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Amp repair where do I start

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  • Originally posted by Helmholtz View Post
    No sense measuring power with master at 3. Must be fully up.
    Agreed.​

    Originally posted by LoveChet View Post
    My modified deluxe reverb is working against dummy load. I get 10 watts out with treble and volume set to 5 and master volume at 3.
    It's time to review the input previously posted in this thread. You should find it very helpful because several of the items you are posting about are repetitive and are in conflict with past advice.

    Comment


    • I came into this as a stranger to using a scope and what to expect from various circuits. It didn't help for sure that I was reading the scope incorrectly and seeing 10 times less than was actual and didn't figure that out until almost 300 posts. And yes, you told me to check the speaker rms with a multi meter which I didn't however at that time my 6v6 were totally bad so yes it might have got me to where I am now earlier. I'm not defending myself and you guys know way more than me. However, after tracing through from front to back I can see that my pp readings vary from the reissue, mainly due to me having a master volume that is ACTIVE with an additional 12AX7 and lots of gain. Its difficult adjusting for max signal and no distortion and trying to match reissue. Again, my goal was to find why the amp didnt work not to find how many watts I get. To that end I found one short, one open, and bad 6v6's.

      AND right now after having getting 10 watts out of it, it doesn't work again so I need to go back in and trace.

      However, i did take your advice by keeping the fx loop out of the equaion and to plug fx input to fx output with guitar cord and now with doing that i get whole lot of gain with the additionaln 12AX7.

      Comment


      • And, per your suggestion I will see what results I get having the master volume at max but because that circuit is active I'm sure that means I will have to back off on the main volume control to keep things undistorted. In practical terms I know that having a.master volume lets you push the preamp more to achieve a tone you like with higher knob settings in that stage, and then be able to control a reasonable volume with the master volume.

        Comment


        • Any master volume limits power output at lower settings.
          From your drawing the master volume is passive.
          As your preamp is heavily modded it has a gain structure different from the Fender RI, so preamp signal levels are not comparable.
          The only signals that should be comparable are from PI input to speaker output.

          And, per your suggestion I will see what results I get having the master volume at max but because that circuit is active I'm sure that means I will have to back off on the main volume control to keep things undistorted.
          It's much better to lower the input signal instead.
          Last edited by Helmholtz; 04-12-2025, 06:26 PM.
          - Own Opinions Only -

          Comment


          • Post #81 shows a schematic of master volume. It uses v5B as a gain stage so it is active.

            Comment


            • Having a gain stage before and after a volume pot is normal and doesn't make it "active".
              Active volume control would the require the pot to be in an NFB path.
              - Own Opinions Only -

              Comment


              • Originally posted by LoveChet View Post
                Post #81 shows a schematic of master volume. It uses v5B as a gain stage so it is active.
                The way to think about that circuit is that the modified amp has an added gain stage and an added master volume pot. The master volume duties performed by the pot are indeed passive. If you turn turn that master volume down, it just reduces the amplitude of the signal driving the power amp section. If one of the previous stages is already distorting, then turning the master volume pot down just reduces the amplitude of the distorted signal. It does not "clean up the output of the amp" unless the power section was also being driven into distortion. That's why we do power testing using a setup the drives the power section with a clean signal.

                Comment


                • Question !
                  With .25v pp amp input I have 150 v pp clean sine at v2B plate by having treble at 10 and volume at 8. But at pin 7 of V5B grid it gets reduced to 10 v pp but very clipped at top. If I bring the volume down to 2 I can get clean sine at pin 7 V5B of 3v pp. The only thing between V2B plate and V5B grid is a 22 meg resistor and a 220 k resistor to ground. I can see why the amplitude gets drastically reduced but I don't see why it's clipped. Does it have anything to do with V5B conducting?

                  Comment


                  • The signal gets clipped at the grid when the grid starts to draw current. This is normal. Turn down your volume control or the input signal until you get the max unclipped signal out on your dummy load. Remember to keep your master volume set at max.

                    BTW, rule of thumb, the closest to "flat" a typical FMV (Fender/Marshall/Vox) tonestack will be is with bass and treble turned to minimum, and mid turned to max. This is typically the easiest way to set up an amp for testing power output short of feeding a test signal directly into the phase inverter.

                    Comment


                    • As said before, I recommend to turn the channel volume fully up and lower the input signal instead.
                      This should minimize distortion.
                      - Own Opinions Only -

                      Comment


                      • Thanks GR. So I was correct that grid drawing current caused the clipping. GR here also !

                        Comment


                        • Final result for my working modified deluxe reverb silverface.

                          24 clean watts out.
                          Master volume10,
                          Volume at 2,
                          Treble at 10
                          fx loop with in connected to out, in and out gain pots set around 3.

                          Fixes:
                          Bad 6V6s
                          V4B mix unsoldered wire
                          Fx loop pot unsoldered wire.

                          The only remaining thing to check is how the speaker sound since it might have been damaged when amp fell over.

                          Thanks all. I learned at lot. Reading scope off x10 spot, not x1 spot.

                          RMS = p-p/2.8

                          Watts = rms volts x rms volts / 8

                          Comment


                          • Originally posted by LoveChet View Post
                            Final result for my working modified deluxe reverb silverface.
                            24 clean watts out....
                            Nice summary and I'm glad to hear of the great outcome.
                            I hope the final sound check with a speaker is good.
                            Cheers,
                            Tom
                            Last edited by Tom Phillips; 04-13-2025, 08:47 PM.

                            Comment


                            • Now if I can get granddaughters boyfriend to make me a new 1 inch fingertip on my left hand middle finger with his digital printing machine I should be able to play a Chet tune again. As I mentioned in a previous post, it was Paul Yandell and Chet Atkins who put me onto getting my silverface modified for an FX loop and mstr volume!

                              Cheers back

                              Comment


                              • Originally posted by LoveChet View Post
                                I have been reading the scope wrong by looking at X1 setting instead of x10 setting. What I thought was 30 mv pp input was actually .3 vol pp input.
                                That puts things in a whole new perspective. I'm glad you got the amp figured out!
                                "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

                                Comment

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