First amp build completed

Discussion in 'Amp Tech Center' started by 8urchevy, Feb 14, 2019.

  1. 8urchevy

    8urchevy Tele-Meister

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    I was able to redo my ground scheme per your advice and realized my 6.3v tap didn't have a center tap. I created an artificial center tap on the one of the power tube sockets. Between the two I should have a quiet amp now.

    My wife and son crashed in the living room by where I do my work so I wasn't able to turn in it on to check the plate voltages. I am hoping that if they are off they will coincide with a different rectifier tube voltage drop. Be a lot easier to correct the voltage that way. lol I wonder if I was getting parasitic oscillations due to the ground scheme? Maybe that played into my tone issue. Regardless tomorrow voltages will be checked.

    Thank you all for your help.
     
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  2. Luthier Vandros

    Luthier Vandros Tele-Meister Silver Supporter

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    The artificial ct for the heaters alone is going to make a significant difference in background noise! Can’t wait to get an update on your results from the grounding scheme. I’m not sure that’s going to impact the tone itself, however. Good work!
     
  3. Lynxtrap

    Lynxtrap Tele-Holic

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    The Bassman circuit does not have a lot of gain in the preamp. Without a master volume, the output tubes are the first to go into overdrive.
    With a post PI master volume turned down, you will have to turn up the channel volume pretty high, and the main overdrive you will hear is from the PI. That is not always a very pleasant sound.

    Also, the "type 3" master volume is the simple crossline with just a pot between the lines from the PI, right?
    Not everyone likes it, and you never really see them in amps with negative feedback such as the Bassman.
    It's better suited to open loop circuits such as Vox (including Matchless and Trainwreck).
     
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  4. 8urchevy

    8urchevy Tele-Meister

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    I removed the negative feedback and I added the second half of V1 into the circuit. I built it with one channel one input. However, I did put the second half of V1 on a switch.
     
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  5. 8urchevy

    8urchevy Tele-Meister

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    The hum is significantly better. Like as in just a single coil guitar type noise, but is present without anything in the input. I will have to redo my input jack as I believe the ground isn't as good as it needs to be. It is one of those switched jacks so it should be dead silent if all is right I presume.

    Here are the voltages and they don't add up right to me. Particularly the volts going into the OT center tap. Trying to figure out where I went wrong and or how to correct the voltages.

    V1 189 193 voltage at cap can 285 332 366
    V2 168 287 cathode bias voltage drop 11.57
    V3 218 218
    V4 357 If I did calculations correctly I am only dissipating 7.6 watts per V5 355 tube
    OT center tap 371
    recifier B+ 373


    Thanks for your help everyone. I didn't get to play it with the corrected ground scheme yet, but I assume by these measurements it will still sound like a turd.
     
  6. Luthier Vandros

    Luthier Vandros Tele-Meister Silver Supporter

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    I’m tempted to tell you to put the negative feedback on a pot. No NFB can make an amp very noisy and create harsh overdrive. I have no experience with tweaking that on a tweed circuit, but that’s been my exp with blackface circuits.

    You’ll also need to get some known good tubes in there! It’s going to be hard to dial out buzz, hum, and tone in general if the things are on their way out. Sometimes it’s very tough to troubleshoot an amp that’s been modded straight away.

    I saw your new post with the voltages. They look odd at first glance, but will try to have a peek tonight. The best you can do is open up a 5f6a schematic and compare your voltages point for point.

    Cheers!
     
    Last edited: Feb 19, 2019
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  7. 8urchevy

    8urchevy Tele-Meister

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    I have a pot available for that. Was for presence but seemed useless so I disconnected it. I will hook it up for negative feedback. The output tubes are 6l6gc and can handle 30w plate dissipation. I am currently at just under 8. I will need to order the appropriate resistor to bring that up. Big factor in my tone issue I imagine. Thank you so much.
     
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  8. Luthier Vandros

    Luthier Vandros Tele-Meister Silver Supporter

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    Last edited: Feb 20, 2019
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  9. 8urchevy

    8urchevy Tele-Meister

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    They are original to the PA. The sockets labeling matches with the tubes . they are all Mullards and one rca from 1961. I am going to triple check my measurements tomorrow to ensure I used the correct procedure to calculate the bias. I used rob's calculator on his site . Hard to believe they are sitting at 8w. If that is true I imagine I will have a great surprise when they get biased properly .
     
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  10. Luthier Vandros

    Luthier Vandros Tele-Meister Silver Supporter

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    Interesting. The original tubes on your amps schem are some oddballs that are def 6l6 compatible. Maybe someone swapped em out a long time ago.
     
  11. 8urchevy

    8urchevy Tele-Meister

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    Don't think so. The labeling on the chassis matches and it appears to unaltered factory stamping. The tube compliment it came with is what provoked the Bassman build . I got the amp for $75 . Couldn't believe it .lol
     
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  12. Luthier Vandros

    Luthier Vandros Tele-Meister Silver Supporter

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    I’ll be darned. Maybe I linked the wrong schematics earlier on.

    Good deal. That would have provoked me, too. Then again, finding a spare tube socket in my parts bin spurred on my last build haha.

    You should try a 5e3 build at some point. A fantastic amp that goes from clean to mean with your guitar’s volume knob!
     
  13. 8urchevy

    8urchevy Tele-Meister

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    Funny you mention it. I have most of what I need for that build right now. Missing tubes and sockets is all. lol
     
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  14. Luthier Vandros

    Luthier Vandros Tele-Meister Silver Supporter

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    DO IT. Don’t you have like 27 unused sockets from all those PA channels? :lol:
     
  15. 8urchevy

    8urchevy Tele-Meister

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    Lol .actually another 12ax7 and a 7247

    Thought about adding tremolo and reverb. Plenty of room, but I probably won't add tremolo since I wouldn't use it
     
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  16. peteb

    peteb Friend of Leo's

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    Those are usable numbers. Bias around 27%. Should get that higher.

    How did you measure current? You are getting about 21 mA per tube? Not too too low.



    What about that 12 v cathode bias voltage? Which stage? That should not be.
     
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  17. 8urchevy

    8urchevy Tele-Meister

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    Ok so I have got the amp biased correctly and the tubes are well matched as in off by .5 watt. I looked over my plate voltages and compared them to the 5f6a schematic to determine the differences. The measurements are on the plates of preamp tubes.

    Actual readings schematic actual difference to schmatic
    V1 189 193 150 150 +39 +43
    V2 168 287 180 180 -12 +100
    V3 218 218 230 235 -12 -17

    My plan to get the voltages more in line are to replace the value of their plate resistors per schematic to the following.

    V1 120k 120k
    V2 N/C 140k
    V3 N/C N/C

    Do these value changes look to be good to get within 20% of the schematic values?

    Thank you
     
  18. peteb

    peteb Friend of Leo's

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    Good!


    What did that do to the sound?








    I think your over voltages on your pre amps tubes are probably ok.



    If you want to adjust your preamp plate voltages to the schematic, I think it is more correct to change the resistors in your power supply. Changing the plate resistors will also affect the signal, but the power supply resistors will not.
     
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  19. 8urchevy

    8urchevy Tele-Meister

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    Unfortunately, I haven't played it yet. Wife has a migraine so that is a no go today. I am going to remove the resistors from the offending plates and check their values out of circuit. I swear I checked every component prior to putting them in, but I guess it is possible I may have gotten inpatient at times during the build. I agree with you about which resistors to change, but if you look at the voltages further up in these posts they are low coming out of the rectifier tube, through all points of the filter caps and high at the plates of the preamp tubes. Well three plates anyway.

    I will play it in the morning and determine if I am happy with it and leave it alone since the voltages are well within the tubes capability of handling.
     
  20. Luthier Vandros

    Luthier Vandros Tele-Meister Silver Supporter

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    I’m with peteb - stoked, as well.

    You should notice a big difference in tone. Tweaking from here will be cake. Do you happen to have a decade box?
     
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