Hello there!
I bought myself an old radio from the 60s, typical AA5 circuit using miniature 7B tubes.
I've changed the smaller caps, as mandatory, but didn't change the large electrolytics as i don't hear hum on the speaker.
As this radio was meant to 110V, it uses a 2uF cap connecting rectifier plate to ground to lower the B+.
I've only listened to it twice as i have the chassis out for cabinet restoration, but today i had it playing for around 1/2 an hourwhile taking care of the cabinet.
The radio starts to fade slowly, and not only the tubes, but the chassis gets
hot! I think i have traced it to the 2uF cap, as 1- it was hot as hell, 2- it has a metal tab connected to the chassis and that area is the hottest. B+ measured around 60V.
I couldn't measure any resistance across the cap using the DVM, but it should be culprit no?
One other thing: the cap says 2uF/220WVAC and the filter 30uFx2/150WV. What is the voltage rating i should ask when buying the caps?
Thank you so much
I bought myself an old radio from the 60s, typical AA5 circuit using miniature 7B tubes.
I've changed the smaller caps, as mandatory, but didn't change the large electrolytics as i don't hear hum on the speaker.
As this radio was meant to 110V, it uses a 2uF cap connecting rectifier plate to ground to lower the B+.
An externally hosted image should be here but it was not working when we last tested it.
I've only listened to it twice as i have the chassis out for cabinet restoration, but today i had it playing for around 1/2 an hourwhile taking care of the cabinet.
The radio starts to fade slowly, and not only the tubes, but the chassis gets
hot! I think i have traced it to the 2uF cap, as 1- it was hot as hell, 2- it has a metal tab connected to the chassis and that area is the hottest. B+ measured around 60V.I couldn't measure any resistance across the cap using the DVM, but it should be culprit no?
One other thing: the cap says 2uF/220WVAC and the filter 30uFx2/150WV. What is the voltage rating i should ask when buying the caps?
Thank you so much
are you sure that the circuit drawing is correct?
Filaments are overheated (valves are going to dead in a short time, if not already gone) and that poor 2uF cap shorting line voltage to ground dissipates about 40 watt.... obviously the chassis goes hot.
Take away the 2uF (you could use a 2nF instead, just to suppress HF interferencies), connect the filament series after the main switch and use a 240/110 step-down transformer, which will act also as a safety device. That kind of radios, connected directly to the main voltage, can be quite dangerous.
regards, Piero
Filaments are overheated (valves are going to dead in a short time, if not already gone) and that poor 2uF cap shorting line voltage to ground dissipates about 40 watt.... obviously the chassis goes hot.
Take away the 2uF (you could use a 2nF instead, just to suppress HF interferencies), connect the filament series after the main switch and use a 240/110 step-down transformer, which will act also as a safety device. That kind of radios, connected directly to the main voltage, can be quite dangerous.
regards, Piero
Can be? IS. This sort of circuit is DEADLY. The reason they aren't made like this anymore is that this arrangement is deadly dangerous, especially when run with the cabinet open. ABSOLUTELY use an isolation transformer- touching the chassis at the wrong moment will KILL. Piero, thanks for adding that warning!
Yes, the circuit is pretty much wired like that, it's just an image i picked up from the web illustrating the "basic" connection of it.
I am aware it is lethal, i replaced the isolating cap from circuit to chassis for that
Will do as suggested. For the filter caps, 220V rating is enough?
http://pw2.netcom.com/~wa2ise/radios/aa5240v.html
I am aware it is lethal, i replaced the isolating cap from circuit to chassis for that
Will do as suggested. For the filter caps, 220V rating is enough?
http://pw2.netcom.com/~wa2ise/radios/aa5240v.html
This looks to me like someone's REALLY BAD idea for powering a 120V AA5 directly off of 230V. The 2uF cap provides a series reactance (reactive divider) that is supposed to effectively drop the filament voltage to 120V. The B+ is derived from the difference between chassis ground and that cap - theoretically if the currents all balance the chassis will be at half the supply voltage. DANGER! It is unlikely to balance, and the circuit will be extremely sensitive to very small variations in the value of the cap. The losses in the 2uF cap theoretically are in the imaginary plane, but the losses from ESR and DA are not. A very large cap of just the right value would be required to make this work properly. I would find the correct schematic reconfigure it for 120V operation and for HEAVEN'S SAKE use an ISOLATION step down transformer. Do NOT use this with the usual auto-transformer, it could kill you. 
Hey, I hope this isn't thread jacking, so i'm sorry if it is
I have an AA5 radio that I jolted myself on once (i should be dead), If I use it with an Isolation transformer will It keep me from getting Jolted?
also, how do I attatch a Third prong to an AA5 radio? because it says on the back of my radio in big letters "DO NOT ATTACH A GROUND TO THIS RADIO"
I have an AA5 radio that I jolted myself on once (i should be dead), If I use it with an Isolation transformer will It keep me from getting Jolted?
also, how do I attatch a Third prong to an AA5 radio? because it says on the back of my radio in big letters "DO NOT ATTACH A GROUND TO THIS RADIO"
Every advice taken into consideration and capacitor removed. One of the paper caps i replaced connected the 35W4 pin 4(filament) leg to ground. Can i do without this one too? It doesn't seem to be doing much, unless referencing the filament to ground?
One of the hardest things will be finding an isolation transformer, as all i can find is autoformers Will probably end asking to custom wound one for me.
One of the hardest things will be finding an isolation transformer, as all i can find is autoformers
Will probably end asking to custom wound one for me.Its more source R, but you can use a pair of any identical stepdown/stepup transformers back to back.
Edit: this just gives isolation, not stepdown. It may be possible to get 2:1 stepdown if the transformers have dual secondaries. Or an autotransformer could be used in series with this. Really low performance/iron though.
Edit: this just gives isolation, not stepdown. It may be possible to get 2:1 stepdown if the transformers have dual secondaries. Or an autotransformer could be used in series with this. Really low performance/iron though.
Hi Simpleton,
You need a step down transformer! This radio must be supplied with 115 VAC. You have overheated all the filaments and stressed the supply caps. I'm going to guess your 2 uF cap is history too.
You may have guessed at all this as previous posts have mentioned these concerns.
So, to begin with, you need a step down transformer with insulated windings between primary and secondary. Look in catalogs for transformer companies. I think I saw some at Hammond as well.
Hi alexmoose,
A 1:1 isolation transformer for you will prevent shocks due to a live chassis.
-Chris
You need a step down transformer! This radio must be supplied with 115 VAC. You have overheated all the filaments and stressed the supply caps. I'm going to guess your 2 uF cap is history too.
You may have guessed at all this as previous posts have mentioned these concerns.
So, to begin with, you need a step down transformer with insulated windings between primary and secondary. Look in catalogs for transformer companies. I think I saw some at Hammond as well.
Hi alexmoose,
A 1:1 isolation transformer for you will prevent shocks due to a live chassis.
-Chris
Simpleton:
See if any of these are available in your area:
Triad N-68X Cheap made in China, I use them with good results
Hammond 185D230
Hammond 185E230
Hammond 186E120
alexmoose:
Connect the hot and neutral leads of the line cord to the transformer primary (wired for 120 volts) and connect the transformer secondary (120 volts) to the radios original power cord. Connect the ground lead on your line cord DIRECTLY to the radio chassis AND the metal frame of the isolation transformer. The original warning is nullified by the use of the isolation transformer.
In the original radio design one side of the line cord was often connected directly to the chassis. If the plug was inserted such that this happened to be the hot side there would be a lot of sparks when you grounded it. That is also why it shocked the **** out of you. The isolation transformer fixes this.
I have seen guitar amplifiers wired this way! I have an old Electrolab guitar amp that has no power transformer. The line cord is connected through a paper and wax capacitor DIRECTLY to the input jack. The cap was shorted! This amp was sitting in a thrift store for "as long as the owner could remember" until I bought it for $2. I added a transformer and changed some caps and it works. It uses the same radio tubes 35W4, 50C5, 12AU6, and 12AT6.
The Triad N-68X is available at Mouser (# 553-N68X) for $11.20. A lot cheaper than a funeral and a whole lot more fun.
See if any of these are available in your area:
Triad N-68X Cheap made in China, I use them with good results
Hammond 185D230
Hammond 185E230
Hammond 186E120
alexmoose:
Connect the hot and neutral leads of the line cord to the transformer primary (wired for 120 volts) and connect the transformer secondary (120 volts) to the radios original power cord. Connect the ground lead on your line cord DIRECTLY to the radio chassis AND the metal frame of the isolation transformer. The original warning is nullified by the use of the isolation transformer.
In the original radio design one side of the line cord was often connected directly to the chassis. If the plug was inserted such that this happened to be the hot side there would be a lot of sparks when you grounded it. That is also why it shocked the **** out of you. The isolation transformer fixes this.
I have seen guitar amplifiers wired this way! I have an old Electrolab guitar amp that has no power transformer. The line cord is connected through a paper and wax capacitor DIRECTLY to the input jack. The cap was shorted! This amp was sitting in a thrift store for "as long as the owner could remember" until I bought it for $2. I added a transformer and changed some caps and it works. It uses the same radio tubes 35W4, 50C5, 12AU6, and 12AT6.
The Triad N-68X is available at Mouser (# 553-N68X) for $11.20. A lot cheaper than a funeral and a whole lot more fun.
Hi tubelab.com,
Thank you. I knew I remembered seeing those in a Hammond catalog!
The alternatives are helpful too.
As for wiring, one side of the AC line was always connected to the chassis. They had to complete the circuit. SY's warning stands for any of these sets (I have many!) that are run without an isolation transformer.
Tubelab, Ive seen band members get one heck of a shock due to this. I have as well. I always carried a meter with me, and that was a prime reason.
-Chris
Thank you. I knew I remembered seeing those in a Hammond catalog!
The alternatives are helpful too.
As for wiring, one side of the AC line was always connected to the chassis. They had to complete the circuit. SY's warning stands for any of these sets (I have many!) that are run without an isolation transformer.
Tubelab, Ive seen band members get one heck of a shock due to this. I have as well. I always carried a meter with me, and that was a prime reason.
-Chris
HELP!
Ok, i did it big this time.
Took the 2uF cap out yesterday, and connected a new filter cap.
I managed to buy an isolation 220/110V transformer.
Switching it on, and nothing. No filaments, no B+.
What did i do wrong? If i connect rectifier plate (ac in) to circuit ground, the filaments come to life, but nothng else.
Here's what it looks like now on the attachment.
Please, help!
My head is about to explode, i can't make sense of it!
Thank you (snif)
Ok, i did it big this time.
Took the 2uF cap out yesterday, and connected a new filter cap.
I managed to buy an isolation 220/110V transformer.
Switching it on, and nothing. No filaments, no B+.
What did i do wrong? If i connect rectifier plate (ac in) to circuit ground, the filaments come to life, but nothng else.
Here's what it looks like now on the attachment.
Please, help!
My head is about to explode, i can't make sense of it!
Thank you (snif)
Attachments
I have no way to draw a diagram at this moment, but I will try an explanation.
Connect one of the leads from your new transformer (wired for 120 volts) to radio ground (this is uaually the chassis).
Connect the other transformer lead to BOTH of your original power leads. (rect plate and fils commected together)
This should work.
Connect one of the leads from your new transformer (wired for 120 volts) to radio ground (this is uaually the chassis).
Connect the other transformer lead to BOTH of your original power leads. (rect plate and fils commected together)
This should work.
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