SVT-CL or SVT-VR output power NOT anywhere close to 300W

[nevetslab]

One of my steady clients sent back the latest Ampeg SVT-CL-Heritage Bass Amp this past week, after I had worked on it, finding it hadn't yet been set up and spent the time to solidify it for years of service as I've done for our large supply of those amps, as well as their's. It came back with the complaint of it having low output power.

I pulled up my service notes on what I measured on it's recent visit to the shop and powered it up to give a listen. I think the last time I had actually measured steady state power on an SVT was an SVT-VR, setting the Balance pot on the rear panel. I normally don't bother measuring output power on these, they having very healthy power supplies and either KT88's or 6550's (being what is supplied from Ampeg). This had a matched set of J/J 6550's in it. When I measured the power output at 200Hz driving 4 ohm dummy load (4 ohm/500W Dales), it was clipping at 206W out. Hello! I then connected it to the speaker (Ampeg BXT115HL4 15" woofer/tweeter) and fired up my B & K 2130 SPL Meter/Freq Analyzer at my desk. 20 ft from the speaker, I was reading an average of 104dB SPL with periodic peak clipping using burst pink noise. Meter readings driving the speaker at those peaks was around 31V RMS, lower than the nominal spec'd 34.6V RMS @ 4 ohms. The burst Pink noise has plenty of LF power, with bass turned up a bit on the preamp. I always have the amp under test powered thru either my Magtrol 4614 100A Power Analyzer or a Valhalla 2101 20A Power Analyzer and variac, maintained at nominal 120VAC. Current/wattage under output power test was 5.8A/580W @ 120VAC typical.

I tried increasing the idle cathode current readings with the bias pots from 23mA/tube to 25mA/tube, which got me a bit higher. 228W steady state dummy load. I replaced the driver tubes ECC82/12AU7's and the front end input tube ECC83S, rebiased, and got a little higher. Then swapped out the power tubes with a new set of 6550's, biased those up at 23mA/tube, and got 261W at clipping THD less than 5%.

I then put up the client's SVT-VR, repeated the same measurements, and it too wasn't anywhere close to 300W. Ampeg's spec calls out 300W Min @ <5% THD.

Power supply voltages for the power tube plate/screens was nominal 660VDC Plate, 365V Screen, 380VDC driver stage plates. I didn't measure voltages under drive conditions, not having any previous reference data for comparison.

So, ok. I agree. Output power IS low. I didn't have any KT88's here in the shop to swap in to see if there was any difference there. IF this IS the nominal output power on these amps, then we have around 20 of them on hand, and the client has at least half that or more. I've never heard any complaints on these amps until now. And assuming KT88's won't offer any further increase in power, that's then pointing to the power supply filter caps as well as the power transformer. Getting into expensive territory if the client is going to be insistent on having their rated power spec met.

Just as a side note, I had just completed repairing one of the client's Eden WT800's, which had both power amp's with shorted power xstrs. Only one xstr shorted on each module, restored order, and later checked output level on the SPL meter driving the Ampeg 15" bass bin in the shop, again at 20ft. I was reading 108dB SPL with burst pink noise. Granted, that amp in bridge mono IS producing more power out than the SVT-CL or -VR, but I was somewhat taken back seeing 104dB SPL nominal and hearing the SVT output clipping reaching a bit above 30V RMS on the meter feeding the scope (Amber 3501a Audio Analyzer).

The client's delivery man today brought me another SVT-CL that's now going into Protect, so I did pass along my findings on this 'low power' complaint on a typical SVT, a brand new one added to their collection.

I'll have a look at the Hiwatt DR201 that just came in from Lon Cohen later this week, it being a 200W 4-KT88 bass amp. And I have the one I built for myself here as well for comparison.

[xtian]

Steve, do you have a young buck to lift these SVTs onto the bench? I'm receiving my fourth one soon, over a two month period, and it's wearing me out!

[nosaj]

At this one lawnmower shop I went to there was this palletjack like device but it was flat like a workbench that goes up and down with the press of a button. That what ya need for those SVT's.
nosaj

[The Dude]

I find this often with modern gear (overrated power specs). In the old days, you could depend on fairly accurate specs. These days, I often wonder how they get the values they do and the test conditions necessary to achieve the rating. I guess 1k tone (not full bandwidth), mismatched load, wind out of the west, full moon, Friday at 11PM, air conditioner off, and car in park?

[g1]

At what frequency does Ampeg spec the 300W? I never test power at less than 400Hz so I wonder if the 200Hz could at least contribute to not meeting spec.
Aside from that, did it sound overly weak to you when you first did a listening test? I'm not sure I would notice 200W vs 300W.

[catalin gramada]

[QUOTE=nevetslab;n1003061

Power supply voltages for the power tube plate/screens was nominal 660VDC Plate, 365V Screen, 380VDC driver stage plates. I didn't measure voltages under drive conditions, not having any previous reference data for comparison.
[/QUOTE]

Not quite same story but same tubes references. Get 36Vpk before clipping into resistive 4 ohm load with same 6x JJ 6550 into same 1.9k Ra-a with 460V plate / 370V screens.biased at -44V. Roughly 160W very clean output from 40 to 10kcps. at 470-460V only. With 660V you should be able to get 100w per paar or close.

[nevetslab]

At what frequency does Ampeg spec the 300W? I never test power at less than 400Hz so I wonder if the 200Hz could at least contribute to not meeting spec.
Aside from that, did it sound overly weak to you when you first did a listening test? I'm not sure I would notice 200W vs 300W.

I found a Test Procedure in my SVT2-Pro product folder and have attached that below.

SVT-2PRO Test Procedure.pdf

They plug line level signal into the Power Amp input jack on the rear panel, and test for 24.5V RMS into 2 ohms, and for 34.6V into 4 ohms at 1kHz, checking for symmetric waveform. They didn't specify where the power tubes were biased up at, only that bias is set for being GRN LED is on and RED LED is not for both halves of the output stage. I normally set them for 23mA nominal thru each tube's 10-ohm cathode resistors and had already matched the tubes for best even current levels at idle, so output stage is well balanced.

They use 1khz as test frequency. I'll have a look at that frequency when I get the Vox AC30CC2 off of the bench and check the SVT-CL Heritage again. I've also never run a Power vs Frequency Response Curve to show what the max RMS Power Output is per load (4 Ohm and 2 Ohm), which at this point is something I'll do to add to my database and publish the curve here in this thread I began.

As to listening to the amp, until I took it to max output level at onset of clipping (seen on the burst pink noise output waveform), it sounded normal. Once I had the scope on the output of my audio analyzer, it began sounding weak entering into peak clipping. Harder I pushed the drive, the more clipping was showing, which of course sounds worse. I haven't plugged my bass into it since the amp came in but will do that as well. I usually plug into any of the bass amps that come thru here after having done all of the repairs to be sure all sounds nominal.

200W to 300W is only a 1.76dB Power Increase, or 3.52dB Level Increase, noticeable but not by a lot. During my burst pink noise testing at high output level, normally below clipping but close to it, my AC power analyzer is showing around 250 to 270W Power consumption at 120VAC mains. If I have the amp at my test bench, then I get to see all three readings...Mains Current, Mains Wattage and Mains Voltage. And can set it to Average over time, which is helpful using burst pink noise.

[Helmholtz]

200W to 300W is only a 1.76dB Power Increase, or 3.52dB Level Increase.

SPL is proportional to power. So 1.76dB power increase means 1.76dB SPL increase.

[mikeydee77]

Following with interest

[nevetslab]

Steve, do you have a young buck to lift these SVTs onto the bench? I'm receiving my fourth one soon, over a two month period, and it's wearing me out!

Wished I did. Now at 74 yrs of age, and rolling over to 75 middle of November, pulling these beasts out of their road case and first lifting it up to set onto the edges of the bottom of the road case, then I can heft it up to get the first pair of cabinet feet onto the bench and then it's pushing the rest onto the bench does let you know you're not as strong as you think you are.

Hefting a Matchless DC30 or Vox AC30 from the road case base to the bench: one of these days I'm gonna not be able to do that, but so far, haven't injured myself. I've started looking at the wheel'd hydraulic lift tables to deal with getting these amps up to the bench. Trouble I have is there is so much stuff on the floor, stacked out from the steel shelving on one side of my 30ft by 10ft shop, with 3ft deep work benches, so the wheeled hydraulic lift tables have to be steered into place to then have it do its' job to get it to bench height, and then push it over to the check-out bench. Not that expensive. Under $500 if I recall for a proper one. Had one at BGW Systems, though they bought a cheaper one and over time, it failed without it getting repaired.

Any time I find someone available (often, as there are three rehearsal studios down the hall from me here at CenterStaging). I usually do the manual work. Keeps me young, I think. End of the day, though, I do feel the stress of too much physical labor. Hard enough just getting the lid off of road cases if they're tall and have a tall combo inside of it.

[xtian]

I have a 9mo-old grandbub. He's a chonk! Gives me tennis elbow fast. Going to need a bionic exoskeletor!

[J M Fahey]

I never ever expect tube amps to _exact_ meet brochure stated power.
In any case, very little audible difference between 200 and 300W and none noticeable by ear between 261 and 300.

Also new tubes often don't meet datasheet specs.
Because they are worse?
Because some old esoteric Tech knowledge was lost?
Because old Artisans retired or died?
No, simply because Manufacturers couldn't care less and quality is all over the place, you get what you get..
Tubes have become a minuscule niche market, nobody expects _growth_, so manufacturers keep making them in old worn 50-60 y.o. equipment until it falls out to pieces.
Nobody is *investing* here.

[Mars Amp Repair]

As for a possible point of reference, I typically get over 300watts when testing the SVT's. Is it possible you're running the amp through a Sencore PR57 or a supply where the AC will dip a bit? I ran into that & had to bypass the Sencore as it's not capable of supplying the current, and hence the proper line voltage, necessary to run a higher powered amp to it's full potential.
Just a though. g

[nevetslab]

As for a possible point of reference, I typically get over 300watts when testing the SVT's. Is it possible you're running the amp through a Sencore PR57 or a supply where the AC will dip a bit? I ran into that & had to bypass the Sencore as it's not capable of supplying the current, and hence the proper line voltage, necessary to run a higher powered amp to it's full potential.
Just a though. g

I have a dedicated 30A 240V service that feeds my 5KVA Topaz isolation transformer strapped for 240V in, 120V out, which feeds a Superior Electric 30A Powerstat, and passes thru a Magtrol 4614B 100A 3-Phase Power Analyzer at one bench, and the other at my checkout bench, has a General Radio W20 0-140V Variac, feeding thru a Valhalla 2101 20A Power Analyzer. So, I'm solid on the AC Mains supply end, and trim up the variac for 120VAC under power testing at either bench. At full output at clip, the SVT-CL was pulling 6.3A/620W @ 120VAC when I got as high as 261W into 4 ohm, onset of clipping @ 200Hz Sine. That was with a new matched sextet of J/J 6550 Power tubes, biased to 23mA eacn (nominal) prior to power testing.

[nevetslab]

I never ever expect tube amps to _exact_ meet brochure stated power.
In any case, very little audible difference between 200 and 300W and none noticeable by ear between 261 and 300.

Also new tubes often don't meet datasheet specs.
Because they are worse?
Because some old esoteric Tech knowledge was lost?
Because old Artisans retired or died?
No, simply because Manufacturers couldn't care less and quality is all over the place, you get what you get..
Tubes have become a minuscule niche market, nobody expects _growth_, so manufacturers keep making them in old worn 50-60 y.o. equipment until it falls out to pieces.
Nobody is *investing* here.

That's somewhat why I never really bothered measuring the power output of the SVT's on any regular basis. Once I've gotten thru with whatever servicing I've done on SVT-VR's, -CL's or SVT2-Pro's, I run them
at onset of noticeable max unclipped level feeding an Ampeg BXT115HL4 15" bass cabinet (4 ohms), first using burst pink noise and finally with my Fender P-Bass plugged in. Under Burst Pink, I see AC Wattage consumption peaking around 300-340W where onset of clipping begins.

I'll see more detail after making a series of measurements, just to say I've done it, have the data to show the power supply voltages sag at full power vs test freq at both 2 ohms and 4 ohms (shouldn't be much difference there using the output xfmr taps, but who knows.

The HP 712C 0-500VDC/0-200mA HV power supply, with 0-300V neg Bias source, 6.3VAC/10A Heater supply I picked up years ago came out of Sylvania. Comes in handy sometimes, though no substitute for the higher current outputs one gets with conventional power supplies as we find in our tube guitar and bass amps.