If you dislike the sound coming out of the 24a power detector here's a simple circuit at converts it to a diode detector. The audio recovery is much cleaner with little distortion. Requires just a few parts and can be packaged to be a plug in unit if you would want to put it back to the old circuit. Connects from the grid clip to the grid cap and needs a single ground wire. I tried it out on my 370 and worked great!
Nice! So you are leaving the filter on the plate of the 24 in circuit. I did the modification previously talked about on my 570, removing these components from the circuit, but leaving them in the radio for future reversion back to original. I may play with this on my 70. The distortion is annoying.
I did think about removing it out but it sounded good before I didn't any under the hood stuff so I left it in place. Was trying to keep it simple. Very little difference in levels between the two circuits so didn't have to adjust the value of the 51K resistor down to pump up the gain. The audio frequency response is better more high and lows. Here it is when I was giving it a test.
The other more extensive mod adds AGC, which I like. But it is an extensive modification.
This can be assembled in a socket of a dead tube, so if one feels like restoring the radio to the original state, just remove this and put the tube in.
Not even that it could be in the small plastic tube with grid cap one one end, a grid clip on the other and a ground wire with an alligator clip. The parts could be a lot smaller than the ones I used, they were just what I had in the junk box. It doesn't connect to anything at the bottom the tube near the base.
I believe the 24 is still in use as an audio driver so where would you put the dead tube? I may haul out my 70 tonight.
Wouldn't this be of benefit on the first RF 24a detector as well?
No that's sort of a misnomer. The 1st detector is better known as the mixer or converter. 1st detector doesn't detect anything but it does mix two signals or converts the incoming rf signal and the local oscillator signal to the IF frequency.
They called detectors all kinds of things.
The only true detector is the one that rectifies the modulated carrier. Diode ones do true rectification, the plate one simply clips it at the bottom being an amp.
The rest are not really detectors.
Don't take this the wrong way - but, after spending hrs. on the finish and restuffing caps, and replating parts, and making replica resistors - I want my 70 to sound raspy, because they did and people seemed to be OK with that.
Good mod though.
You know, mine only sounds raspy on strong stations (it might be weak stations, but it has been a while).
Now be nice. I could have said " If you want it to sound good, trade it for a Scott". But you know that I would never say that.
That "raspy" is a part of history now.
This is why I try to not improve the original schematics, add AGC and such. I want my radios as close to original as I can get them. But I am OK with folks trying to improve things.
After all, 20, 70 and 90 do sound a bit raspy.
I even think, one could take a 24A dud, extract the envelope, cut the bottom, assemble a SS detector in the base, pot it and glue the envelope back.
A bit of raining on the good sound parade though: same as achieving better quality of sound by reducing the cathode feedback cap from, methinks, 0.5uF to 200pF, any such technique results in killing the PD gain. PD is, after all, a Class B amp. If it no longer amplifies, the sensitivity of the set will be noticeably reduced.
I see both sides of the story. I enjoy listening to my old radios....unless they sound like crap. My 70 has been lingering on the shelf with no attention. That's not fair to it either. So an easily reversible mod to improve its sound brings the radio back to life in my collection. I think it deserves that. And for the purist that someday buys it, I can put it back.
Will try this when I finish other stuff, but meantime I bumped up the tone control to .05, and .1, you end up with that smooth old timey sound - smooths out the sound and filters stray out, might put in a potentiometer to vary it.
.1.......that will reeeeally roll off your highs.