[etech]

Hi all,

Let me say that I would probably not make any permanent changes to my early model 70, but it would be a fun project to see if avc could be easily added to the model 70. I have noticed the similarity in the circuits of the early model 70 and the middle model 90. Other than the early 70 using a plate type of detector, they are pretty similar. Here are the schematics.

Early 70 - http://www.philcoradio.com/tech/images/70a.jpg

Middle 90 - http://www.philcoradio.com/tech/images/90b.jpg

Here are my thoughts as things necessary for a conversion to avc operation.

1 Remove original volume control and install fixed 250 ohm resistor between points G & H.
2 Tie cathodes of RF and IF tubes to point G.
3 Lift secondaries of RF coil #2 and IF coil #18 and bypass each to ground with a .05uf cap.
4 Attach a 470K resistor to each lifted secondary and tie them to a newly created avc buss.
5 Create new avc buss by using a signal diode (either a 1N34 or a 1N914) with cathode connected to the hot (top)side of the detector coil #20 secondary. Load anode with a pair of 51K resistors in series (or a 100K resistor) to ground.
6 Install a new 250K (or 500K) volume control pot in place of resistor 32 at grid of 47 tube.

That's it. What do you model 70 and 90 mavens think? All opinions will be appreciated.

Thanks,

Ed

[BDM]

Interesting idea Ed. I'd like to see if this works.

[Ron Ramirez]

Yes, it can be done. And, no, I am not really satisfied with the results.

I have a schematic around here somewhere which shows the changes I made to a Model 370 Lazyboy that is in my living room. It has an integrated volume control and off-on switch that is pretty much unobtainium. And, as luck would have it, the volume control in mine was destroyed; hence my reason for modifying mine.

Oh, it works okay, but volume is low unless I hook up a longwire antenna, which makes it shine. Early Model 70 sets (unmodified) will pull in lots of signals with a fairly short piece of wire. Not mine, however. And, yes, all paper and electrolytic caps have been replaced, all tubes checked, and it has been thoroughly aligned.

Ed Locker has also done this mod to one of his 70 cathedrals. Perhaps he will chime in with his thoughts.

[etech]

Oh, it works okay, but volume is low unless I hook up a longwire antenna, which makes it shine.

It might be that the model 70 does not have enough overall gain to support an avc operation, being only a seven tube set and especially with no multi-function tubes. I suppose I could replace the 24 detector tube with a 55 tube and install a conventional diode detector and triode amplifier as was used in later designs with the 6Q7 or 6SQ7. But then I would have to change out the tube socket also. That is not an option which is easily reversible.

Ed Locker has also done this mod to one of his 70 cathedrals. Perhaps he will chime in with his thoughts.

Yes, it would be interesting to get Ed's take on this, being that he has done a conversion on one.

I would also be very interested in any else's opinion on this topic.

Regards,

Ed

[Ron Ramirez]

It might be that the model 70 does not have enough overall gain to support an avc operation, being only a seven tube set and especially with no multi-function tubes. I suppose I could replace the 24 detector tube with a 55 tube and install a conventional diode detector and triode amplifier as was used in later designs with the 6Q7 or 6SQ7. But then I would have to change out the tube socket also. That is not an option which is easily reversible.

I disagree, the Model 90 with AVC and a single 47 output tube has the same front end as the early 70, just modified to use AVC.

In my mod, I switched the RF and IF tubes (originally type 24) to type 35/51 as these, being remote-cutoff tetrodes, are designed for AVC use. I believe Ed used 35/51 tubes in his mod.

I'll try to post a schematic later.

[etech]

I disagree, the Model 90 with AVC and a single 47 output tube has the same front end as the early 70, just modified to use AVC.

I was thinking the overall gain including the audio stages might be somewhat less, thus causing the radio to play with less volume when using the avc circuit. I did notice that both the later model 70's and the model 90's with avc have an extra audio stage. I don't know how much that will affect the overall audio output.

I'll try to post a schematic later.

I would be very interested to see your circuit if you can find the time to post it.

Thanks,

Ed

[Ron Ramirez]

Here's the schematic:

[Image: http://i326.photobucket.com/albums/k420/...mod_sm.jpg]

Larger image here:

http://i326.photobucket.com/albums/k420/.../70mod.jpg

[etech]

Thanks for the schematic, Ron. That is a very similar approach to what I was thinking. The only real difference is that I was planning to control the gain of the RF and the IF tubes with the avc. My idea was to create a separate avc buss and detector only for the gain control and continue to use the plate detector for the audio demod and driver. Who knows, maybe it won't work out, but it would be interesting to try it. I agree, it would make sense to swap out the 24's with 35's, at least in the RF and IF stages.

Ed

[Ron Ramirez]

Ed

If you use a diode as your 2nd detector, you need to do something with that tube that served as a "power detector." In my mod, I converted it to an audio amplifier, same circuit as used in the 2nd version 70. This drives the 47 audio output.

You could easily add AVC to the IF stage in my mod; in fact I had AVC on the IF stage of mine at first. But being dissatisfied with its overall performance, I changed it so the IF amplifier is running "wide open"; AVC is fed only to the RF amp in the schematic.

[etech]

Very interesting, Ron. I see that your circuit is nearly identical to the later model 70 circuit. So, that makes me question the performance of the later model 70's with avc. That is, I wonder if they are somewhat lacking too?

Ed

[etech]

If you use a diode as your 2nd detector, you need to do something with that tube that served as a "power detector."

My idea was to continue to use the power detector as is. I would add a IN34A as a second 'avc only' detector. I'm thinking it would probably work OK.

However, your circuit would probably sound better. From what I have read, the power detector circuits are prone to more distortion.

Ed

[Texasrocker]

I tried the same thing on a 1936 Crosley 516 before. As Ron stated, and I agree, there really wasnt any noticable difference in performance after the hassel of adding AVC. However, I guess some sets could be improved, and if nothing else, its a good experiment, and the end results will bear their fruits of labor. Good luck with yor Philco retro-fit!

[Ron Ramirez]

My idea was to continue to use the power detector as is. I would add a IN34A as a second 'avc only' detector. I'm thinking it would probably work OK.

However, your circuit would probably sound better. From what I have read, the power detector circuits are prone to more distortion.

Very, very true - the "power" detectors do introduce a fair amount of distortion. Making that tube into an audio amplifier is a better way to go.

The late 70 sets with AVC work very well. Must be something I did wrong - I don't think Ed Locker had the same problems that I did with mine...I believe his works better? Maybe he will chime in soon with some comments :

[Ed Locker]

Hi All

Sorry I didn't chime in sooner, I just saw this post this morning.

I'm pretty rusty on this, it's been years since I modified my 70's (3) to AVC.

Let's see: I used a 1N34 for the detector, removed the grounds from the IF and RF tube signal grids and ran them back to the 1N34 through a 2.2 meg, removed the original dual volume control and installed a 350K pot. for audio control and detector load. Converted the original 24 detector stage to a audio amp.

The audio fidelity is excellent. The RF gain is excellent. The "Plate detector" distortion is gone, which is the reason I did all this.

If you need the schematic, email me and I'll send it to you.

Ed

[etech]

Ron - Thanks for the information and guidance on this project. I guess there are a number of ways to complete this project. I am just trying to ascertain the best method.

Ed - Thanks for checking in on this topic. Yes, I would be very interested in comparing your method with Ron's. As Ron mentioned, he was not satisfied with the performance of his set after the avc conversion.He feels that the set lacks sensitivity unless he uses a longwire antenna. Apparently, you did not experience this difficulty with your avc converted 70. I will drop you an email for the schematic if you would be so kind.

Regards,

Ed