5f2a low volume, different plate voltages V1a vs V1b

Discussion in 'Amp Tech Center' started by Jonp, May 23, 2019.

  1. Jonp

    Jonp TDPRI Member

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    Hello people,
    I have here a 5F2a clone (these chinese clones), which produces a volume about half of what it should be.
    By connecting different parts of the tone path between this amp and another 5F2a I have located the issue somewhere between the grid of V1b, and the grid of the 6V6.
    I think these components are of influence, am i correct?
    -1,4k V1b cathode resistor
    -22Nf coupling cap
    -100k plate V1b plate load resistor
    -220k 6v6 grid leak resistor
    All resistors measure fine, cap is also ok (even replaced it).

    I have also replaced the 12ax7 tube to no avail.

    One strange issue seems that the plate of V1b has a higher voltage (215v) than the plate of V1a (around 150v). (I must note that in these amps, suck a low a plate voltage of 150 is normal, due to a strange chinese rectifier tube.).
    Out of the power section I get 232v, both go to a two 100k (measure closer to 85) resistors, respectively to the plate of V1a and V1b, but for some reason I get a much higher voltage at the latter.

    I am not an amp tech, but using deduction and measurement I got this far. However, I'm out of options right now.

    What should I be looking for next? Should I recheck some components? Check wires?

    THanks in advance for your kind help.
     
  2. dan40

    dan40 Tele-Afflicted Silver Supporter

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    A really bad v1 tube could cause a difference in plate voltage readings but yours has quite a bit of difference. Another thing to check would be the cathode connection for v1b. If the cathode is not properly grounded through it's cathode resistor, that half of v1 will not conduct current and the plate voltage will rise. Check the wiring and all solder joints from pin 8 all the way through to where the cathode resistor is grounded. Also be sure that the socket is making a firm connection with the pins of the preamp tube.

    Have you tried a new preamp and power tube yet to rule that out first? New tubes are always the first step in the troubleshooting plan.
     
  3. Jonp

    Jonp TDPRI Member

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    Dan, Thanks for the pointer to the cathode connection, if this is faulty it would also explain the low volume, correct? Will try it tonight.
    Yes I tried different tubes, no change.
     
  4. D'tar

    D'tar Tele-Afflicted

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    +1 with Dan on your cathode connection. Easiest way to confirm,IMO, is to disconnect the nfb tap from the circuit then measure v1p8 to ground. While youre at it measure v1p7 to ground as well. Rotate the volume knob also while taking readings and note any changes on either pin to ground. Pictures may help if your issue does not become obvious during this check. Ive seen a few new builds with pin 7 and 8 on v1 mixed up when routed under the board.
    Best of luck. Be safe. Have fun!
     
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  5. Jonp

    Jonp TDPRI Member

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    Dan was right. Resolderd all the connections from cathode to ground, and now I have a decent amount of volume, and plate voltage on V1b is the same as V1a!
    Pin 7 and 8 where connected right D'tar, but thanks for the suggestion.

    Thanks a million Dan and D'tar!


    Now I'm faced with a noise (not hum of hiss, more buzz). My first guess would be somewhere in the input, and only now became audible enough to be annoying, since there is now more amplification of the signal.

    So i'm going to check what happens if I disconnect the input, and start from there.

    Keep you posted.
     
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  6. Wally

    Wally Telefied Ad Free Member

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    Armchair quarterback observation... a complete voltage chart would have pinpointed this particular problem.
     
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  7. Jonp

    Jonp TDPRI Member

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    Hi wally, apologies I'm still a bit new to this. What does a voltage chart consist off?
    Voltages on all the pins of all the tubes?

    Still have the buzz btw. The input was very badly soldered, so I started with resoldering that, but that didn't solve the problem.

    Tried to disconnect the wire from the input to the grid stop resistor (just after the 1M grid leak resistor) to check whether the buzz was coming from the input, but then I still get buzz, plus motorboating on higher volumes.

    If I can provide input (photo, sound, measurements,...) that can be of help, let me know/

    Thanks again guys.
     
    Last edited: May 24, 2019
  8. Jonp

    Jonp TDPRI Member

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    Hi, went ahead and made a video, with the buzz and zooms on the inside of the amp. Maybe this helps.

    I know wiring is a bit of a mess, I have another amp, the same one but with a larger speaker, but circuit is identical (I checked), and that one is silent, although wiring is also a mess.

    But I don't want to imply that wiring in this one couldn't be a problem.

    I tried to film the whole inside, maybe you see something.

    https://photos.app.goo.gl/PqmqiBW31sm3MteP7
     
  9. Jonp

    Jonp TDPRI Member

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    Forget to tell: I have also chopsticked the wires, moved everything around, but no improvement.
    I have found a similar thread here with the same kind of buzz. Turned out to be a grounding issue. So I'm going to double check all ground connections.

    http://www.tdpri.com/threads/5f2a-noise.497770/

    Keep you posted.
     
  10. Jonp

    Jonp TDPRI Member

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    Ok already found two issues. Heater wires etc. are not twisted. Also the power plugs brown and blue wires are mixed up. Brown goes to the switch, blue goes to the fuse!
    First I need to fix that.
     
  11. Wally

    Wally Telefied Ad Free Member

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    Hello, Jon....no apologies necessary. As noted earlier, the plate voltage and the cathode voltage are related to each other. That high plate voltage indicated a probable problem. A check of the cathode voltage would have shown the bad connection there. When at a tube socket with a volt meter when trying to find a problem, one old want to check all voltages there...not just the plate. One can make a chart or just make mental notes, but P8/V1 would have been notable. If a person were curious, one could lift the connection that was bad and take a reading of that pin 8. Then, resolder the connection and look at that pin again with the meter.
    Here you will find an example of a voltage chart for a tube circuit....
    https://www.google.com/search?q=vol...8&hl=en-us&client=safari#imgrc=CvLpKaIehxnH3M:


    Re: Ac power supply. I am not an expert in EU electronic codes, but as I understand it both the brown neutral and the blue line are supposed to be switched. The line..hot...should be fused......I don’t know if EU requires that the neutral be fused????? I don’t think so, but I don’t know. And, hey, ‘tis is a build, right? Line/Hot Blue through a fuse through the switch to the primary and brown neutral direct to the primary????
     
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  12. dan40

    dan40 Tele-Afflicted Silver Supporter

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    Does the buzz sound happen with a guitar plugged into the input jack or with nothing plugged in?
     
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  13. Silverface

    Silverface Poster Extraordinaire Ad Free Member

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    I have a an immediate concern - kit, purchased amp or whatever - do you understand the safety rules for working inside an amplifier? Do you know how to discharge filter capacitors?

    As Wally noted - is this a kit? If so, please post:

    The schematic

    The wiring diagram

    One or more photos (not video) of the chassis. Videos are too difficult to see detail in and refer to later.

    Look at the schematic and wiring digram for the 5F2a - then search using Google for some kits and typical voltages. That will help guide you as far as what needs to be on a voltage chart.

    But if it's a kit there should be test voltages provided so you can verify proper operation. If you know to check plate voltages for the tubes do you not know how/where to check other voltages in the circuit? You mention not being an amp tech - how much electronics knowledge do you have? Do you understand the basics or voltage, resistance, current, impedance, signal path?

    Another point - late in the thread you mention the filament wires were not twisted. This is standard in every tube amplifier. If it is a kit were there not directions to twist those - and even without instructions to do so, am I right in assuming you didn't know it was necessary?

    These questions are why I'm asking for the schematic, wiring digram and wiring images - it sounds as if the questions you asked at the very start of the thread may have been far over your level of expertise, and we can help you far more by seeing what has/has not been done and what tools you had to work with.

    This isn't uncommon. Unfortunately (at least in my opinion) many people undertake "assembly" or kit amplifiers without the basic electronics knowledge...or worse yet, the safety requirements...necessary to build one that's trouble-free and can operate properly and safely.

    Accordingly, questions get asked about plate voltage issues but basics like twisting or filament wires were neglected, and (probably, form the description ) the main power supply was wired in violation of basic electronics (not just amplifier construction) safety rules.

    I'm not being critical - I just want to help ensure you end up with something completely functional - and safe.
     
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  14. Jonp

    Jonp TDPRI Member

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    Thanks for your concern. It's not a kit I built myself, but an amp I aqcuired via a trade. So I didn't forget to twist wires or connected the power incorrect myself :).
    I'm no expert, but I think I can honestly say I know the basics.
    If I service an amp, I unplug it and drain the caps. Or I use a light bulb current limiter when working on a live circuit. And use the "one hand in pocket" rule,...
    Also, I have been working with guitar electronics for around 15 years, built dozens of pedal kits, modded or repaired about 15 tube amps, watched and understood most of Uncle Doughs youtube video's, read/studied some amp books and generally read everything I can find online about tube amps (especially forums like these, which are little goldmines).

    But it's good that you ask. Above is not to be defensive, but to mitigate your concent. Don't worry, I am cautious and not the overconfident type :).

    There is no officiel schematic, but I've traced the circuit out myself. Only, I am terrible are writing and drawing, so apologies for that. Let's see of I can post it here.
     
    Last edited: May 25, 2019
  15. Jonp

    Jonp TDPRI Member

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    Last edited: May 25, 2019
  16. Jonp

    Jonp TDPRI Member

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    Anyhow, first I will twist the heater wires, see if that solves it, and if not I'll come back.
     
  17. Wally

    Wally Telefied Ad Free Member

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  18. Paul G.

    Paul G. Friend of Leo's

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    Not to mention all those Fender amps that only have one filament wire and send the other leg of the circuit through the chassis.
     
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  19. D'tar

    D'tar Tele-Afflicted

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    Looking at the drawing, this is essentially what is going on here. I would lose the heater ground at V1 and install an artificial center tap at the pilot light or elevated tap at one of the power tubes.
     
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