Sonaro question - weirdness abounds

[urbanspaceman]

Hello, I've been a proud owner of a '70/'71 (stamped Dec 1970) Sunn0)) Sonaro Bass head for nearly a decade. I've gigged all over the country with it and have had very, very little problems with it. Recently, however, it started making some 'strange noises' at certain frequencies. So, i took it to the local vintage electronics guru - who did some previous work on the capacitors when i first bought it - to see that was going on.  I just got back from picking it up, and have yet to try it out. Aside from a few connection issues, it appears that the tubes will need to be replaced. The amp I have has 2 Svetlana 6550's in it, which is not uncommon for this model/year. What is strange is that all of the Sonaro's (this one is labeled 190B, but the sticker was peeled back prior to my purchase revealing the sonaro name on the plate) I have seen on this forum have a different wattage then mine. The amp I have is listed at 140watts. It is way louder than any of the other sonaro's I've seen. When I bought this there was another Sonaro in a local pawn shop, that sound very weak in comparison to mine. So I know it is 'special'. I mentioned this to my electronics guy, and he agreed that the transformers looked very big for an amp with only 2 power tubes. What gives? I do not have access to a digital camera at the moment, or i would post pictures. I will do so, hopefully, this weekend.

Also, since I will have to replace the tubes on this, do you all have any recommendations?

[Soundmasterg]

The 140 watts listed is the power consumption, and not the actual power output.

From the vintage of the amp, I can tell that it was a 2x6550 Sunn, which means that it would have had the larger transformers intended for use with 2x6550 as opposed to some earlier Sunn amps that would have been intended for use with 2xEL34. A 2x6550 or 2xKT88 Sunn will put out about 60 to 70 watts RMS, and with the right speaker cabinet can really move some air.

For tubes, any of the modern manufacture 6550 or KT88 will work, but I would suggest to go with the JJ KT88, SED/Winged C 6550C or KT88, or the EH KT88. The KT88 will make a little more power in your amp than a 6550 will because it is a higher dissipation tube. Personally, I use the JJ KT88 in my Sunn stuff and it sounds great and works very well.

[urbanspaceman]

Thanks! This amp definitely moves some air. The space where my band practices is largely populated with neu-metal/emo/pop punk bands (we are a mod-esque, garage combo), and they complain about (compliment) the volume of my rig - and they use alot of SVT's.

Also, the capacitor on this amp is a little strange. My electronics guy commented on it, when he fixed it years ago. There are four connections, but only three are wired. (I probably have the technical stuff wrong). Is it supposed to be like this? If not, what would happen if I wired all four?

I'll still try and post some pictures of this amp soon.

Thanks!

[Soundmasterg]

If you have the original Sunn cabinet with the 2x15 setup, it is a rear-loaded, ported setup. What this means is that the cabinet is loud, but not up close as compared to how loud it is far away. The SVT cabinet is loud up close more than far away because of it's design.

It sounds like you're talking about the can cap in this amp? If so, then that is actually a can with 4 sections, which means that there are 4 caps inside that one can. With today's wall voltages, that can should be disconnected and discrete (individual) caps should be wired up in series with parallel resistors to get an appropriate voltage rating for what the amp really has. That can cap is not rated high enough to take the B+ voltage of this amp and will fail sooner rather than later. If you read though the posts here, you'll see some discussion about mods I made to my 200S which is similar to your amp, and how I wired up discrete caps in series to get an appropriate voltage rated cap.

Your electronics guy should be familiar with can caps and with not using all of the connections in the can if he is at all experienced with old electronics. Nowadays can caps are more expensive than discrete caps, and are less available. Back when your amp was made, they were cheaper and widely available, so that is why manufacturers used them. There is no reason for you to wire up the other connection on the cap.

[EdBass]

I can't imagine a scenario where a 2 X 6550 Sunn amp could compete in volume with a 6 X 6146 or 6 X 6550 SVT at any distance.

[urbanspaceman]

Thanks for the info.
As far as the cap goes, that's what my electronics guy said years ago. Instead of replacing the cap, he just wired up the one that wasn't in use.

I tried it out this evening at practice and the 'noise' was still there. But, i switched the tubes around and the noise disappeared. I know that there are problems with both tubes, one looks like it's getting ready to die and the other has a loose fitting. I just want to put off replacing them for as long as I can. I love the way my amp sounds right now and really don't want to change it - which will happen with a tube job.

I don't understand what the kids at our practice space are talking about either, concerning the volume.

[Isaac]

I'm just guessing here, but I think the Sunn setup has a lot more low end than the SVT's do. Low frequencies are notoriously difficult to soundproof, so the bottom octave from the Sunn, missing(?) from the SVT's, may be penetrating the walls of the practice facility and making itself known.

[Soundmasterg]

I can't imagine a scenario where a 2 X 6550 Sunn amp could compete in volume with a 6 X 6146 or 6 X 6550 SVT at any distance

I didn't mean to imply that the Sunn will be louder than an SVT! The Sunn cabinet design throws the sound out more than the SVT cabinet design, but the SVT is just way louder overall. I don't know about he low end thing either because my SVT will shake and rattle the floor at 2 if I turn the bass up. Maybe these SVT guys are using round wounds, with a bright bass sound or something?

Wiring up the cap that wasn't in use isn't a good idea because electrolytic caps fail quicker with non-use then when in use, so that cap he hooked up was probably in worse shape than the others in the can.

If your worn out tubes decide to fail, it could take the output transformer with it when they go, and that would certainly change the sound a cost a lot of money to repair! Tubes are cheap. I'd suggest to get some new tubes and rebias and call it good. The amp will probably sound better with more highs and more lows with new tubes in it.

[JoeArthur]

I can't imagine a scenario where a 2 X 6550 Sunn amp could compete in volume with a 6 X 6146 or 6 X 6550 SVT at any distance.

I believe it could.

If we take 60 watts for the Sunn and 300 watts for the Ampeg, that would be a power difference of... 5 times. Assuming the same speakers, that would be a SPL difference of just a hair under 7db.

If you use the Sunn with a 7db more efficient speaker system than the Ampeg, the SPL would be the same. Not out of the range of possibilities.

[EdBass]

I can't imagine a scenario where a 2 X 6550 Sunn amp could compete in volume with a 6 X 6146 or 6 X 6550 SVT at any distance.

I believe it could.

If we take 60 watts for the Sunn and 300 watts for the Ampeg, that would be a power difference of... 5 times. Assuming the same speakers, that would be a SPL difference of just a hair under 7db.

If you use the Sunn with a 7db more efficient speaker system than the Ampeg, the SPL would be the same. Not out of the range of possibilities.

OK, Mr. Smartypants. I’ll amend my post to read; 

I can't imagine a practical scenario where a 2 X 6550 Sunn amp could compete in volume with a 6 X 6146 or 6 X 6550 SVT at any distance.

Let me also add that I personally prefer the tonal qualities of the Sunn amps over the SVT. I think that a properly set up Sunn tube amp is very hi-fi, but can get a nice thick growling saturation, easily controllable with a passive Bass, that the big Ampeg doesn’t quite attain at reasonable levels.
Sort of like a Hiwatt.

No disrespect to the mighty SVT, it unquestionably deserves it’s icon status among tube bass rigs!

[basiklybass]

Just watch those preamp tubes on the SVT. They like to generate very high frequencies which can burn out a bass speaker...personal experiance....at the cost of 4 Altec 421's. First 2 went, replaced them...then those 2 went. New preamp tubes and never had another problem.

[JoeArthur]

OK, Mr. Smartypants. I’ll amend my post to read; 

Na na... na, na, na

Yeah, I know, very childish of me, but at my age I can't resist!!

Just watch those preamp tubes on the SVT. They like to generate very high frequencies which can burn out a bass speaker...personal experiance....at the cost of 4 Altec 421's. First 2 went, replaced them...then those 2 went. New preamp tubes and never had another problem.

That's very strange. The power amp of the SVT has such a severe high frequency roll-off, and there isn't much of a feedback path in the preamps for it to develop such a high frequency oscillation.

I don't dispute what you are saying, but there must be something else happening besides tube quality or tube age to cause something like this.

[EdBass]

Just watch those preamp tubes on the SVT. They like to generate very high frequencies which can burn out a bass speaker...personal experiance....at the cost of 4 Altec 421's. First 2 went, replaced them...then those 2 went. New preamp tubes and never had another problem.

Huh?
Want to sell those "burnt out" 421's cheap?

[basiklybass]

It was in 1974, in Iowa City and I had to drive to Chicago to get a new set of speakers...who knows...they still might be out there...

Good point....I don't know the answer...just that when the preamp tubes were replaced...from then on I then replaced them every 6 months or so....I never burned out another speaker. We played 5 nights a week for a couple of years.

I do have a couple of Sunn SPL 916's I am going to sell....care to make an offer? There is a picture of them here:

http://sunn.ampage.org/sdp/index.php/topic,3413.0.html

They look a lot like the 421's..not exactly...but close.