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Thread: 5F2A Tweed Princeton Assistance

  1. #1
    Senior Member
    Join Date
    May 2011
    Out West, Canada

    5F2A Tweed Princeton Assistance

    H folks, this week I have been working on a 5F2A clone that I built a few years ago. I recently did some work. I replaced a few resistors that had been overheated when originally installed. They had signs of cracking and I found them chirping when I did a chopstick test. I replaced with same values. I also added some Zener diodes to get the voltage down to vintage schematic values. That all seemed to work as I am at a steady 364V B+ now.

    When I fire up the amp now, the amp makes noise with a guitar but it's quiet. It almost sounds like the OT signal is grounded or something. I have been racking my brain for hours over this symptom. Trying different things to no avail. I have got myself worked stupid over it and need to take a break and a fresh new look after a rest.

    I've tried swapping in known good tubes.
    Lifted board several times to look underneath and inspect & check wiring
    Checked and rechecked all wiring many, many times.
    I replaced 2 coupling caps and the 2 cathode caps.
    Checked all my contacts for grounds or not as required.
    I have tried different speakers and cords and guitars to eliminate those variables
    Chopstick entire circuit when on, it's all nice and quiet
    I have switchable NFB. I hear a little crackle when I switch it on and off. Same as before. I also removed NFB completely to eliminate that option. No improvement.
    I lifted board and tested amp. To make sure nothing under the board was touching ground. No improvement.
    Tone and volume knobs work as they should.

    Just the amp is quiet. It breaks up, as I crank the volume, like it used to. It just puts out what sounds like about a 1/4 of a watt of power.

    Schematic and voltage checks are next. think I need a break for a bit.
    Any tips on getting to the bottom of this are appreciated.
    Thanks. Keith

  2. #2
    Supporting Member
    Join Date
    Dec 2009
    Checking your DC voltages are correct is a good idea and it's usually my starting point. Check the plate (around 195v) and cathode voltages (1.4v) on V1a/V1b, as well as the screen (340v) , plate (350v) and cathode (20v) voltages on the 6V6 socket then post them back here.

    Which resistors did you replace, and where did you install the Zener dropper?

  3. #3
    g1 is offline
    don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Canada, somewhere north of Fargo
    I don't think you mentioned it, but I assume you verified the speaker? And bypassed any spkr or ext. spkr jacks if installed?
    Certified Dotard

  4. #4
    Senior Member
    Join Date
    May 2011
    Out West, Canada
    Hi folks thanks for responding. This 5F2A got set on the back burner for a little bit. I will get back to it soon.
    A friend asked me to look at his 1978 Twin Reverb. I have it on the bench now, bumping ahead of my 5F2A. I have some Q's about the Twin. I will post a new thread. Then get back to this thread later. - K
    g1 likes this.

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