I am working on a Magnecord 1048 Tube Reel to Reel. It has a tube preamp stage which include a 7025 and a 12AT7. The issue is that no sound coming out of 12AT7 pin 3 (the output). If I manually feed audio to the input of 12AT7 (Pin 7), it is all good. But if I feed audio to the input of 7025 (pin 7), then I get nothing even if I turn the volume to the max.
After some test, I noticed that the Filament voltage of the 7025 (between Pin 4 and pin 5) is only around 4v. So I adjusted the variable resistor to increase the 7025 filament voltage to around 6v. Now, there is hum coming out of the 12AT7 output and it is increaseing/decreasing with the turning of volume control. So I think the 7025 still has some issue. Any suggestions?
After some test, I noticed that the Filament voltage of the 7025 (between Pin 4 and pin 5) is only around 4v. So I adjusted the variable resistor to increase the 7025 filament voltage to around 6v. Now, there is hum coming out of the 12AT7 output and it is increaseing/decreasing with the turning of volume control. So I think the 7025 still has some issue. Any suggestions?
Only 4V on a 6,3V heater suggests there is something wrong with the tubes heater or adjustable resistor (unless someone fiddled with it and set it to such an unusual low voltage). There can be some advantage by running a input tube at lower heatervoltge but with a limit. 4V seems to be below that. Do you have an other 7025, or equivalent, to try out? What are the dc voltages across R46, R49, C26? Measure the resistance between the heater and the cathodes of the cold (pulled!) 7025, there should be very high resistance, if any. Try if tabbing the tube makes any difference.
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Try cleaning the 25R adjustable resistor in the filament circuit. The wiper may be dirty,
or it may be misadjusted.
or it may be misadjusted.
The 7025 filaments are in series for 12 volt operation from the looks of it. a tube is dragging it down as you should see 12 volts on pins 4&5. Something is either bad in the adjustable resistor or the tubes. Pull the tubes and measure the voltage..
Also the two filament capacitors could be bad, which would lower the voltage as well.
Measure the DC voltage on each capacitor.
If the first capacitor were open, the DC voltage would drop by around half.
Measure the DC voltage on each capacitor.
If the first capacitor were open, the DC voltage would drop by around half.
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Good point, did not cross my mind it could be 12V since he said he lifted from 4 to 6V. 6V is not nearly enough, must be much closer to 12V, propably at least 10V for proper operaation.
Is this guy old enough to use selenium rectifiers? These are big structures with fins. Is so, they need to be replaced for safety reasons, and they degrade over time and may be causing your voltage issue. Replace with modern silicon rectifiers.
All good fortune,
Chris
All good fortune,
Chris
Hi, Yes. The bridge rectifier is sqaure in shape like 4 thick papers stuck together. I think that is what you are talking about. For replacement, do I simply replace it with a morden single bridge rectifier or I have to replace it with 4 diodes?
Yes, a modern bridge is ideal. And also, as rayma has mentioned, replace the old electrolytics and clean the pot. Caig D5 works great, just like on carbon track pots.
All good fortune,
Chris
All good fortune,
Chris
The problem may be a little of everything, and the parts are not expensive.
I'd up the caps to around 2200uF each, as long as you're replacing them.
I've even used 10,000uF each in a circuit like this, with higher current diodes.
Otherwise, 1N4005 is fine, or the usual bridge type.
I'd up the caps to around 2200uF each, as long as you're replacing them.
I've even used 10,000uF each in a circuit like this, with higher current diodes.
Otherwise, 1N4005 is fine, or the usual bridge type.
I've been using F5 instead, but I'll look into D5. I find it works very well though. I've fixed many crackling / bad pots with it.
Thanks.
EDIT: It looks like F5 is a more advanced product judging by the description and price 🙂
While trying all the test, I suddenly found something unexpected. The C77 1000uf filter capacitor was replaced with a 8200uf capacitor. I knew that someone before me replaced many capacitors but did not expect this. This capacitor is inside a hole and usually not visible so I did not pay much attention.
Ok if you replace the selenium stack with a power bridge. But I'd not use the 1N400x series diodes with 8,200uF.
Maybe the 1N5400 series, but a bridge module is easier and can be mounted in the same hole.