fender reverb pot acting like drive pot? Solved! Fender installed incorrect capacitor!

Discussion in 'Amp Tech Center' started by texwest, Sep 4, 2020.

  1. Jon Snell

    Jon Snell Tele-Holic

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    If you inspect a Fender valve amp that is north of 15years old, look at the electrolytics carefully. If there are signs of leakage on or near the positive lead , with double ended types, it is time to replace them. Snap fit and single ended capacitors always show signs at the business end. Some say they are swelling when the plastic cap on the base (the non business end) swells but that is due to the plastic ageing, nothing more sinister.
    Visually inspecting coupling capacitors is more difficult. They all look good even when they leak, electrically.
    The old school way to check capacitors is to use an AVO model 8 on high Ohms range and check for leakage. This sometimes doesn't work though as the meter only uses a 15volt battery on high Ohms.
    A DVM is no good at all.
    A valve tester is nice to have but very expensive unless you can warrant the expense of course and then an accurate one is hard to find. My AVO 160 will read the beta values as well as current flow for a desired bias voltage etc and is very accurate for balancing sets of output valves.
    The Blues Junior, for instance had a bad batch of electrolytics and they physically leak after a few years. Then again one gets to know these things with the experience of time.

    To find an incorrect value component is one of the most difficult things with regards to stock amplifiers, you have very observant eyes.
    Best regards and keep up the good work.
     
  2. BobbyZ

    BobbyZ Doctor of Teleocity

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    Checking the cap can in a 1965 Princeton is easy. Pull it out and look at it closely whilst tossing it the trash. Put in a new one.
    The one in my 64 was an Astron. Amp didn't hum and all the electrolytics looked like new. In fact the only issue seemed to be that the CTS 10"alnico from 1971 it had was getting weak.
    I hated to change anything, it was that clean, but with past experience with E-caps that age I did um all.
    That "weak" speaker was brought back to life in the processes, still in it today.
    You can test the things eight ways from Sunday with 9 volt testers but the only real test is substituting a new one onto the circuit. There are testers that check them at operating voltages but almost no one has those. And like tubes and women, good looks don't mean you can trust um.
    Running old E caps in a Princeton, especially a blackface, is gambling with an already hard working power transformer.
     
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  3. texwest

    texwest TDPRI Member

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    I've been working on my Princetons for a long time and I have taken to not using cap cans at all and using F&T's hard wired inside the amp. It's much cheaper and I figure it's a more high quality capacitor.

    Here's a pic of the amp I just finished. It sounds absolutely killer now. Someone pulled all the blue caps in it before me! They had OD 715's in it, so I replaced those with the Sozo's. It's the first time I used the blue sozos and this amp sounds so good I might use them again. I installed the new bias circuit and all the new caps and resistors.

    On this amp I used TAD caps and the OD's are 6ps which I have found sound really good in the PI. I also have source at the university near me where I can get NOS carbon comp resistors so I can update the ones that are out of spec.

    The burn marks around the filter caps are from the last tech who installed a new can cap back in 1998.




    IMG_20200905_141739223.jpg
     
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  4. BobbyZ

    BobbyZ Doctor of Teleocity

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    He use a blow torch?
    I use a 200 watt iron and the C&E caps, zero problems with them in old Fenders. "Off brand" amps I might do individual caps, depends on the amp in question though. If a can is available I'll use it, not always the case unfortunately. Just seems to me individual caps often end up looking like a hack job, unless you fit them to a board or tag strips and that takes drilling.
     
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