Ad Widget

Collapse

Announcement

Collapse
No announcement yet.

6g2 tremolo issue

Collapse
X
Collapse
 
  • Page of 1
  • Filter
  • Time
  • Show
Clear All
new posts
Previous template Next
  • #1

    6g2 tremolo issue

    Hi!

    I finished building a 6g2 brown princeton clone today and it sounds good, and way bigger than I'd expected. I built the amp because I wanted a small amp with bias tremolo, and tremolo does throb away nice and swampy and everything is hunky dory until I hit the strings on my SG Jr. pretty hard. When I do that the tremolo stops oscilating for a second or two before it starts up again. Is this normal, or is it time to start debugging (as is most often the case with my builds)?



    Thanks!

    Chris
    Tags: None
  • #2
    Does it do this at any trem intensity or just with the trem at full intensity?

    My guess is that the higher output of the SG is driving the amp hard enough to cause some grid conduction at the power tubes. This grid conduction would cause a voltage to build at the grids and mess with the trem/bias relationship. Three possible solutions, or a combination of them, would be to change the .1uf PI coupling caps to .022uf, increase the grid stop resistors (not sure how much it would take to make a substantial difference) and it could be that your bias is off too. Check your bias current. You may need to use more -V at the bias node.
    "Take two placebos, works twice as well." Enzo

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "If you're not interested in opinions and the experience of others, why even start a thread?
    You can't just expect consent." Helmholtz

    Comment

    • #3
      It does it anywhere at the Intensity dial. I also put new tubes in V1 and V2 to see if it made any difference, but no luck. The bias voltage reads around -35 volts, and the tremolo problem is not affected by change in the bias current within the range of the bias pot. So I guess my next move is to change the PI coupling caps and the increase the 1500k grid stopper resistors.

      Comment

      • #4
        Can you tell us what the sweep of the bias pot is please (in Ma) and the plate voltage at the 6V6s. Negative grid voltage alone tells us nothing about how the amp is biased.

        Add some 470ohm, or slightly larger, screen grid resistors whilst you are there too.

        Do you have any issues when the tremolo is turned off (not just turned down on the pots)?

        The new tubes you treid in V1, were they still 12AX7, maybe try a 5751 or a 12AY, it might cope with the SG's pick ups better than a 12AX7.

        Post some picks if you can.

        Comment

        • #5
          http://www.flickr.com/photos/donmedium/5805869608/The bias sweeps from 17mA/414v to 39mA/379v.

          There is actually another issue with the amp; sometimes when I turn the tremolo of with a footswitch there is a noise like a nasty oscillating fart that doesn't stop until I stomp the footswitch a few times. I tried an other footswitch, but the same thing happens.
          Also, I plugged a Tele into the 6g2, and while the problem remains, it is less pronounced. Maybe because of the Teles lower output. I haven't tried other tubes like 12ay7 since I don't have any. I did try a 6sl7 in the auxillary octal preamp socket, but that didn't change anything concerning the trem issue.

          I'm a bit confused about where I should put the 470 ohm screen grid resistors in this build. Please advice!

          Here is a link to a cuople of pics, I'm unable to embed them in the post. It's is a parts bin special with homemade chassis and cab.
          http://www.flickr.com/photos/donmedium/5805871348/

          Comment

          • #6
            I'd start out with the idle current around 25mA (set with trem turned off).

            If you have an issue with the trem turned off, how do you know that the issue is specifically trem related, or just a btproduct of a deeper issue that is exacerbated by the trem? Eliminate the issue whentthe trem is off. I can't see your pics for some reason, check grounds are solid & that pots & jacks are secure.

            The 470 ohm resistors are typically fitted between pins 6 & 4, pin 6 is just a mounting point the screen supply voltage is applied here.

            Comment

            • #7
              OK, got the pic now...you need to shorten &/or shield your grid wires, the octal preamp socket with all the extended wires to the noval socket is less than ideal.

              Comment

              • #8
                The problems are solved. I tried lots of stuff; first I increased the value of the screen stop resistors and lowered the PI cuopling caps to .022 uf. I also added a 470 ohm grid stop resistors at the power tubes. But problems remained. Out of desperation I then pulled the caps in the trem circuit and replaced them, and that cured the failing trem. Don't now if it was a bad cap or a bad soldering job, both are likely explanations, the last the most likely... As for the nasty fart-sound when I switched of the tremolo, it occured to my senile brain that I did the tube swapping when I first discovered the failing trem, but that was before I'd even plugged in a footswitch into the trem jack and found that I had another problem. So I swapped the shiny new chinese 12ax7 in V2 with a well-used tube from an other amp, and the nasty sound was gone. Seems the tube couldn't handle trem and Pi duties all at once. The tube works fine in any other applications, but apparently it can't handle V2 in a 6g2.
                Well, it's been a learing experience and I really appreciate the input and advice I've received here in the forum during the debugging process. Thanks!

                Comment

                Previous template Next
                Working...
                X