1200s HUM question

[usersunn]

My 1969 7 knob Sunn 1200s head has a hum issue. When powered on, (nothing plugged in/volume down) it has a slight but not bad 60 cycle hum> Im wondering if it is normal to hear a little hum, or if this is a problem.... But heres the other issue... For no apparent reason, all of a sudden,  the hum gets about 3 times louder, and then goes back to the lower hum. By turning the standby on and off, I can get it to go back to lower hum, but sometimes the hum stays loud, other times I dont hear it doing it at all. Could this be a tube? Im not a tech, but I opened the amp up to look inside... The only visible thing wrong was one cap (on the circuit board with the transistors-tremolo?) was bulging out at the end a little, and one big brown resistor across the bottom of one of the tube sockets seems darker, maybe burned a little. Could this be the prob?  All power tubes are old RCA 6550's, the rect tubes are both lit, and both Amperex, the preamp tube is an old Mullard, the middle small tube is a new EH, and the other tube has no brand or markings..theyre worn off. Having said all this, the amp sounds loud and strong and GREAT as a bass amp, so huge and hi fi sounding.. Its very strong and clear, I dont want to lose the sound it has now by replacing power tubes, etc... any help would be great!

[Isaac]

My first thought is that the caps in there are probably 40 years old, especially if it still has the original tubes, which it might. Your comment about the bulging cap supports this. As it is capacitors which keep the hum down, that's where I'd look first, starting with the one that already looks bad.

As you are not a tech, I'd suggest taking it to someone who is. Messing around inside of electronic equipment without knowing what you're doing can be dangerous, for both you and the amp. Also, be forewarned that fixing this problem very well might change the sound of the amp, but that not fixing it might result in nothing but hum, or no sound at all from the amp.

[usersunn]

Thanks Issac for the info..my tech said he'd do the job for around $100 bucks so Im thinking of going for it. One more question if I may- Do you know how many watts are consumed by the 1200s while idling, and also while in standby mode?? I think the 280w consumption is when you're pumping it, right?...thanks! - usersunn

[stanner]

get a three pronger

[noel]

I don't know the number off the top of my head regarding the power consumption but my understanding is that the 280W will be when the amp is powered on (being played).  The standby switch in tube amps leaves the heaters on so that the tubes are warm and ready to start emitting a beam of electrons when you hit the switch (standby off).  The heaters require 6.3V and I think the fuse is a 5A...in other words the amp cannot pull more than 5 amps ever so... W=VA, so I imagine the standby power consumption is less than 30W.

Regarding your other problem, recapp that thing!  When the caps have swelled that is a dead give away that they need to be replaced... and that is if you're going to ignore the fact that the amp is old.  Not to start a whole other thread here but any amp from this era must be recapped.  Original tubes and "stock" condition seem to be selling points for vintage stuff but that seems ridiculous to me.  Original tubes could be fine if the amp had never really been played (lucky but probably not gonna find many of those) but original caps are going to be out of tolerance or totally destroyed by now no matter what.  As long as the amp is restored properly it will be more like it was when it was new (and safer) than in the true "original" condition but 40 years down the road.  An of course as someone has already mentined, put a three prong grounded iec socket in this thing and remove the polarity switch...unless you want to die.  Electrical standards back in the day are kind of crazy by comparison to what we have now...especially with those voltages!!!! yikes

After all that enjoy that 1200s!  I'm a 2000s man but the 1200s is a cousin of that amp so you know its gonna be awesome!
Hope that helps.

[usersunn]

*** Thanks guys for all the great info/advice so far,  I think Im gonna go for recap,  but just not yet....the hums been behaving Ill enjoy it a little more like this in case I like the "before" better... There is SO much bass on this head..and Ive played just about eveything, vintage Sunns being the last to try (and the best!) I feel like I wasted the first half of my life playing inferior amps!  **QUESTION**    IS the 1200s supposed to be such a great bass amp?  Its so bottom heavy, with a glassy top end though, I use the brite input most of the time! Excellent..*******

[noel]

No offense, but not recapping this thing is really dumb.  If you recap it and don't like the sound I'd be shocked ...as would every other member here I"d be willing to bet.  Here again, recapping it doesn't change the sound, it just makes it sound the way it does when it was new.  Anyway, the main thing in a recap is to rebuild the power supply.  If a cap in the tone stack or a less essential area of the amp drifts/fails you're probably looking at anything from a different a distorted sound to more minor failures.  If your power supply is not filtered properly you got tons of high voltage going into the amp...say hello to big problems.  You have way more to loose by operating electrical equipment out of spec. than you have to gain.  Anyway, don't cry to the board if you have problems later because a couple guys wasted quite a bit of time answering your question.  You can lead a n00b to water...

To waste a little more time, the 1200s was designed to be a guitar amp.  But as some others have discussed already, Sunn made primarily bass amps.  Like any good company they took a popular design, made minimal changes, repackaged it and sold it.  The 1200s is a 2000s derivative as far as I know.  Again, there is no inherent difference between a guitar and a bass amp... just the tone stack really.  Plus if you see a reverb or tremolo circuit that's a dead give away that the designer was thinking guitar.

Sunn made nothing but cool amps...just don't kill yourself playing them the better part of a half century down the road.

[usersunn]

No offense taken!! I TOTALLY appreciate you guys taking the time to come on here and help out........Im very excited about having a good amp like this, and I do plan on doing the recap.... Noel convinced me to do it now and not later ( although Isaac says in his reply that fixing the problem may very well change the sound of the amp) but I think that Id rather have a slightly changed amp than a dead one.... And yes, I do believe the 1200s and 2000s are the same sound, maybe the midrange freq is different? I really like the 1200s , it's got full frequency range to my ears. The transformers are gigantic , which is I think is where all the bass and huge sound is coming from..

[noel]

Glad I convinced you  To Iassac's point, yes it will "change the sound" but like I said, this change will be more like the way it was new.  Audio myths definitely die hard so I'll conceed that the difference (although I do not believe it is noticable) stems from the fact that caps are made differently (in some cases).

If you want to get as close as humanly possible put some money into nicer caps...nrrrrr  I always love guys bitching about this topic that would rather play a shitty borken down amp than paying the extra buck it costs for some nice sprague atoms or something.  Anyway, Sunn didn't really use many "exotic" (if you can call them that) caps like you might find in a 60's marshall or hiwatt (mustards).  So you'll be totally cool with sprague's in the power supply and some orange drops if you want to do the rest.  

All this stuff about changing components is just retarded though...  Analogue stuff usually has a pretty sizeable tolerance (+/- 10%).  Its all about getting it in a condition that is safe so that you can play it for another 20 years.  If you do a double blind test of a couple amps I would bet that any difference the biggest audiofile could notice has more to do with component choices and tolerances than anything.  Think about an ideal cap in an RC circuit (a tone control here).  The tolerance is going to effect the frequencies that the filter controls.  I could go all day but this shit just doesn't matter hahah.  Do a nice job on the recap wiht good components and your amp is going to sound awesome and be reliable.
Cheers!

[usersunn]

Yeah, Ill probably love it even more after the recap. I will let you know for sure! Thanks again for the info/advice!

[Isaac]

The changes I can imagine happening as a result of changing the power supply filter caps (the most likely culprits in this scenario, in my mind) is that it will tighten up the power supply. It will, as noel says, restore the amp to it's original condition, or at least bring it closer. Which might mean that there will be a lot less sag in the power supply, and as a result of that, less compression and more total output. In general, this is a good thing for bass, not necessarily so for guitar.

Bottom line, I agree with noel. Change out the caps. Any changes in tone, if there are any at all, will likely be minor, and a slight tweek of the tone controls should take care of it.

[hawaiitone]

I have a sentura I that with a two prong plug and the thing was humming like a... loud humming thing (laugh)... anyhow, if you are on two prong - reverse the way it's plugged into the socket - helped a lot on my amp. Next, mine was still humming, thought I needed re-cap - changed the preamp tubes to some good low noise tubes and... hush... the amp is silent - no hum. Cheaper and less invasive than a recap (depending on the tubes you use). Good luck! Sometimes people jump into the recap a little early - if you have old tubes that could very well be your problem.

[knw]

My 69 1200S had 4x 6550's RCA.

preamp:

V1 = 12AX7 EH
V2 = 12AU7 EH
V3 = 6AN8A Svetlana made in USA.

I had a slight hum at around 4 volume and it would increase with more volume I play bass trough this amp at around 6 or 7.

The hum would only be there if I had my bass idle.

After having my Tech clean it and Retube it with.

4x6550's Tung Sol.

preamp:

V1 = 12AX7 Tung Sol
V2 = 12AU7 JJ
V3 = 6AN8A Svetlana made in CANADA.

There was a slight increase in Volume (not that big tho) and the hum is gone even with the volume set at 10.

Tone wise, the amp sounds much Crisper and Cleaner to my ears.  Didn't lose any bottom end at all  I can make the amp growl if I dig into the strings but if I play softer it sounds super clean. The glassy high as still there and I think they sound much better after getting it retubed.  There is nothing I dislike about the Tone of this amp.  I spent close to 300 bucks on the retube/tech.  I don't regret one bit of it.

Recapping the amp would only improve it wont' make it worst but it is pricey to get it re caped.

I prefer this amp over a Model T.

[usersunn]

Thanks for all that excellent info everyone!! I'm glad you are happy with your 1200s retube/recap,  KNW....  Just to give you guys an update, the hum on my '69 1200s is now at a steady volume, not going up and down; I have now stopped using the amp just to be safe (although it still works and sounds super) as per Noel's advice..I will be bringing it in for the recapping soon, when I have a little extra $$$.... I will let you guys know how it turns out for sure!! In the meantime, I'm using my 35 watt Oliver tube head to hold me over until then....