After replacing the filter caps (50/30uf) with new 47/33uf to clean up the blaring AC hum, I then noticed after getting well warmed up,... some serious audio distortion as well as a bit of a sharp, harsh-tinny sound to the audio too.
I was speculating on what it might be that changes after getting hot to cause this distortion.... All the old paper caps were just oozing gobs of wax all over everything... so I figured those that are closest to hot things might be doing it or maybe some cap is just "breaking down" after a few minuets of operation... or it could even be a bad tube too.
I measured all the voltages too ...which all seemed a bit higher (my AC is 127v) ...but "close enough" to work alright I figured.
Well then I could have just started swapping tubes to see... but with such a mess of old caps that I'm going to replace anyway... I decided to just start re-capping one by one in the order of the areas I most suspected and then testing the audio after each cap change to see my results.
Well after every cap was replaced... there was nothing left ... yet the distortion was the same... lol
I swaped out the 50B5... and the distortion went away. Totally! lol... see just a tube after all!!
Now I'm really glad I DID have that ONE spare 50B5 on hand... because otherwise I might have tried to troubleshoot it till I was bald from pulling my hair out.
Now I need another good spare 50B5 for my tube-treasure-chest.
I also added a .02uf cap from plate to cathode of the 1st audio 12AT6 which reduced the harsh sounding audio... and then replacing the 12BA6 IF amp helped the tinny sort of microphonic sound too.
Then I put in the .02uf cap across the output transformer primary because it was missing... but I didn't notice anything when I did that. I also put in a 22uf cathode-bypass cap that isn't on the schematic.
Now it sounds very nice... but I have 3 questions.
1.)
The two antenna leads from the loop antenna make a BIG difference if reversed. Why is this?
The antenna loop is 30 turns on cardboard. Nothing else... no trimmer caps, no ground clips.. nothing... just one continouos winding that is wound flat from the center outwards...each wire stacks above the next as the loops get larger for 30 turns or so. Thats all.
The innermost turn NEEDS to be connected to the input grid (pin 7) of the rf amp tube 12be6. If I reverse the 2 leads the signal drops in half or more!
Why should it matter which end of the antenna coil feeds the input? I mean all we are doing here is making a tank-circuit that runs best at the selected RF freq... no?
2.)
My next question is why does the AVC seem to show nice voltage variations from ~ -1 volt on weak stations to as low as -2.3 volts or so on strongest stations.
Yet.. as I dial across those strong stations it seems they are "too" strong to my ears and I feel a need to turn down the volume!... so much for Automatic Volume Control!!..
Is the AVC really doing it's job or not? And how can I disable the avc easily to see how it works without it?
I tried lifting the .05uf AVC cap #20 to see... but then I get nothing... just a little static-y hum.
3.)
The schematic shows an anticipated 117v AC input to the 35W4.
But it shows only 118V DC on the cathode as it enters the 1st section of the pi-filter at the 50uf cap.
I have 127v AC as input and my DC on the cathode is 141 volts!
Ok so I'm 10v higher on the AC side but 25V higher on the DC side. Why is that?
Is the schematic wrong?
My filter capacitors are very close to original values.
The print indicates that I should only have 110vdc on the 50B5 output's plate but I'm getting 135v dc.
Even the B+ is higher too. The print shows olny 85v and I've got 95V... and it would be even higher yet but the 1.2k ohm restistor in the pi-filter has drifted high and is at 2.4k
Curious.
I invite your thought's gentlemen.
Peter
:Hello Artist.
:
:In its inquiries, before anything else, also change a capacitor that is among the plate of 12AV6 o'clock for ground. He is a filter of RF and helps to attenuate high frequencies in the audio band.
Ok..I think you mean the 12AT6?
also ALL capacitors were already replaced even the micas.
That cap that you refer to is #26? (250pf)... that was replaced with a silver-mica(300pf)
:In its 2nd question, know: AVC needs several parameters pair to work well. From a perfect alignment (main), until the correct polarization of each valve in the following apprenticeships.
:If the radio was gauged with precise generator,
YES I used a frequency counter to measure 455kc from my signal generator for IF alignment. (very carefully aligned too)
if vc changed the capacitores for elements of same value,
The volume control was not changed.
Maybe the AVC "is working" correctly I am just not sure... but to me I feel that the strongest station still requires me to lower the volume.
there is not the because not to work right. Observing the outline, I say to you. There are been like you to measure capacitancias, check: 24(Cap. 250pf);
#24 was replaced with a 270pf mica
13(Resistor 10Megohns)
Yes this is within 10%
and 10(Resistor of 3.3M).
Ok that is within 10% too.
:Observe that for 12AT6, it just uses a diode as detecting and as filter of AVC. It is one he/she practices not very common.
:If the induction of the antenna reel seems poor,
I didn't say the reception was poor... it is great!. The question is WHY if I REVERSE the two wires from the antenna then the signal drops to half. ??
see the state of 7 (Resistor 22K).
#7 is within 10%
:If the audio doesn't still seem it pleasant, see 15 (Resistor 470K).
Okay here this resistor 15 measured high at 625k...
So...I have NOW replaced it with a 470k but the sound is still very sharp
It is less sharp only with my modification of adding a .002uf cap from plate to cathode of the 12AT6.
:If you want a more pleasant sound in the low frequencies, add a capacitor of 25UfdX40Volts close to the cathode of 50B5.
I had already added a cathode by-pass cap of 22uf.
Peter
Regarding AVC, if you want to see how the set operates without it, connect a wire from the .05 MFD condenser to B-. This will show you what the radio is like without it. Strong stations will distort beyond belief, and it will be necessary to adjust the volume for each station. If you want to see what the radio would be like without AVC, and would like the stations to come in clearly, take a 500K potentiometer and connect the AVC lead to the center terminal. Disconnect the 2.2 MEG resistor from the AVC circuit so that it doesn't affect it. With the terminals facing down, and the shaft facing towards you, take a 9 volt battery and connect it so that the negative terminal is to the left terminal on the potentiometer, and the positive terminal is to the right pot. terminal. Connect the right pot. terminal to B-. Be sure to do this experimentation on an insulated surface (like carpeting or something like that). The 9 volt battery should give you some RF bias regulation, though AVC voltages can climb much higher. Most of us do not have sensitive enough meters to sense the higher voltages, and so we usually see only about -7 volts on a strong station. Anyway, turn your regular volume control all the way up and use the battery control to control your volume. If the battery control doesn't turn the volume all the way down, you'll have to use the regular volume control, too. With this battery control, though, you are manually controlling the bias of the RF amplifier grids, and will have to adjust the control for each station in order to receive all stations at the volume which you prefer.
Regarding AVC poor operation, if you suspect poor operation, be absolutely sure that the AVC resistor is on value, and be absolutely sure that the AVC condenser has ABSOLUTELY NO LEAKAGE. It cannot have any or it will bog down the negative AVC voltage, causing local stations to blare and even distort. Some radios just have poor AVC regulation even with good components. I have ways of modifying this, especially the dumb radios which distort on distant stations (imagine that!).
Regarding your harsh audio, here's a simple negative feedback system that'll work wonders for you: Connect a 50K resistor between the 12AV6 plate resistor and B+. To where the 50K and the original plate resistor join, connect a .01 MFD condenser. Connect the other end of this condenser to the plate of the output tube. Check audio. Connect a .001 to .002 MFD condenser to where the 50K and original 12AV6 resistors join. Connect the other end of this condenser to B-. This condenser will re-establish some of the high treble, but not the middle end treble. Experiment with resistors in series with the .01 MFD condenser. You can go as high as 100K. See which values put in just the right amount of mid-range muting. You may also want to change the value of the condenser which you have going from the 12AV6 plate to B-, or the one which you have going from the output plate to its cathode. Perhaps use a smaller value like .005 MFD for the output tube.
Once you have established a nice sounding negative feedback system, you can do other nifty little things. If there was a way of adding a switch to the radio or a potentiometer, you could do two things. With a switch you could connect the switch to B-. The other side of the switch could go to a .01 MFD condenser which could then go to where the 50K and 12AV6 resistors join. Turning on this switch would place the .01 MFD filter condenser at the 50K resistor. This would nullify all negative feedback and bring back your harsh audio. This switch could be labeled "speech" and "music". If you ever build a radio with such a feature employing this circuit, you will be amazed at how the switch makes music so sweet and speech so clear. Also, replacing the 50K resistor with a 50K potentiometer, that puts the .01 MFD condenser, that runs from the output tube plate, to its center terminal, will give you a variable "presence" control.
Thomas
On the AVC... are you suggesting just jumpering across the avc cap as a test? ... I tried that and it just kills everything... no reception.
But I'll try all the AVC hints as well as all the various audio improvement suggestions as a bench experiment and see what it produces. I love the deeper rich, round tones but without the audio sounding muffled in anyway.
But the quick, rather satisfying, path to that, I found was puting that "feedback?" cap .002uf from plate to cathode on the 12AT6.
That cap exists exactly that way in my Gilfillan 56A.
REF(http://www.nostalgiaair.org/Resources/572/M0008572.htm)
I really love the nice rich sound I get from what is otherwise just another "5 tuber" Gilfillan 56A...
and so, I've tried to determine the things that contribute to that particular difference compared to other 5-tubers that sound harsh sharp and tinny like this Crosley.
REF:(http://www.nostalgiaair.org/Resources/665/M0003665.htm)
So I made another change tonight that helped bring the sound much closer to that of the Gilfillan 56a.
I changed the .003uf cap#25 to a .01uf from the volume control to the grid of the 12AT6.
So .. I've now copied 3 key audio enhancement elements from the Gilfillan 56a that are now incorporated into the Crosley as well.
1.) Changed c25 vol contorol cap from .003 to .01
2.) Added a .002 from plate to cathode in the 12AT6 ..same as c8 in the Gilfillan
3.) Added a 25uf cathode-by-pass cap ... same as c12b in the Gilfillan.
These all helped a lot but the most significant change was step 2... and step 1 just enhanced it a bit more.. Somewhat "richer" is all I know how to describe it.
Peter
Regarding changing the condenser which goes to the 12AV6 tube, inserting a larger value will pass more bass. It won't necessarily mute harsh tones, but it will pass more bass. In some circumstances this can cause the audio to distort prematurely. For instance, in my Zenith 5-G-500, which can only put out about .25 watts, a .002 MFD (or there-abouts) condenser is used. Using a larger value increases bass, but you can't turn the audio up as loud because it distorts more easily due to the strong bass notes, which easily throw off tube bias.
You can also increase bass by increasing the value of the grid resistor. I find that an ideal set-up is when a value from .002 to .005 MFD is used as the grid condenser, and then a value from 5 to 10 MEG is used for the grid resistor. In using a larger value condenser, a lower value grid resistor can be used to conpensate for excessive bass, but this also ruins the sensitivity of the audio circuit.
Placing a condenser from the plate to the cathode of the 12AV6 tube is negative feedback, but it sends a rather broad band of frequencies through, since you are simply using a condenser. The circuit I described tends to pass more of the middle tones due to the use of a .01 MFD condenser, and then a resistor in series. Also, inserting this after the 50K resistor and not directly to the plate of the 12AV6 tube tends to insert it in a controlled way so that it doesn't sharply mute everything. If this was any other day, I could explain my negative feedback circuit a little better, but right now I'm tired and my brain is fried for whatever reason.
Another thing you can do to reduce harshness of the audio is to place a filter in series with the 12AV6 grid condenser, or the output tube grid condenser. Placing the filter in series with one may have more of a dramatic effect than with the other due to impedance variations in the two circuits (and other things). Choose a condenser from .0005 to .002 MFD, and a resistor from 500K to 10MEG. Place them in parallel and place them in series with the grid condenser of your choice. Resistors tend to pass bass notes better than treble notes, so increasing the resistance of the resistor will lower the "cut-off" point of high frequency passage. High frequency passage actually tapers off...it doesn't cut off, but I use the term for simplicity. Condensers, as you already know, pass higher frequencies better than lower ones. The smaller the condenser value chosen, the higher the low frequency cut-off. By trial and error you can find the right combination of resistor and condenser that tends to not favor the mid-range notes which bother you. This simple filter does wonders. I much prefer my negative feedback circuit, as I can play around with it a lot more, and it has much more of a dramatic affect on the audio. Both are nice, though, and you can even use both if you wish. Another nice thing about the negative feedback circuit is that it clears up some distortion much like push-pull amplification reduces distortion.
Thomas
I invite your eyes and thoughts:
http://www.pbpix.com/crosley9-120w/
Thomas