5f6a build needs help

Discussion in 'Shock Brother's DIY Amps' started by fwwymore, Aug 3, 2019.

  1. fwwymore

    fwwymore TDPRI Member

    Posts:
    18
    Joined:
    Sep 15, 2018
    Location:
    San Francisco
    This build is a bit different from the standard 5f6a. The amp is in a tweed-super size chassis, and will fit into a 2x10 tweed-super cab - to reduce the size and weight. The circuit has a few mods: 1 normal input jack and 1 bright input jack, the low input jack circuit connection are jumped to the high input connection, a master volume, an adjustable bias and an IEC power jack. The amp works but with very low volume - even with the volume controls turned all the way up and the sound I do get is very distorted. So far, I've tried:
    1) The speaker works fine with another amp
    2) Inspected carefully for shorts or other bogus stuff.
    3) Probed with a chop stick for loose/bad connections
    4) Measured the voltage on all tube pins - seemed reasonable. A voltage chart follows.
    5) Used my (homemade) signal tracer to trace the input signal. Volume seemed good but
    had become very distorted going into the power tubes

    Any ideas? Please help. Thanks - fw

    Voltage Chart:
    1 2 3 4 5 6 7 8 9
    V1 158 2.5 156 2.5
    V2 176 1.1 182 176 177
    V3 247 25 242 0 41.7
    V4 .02 418 417 -46.2 416 .02 X
    V5 .02 417 415 -46 417 .02 X

    Pix:

    2277.jpg 2278.jpg 2279.jpg 2280.jpg 2281.jpg 2282.jpg 2283.jpg 2284.jpg 2285.jpg 2286.jpg
     
  2. fwwymore

    fwwymore TDPRI Member

    Posts:
    18
    Joined:
    Sep 15, 2018
    Location:
    San Francisco
    Evidently, the auto formatter removed critical spaces. Here is the voltage chart again.
    V1

    pin 1:158

    pin 2:

    pin 3:2.5

    pin 6:156

    pin 7:

    pin 8:2.5

    V2

    pin 1:176

    pin 2:

    pin 3:1.11

    pin 6:182

    pin 7:176

    pin 8:177

    V3

    pin 1:247

    pin 2:25

    pin 3:41.8

    pin 6:242

    pin 7:0

    pin 8:41.7

    V4

    pin 1:.02

    pin 3:418

    pin 4:417

    pin 5:-46.2

    pin 6:416

    pin 8:.02

    V5

    pin 1:.02

    pin 3:417

    pin 4:415

    pin 5:-46

    pin 6:417

    pin 8:.02
     
  3. corliss1

    corliss1 Friend of Leo's

    Posts:
    3,425
    Joined:
    Sep 13, 2008
    Location:
    Lansing, MI
    At what point did your signal tracer either lose volume or get distorted?
     
  4. fwwymore

    fwwymore TDPRI Member

    Posts:
    18
    Joined:
    Sep 15, 2018
    Location:
    San Francisco
    I'm getting a strong signal right up to the output jack - though quite distorted at that point. The sound/signal is quite weak coming out of the speaker. This caused me suspect the speaker so I tested it with another amp - sounds good. The distortion seems to start in the phase inverter area. Confusing...
     
  5. corliss1

    corliss1 Friend of Leo's

    Posts:
    3,425
    Joined:
    Sep 13, 2008
    Location:
    Lansing, MI
    Half of your phase inverter seems wrong, which would result in major distortion. I'd expect to see the same voltage on pin 2 as I would on pin 7, give or take. What does the audio on those two pins of the PI sound like?
     
    D'tar and dan40 like this.
  6. dan40

    dan40 Tele-Afflicted

    Posts:
    1,341
    Joined:
    Aug 19, 2015
    Location:
    Richmond Va
    A very common mistake made in this circuit (and Marshall style also) is using a 470k resistor instead of the proper 470 ohm in the phase inverter. The main symptom is a normal but very quiet sounding amp. The resistor I'm referring to is located between the two 1 megs on the board in the PI section.
     
    D'tar likes this.
  7. fwwymore

    fwwymore TDPRI Member

    Posts:
    18
    Joined:
    Sep 15, 2018
    Location:
    San Francisco
    V3 pin 7 measured 27 volts this time.
     
  8. fwwymore

    fwwymore TDPRI Member

    Posts:
    18
    Joined:
    Sep 15, 2018
    Location:
    San Francisco
    The middle resistor flanked by a 1 Meg resistor on either side measures 468 ohms - yellow, purple, brown.
     
    dan40 likes this.
  9. D'tar

    D'tar Friend of Leo's

    Posts:
    2,208
    Joined:
    Jan 11, 2013
    Location:
    WNY
    Can you explain the speaker/output jack wiring? On the unused jack the green and black wires and the white. And on my phone I cant tell if the shunt is connected to anything.

    Are your V3 voltages stable now?
     
  10. fwwymore

    fwwymore TDPRI Member

    Posts:
    18
    Joined:
    Sep 15, 2018
    Location:
    San Francisco
    The V3 voltages appear to be stable. I had suspected the output jack wiring - there may be a short there somewhere. I took apart the jack and rewired it as simply as possible - white to tip, black to sleeve - ground, yellow to tip - negative feedback. I left the shunt unconnected. I didn't wire the other jack.
    No change in the output.
     
  11. D'tar

    D'tar Friend of Leo's

    Posts:
    2,208
    Joined:
    Jan 11, 2013
    Location:
    WNY
    Looking at the schematic and your voltages, v2b plate v seems quite low. Can you confirm that. It should be equal to B+4 node. I'm guessing a typo since the rest of the preamp numbers look good.

    So what master volume are you using?
     
    Wally likes this.
  12. Bob Arbogast

    Bob Arbogast Tele-Afflicted

    Posts:
    1,591
    Joined:
    Mar 16, 2003
    Location:
    Lowell, Michigan USA
    It looks like your extension speaker jack is shorting the signal to ground. The tab for the shorting contact on that jack should NOT be connected to the ground tab on the jack!
     
    Snfoilhat likes this.
  13. fwwymore

    fwwymore TDPRI Member

    Posts:
    18
    Joined:
    Sep 15, 2018
    Location:
    San Francisco
  14. fwwymore

    fwwymore TDPRI Member

    Posts:
    18
    Joined:
    Sep 15, 2018
    Location:
    San Francisco
    Bob Arobogast, you get the gold star! Even after I overhauled the output jack, I had mistakenly left the tip wire in place. The shunt on the extension jack was causing the signal loss - I should have known better. Big thanks to all.

    On to the next problem: with the master volume all the way up (or most of the way up) I get a loud, low frequency continuous sound - even with the preamp volume(s) all the down. Any ideas why this might be happening?
     
  15. D'tar

    D'tar Friend of Leo's

    Posts:
    2,208
    Joined:
    Jan 11, 2013
    Location:
    WNY
    Bob, I think he has rewired the outputs and only has one wire up.

    Fwwymore please confirm.

    That MV is easy enough. Cant really see it in the photos. You may like to disconnect it until you have quality sound out. Also try with nfb disconnected.

    Your voltages all appear inline as expected. May like to get more detail with your signal tracer or a Multimeter and measure the signal as it gets amplified through the circuit.

    Quick check on your PI.... with nfb disconnected (although voltages imply correct values)measure ohmz, each grid of V3 to its cathode then measure cathode to ground. Sweep the presence pot while checking cathode to ground. Should not change your reading. Good time to reverify nfb r value as well.
     
    Last edited: Aug 4, 2019
    Snfoilhat likes this.
  16. Snfoilhat

    Snfoilhat Tele-Afflicted

    Age:
    38
    Posts:
    1,119
    Joined:
    Apr 8, 2016
    Location:
    Oakland, CA
    Sometimes an ordinary old negative/positive swap in the feedback loop manifests not as shrieking highs but as a lower frequency sound. If breaking the global negative feedback loop by lifting a part of it makes the sound disappear, then swap the OT secondaries and reconnect the NFB.
     
    D'tar likes this.
  17. fwwymore

    fwwymore TDPRI Member

    Posts:
    18
    Joined:
    Sep 15, 2018
    Location:
    San Francisco
    I disconnected the presence pot. This resulted in a very high pitched whistle which would change frequency as the changed the volume. The NFB resistor measures 26k - red purple orange.

    I disconnected the master volume which resulted in the very loud, low frequency noise ALL the time.
    Ideas?
     
  18. fwwymore

    fwwymore TDPRI Member

    Posts:
    18
    Joined:
    Sep 15, 2018
    Location:
    San Francisco
    Oh yeah, with the presence pot connected (and the MV connected) and turned way down, this produced a very high pitched whistling. The whistling would go away as I turned the presence pot up. With the MV disconnected, I get the loud low frequency noise all the time.
     
  19. D'tar

    D'tar Friend of Leo's

    Posts:
    2,208
    Joined:
    Jan 11, 2013
    Location:
    WNY
    Fwwymore, my last post got out of order, you posted before I finished typing and was more o your original issue.

    Disconnecting the presence pot will add issues. Try disconnecting the nfb feed from the jack tip or some other convenient spot without disrupting the rest of the phase inverter tail.

    If its more convenient you can simply swap the power tube plate connections and retry
     
  20. Snfoilhat

    Snfoilhat Tele-Afflicted

    Age:
    38
    Posts:
    1,119
    Joined:
    Apr 8, 2016
    Location:
    Oakland, CA
    Remove the power tubes, desolder the OT secondaries (typically blue and brown, but your colors may vary) from pin 3 of each socket, and swap them. Resolder, retest.


    Presence is a filter for the highs in a working negative feedback loop. And maser volume affects the entirety of the NFB. Neither sounds like a root issue, but each may have been masking the underlying problem of positive feedback.

    Edit: doing what D'tar suggested first is probably best, for thoroughness' sake. What I'm suggesting would come next depending on what that test uncovers.
     
    D'tar likes this.
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.


  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.