Can someone tell me what the tubes are in a Heathkit model A7 amp which has a 12J5 as the 2nd tube in the signal path? Thank you, Chuck.
Looks like someone substituted a different tube.
http://www.kta-hifi.net/projects/amp_page/a7amp/a7.htm
The tube complement is one 12SL7 (phono version only), one 12SQ7 utilized as a tone control amplifier, a 12SN7 as a voltage amplifier,
and cathodyne phase inverter, and a pair of cathode biased 12A6/VT134A beam power pentodes in an ultra-linear connection.
Output power is estimated at about 7 watts, and although the original output tubes are metal, the glass 12A6GT can be substituted.
Tube voltages
I found the correct schematic for my Heathkit A7C amplifier. It uses a pentode (12SJ7 or12SH7) as the preamp, a12J5, a 12SL7, and 2 12A6's, with a 5Y3 rectifier. The schematic does not show pin voltages for these tubes. Does someone have the voltage information for my amp? Chuck
I found the correct schematic for my Heathkit A7C amplifier. It uses a pentode (12SJ7 or12SH7) as the preamp, a12J5, a 12SL7, and 2 12A6's, with a 5Y3 rectifier. The schematic does not show pin voltages for these tubes. Does someone have the voltage information for my amp? Chuck
Why do you want to know the voltages?, valve amps are crude and simple with the voltages really pretty obvious.
Is it faulty?.
tube voltages
The amp is working to a point, but I can't believe that there is only about 20 volts on the plate of some of the triodes. I just want to compare what I have to what the voltages should be. Chuck
The amp is working to a point, but I can't believe that there is only about 20 volts on the plate of some of the triodes. I just want to compare what I have to what the voltages should be. Chuck
That can't be right. There must be a power supply problem, likely the filter capacitor.
If the capacitor just after the rectifier is open, there will be high hum and low B+ voltage.
Also check the rectifier tube.
According to the schematic, the filter capacitor voltages should be 390V, 360V, and 180V.
Also check the resistor values, these can drift a lot.
One of the most common faults on valve amplifiers is the anode(plate) loads going high or O/C - 20V sounds too low, check the values of the load resistors.
Assuming you find them high, then replace them all - and I would suggest metal film 1W resistors, and they will never give problems again 😀
multi-value capacitor
Thank you folks, for your help. I would like to replace the filter capacitor; are large can multiple value capacitors still available? I would need a 40-30-10 mfd @ 450 volt chassis mounted aluminum can capacitor. Where can I get one? Chuck
Thank you folks, for your help. I would like to replace the filter capacitor; are large can multiple value capacitors still available? I would need a 40-30-10 mfd @ 450 volt chassis mounted aluminum can capacitor. Where can I get one? Chuck
Are you using this as a guitar amp? If not it really ought to be in tubes / valves - let me know and I will move it.
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