I just finished doing a repair on my 1969 Guild Thunderbird 2x12 amp and thought I would post a quick reminder of how important it is to replace all the electrolytic capacitors in any vintage tube amplifier. Tubes are always the first thing people think of when tube amp is not preforming well but they can be an expensive and unnecessary solution. Especially in the preamp section. Vintage preamp tubes very often have a lot of life left in them. So... look to the electrolytics as well!
I know many of you are familiar with this subject but I also know more and more of people are buying these great old tube amps, not just Guild amps, and may not be familiar with how the safety and sound of these amps can be affected by deteriorating electrolytic caps. There are many great articles written on this subject as well as other posts by much more knowledgeable people than myself but I don't think it can be repeated too often. Replace ALL electrolytic capacitors in vintage tube amplifiers!
One of the best ways to protect your investment in a vintage tube amp is to as soon as possible replace all the electrolytic caps in the amp. Either yourself if you are familiar with working on high voltage equipment like tube amplifiers or have a trusted tech do the job for you. Typically techs will replace all the high voltage power supply section caps, which is great. At the same time I would have all the electrolytic preamp section bias capacitors replaced as well. They are very inexpensive and when done while the amp is apart anyway only takes a few extra minutes.
Back to my recent repair...
When I bought the Guild Thunderbird 2x12 amp I replaced all the electrolytic capacitors "can caps" in the power supply section of the amp. A previous owner had apparently replaced most but not all of the electrolytic bias capacitors in the pre amp section, but not the power supply electrolytics. Why, I have no idea. Anyway, I left it at that and did not replace the lone old electrolytic bias capacitor in the reverb section of the preamp (my mistake as you will see) and only replaced the caps in the power supply. I was fortunate enough have a decent tube tester available to then verify that the all the tubes were good, most still strong. They were dirty but just fine.
So after replacing the electroltyics I fired it up and was in Guild tube amp heaven. Until a few weeks ago. The reverb on the amp started to slowly fade in strength to the point were I had almost none. My first reaction was to check the reverb tank wiring for a loose connection, nope Then pull and test the preamp tube that handled the reverb function, nope. Now pull the chassis and look for loose connections, bad pots, and the like but nope again. I even check the reverb transformer, nope again. Then it dawned on me. I never replaced that silly little 10uF/25V electrolytic bias cap when I was in there months ago. So five minutes later after installing a new electrolytic capacitor I had a full and rich tube driven spring reverb sound again.
Lesson learned.
I know many of you are familiar with this subject but I also know more and more of people are buying these great old tube amps, not just Guild amps, and may not be familiar with how the safety and sound of these amps can be affected by deteriorating electrolytic caps. There are many great articles written on this subject as well as other posts by much more knowledgeable people than myself but I don't think it can be repeated too often. Replace ALL electrolytic capacitors in vintage tube amplifiers!
One of the best ways to protect your investment in a vintage tube amp is to as soon as possible replace all the electrolytic caps in the amp. Either yourself if you are familiar with working on high voltage equipment like tube amplifiers or have a trusted tech do the job for you. Typically techs will replace all the high voltage power supply section caps, which is great. At the same time I would have all the electrolytic preamp section bias capacitors replaced as well. They are very inexpensive and when done while the amp is apart anyway only takes a few extra minutes.
Back to my recent repair...
When I bought the Guild Thunderbird 2x12 amp I replaced all the electrolytic capacitors "can caps" in the power supply section of the amp. A previous owner had apparently replaced most but not all of the electrolytic bias capacitors in the pre amp section, but not the power supply electrolytics. Why, I have no idea. Anyway, I left it at that and did not replace the lone old electrolytic bias capacitor in the reverb section of the preamp (my mistake as you will see) and only replaced the caps in the power supply. I was fortunate enough have a decent tube tester available to then verify that the all the tubes were good, most still strong. They were dirty but just fine.
So after replacing the electroltyics I fired it up and was in Guild tube amp heaven. Until a few weeks ago. The reverb on the amp started to slowly fade in strength to the point were I had almost none. My first reaction was to check the reverb tank wiring for a loose connection, nope Then pull and test the preamp tube that handled the reverb function, nope. Now pull the chassis and look for loose connections, bad pots, and the like but nope again. I even check the reverb transformer, nope again. Then it dawned on me. I never replaced that silly little 10uF/25V electrolytic bias cap when I was in there months ago. So five minutes later after installing a new electrolytic capacitor I had a full and rich tube driven spring reverb sound again.
Lesson learned.
