Hi All,
I picked this up from my repair man and took it to another repair man and he said..... its in fine shape. Just find the pre amp and it will work.
So, I have searched hi and lo...... does anyone know where to locate off amps and pre amps besides the obvious.
I am searching for a Waveforms inc. C-5 Pre Amplifier. The power amp is Model a-20. Sometimes the combo is called A-20-5.
Thanks.. Ill find a pic
I picked this up from my repair man and took it to another repair man and he said..... its in fine shape. Just find the pre amp and it will work.
So, I have searched hi and lo...... does anyone know where to locate off amps and pre amps besides the obvious.
I am searching for a Waveforms inc. C-5 Pre Amplifier. The power amp is Model a-20. Sometimes the combo is called A-20-5.
Thanks.. Ill find a pic
Oooops, I did not read all the posts before I posted this:
My original post: One 6.3V filament winding is rated at 1 Amp. It probably is for powering the external preamp's 2 12AX7 tubes, 0.6A total.
The other 6.3A filament winding is rated at 4 Amps. It probably is for powering the 2 6L6GA/5881 and the onboard 12AX7 tube. 0.9A, 0.9A, and 0.3A = 2.1A. Your picture shows 2 KT66 output tubes (1.3A + 1.3A filaments = 2.6A) + 0.3 for the 12AX7 = 2.9A total, should be OK for using KT66s.
But you may have to adjust the bias to the KT66s to the same cathode current as was intended for the original 6L6GA/5881 that would have been in the amplifier. (there is no fixed bias, and no fixed bias potentiometer). You would have to change the cathode self bias resistor to get the current right.
You might look for another technician, if he does not understand the power transformer specifications, including the exact Stancore Corp. part number. But before you purchase a new power transformer, be sure that the Output Transformer is in good working order. If the output transformer is also bad, you will be spending a lot of money, and still have to replace other parts, such as Coupling Caps, Electrolytics, etc.
My post add on's:
It should be a nice amp if you replace the old coupling caps and electrolytics, and get new matched output tubes, and new rectifier and 12AX7 if necessary. The real test of the old output tubes is in the amplifier, not a simple tube tester. Use a DMM and some 10 Ohm plate resistors to see how much plate current there is, and how well they match. But as always, be super careful with powered amplifiers. The amp may not even have bleeder resistors to discharge the electrolytics after the power is turned off, and the power cord is unplugged. Get a technician to check, and install some bleeder resistors if there are none there. Even then, it can take a minute or two to get the voltage discharged to a safe value.
Then, even if you can not get the preamp, you can use a CD player and a Potentiometer (Passive) Volume control. You should be able to listen to the amp that way . . . enjoy!
My original post: One 6.3V filament winding is rated at 1 Amp. It probably is for powering the external preamp's 2 12AX7 tubes, 0.6A total.
The other 6.3A filament winding is rated at 4 Amps. It probably is for powering the 2 6L6GA/5881 and the onboard 12AX7 tube. 0.9A, 0.9A, and 0.3A = 2.1A. Your picture shows 2 KT66 output tubes (1.3A + 1.3A filaments = 2.6A) + 0.3 for the 12AX7 = 2.9A total, should be OK for using KT66s.
But you may have to adjust the bias to the KT66s to the same cathode current as was intended for the original 6L6GA/5881 that would have been in the amplifier. (there is no fixed bias, and no fixed bias potentiometer). You would have to change the cathode self bias resistor to get the current right.
You might look for another technician, if he does not understand the power transformer specifications, including the exact Stancore Corp. part number. But before you purchase a new power transformer, be sure that the Output Transformer is in good working order. If the output transformer is also bad, you will be spending a lot of money, and still have to replace other parts, such as Coupling Caps, Electrolytics, etc.
My post add on's:
It should be a nice amp if you replace the old coupling caps and electrolytics, and get new matched output tubes, and new rectifier and 12AX7 if necessary. The real test of the old output tubes is in the amplifier, not a simple tube tester. Use a DMM and some 10 Ohm plate resistors to see how much plate current there is, and how well they match. But as always, be super careful with powered amplifiers. The amp may not even have bleeder resistors to discharge the electrolytics after the power is turned off, and the power cord is unplugged. Get a technician to check, and install some bleeder resistors if there are none there. Even then, it can take a minute or two to get the voltage discharged to a safe value.
Then, even if you can not get the preamp, you can use a CD player and a Potentiometer (Passive) Volume control. You should be able to listen to the amp that way . . . enjoy!
Last edited:
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.