[462ron]

Ditto Ed! I have a 70 that I haven't touched yet! 

Ron

[Ed Locker]

462ron, send me your email address as well.
Man, I'm going to be busy!

[Ron Ramirez]

To answer the requests for the 70 MOD schematic, here it is:

   

Right-click on the image above and select View Image to see it in full size.

Then right-click on the full size image and select Save Image As to save it to your computer.

Many thanks to Ed Locker for furnishing this image.

[Ed Locker]

Thanks for posting it, Ron.

[462ron]

Thanks Ed and Ron for posting!

Ron

[Phlogiston]

But crunchy audio is the only way I can tell that it is working right!

(because some loss of gain in any stage will make a 70 or 90 sound better) still - - nice to see.

[Jake Blake]

" . . . crunchy audio, . . . " ???  

[rfeenstra]

Thanks Ed.  Thanks Ron for posting it.  I think that will be tonight's project for the 570.  If it's a great improvement,  I'll do it to the Cathedral as well!  That slightly fuzzy sound drives me crazy.  AM is bad enough without that!

[rfeenstra]

After closer examination of the mods, I see that it is quite major!  I don't think I'll accomplish that this evening.  I like getting rid of the volume control system, especially since mine is scratchy even after disassembly, cleaning and lubricating.  Do the 35's make a big difference over the 24's?  Thanks!


Edit: Methinks I see AGC, hence the 35's.

[Ron Ramirez]

Yes, Ed's mod adds AVC (AGC) which necessitates the 35/51 tubes for best performance.

I have an unrestored 70 cathedral, to which I plan to add Ed's mod.

[rfeenstra]

The AVC by itself is worth the mod.  Certainly won't be original any more, but should perform much better.  I have no 35's so I just emailed Bob to see if he has 6!  Also need a couple of 500K pots.  Just did some reading on diode detectors in tube radios.  they can be made to perform better than the original tube diodes but it takes a bit of work with forward biasing and temperature compensation.  Too much for this purpose.  I'll just stick with the germanium diode though this guy said it was not the best for tube radios.  He had a lot of math to explain why.  More than I wanted to plow through!

[Ed Locker]

I used a volume control from a junker 89. You want the one without the switch. The shaft length is right as is the shape of the shaft where it will accept a Philco knob.

Ed

[rfeenstra]

I have the modification nearly complete.  Waiting for 35's from Bob.  The radio works with the 24's quite well except for very strong stations.  The AVC voltage gets to about -13 volts and I think it is starting to cut out the 24's.  AVC is working and I do believe the fuzz is gone from the audio.  Final judgement will be made when the 35's arrive.  I think it's going to be a major improvement.  My local electronic supply store had a 500K pot which fit well.  Unfortunately, it is linear taper, but still works acceptably.  I had to grind a flat spot on the shaft for the knob.

I'm a bit torn about making so many changes to the radio, certainly not very original. But, I'm thinking the performance improvements will make the radio more enjoyable to me.

Thanks Ed!!

[Ed Locker]

Yes, the late model 70 is what I based the mods. on.

That power detector distortion will drive you crazy!

Ed

[Ed Locker]

-13V AVC? Wow. You must be on top of a real flamethrower! WJR? They blow smoke because I can hear them here in the daytime.

Unfortunately, there are no 50,000 watters in this area for comparison. The closest 50,000 watter is in Cleveland (90+ miles). There is a 23,000 watter in Columbus, but they are 30 miles away. 

There is a local 1000 watter, but it can't compare with the strong RF in your area so I have no overload problems with any of my radios.

Hopefully, the 35/51's will cure it.

It's all worth it because it eliminates that nasty power detector distortion, makes the early 70 sound very good!

Ed