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Thread: measuring output power

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    measuring output power

    I must be missing something, I tried two different amps with a scope signal Gen and Dummy load,
    In both cases the power seemed to be about 1/2 of what it should be.

    One amp was cathode bias 5881 push pull about 387 on the plates 4K opt
    16 ohm 200W dummy load about 13.8 VAC measured clean sine wave,

    gives me about 11.9 watts?

    I would guess about twice that 23- 24 Watts would be expected as it is cathode bias

    I also measured a small 6v6 push pull amp and it came out to like 6 watts, maybe 12 is more like it.

    I am using V-squared over R for power, but it seems like it needs to be about times 2

    Sorry if this question had been asked before I did some searches.
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    Lifetime Member Enzo's Avatar
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    Did you apply your test signal directly to the phase inverter grid? or did you run through the preamp? If you ran through the preamp, you have to make sure the signal is not distorting there. If the power stage receives a distorted signal, it has no way to clean it up.
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    Old Timer J M Fahey's Avatar
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    If you want to live happy donīt read your girlīs stored messages and donīt measure Tube power amps

    Seriously now, power ratings are "optimistic" , specially with modern tubes (which are weaker) and in modern amp versions (which lower voltages precisely because modern tubes are weaker to begin with)
    In the old days Fender reached 500/530V and some Marshall reached hair rising 560V , today most are tamed to 400/420V or so under penalty of replacing lots of tubes under warranty, a losing game.

    If I measure 80/85W *clean* out of a Fender or Marshall I am more than happy, in any case thatīs LOUD.

    Only amps which regularly meet or surpass official ratings are old Ampeg (think V4 and such), 6550 equipped Marshalls . both of which are based on "stronger" tubes such as 7027 or 6550, not your run of the mill 6L6 or EL34, and MusicMan which feed their tubes 600 to 700V and cathode drive them balls to the wall and beyond.

    Yours should put out somewhat more, but nothing groundbreaking.
    Test fresher tubes, vary cathode biasing resistor, modify load up and down, you might improve things.

    EDIT: there might also be some measurement procedure error:
    such as:
    * if you are using 1kHz signal and measuring with a standard multimeter, many donīt reach that high so display less than what you actually have.
    Try 250 or 440 Hz from: http://www.mediacollege.com/audio/tone/download/

    get the 30 second MP3 and set player to Loop or Repeat 1 to have a continuous tone.

    * if you are measuring by injecting 60Hz (many do, as you have it "free" straight from a filament tap) Guitar amps usually lose power/efficiency that low.
    A Bass amp "should" play 60Hz well.
    Last edited by J M Fahey; 02-27-2018 at 03:47 PM.
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    What are you using to measure the 13.2Vac?
    Cheap meters have a limited freq range, may not cope >200Hz, and only good for low distortion sine waves, so will mis-read tube amps that don't have negative feedback. True RMS meters are preferred.
    The plate voltage at idle (eg 387Vdc) is kinda irrelevant. What matters is the Vdc between the cathode and the HT (eg at OT CT) at max signal output.

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    Quote Originally Posted by Enzo View Post
    Did you apply your test signal directly to the phase inverter grid? or did you run through the preamp? If you ran through the preamp, you have to make sure the signal is not distorting there. If the power stage receives a distorted signal, it has no way to clean it up.
    I ran the signal through the preamp but was on the load with the scope and watching for clipping

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    Quote Originally Posted by pdf64 View Post
    What are you using to measure the 13.2Vac?
    Cheap meters have a limited freq range, may not cope >200Hz, and only good for low distortion sine waves, so will mis-read tube amps that don't have negative feedback. True RMS meters are preferred.
    The plate voltage at idle (eg 387Vdc) is kinda irrelevant. What matters is the Vdc between the cathode and the HT (eg at OT CT) at max signal output.
    I was using a fluke 87 on Ac volts

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    Lifetime Member Enzo's Avatar
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    OK, you were watching the output for clipping, but where did that clipping occur? It is easy to overdrive the preamp into distortion, which will make the output appear weak.
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    Quote Originally Posted by Enzo View Post
    OK, you were watching the output for clipping, but where did that clipping occur? It is easy to overdrive the preamp into distortion, which will make the output appear weak.
    I guess I was trying to make sure the sine wave was clean or not clipped, and measure before any distortion or clipping happened.
    I think there is a flaw in my method somewhere

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    Supporting Member Jazz P Bass's Avatar
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    Try again with an 8 ohm load.
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    Lifetime Member Enzo's Avatar
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    What I am clumsily trying to say is that your tubes may well be capable of greater clean power, but your preamp stages may be limiting how large a signal can get to them before distortion occurs. You are looking for onset of distortion in the output, but I THINK you are assuming the clipping occurs there. It may or may not.

    Think of this, what if we ran a fuzz pedal before the amp, so no level of signal came out the speaker undistorted. it is real distorted no matter what because of the pedal. It would not be fair to the power tubes to claim they had zero power.
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    Old Timer Tom Phillips's Avatar
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    To support what Enzo is getting at, please tell us the amplitude and frequency of the test signal you are applying to the amp's input.
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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by amptweeker View Post
    I guess I was trying to make sure the sine wave was clean or not clipped, and measure before any distortion or clipping happened.
    I think there is a flaw in my method somewhere
    No flaw, what you measure is truth, anything else is smoke and mirrors.
    That said, amp might be improperly set, biased, loaded.
    For example, you didnīt state cathode voltage, idle current.
    Maybe 4k Zaa is too low under your conditions.
    Maybe tubes are worn.
    >>>Are you connecting 16 ohm load to 16 ohm tap?<<<<
    Less than a Month ago, a colleague in an all Tech closed group had a similar problem, he was, say, using a 16 ohm load into a 4 or 8 ohm tap and stubbornly insisted that "amp should put out full power anyway" ...... finally he properly matched load and, no surprisingly, expected power was there .
    Ok, a little less , but reasonably close.
    Please post schematic.
    Last edited by J M Fahey; 02-27-2018 at 06:03 PM.
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    I was using a 1khz sine wave I did not measure the input signal, I guess I will perhaps try again and gather more details
    I was assuming there was a "standard way to measure the output power of a tube amp, at this point all I know
    is it is a very loud cathode bias amp perhaps the loudest I have heard, I was just curious if I should call it 20W or 25W
    or even 30W, I guess it does not matter, I was just curious

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    Quote Originally Posted by amptweeker View Post
    I ran the signal through the preamp but was on the load with the scope and watching for clipping

    I guess I was trying to make sure the sine wave was clean or not clipped, and measure before any distortion or clipping happened.
    I think there is a flaw in my method somewhere
    Amplifier power output is quoted at a certain distortion level (5% or more) so you don't want a perfectly clean not clipped output. Turn it up until there's a little flat on the peaks (like the plot below which measures 5% THD) then make sure the pre-amp isn't clipping by scoping the PI input.

    The plot below is an amp similar to the one in the first post i.e. 2 x 5881 cathode biased 4K OT, B+ 375V. It measures 25W @ 5% THD.

    j30-25w.png
    Last edited by Dave H; 02-27-2018 at 08:49 PM.

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    Quote Originally Posted by Jazz P Bass View Post
    Try again with an 8 ohm load.
    Exactly. I have 4, 8, 16 ohm dummy loads, when i build from scratch or a clone, I will find 1 impedance where you get more power out.

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    Senior Member nevetslab's Avatar
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    Ditto on what Dave H has seaid and shown on his scope screen. That visual clip is typically where guitar amp manufacturers spec their power output. I assume you have enough headroom in the preamp to turn the input level down some, and make up for the level with the Volume. Tone controls set flat., or as much as one can get. (usually have a midrange 'scoop' in the response if typical 3-control tone stack. Also, if you look at, say 50Hz or lower, you'll see even more clipping.

    In comparing performance notes with one of the members working on an Ampeg SVT II , where I have one of the rental amps on the bench, it should produce 300W output. It barely makes 200W without clpping, and 225W clipping like the waveform Dave H shows above. None-the-less, the amps sound strong and loud, and we do get handed smoke and mirrors by manufacturers, assuming old tubes, weak power suply, AC mains voltage is AT specified line voltage.

    Do you have a Line Voltage meter to be sure you're at 'nominal line? Low line will ot produce as much power.
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    Quote Originally Posted by nevetslab View Post
    In comparing performance notes with one of the members working on an Ampeg SVT II , where I have one of the rental amps on the bench, it should produce 300W output. It barely makes 200W without clpping, and 225W clipping like the waveform Dave H shows above. None-the-less, the amps sound strong and loud, and we do get handed smoke and mirrors by manufacturers, assuming old tubes, weak power suply, AC mains voltage is AT specified line voltage.
    Interesting about the power output of the SVT II. I used to have an early MCI SVT from the early 80's that put out 240 watts RMS without clipping. That was still using the 12DW7 circuit rather than the 12AX7 circuit they later went to. A friend had a later MCI with the 12AX7 circuit and his made around the same power as yours, and he also had an early blue line SVT from the late 60's using the 6146 power tubes, which made 330W RMS without clipping and had a more midrangey sound. We never did measure a Magnavox era amp though....but I would suspect it was probably similar to the SVT that I had.

    Greg
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    Better Tone thru Mathematics bob p's Avatar
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    SVT are damned powerful amps. I have a pair of SVT-II Pro (one SLM one Asian) and they easily put ot 300W ... as long as the driver tubes are fresh, and depending on where I set the threshold on my distortion analyzer. As a rule of thumb, most people rate tube amps at 5% THD when measuring power.

    I haven't read every post in the thread, but if the OP is having problems with lower than expected power output, consider the following:

    1. Don't measure power/distortion at the output section based on a signal input at the front of the amp. Guitar preamps distort signals. Inject a clean signal directly into the PI and measure output at the OT secondary.

    2. Be sure you have a meter that has good frequency response. Many meters are actually pretty bad.

    3. Use a purely resistive dummy load.

    4. Be sure that you're measuring V RMS and not V P-P. The conversion factor is 0.707.

    5. Eyeballing distortion is notoriously inaccurate. If you really want an accurate measurement, a distortion analyzer really helps. For tube guitar amps most manufacturers (like Fender) rated them at 5% THD, which is probably higher than you might expect.
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    Old Timer olddawg's Avatar
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    You can also try injecting the signal at the "Main Amp In" or effects return. If you do use the front panel input pay careful attention to you tone controls, MV, various boost/cut switches, etc. I use a 2 channel tube amp on stage. The "dirty" channel really compresses the signal at the preamp. I can set it up with the clean and dirty channels having the same volume straight in. If I hit my TS clone pedal while on the clean channel it can take your face off even with the master at the same setting. But using the same pedal on the dirty channel just makes it dirtier. I've really annoyed my bass player a few times, lol! But yeah.. the preamp architecture can really mess up your measurements. Best to go straight into the power amp.

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    Senior Member nevetslab's Avatar
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    The SVT II mentioned is the older NON-Pro amp. I brought it over from our Rental inventory to aid in comparing notes with the SVT II that's over in the UK having output problems. I haven't serviced the amp since 2014 or 2015, and haven't yet made a project out of it, apart from using it as a bench-mark for the time being. You've seen the thread and the data I posted. AC mains is via variac & 100Amp Power analyzer, AC output measured on True RMS Audio Analyzer Amber 3501a that is accurate.
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    Signal IS being injected at the Power Amp input
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    Better Tone thru Mathematics bob p's Avatar
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    Hijack:

    It sounds like you've got some nice gear in Burbank. Whenever I see an SVT that seems on the low side, one of the first thing that pops up in my mind is worn out driver tubes. As far back as the II era they started using 12AU7 drivers instead of 12BH7. I can't tell you how many times I've seen a worn-out 12AU7 cripple an SVT. That tube tends to have a short life in that amp. I change mine regularly.
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    Senior Member nevetslab's Avatar
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    Quote Originally Posted by bob p View Post
    Hijack:

    It sounds like you've got some nice gear in Burbank. Whenever I see an SVT that seems on the low side, one of the first thing that pops up in my mind is worn out driver tubes. As far back as the II era they started using 12AU7 drivers instead of 12BH7. I can't tell you how many times I've seen a worn-out 12AU7 cripple an SVT. That tube tends to have a short life in that amp. I change mine regularly.
    Ah....good bit of information. We just got fresh 12AU7's in, so when I get that amp back up on th bench, I'll see how that dials in, after re-biasing. Out of curiosity, how did the 12BH7's fare as Drivers in that circuit? Their characteristics look very similar, though the 12BH7 has a bit higher dissipation capacity and peak voltage rating.
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    Better Tone thru Mathematics bob p's Avatar
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    I don't profess to be an SVT expert, I'm just a guy who has fixed a few and has figured out that they eat their 12AU7 drivers.

    The 12AU7 driver tube seems to be a very common problem. I know that they designed the first generation SVT using the 12BH7 driver and when the 12BH7 became hard to find they started replacing them with 12AU7.

    Unlike the typical preamp tubes that live unstressed lives and tend to last forever, the 12AU7 in the SVT driver position tend not to last because they live a hard life. I honestly don't know if the 12HB7 is more robust or not. Maybe the same is true for the 12BH7.

    There are other guys on this forum with lots of SVT experience, like Enzo and Leo. Maybe they could give you a better answer on the 12BH7 in that application.

    What I have figured is that the SVT operates both of those tubes near the limit of their cathode-to-heater voltage spec, which is 200V. In my II-Pro the difference is 6.3 v - (-180v) = 186.3V. That's pretty close to 200. So we know that the voltage is near the spec limit. I haven't measured the cathode current. Looking at the data sheet the 12BH7 does have a marginally higher peak cathode current spec than the 12AU7. Is that significant? I don't know.

    I've often thought about whether I'd be ahead to substitute a 12BH7 in place of the 12AU7 in my amps, now that 12BH7 are back in production. But every time that I feel the need to replace them, I never have a 12BH7 in stock and I always have a pile of 12AU7 at hand. The 12AU7 are cheap and plentiful, so I've never really worried too much about it, I just keep retubing the drivers, like it's an oil change.
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    Lifetime Member Enzo's Avatar
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    I have to say that the 12AU7s are the first thing I check when I have SVT problems. Weak ones screw up the whole system. Plus I also find amps with 12AX7s in their place. Or they have the small tubes swapping places.

    I suspect they dropped the 12BH7 because 12AU7 was in production but 12BH7 was a fringe tube.
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    Better Tone thru Mathematics bob p's Avatar
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    The 12BH7 was originally intended as a horizontal or vertical deflection amplifier in TV sets, right? Once TV sets started using SS components for those applications then the demand for the special purpose tube went away. Only a few HiFi amps needed them. I think the Big Macs, for example.
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    Quote Originally Posted by bob p View Post
    SVT are damned powerful amps. I have a pair of SVT-II Pro (one SLM one Asian) and they easily put ot 300W ... as long as the driver tubes are fresh, and depending on where I set the threshold on my distortion analyzer. As a rule of thumb, most people rate tube amps at 5% THD when measuring power.

    I haven't read every post in the thread, but if the OP is having problems with lower than expected power output, consider the following:

    1. Don't measure power/distortion at the output section based on a signal input at the front of the amp. Guitar preamps distort signals. Inject a clean signal directly into the PI and measure output at the OT secondary.

    2. Be sure you have a meter that has good frequency response. Many meters are actually pretty bad.

    3. Use a purely resistive dummy load.

    4. Be sure that you're measuring V RMS and not V P-P. The conversion factor is 0.707.

    5. Eyeballing distortion is notoriously inaccurate. If you really want an accurate measurement, a distortion analyzer really helps. For tube guitar amps most manufacturers (like Fender) rated them at 5% THD, which is probably higher than you might expect.
    What is a decent distortion analyzer that measures audio frequencies and don't cost a ton Bob? In the past when we measured the SVT's for power output we went into the power amp and brought up the waveform on the scope, but we measured it when clean instead of the 5% distortion, so that is likely why the power was 240 watts instead of the claimed 300. I've also had a Vox AC100 build that seemed to be reading lower power than you would think it should, but again, I was likely measuring it with too low distortion. A distortion meter would help a lot with accurate readings I think.

    Greg

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    Senior Member nevetslab's Avatar
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    As for affordable distortion analyzers, the HP 334A Distortion Analyzer is about the most affordable on the used market. Manually tuned, requires a separate low distortion oscillator so you don't introduce errors. A Function Generator is NOT a low distortion source. I just looked to see what' on ebay at the moment, and see some 334A's in the $100-$200 range. I personally own a Tektronics AA501 Distortion Analyzer, with the com[panion SG505-01 and SG505-05, and a couple Amber 3501a's, one on the bench, one for portable job-site use. you can find Sound Technology 1700's and 1710's ou tthere. I've never been a big fan of those, as it push-button-frequency selection, and they dont' hold up over time. I paid $175 for my first Amber 3501a, had to repair it. I bought 3 more over time, cobbled together a second instrument using the parts from the other two, so the second working instrument really cost me around $400 all said and done. Small, accurate, allows the frequency tuning analyzer to become a 1/3 oct manually tuned spectrum analyzer as well as a variable LP filter. True RMS reading, the meter uses a log converter, so the db scale is linear, which I like. There are a couple of those on ebay, but are priced at or over $1000.

    An HP 3581A Wave Analyzer is an alternative. It's a metered' spectrum analyzer, very similar to the HP 3580A which I also own. Both will display individual frequency harmonics of the fundamental. I picked up a beautiful HP 3581A for around $100, and the 3580A I think cost me $300. The Amber 3501a is my daily dirt instrument, though I'm normally using pink noise or burst pink noise as a signal source and my ears to judge what I"m hearing.
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    Thanks very much for the info. Gives me somewhere to start looking.

    Greg

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    Old Timer olddawg's Avatar
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    Back in the day manufacturers demanded that you have to have a Distortion Analyzer in shop to attain warranty status. Yamaha especially had a hard on about it. Honestly I think I used one maybe 3 times in 30 years.

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    I have decided to go back the idea that this amp puts out about 25 watts in this example
    and I have lost the drive for "accurate measurement of power.
    Thanks to all for replies

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    Sometimes I find that oddball amps run an impedance mismatch, perhaps for less distortion. The trade off is less power output than you would expect.
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    Senior Member nevetslab's Avatar
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    And, in spite of having all the gear needed to be clinical about it, I rarely bother measuring power output, let alone distortion. Normally, it it sounds 'nominal' lke all the others of the same model, and, if a tube amp, the plate current readings all make sense & the amp sounds good, I'm happy.
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    Quote Originally Posted by nevetslab View Post
    And, in spite of having all the gear needed to be clinical about it, I rarely bother measuring power output, let alone distortion. Normally, it it sounds 'nominal' lke all the others of the same model, and, if a tube amp, the plate current readings all make sense & the amp sounds good, I'm happy.
    I built a Vox AC100 clone several years ago that I added a second higher gain channel to. I haven't finished it and have to gut it and start over with a new layout to fix an issue, but when it was functioning the power output was less than the 100 watts RMS or the 80 watts RMS that Vox claimed for the fixed bias or cathode bias versions respectively. I was only getting about 65 watts RMS with fixed bias or a bit less on cathode bias with about 480V B+ and a correct impedance Heyboer OT and running into a correct resistive load. I haven't been able to get back to the amp to troubleshoot further but it would be nice for it to make the expected power. After reading this thread I realize that some of it could be because I was measuring the power with a completely clean sine wave on the scope instead of the 5% distortion as others have mentioned....but that still doesn't explain the excessively low power output. If I had a distortion analyzer that would help to get the 5% that I am supposed to have when measuring power output. After I finish some other things I hope to get back to that project and after I rebuild it with the new layout, get back to finding out why the power output is lower than expected.

    Greg

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    Supporting Member Jazz P Bass's Avatar
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    When faced with an output power issue I usually attempt to narrow it down by observing the input voltage to the output tubes,

    As a general rule a 50 watt amp you will see a 50Vac RMS signal at the grids.
    A 100 watter will need 100 Vac RMS.

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