Two Ampeg SVT's, One is Louder

[SeawolfAudio]

The guy that owns the studio that I work in part time as a tech also plays bass in a touring band. His live rig consists of a 1969 SVT and a 1971 SVT. His preffered setting on the 69 is around 10 o'clock. The get the same volume out of the 71, the volume has to be set at about 1-2 o'clock. He wants to get them set up so all the controls are closely matched as possible. He's not a technical geek like me so I didn't bother going into the long explanation of all the little intricacies that determine volume, tone, eq, headroom, etc. so I just told him it's nearly impossible to get them identical but we should be able to get them pretty close. Next time I'm over there I'll be taking them both apart to get some readings. I told him I could easily figure out how to do it regardless, but depending on what's causing the difference, it may not be a permanent solution. The 69 is all original, clean and spotless inside. As far as I know nothing's been replaced. Being NOS tubes, it's not impossible that they're still running strong. I'm much more skeptical that the original electorlytics would allow this thing to be as loud and clean as it is. Absolutely a museum piece example of an SVT. The 71 looks like a typical 71 inside. Cobwebs, etc. Not in bad condition at all, just needs a little cleaning. He keeps all his records and I know some of the 5w resistors were replaced and I'm fairly certain it was re-capped as well. It was retubed and re-biased and I think converted to 6550 output tubes. Since I haven't opened them up yet and printed up all my normal pre diagnistic specs, come up with a checklist, etc, I'm going to assume for now that the 69 still has the original 6146 output tubes. So, here's a rough list of some possible causes starting with what i think is the simplest.

1) 6146 output section in the 1969 vs 6550 output section in the 1971.
2) Volume pots for both should be B1M but, they could be within that 20% tolerance and within spec and one amp's pot could measure 800k and the other 1.2M
3) Old leaky electrolytics in the 1979. That's usually the first thing I check in an older amp. In my experience that's by far the most common issue. The part that interest me though is if he's correct that the 69 caps are original, it seems unlikely that they wouldn't be causing issues too. And that thing is mind numbingly loud and still clean with the volume at 9 o'clock.
4) 6550 tubes in the 1979 are nearing the end of their useful service life, needs a bias adjustment, or some other output tube related issue.
5) Assuming there are a lot of carbon comp resistors in the 79, some of them have probably drifted out of spec
6) A worn down ouput tranny in the 79.
7) Some other thing I haven't thought of yet since I've only been working on tube amps for about 3 1/2 years and a lot of you have forgotten more about amps than I'll ever know in my lifetime.

When I go into the studio again some time this week, we're going to measure the actual dB difference between the 2 of them at the same volume pot setting .

Some other important things to note. He's absolutely thrilled with how the 69 sounds and he's very hesitant to let me change anything in there, and I get where he's coming from. Sometimes that lucky magical tone comes from old parts having wide tolerances, or have drifted out of tolerance. I replaced a bunch of resistors that had drifted wayyy out of spec in an old 50s gibson guitar amp once and it made a pretty big difference in the tone. It sounded a bit more sterile, lost its beautiful warmth and smoothness. But I feel like at least the power filter caps would be safe to replace. So I'm going to try my damndest to do all or most of my tweaking to the 71. I figured it would be nice to get some input from guys with more experience before I start wasting a ton of time whacking away at these things. They're usually permanently mounted in the flight case and are a huge pain to pull in and out since they weigh 97 lbs each. I was hoping to come up with a game plan, take as many measurements as possible on both amps, then decide on plan, gather any and all possible parts I think I'll need and try to do all the work in one weekend. I'm sure once I report back with a list of readings for both amps, that will help you guys give me more specific advice. Thanks again in advance for anybody willing to take the time to share their knowledge and experience with me.

[Leo_Gnardo]

Not just volume pot tolerance - one might be linear taper, the other log taper. That alone could account for a large disparity between loudness & volume pot settings.

Also - in years when 12DW7 tubes & their equivalents were scarce to nonobtainium, many SVT repairs swapped in 12AX7 instead, and that can account for big differences in gain. Some amps were modded to accept the AX's, others weren't. And some of those modded to accept the AX's later had DW's or maybe newer JJ ECC832 installed but did not get modded back to original. Those & other opportunities for gain hijinks might explain why one amp is one way, & the other seems different.

I'd look at the "small" tubes first, make sure they're the right ones in the right places. And check the circuits for mods made &/or unmade. And the volume controls for log/linear.

Also keep in mind old played-out power tubes sound mushy and suffer reduced gain & power delivery when really worn to a frazzle though they may keep running. One SVT I examined years ago had only one out of each triplet of power tubes still operating, you can imagine how lousy it sounded though it still "worked." Because of the impedance mismatch with only one frazzled tube operating on each side, it turned out a very distorted 45 watts into its 8x10 cab. Miles off from the typical 240-260W one sees with a fresh sextet of 6550 or KT88.

Output transformers don't lose power when "old and worn out" so that's one less worry. I'd expect an OT with shorted turns would be a sure fire fuse popper. Can't say I've ever seen an SVT with any bad transformers. That's not to say it's impossible, just not likely. There's lots of other places as you well know, to have SVT's go off the rails. You'll figure it out, I'm sure.

[nickb]

I suggest you inject 1KHz (say 100mv RMS) and measure the output of each stage in both amps to see where the differences are. Repeat at 200Hz and 2KHz to check for frequency response errors.

[Justin Thomas]

Not that they're calibrated to the same standard, and not that a 6550 is a slouch, but it seems the 6146 might have a little bit more oomph to it than the 6550.

Justin

[nevetslab]

You also have the two bass cabinet's present effective sensitivity in the equation. I'd also look at swapping cabinets/cables in the assessment to verify the relative sensitivity of each cabinet has in the set-up.

I haven't yet stopped to look and see if these early SVT amps have Preamp Output/Power Amp Input loop. The simplest thing in my mind, with both amps set up for the same control settings, assuming each amp at a commonly used setting sound the same (what's the chance in that), would be to tweak the signal path at that input to the power amp section in the amp/spkr rig that is lower in SPL level than the other. YES...I WOULD be looking at this acoustically as well as electrically. I'd also suggest spot level checks at 40hz, 80Hz, 160Hz.....octave steps up in the fundamental range of the bass.

Definitely an interesting project. Thus far, I haven't had to do this for any of our clients. I've also never had one with the 6146 Output Tubes. Are those still available? I'll do some digging in my archives to see what other thoughts I have on the project.

[g1]

I haven't yet stopped to look and see if these early SVT amps have Preamp Output/Power Amp Input loop.

The older units have the 'ext.amp' jacks, but they will function as pre-out or pwr.amp-in.
So I think it would be good to start with putting a signal in there and seeing if both power amps are behaving the same.

Multi-model schematic package attached.

[Leo_Gnardo]

I've also never had one with the 6146 Output Tubes. Are those still available? I'll do some digging in my archives to see what other thoughts I have on the project.

Good of you to mention comparing bass cab sensitivities - it's possible they may differ radically. And - are all the speakers working?

6146 now a bit rare. Time was, they were common as dirt. A very tough transmitter tube that saw service in gazillions of taxi two-way radios back in the 50's & 60's.

[soundmasterg]

The 6146 amps make more power than the 6550 amps, though more of the power is concentrated in the mids on those amps and the 6550 amps have a more even frequency response with bigger bottom end. A friend I used to know had a very early 6146 amp, and a mid 70's 6550 amp, and an 80's MTI Ampeg SVT made in Japan. I had an early MTI SVT myself that had US transformers but Japanese parts. The 6146 amp made 330 watts RMS. The mid 70's SVT made about 295 watts RMS. My MTI made 290 watts RMS, and the later MTI that he had made 260 watts. These were all up tp spec with good tubes and what not. All amps except for his later MTI SVT used the 12DW7 tube in the preamp. His MTI also had Japanese transformers, which had about 5% less B+.

Greg

[Enzo]

The difference in loudness between 330 watts and 295 watts would be tiny, less than a decibel.

[nickb]

The difference in loudness between 330 watts and 295 watts would be tiny, less than a decibel.

Quite so. We need measurements else we're just shooting the breeze.

[nevetslab]

Looking thru the revisions of SVT power amp and preamps schematics that g1 posted, I see the same output xfmr used, only one of the power amp has a different plate resistor on the power tubes (10 ohm/5W vs 3.3 ohm/5W), while the feedback values are all the same, as are the plate/cathode resistors in the front end of the amp ahead of the driver stage. So, I'd guess, apart from component drift over time, the gain of the power amps between these models would be the same.

Likewise, looking at the revisions of all the SVT Preamps, they all have the same plate & cathode resistors in the gain stages, the input network to the first stage on both channels as well are the same. I didn't closely check the EQ circuits for any differences. I'd guess the biggest difference would be in the tolerance of the EQ controls and the Volume Pots, but apart from that, I'd think the gain on all of them to also be the same....component drift over time being the variable.

I'd think trimming the input network to the power amp stage between the two amps might be the easiest way to trim up gain differences. Acoustically, as I suggested before, you do have the age factor of the speakers in the two 810 cabinets, so their sensitivity is all part of the overall difference.

In the event that an accurate SPL meter isn't available, you do have what's needed at the studio, using a decent condenser microphone, and a channel strip to feed the console's meter, along with either the consoles oscillator, or an external oscillator or signal generator. Moderate acoustical level would be adequate for the relative level comparison. I'd do it at several frequencies in the bass range (50hz, 100Hz, 200Hz). If you have meter damping, pink noise also works well, though you have LF flucuation to contend with in the reading. Set each amp/cabinet at the same distance, and set the mic for omni mode rather than cardioid.

[g1]

Before modding, it would be nice to know if one of them has a problem.
And it's not necessarily the quieter one. There are faults that can make an amp 'louder' comparatively.
If it is decided that they are both working properly with no faults, then you will have to decide which one to mod, the one that breaks up at 10 o'clock, or the 2 o'clock?

[The Dude]

Agreed. Maybe one of them is not working 100%. For example, an SVT will still work if you have a couple burnt screen resistors. It will just be working with fewer output tubes. Before doing any modification, it should be determined whether the amps are fully working.

[loudthud]

Make sure those 8x10 cabs are firing on all 8 cylinders.

[soundmasterg]

The difference in loudness between 330 watts and 295 watts would be tiny, less than a decibel.

Yes, it is barely any difference. The early amps do sound louder and more aggressive, but I think it is just because of the slightly different frequency response and distortion character.

Greg