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Thread: amp not outputting full wattage

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    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??

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    Supporting Member Randall's Avatar
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    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

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    It's weird, because it WAS working fine.....

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    Meter battery good?

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    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?

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    Lifetime Member Enzo's Avatar
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    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.

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    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.

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    Lifetime Member Enzo's Avatar
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    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.

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    Old Timer J M Fahey's Avatar
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    Quote 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.

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    Last edited by J M Fahey; 08-08-2018 at 10:08 AM.
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    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.

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    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. 

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    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...

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    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.

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    Last edited by Helmholtz; 08-08-2018 at 11:37 PM.
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    Bent Member Chuck H's Avatar
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    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.?.

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    Last edited by Chuck H; 08-09-2018 at 01:47 AM.
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    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.

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    Senior Member nevetslab's Avatar
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    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.

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    Last edited by nevetslab; 08-14-2018 at 07:06 PM.
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    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?

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    Check that your line voltage is holding at 117V inside the amp while the amp is running at full power.
    The schematic shows 410V B+ to be correct, so 80W or so may be all you can expect to get for clean power output.
    Attached Files Attached Files

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    Thanks g for providing the schematic. Note that the bias on John's amp is icy cold compared to the schem.?. Ordinarily I wouldn't expect this to affect output power, but at -13 volts colder I don't know. The only other anomaly would be that John's amp shows the screens at 1V higher than the plates where the schem shows the screens at 15V higher. Regardless of the small B+ difference overall I wonder about that too. Could the OT be compromised with shorted winds? It seems unlikely that both sides would be shorted evenly. And, of course, all the major manufacturers absolutely DID fudge the actual wattage of their products favoring the high side.

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    Indeed,G1, thanks for the amp schematic. Chuck, I'm not surprised to see you say that Fender fudges the wattage output to the high side. I think a lot of amp companies do that. I also suspected the output transformer, so substituted with a known good one with the same results. Further, I did a pure resistance measurement of the output transformer, and both sides read identical. I feel confident the transformer is OK. OK, I'll look into the line voltage at the amp during full output, and will also investigate the screen versus plate voltages. Thanks for those tips. I'll report back soon with those results.

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    Did some more work on the Fender super bassman. Today, the lines voltage at the amp is 120v. This is with the amp running at full power with output waveform slightly distorted. I also decided to measure voltages with no signal as well as full output. All voltages drop at full output, which is no surprise. The schematic indicates such. Today I am getting 15vac at the output at full power with slight distortion and clipping. Doing the math = 56watts into 4 ohms. This is definitely low for a 100 watt amp. I understand that full 100 watt output is not expected, but 56? I would say this is low, especially considering today we have 120vac lines input. I am still at a loss what to do with this amp. Any further help?

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    Supporting Member Randall's Avatar
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    This would be a great time to have a 4 ohm 100 watt OT on hand to clip in to see if yours is functioning properly or not.

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    It's weird, because it WAS working fine.....

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    re: the B52

    Quote Originally Posted by johnhoef View Post
    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.
    Did you confirm the output wattage after the cleaning? "loudness" is too subjective.

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    I'm pretty dumb about a lot of this stuff but I have to ask if anyone has actually measured 100 watt at the speaker with an amp that is running 4 6l6's? With a max anode dissipation of 30 watt per tube That gives you 120 watts total..My old rule of thumb has been that output power is actually about 60% of the total. I know that Fender, Marshall and other say 100 watts and I have never tried to measure output power but I doubt the 100 watts with 4 6l6's same as I doubt the 60 watts from two EL34's.

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    Have we checked that all tubes are conducting? An amp will work with an open screen resistor, but you'll be short the associated tube.

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    Have you scoped the grid one's of the 6L6's to check that the clipping is not happening prior to the final stage?

    Having a cold bias of -56.7V means the Gm of the 6L6's is lower and that means a bigger voltage swing on those grids for full output. This begs the question: what is the bias current set to? Do you see crossover distortion on the output?

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    Thanks so much for your thoughts. I have another unit on the bench at the moment, so haven't yet gotten back to the bassman.
    Randall, Thanks for your idea, but have already tried that. No change. I do indeed have a spare output transformer here that I use for this very purpose.
    Chuck H, The cleaning was with the B52 which has been returned to the customer. On this bassman, I did do a cleaning of all controls, switches, closed circuit jacks, and tube sockets. No change.
    Mac Dillard, Indeed, I wonder the same thing. Several thoughts have been written here including drops in line voltage, and over specifying the power output by the manufacturer. What I have learned is that we can not expect 100 watts out of these amps.
    The Dude, Indeed, all four 6L6s are new, and the driver 12ax7, I have swapped out with several known good tubes, no change.
    Nickb, Good thought. That has not occurred to me to check the amount of drive necessary to get full output. Perhaps my next step is to build a simple adjustable bias circuit, and experiment with that. And no, there is no crossover distortion.

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    Bent Member Chuck H's Avatar
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    Do understand that the 100W rating is fudged by the MFG for good reason. Tube amps can peak well beyond what most SS amps will do and well beyond what the MFG rated them at! In the wash most tube guitar amps rated at 100W are notably LOUDER than equivalent SS counterparts. Some with peaks over 200W. Suffice to say that amplifying a sine wave can be a very different thing than playing a set with rests, breaks, dynamics, etc. (musical power) I'd put most 50W tube amps up against any 100W SS amp in a live, playing scenario any time.

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    Fender declares max output at 10% distortion and that is pretty squashed waveform

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    Quote Originally Posted by alexradium View Post
    Fender declares max output at 10% distortion and that is pretty squashed waveform
    For this amp? I've seen Fender spec at 5% but not 10% distortion. Is that what they are using now?

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  29. #29
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    Hello, fellow techs,
    I'm finally back, with the Fender Bassman on the bench. The first thing I did was install my tube extender to check bias. The extender places a 1 ohm resistor in series with the cathode of the 6L6. The bias was a very cold 12ma! Such was the case with all four tubes. I decided to place a 10k pot between the negative supply and the center tap of the bias centering pot. Contrary to the schematic, there was no 3.3k resistor there. Setting the bias is now a very easy task, and I set it to 45ma, which is in the "warm" vicinity. This did not effect the total wattage output of the amp. The low output persists. Checking the line voltage today, it's 115volts. I also checked the input to the phase inverter at the .002mfd capacitor, thinking that maybe the sine wave was being chopped there....no, sine wave stayed pure well into square waves on the output. Thought I would play around with the bias, now that I have the pot there, while observing the slight square waves at the output. No change as far as trying to increase the output amplitude. If I crank up the volume, and squash the output to almost square waves, I can get around 70 watts, but really, this doesn't seem right to me. I realize Fender specs these amps with a high distortion rating, but still, practically square waves? Is there still something wrong with this amp????

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  30. #30
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    By the way, thought I would mention, there is a 12AX7 for the phase inverter. The schematic calls for a 12AT7. My understanding is that the 12AX7 has more gain, so this should not be a factor?

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  31. #31
    Supporting Member mozz's Avatar
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    I have seen 12at7 help the output tubes put out more wattage more than once. How is your screen voltage compared to the plate voltage?

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    Supporting Member Jazz P Bass's Avatar
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    You set each tube to 45ma's quiescent current?

    That is way too high for one tube.
    How about shooting for 25 or so.

    I have seen these amps do 80 watts pure clean.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Jazz P Bass View Post
    You set each tube to 45ma's quiescent current?

    That is way too high for one tube.
    How about shooting for 25 or so.

    I have seen these amps do 80 watts pure clean.
    Great to have an actual number I can only assume they don't make a true hundred watts because most "100 watt" amps don't.

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    "Never bet your life on somebody else doing their job." SoulFetish's good friend

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

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

    "Back to the amp. It makes horrible sounds when I play my guitar thru it... because I suck at playing guitar." Mike6158

  34. #34
    Senior Member nevetslab's Avatar
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    What frequency are you testing at? OH, you mentioned testing at 400Hz early on. The lower you go below, say, 50Hz, the power output starts rolling off some....in part from the O/T beginning to saturate, in part from now being below the charging frequency of the power supply. But still, 80W out into a known load is typical What IS your load resistance? OH, you did state that earlier, having your load in 1 ohm/100W steps up to 8 ohms. 4 ohms?

    I agree with mozz with regards to the driver tube. The 12AT7 should be installed in place of the 12AX7. More for the plate current ability than the gain factor.

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    Last edited by nevetslab; 11-05-2018 at 01:44 AM.
    Logic is an organized way of going wrong with confidence

  35. #35
    Old Timer J M Fahey's Avatar
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    Please retest with higher and lower load impedance.
    If 4 ohm nominal try 2 and 8 ohm and post results.

    My point being I want to know what´s limiting your power: lack of adequate voltage or adequate current.

    Another separate experiment: remeasure power into the nominal load but *without* NFB, I want to make the Power amp "easier to drive".

    All tests still with 400 Hz signal or thereabouts and just surpassing clipping on both peaks, usually one clips before the other.

    Allow for a little clearly flat area on top and bottom peaks, as Enzo says , this is a Guitar amp, not an Audiophile one and even less a NASA thingie.

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    Juan Manuel Fahey

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