Announcement

Collapse
No announcement yet.

amp not outputting full wattage

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • amp not outputting full wattage

    Hi everyone, I just now registered, so totally new to this forum. I run an electronics repair shop here in Portland Maine, and currently have two amps with the same problem, and with both I am stumped as to what the fix is. One of the amps is the B-52 AT-100, and the other is a Fender super bassman. Both amps use four 6l6s, that should produce 100 watts output. Not the case! I can drive both amps to the verge of clipping into an 8 ohm load, with the resultant output being around 50 watts. Reducing the load to 4 ohms, resultant output is less than 50 watts. All four 6l6s are brand new, as well as the driving 12ax7. All voltages are correct, including bias. All resistors in the power amp section check good, and darned if I can find any bad components. I am observing the output with my scope, and have my fluke multimeter in AC mode measuring voltage output. E squared, divided by output load ohms should give me wattage output. Am I doing something wrong here? These power amp circuits are not complicated, leading me to believe that I am doing something wrong??? Anyone??

  • #2
    Welcome. I am from Portland, ME, was the amp shop guy there until 4 years ago. I'm in SW FL now. Glad to hear you are filling what I know was a void there. I miss HR Dist. on Hanover St,

    With what you describe, I would want to verify those RMS measurements with another meter. I would also do the math with what you see with your peak to peak scope measurement. (p - p /2) x .707 = RMS
    It's weird, because it WAS working fine.....

    Comment


    • #3
      Meter battery good?
      My band:- http://www.youtube.com/user/RedwingBand

      Comment


      • #4
        Agree with above, you need to verify your meter. And your load resistance.
        Have you tried other amps and got correct results using same meter and load?
        Also, what load is the Super Bassman expecting? And the B-52, you changed the impedance switch to match the loads, yet got different results?
        Originally posted by Enzo
        I have a sign in my shop that says, "Never think up reasons not to check something."


        Comment


        • #5
          Use a sine wave and no higher than 400Hz, you meter on AC ought to read that. Trying to measure music with a meter will likely skew results.
          Education is what you're left with after you have forgotten what you have learned.

          Comment


          • #6
            Thank you so much for all your responses.
            Enzo, thanks for steering me over to this area of the forum. Using a 400hz sine wave..NOT music.
            Randall, looks like I stepped in just around the time you stepped out of Portland and I'm happy to report, my new electronics biz is booming. Having trouble keeping up with the demand! Indeed, HR Distributers....Chris and Paul continue to do a good job!
            OK, I am using a fluke true RMS meter. Sticking the probes into the AC line, I get 112 volts, as expected. I felt this is sufficient to prove it's in good working order? pdf64...The battery? That hadn't occurred to me. I would think a failing battery would be more obvious, however, OK, I will replace the battery.
            G1....indeed, selecting the 4 ohm position on the B-52, and setting the dummy load to 4 ohm, I do get a lower wattage output. Indeed, I would expect close to the same measurements with 4 ohms, thus doubling the wattage output.
            Meters, scope, etc aside, I had the customer here, concerning the B-52, and he complained of a very noticable reduction in output loudness, leading me to believe my equipment is sound, and that there is indeed an issue with the amp.
            By the way, concerning the B-52, looking at the schematic, I see there is no voltage labeled for B+1, B+2, B+3, B+4. Any of you can fill me in on what these voltages should be?
            Guys, you do state a few of the obvious, however, I do appreciate that. After all I am human, and have been known to make a silly mistake now and then.

            Comment


            • #7
              100 watt amps generally run on around 500v. 485-520 seems reasonable to me. The problem is the voltage is not going to be stable. If you ever biased an amp, you know that as you change the tube currents from 35ma to 45ma, your voltage drops. ALso at 480v B+ and 120v mains, every volt your mains changes, your B+ changes by 4v. SO a 5v mains shift means a 20v B+ shift. Expecting exactly 493v or whatever is unrealistic. If in doubt, measure the HV AC, and it is simple arithmetic to determine what the B+ ought to resemble.

              Look at your B52. B+1 to B+2 is just a choke, so not much voltage drop. The remaining B+ nodes are for the small tubes. MY very crude rule of thumb is roughly 1ma per triode. Some draw more and some less, but it is close enough for expectations. I see eight triodes on B+4 and four more on B+3. That means roughly 12ma through 1k2 R137. V=IR=0.012x1200=14v across R137. Then 10k R140 has only 8 of the 12, so 8ma. V=IR=0.008x10,000=80v across R140.

              These are just ball park figures, but unless I screwed up the math, I bet I am not far off.

              One other factor is the selectable tube or solid state rectification. There will be substantially different B+ voltages depending upon that setting.
              Education is what you're left with after you have forgotten what you have learned.

              Comment


              • #8
                Originally posted by johnhoef View Post
                Sticking the probes into the AC line, I get 112 volts, as expected.
                Well, yes and no.
                112V mains is fine meaning amp will work well, last forever, etc. but maximum power output will suffer.
                USA has not had 112V "official" mains for decades.

                Fender schematics from the 60's already stated 115 or 117V, go figure.
                Current average is 120VAC and I bet the B52 schematic states that.

                (112/120)^2=0.87 so that alone turns 100W into 87W.

                G1....indeed, selecting the 4 ohm position on the B-52, and setting the dummy load to 4 ohm, I do get a lower wattage output. Indeed, I would expect close to the same measurements with 4 ohms, thus doubling the wattage output.
                Not sure what are you writing here. No "wattage doubling" at all, this is not an SS amplifier-
                Tube amps put out exact same power into different loads, as long as they match load to tap.

                WE know nothing about your load box so please describe it in detail, show the circuit and a picture or two.

                Also post B52 impedance selector.

                As a side note: tubes have no clue about speaker load, they always see 3400 or 3200 or whatever the OT transforms speaker load into.
                Each transformer tap you select must be loaded with the proper matching load, period.

                Another detail: tube amps generally do not clip symmetrical, so an unwritten convention is to rise volume until *one* peak starts clipping (I bet you stop there) , and then keep raising level until the other peak starts visibly clipping.
                Please measure there with your Fluke multimeter.

                Almost forgot: set your Variac to 120VAC for any further testing.
                Last edited by J M Fahey; 08-08-2018, 09:08 AM.
                Juan Manuel Fahey

                Comment


                • #9
                  Where is the signal being injected and what are the tone and volume control settings? Scope the input to the PI to make sure it's not clipping in the preamp before the power amp.

                  Comment


                  • #10
                    I'm throwing this info out there...as a FYI...
                    I once had a friend repairman with a similar scenario....He called me and we went over everything we could think of and nothing was making sense.... Finally when I was at his shop I took a look at his mains voltage....
                    The Fluke 87 meter shows around 120V AC RMS ....... However, when I scoped the mains it showed a different story.... The Pk-Pk was 324V , I attached the actual photo... Idealy a 120V RMS sine wave should have a 339 Pk-Pk ....
                    The power supply in these tube amps is your typical rectifier and filter caps.... These power supplies output voltages are based of the Pk input voltage, not RMS....Click image for larger version

Name:	120V_RMS.jpg
Views:	1
Size:	1.43 MB
ID:	851349
                    This waveform was heavy in Harmonics..
                    His amps B+ voltages were about 20 volts lower than normal...his amps were putting out less than expected power...
                    A few weeks later I was able to get him a BELLMAN power supply....this is a programmable AC power supply ..you can dial in the Hz and the output voltage.....
                    Once he applied clean 60Hz 120V RMS sine wave...his amps were back to running good again at full power output..
                    As for the Utilities power company .....we notified them and they could care less....
                    In the US the harmonic limits for mains power are defined in document IEEE 519-1992 ...clearly this violation of these limits......

                    His example was power output being off by roughly 20% .... However your measuring 50% off from what I can see....
                    Be careful of ground loops between scope, amp and signal generator...

                    Comment


                    • #11
                      As for the Utilities power company .....we notified them and they could care less....
                      Utilties cannot provide zero mains impedance. And they are not responsible for power distribution within buildings/facilities. As long as most equipment is not drawing sine current, the result is voltage distortion. All of the many power supplies with capacitor loaded mains side rectifiers draw their pulse currents at the same time: around the mains voltage peaks. This adds up and causes flattened voltage tops.
                      Last edited by Helmholtz; 08-08-2018, 10:37 PM.
                      - Own Opinions Only -

                      Comment


                      • #12
                        John, be aware that your Fluke meter IS giving you a RMS reading. If your formula needs to start with a peak to peak figure then you're already low. Do like Randall said and use the scope measurement OR just omit the part of your formula that converts to RMS.

                        Also, 112V mains? Now I'm questioning either your mains or your meter. I haven't actually been to your locale but as a westie I haven't seen mains below 117VAC and usually between 120VAC to as high as 127VAC. And a couple of volts low on your meter for an amp output will make for considerable loss in the calculation.

                        EDIT: I just recognized that your formula doesn't adjust to RMS. So I'm just wondering about your meter or mains. Maybe get another meter and confirm.?.
                        Last edited by Chuck H; 08-09-2018, 12:47 AM.
                        "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


                        • #13
                          Again, thank you all for your responses. You do have me wondering about my meter. I decided to again measure the mains, and this time I get 117.6vac. This makes a lot more sense, and I have a much better feeling that my meter is functioning properly. No idea why I got such a low reading last time. My dummy load set up is eight 100 watt resistors in series. I set it up this way so it's easy to select any resistance in one ohm intervals.
                          I didn't get a chance to measure the B+ voltages. I decided to do a thorough cleaning of all pots and switches and tube sockets, etc, and the amp is back to it's full loudness. The customer came over today to pick up the B52, and he was thrilled with how well it worked.
                          So, I am left with the Fender bassman, which is definitely not outputting full power. I'll put it back on the bench later, and go through the measurements, including B+ voltages, which, in this case, are labeled on the schematic.

                          Comment


                          • #14
                            I too am questioning the AC mains. It also wasn't stated whether that 112VAC reading was taken with the amp being driven to clip or under the 'static' mains reading, with whatever is on the test bench is powered up. If it was static reading, it may be dropping even lower when the amp is driven to full power. Many of us have dedicated AC mains circuits just for amp under test, with metered variacs to maintain 120VAC, and being able to read current/wattage being drawn by the equipment under test.

                            All of us will see slight variations of line voltage over the course of the day/evening, as everyone is drawing from the grid. Just in my shop, I've a vintage Weston Line Voltage meter...one of the nice wooden boxed units, with a 6" mirrored scale that monitors the one of the branch circuits that feeds test gear, as well as my desk computer and coffee pot. I'll see a 2-3 volt drop in voltage when the coffee pot clicks in to re-heat the burner briefly.

                            It could be your AC mains at the bench is 'soft', having more line drop than expected.

                            Also, as Enzo pointed out, you would want to be seeing visible clip on both top and bottom halves of your waveform. On Fender Twin Reverbs, just at the onset of clipping with a 4 ohm load, I won't be seeing 100W...more like 90W or a tad under. Usually 100W is around 5% THD, or with a bit of the peaks squared off.

                            If your line voltage is low at this measurement, that could account for some of the error. How accurate is your dummy load? An assumed 8 ohm load measuring 8.5 ohms instead of 8 ohms will only yield 94.2W instead of 100W.

                            Using your scope for voltage measurements....is it accurate? Usually there is a calibration source on the panel so you can trim your X10 probe's compensation with a 1khz Square Wave, and is a 'known' pk-pk value stated at that connector. You can verify or trim the sensitivity of the scope channel with the Variable control if it's NOT measuring the prescribed pk-pk value on the screen.

                            I'd expect your Fluke True RMS DMM is probably at least accurate within 2%. You can always look up it's spec if you don't have the owner's manual, where it will state it. Having more than one accurate instrument helps a lot in trying to sort out low readings that aren't making sense. In lieu of that, one is usually limited to DC-measurements, such as known resistor value, say 1% resistors. That won't quite help telling you the accuracy of the RMS function, but.....it will tell you the scale accuracy of the meter's ranges. Fresh batteries also provide a reference. 1.5V and 9V batteries, brand new/unused are NOT those values, but a bit higher. 1.56V, 9.6V are closer to the true value....battery data sheets will yield a bit more accuracy than this, but they can help qualify your relative meter accuracy.
                            Last edited by nevetslab; 08-14-2018, 06:06 PM.
                            Logic is an organized way of going wrong with confidence

                            Comment


                            • #15
                              OK, I'm finally back, and have the Fender Super Bassman on the bench. I am measuring at best, and this is with slight overdriving so some distortion, around 70 watts RMS into 4 ohms. On the back of the amp, it says 100watts. So, wondering if this is normal output, and Fender simply overrates their amps, or is there some problem? As discussed above, my meter shows 117.6 volts when measuring line voltage, so I believe the meter is reliable (Fluke). Measuring voltages, I find around 410 volts on the 6L6 plates pin 3, 0v on the cathodes in 8 since they are grounded. Bias voltage on pin 5 is negative 56.7v. The grid pin 4 = 411v. Any thoughts as to what could be the problem (if any). Incidentally, looking at the bias circuitry, I see an adjustment only for balancing the bias, NOT changing it. Seems to me, wouldn't it be batter to provide means for VARYING the bias voltage?

                              Comment

                              Working...
                              X