While this amp looks like it’s lived through a bomb, there’s some beautiful tones to be had in these when they are working correctly.
This is the pre op photo of the main board. All original, but as we’ll find out, some of these components were not functioning correctly. The tubes were loose,and socket connections were intermittent. Re-tensioning and cleaning of the sockets took care of that issue!
Sometime in the past, the power transformer had been replaced. The voltage specs were right on but the mounting system was not. One bracket had cracked and the rest were very shaky. This is the heaviest component of the amp, and it’s important that it stay where it’s supposed to!
Original output transformer
Serial number detail
This ground connection wasn’t good at all. While it was holding alright, there was some loose wire and quite a bit of movement. A hot soldering Iron took care of that!
These are the main B+ filter caps for the amplifier. All showed signs of leaking, and the one in the center was noticeably compromised. (leaking electrolytic) These ALWAYS need to be discharged before you touch them as they store very high DC voltage which can kill you. Close to 400v in this particular amplifier.
Here’s a picture with the main filter caps and power supply resistors removed
I replaced the old electrolytics with a high quality German F&T capacitor. I chose to use a slightly higher UF rating for the main filters as these are readily available, and will not only tighten up the Low end on this amp, but give it a slightly firmer response when you really dig in and drive it.
Testing Coupling capacitors. The meter is an insulation tester. It puts a large DC voltage through the capacitor and then tests for any leakage. A brand new capacitor will show a reading above 200 megohms which shows the capacitor is doing it’s job properly. all the caps in this amps showed between 8-13 megohms which was much too great. even just a few volts of leaking DC can completely throw off the bias in the whole amplifier, and cause it to lack power, and sound terrible. It’s a shame, because these are fantastic sounding capacitors when they are working properly.
So out they come! I chose to use mallory 150’s for the coupling capacitor replacement. I also chose to replace the cathode bypass capacitors (larger orange ones in the picture above) While these were technically still preforming pretty much within spec, It’s a gamble to leave 50 year old capacitors in an amp. In talking with the owner of the amp, we agreed it would be best to replace these as well while I was in there. I replaced them with Japanese 25uf caps that the owner had supplied for use.
The completed board. While I had all the capacitors out I tested the resistors to make sure they were still within tolerance. Many times these old carbon comp resistors drift much higher than their intended value, especially in the areas of the amplifier that are under the stress of higher voltages. This amp however passed this test with flying colors. No drifts at all!
The tone capacitor was also leaking. In this section of the amp, when the capacitor leaks DC, it manifests itself as a “crackling” sound when you turn the tone control pot. Many times, people will mistake this for a dirty pot, but obviously, cleaning the pot will have no effect on the crackle in this case
I was fortunate enough to have a vintage AJAX blue molded that I had salvaged out of a similarly aged Conn organ. This cap tested as new with no issues, and made a perfect replacement. After the cap was replaced, the crackling sound in the control cleared right up.
I manufactured some mounts out of a much thicker gauge steel for the sloppy Power transformer. Now it’s solid as a rock
I replaced the worn out handle as well with an OEM replacement. See how much the tweed has changed color over the years
Tube chart dates the amp to being manufactured in December of ’54
This speaker is not original to the amp, although it is a fender driver. Made by oxford in the 15th week of 1965
It was very loose in the front baffle so I secured it properly. If a speaker is loose, or not mounted correctly, the sound of the amp can suffer greatly.
Thanks for checking out this repair!