
Input, Output, grid-plate 12AU7 1.8 2.0 1.5
Input, Output, grid-plate 5963 1.9 0.5 1.5

I'm going to try a 12AU7, unless somebody tells me otherwise.

:

::I have a tube question. The radio is a mid-60's AGS, Japanese AM/FM, similar to a Rincan, that has water damage, I wonder if it floated, but is complete. It is a transformerless design that uses 2 12DT8 9-pin tubes in the RF amp stages. One tube is cracked, so I need to substitute, at least for now until I know it's worth continuing. :>)
::
::From my Marconi Tube Characteristics Manual, the pinouts for a 12AX7 are the same as the 12DT8, except for the filament center tap, which I won't need. However, there are differences as follows:
::
::12DT8 Max plate Watts 2.7
::12AX7 1.0 W
::12AT7 2.5 W
::As you can see, a 12AT7 would be a closer match, but I am not sure if I have one handy.
::1) Is the plate watts rating a serious factor? Would it be foolish to try a 12AX7?
::It looks like one side of the tube will be AM and the other side FM.
::2) Should the substitute go in the first socket, at the antenna input, or the second, after the 1st IF transformer?
:
:Forget the 12AX7. And I don't have a 12AT7.
:What I found in my boxes is 12AU7's and a couple of 5963's, I think they're a milspec twin triode. Here are the specs:
:
:Max plate Watts
:12DT8 2.7
:12AU7 2.75
:5963 2.5
:
:Capacitance in pfd
:Input, Output, grid-plate 12DT8 2.7 1.6 1.6
:Input, Output, grid-plate 12AU7 1.8 2.0 1.5
:Input, Output, grid-plate 5963 1.9 0.5 1.5
:
:I'm going to try a 12AU7, unless somebody tells me otherwise.
:
::
:
:
I know, I'm posting to myself again. I see here the 12DT8 is a High Mu tube. The tubes I've chosen are low Mu, factor of about 20. Should it go ahead of, or after the IF?

A 12AT7 would be the one to use. A 12AU7 should work also, but not as good as the 12AT7. Try it for the RF amplifier. Then try it for the IF. Where ever it works best, leave it there.

:A 12AT7 would be the one to use. A 12AU7 should work also, but not as good as the 12AT7. Try it for the RF amplifier. Then try it for the IF. Where ever it works best, leave it there.
:
I agree with Warren.
At this point you just want to see if you can get sound. The 12AT7 woudl be better, but the 12AU7 should still get you some local stations.

Once it is working and looks worth it, then get your 12DT8. Until then make sure pin 9 is disconnected.

Best Regards,

Bill Grimm

::A 12AT7 would be the one to use. A 12AU7 should work also, but not as good as the 12AT7. Try it for the RF amplifier. Then try it for the IF. Where ever it works best, leave it there.
::
:I agree with Warren.
:At this point you just want to see if you can get sound. The 12AT7 woudl be better, but the 12AU7 should still get you some local stations.
:
:Once it is working and looks worth it, then get your 12DT8. Until then make sure pin 9 is disconnected.
:
:Best Regards,
:
:Bill Grimm
:
Thanks for the backup, Warren and Bill. I dug deeper into the inner sanctum of my junk bin and found a pair of 12AT7's. Plugged one into the first socket and brought the voltage up slowly. So far, just crackle and noise from the speaker, but the will to live is there. I will carry on when time allows.

I tried a 12AT7 in the 1st position and got snap crackle pop from the speaker - all is well. Pin 9 is grounded on this radio, shorting the heater center tap of the sub tube, so I shut her down. Rather than desoldering the grounded pin, I stopped by Pacific TV, and picked up a NOS 12DT8. Here's a link to his website. There are lots of hidden treasures in John's basement workshop. :>)

http://pacifictv.ca/main.htm#NOS

:I have a tube question. The radio is a mid-60's AGS, Japanese AM/FM, similar to a Rincan, that has water damage, I wonder if it floated, but is complete. It is a transformerless design that uses 2 12DT8 9-pin tubes in the RF amp stages. One tube is cracked, so I need to substitute, at least for now until I know it's worth continuing. :>)
:
:From my Marconi Tube Characteristics Manual, the pinouts for a 12AX7 are the same as the 12DT8, except for the filament center tap, which I won't need. However, there are differences as follows:
: Max plate Watts Capacitance in pfd
: Input Output grid-plate
:12DT8 2.7 2.7 1.6 1.6
:12AX7 1.0 1.8 1.9 1.7
:12AT7 2.5 2.2 1.2 1.5
:
:As you can see, a 12AT7 would be a closer match, but I am not sure if I have one handy.
:1) Is the plate watts rating a serious factor? Would it be foolish to try a 12AX7?
:It looks like one side of the tube will be AM and the other side FM.
:2) Should the substitute go in the first socket, at the antenna input, or the second, after the 1st IF transformer?
: