measuring output power

[nevetslab]

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.

[soundmasterg]

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

[bob p]

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.

[olddawg]

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.

[nevetslab]

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.

[nevetslab]

Signal IS being injected at the Power Amp input

[bob p]

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.

[nevetslab]

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.

[bob p]

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.

[Enzo]

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.

[bob p]

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.

[soundmasterg]

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

[nevetslab]

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.

[soundmasterg]

Thanks very much for the info. Gives me somewhere to start looking.

Greg

[olddawg]

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.