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Sunn 300t overheating

xjeremiahx

xjeremiahx

Apr 19, 2009
1,112
7
Chicago, IL
www.encrustmusic.com
Disclosures
No longer a slave to GC.
#1
So, I've got a sunn 300t/fender bassman(whatever you want to call it) and I run two 8 ohm cabinets.

whenever i run the head at 4 ohms, it gets really hot and my tubes get kind of smelly - not really a burning smell...just very warm(hard to describe...i've been told it's just the smell of the tubes being hot?)
but anytime i have the ohm switch in 8 ohms, everything works & sounds fine...my head isnt 1000 degrees and no smell.

is there something i'm just unaware of? i KNOW i should run it at 4 ohms, but when i do, i get all nervous that something's going to happen to my amp...just get a bad gut feeling about it.

help would be great.
and for those who read my last thread, i took care of my EQ problem, thanks for the help :)
J
 
#5
Smelly/hot all the time or just when you play loud?

Are the tubes' plates glowing in either position? If they are, does it happen more in one switch position than the other? Is there consistency among tubes in this regard?

How many hours on the amp since the last tube replacement?

Did this just start happening?

I know zero about this amp specifically, but these are some things you can do to get started.
 
#8
Well could be tech time, but you are also driving two cabinets when in four ohms. Like one of the other posters mentioned and these are good questions...

Does anything turn red?
Do the tubes have an equal blue haze between them when viewed in the dark?
Are you cranked up?
Is the amp sounding good while its making stink?
Is the output transformer super hot?

When I had my VOX Foundation and played that with two bottoms, you could light cigars off the head, I always hated that.

First... I would remove each final one at a time, and gently wipe them with windex and a soft cloth removing all the bar crap and fingerprints from the surface and replace.

Does this head have a fan? I would have a blower pushing air across the tubes.

Then check the bias. BOB
 
#10
Nighttrain1127 said:
+1 What was said about a blower/Fan put it behind the head and let it blow through. When you run at 4 ohms the amp passes more current creating more heat. The fan should take care of this and help keep things cooler.
Click to expand...

Well yes and no. In a tube head the multi-tap output transformer transforms the tube plate load impedance from a very high number to the low impedance of the speakers.

Unlike a SS amp that runs more power with a lower speaker load the tube head does not.

However the lower impedance does raise the speaker current and lowers the speaker voltage. While the head still makes 300 watts its more like putting the 4X4 to the floor in low range... You still make 300 horsepower (watts) but the tach is screaming at a different RPM.

Where there is a difference is when he's running two cabs, he's driving two cabs. When I'm driving two cabs I'm making more volume... This means cranking the amp up higher.

While there may possibly be some shorted winding in the output transformer... My guess is the amp is just cranked. If that is the case then a few small changes or "Cares" will allow the amp to run cooler.

This is the same problem I had with my FOUNDATION. We hung the second speaker on it and turned the big go knob up to "LETS DO ROCK AND ROLL..." Without a blower in the head and only GRAVITY COOLING, the heat coming out the top was so intense a guy thought the envelopes of the tubes was going to melt. Thats a nasty smell and I KNOW the OP is smelling that smell!!!

BOB
 
#11
Wow, thanks to the mod for changing the name of the thread. This is definitely the help i was looking for :)

Ok, so the tubes - NOT entirely sure how many hours are on the tubes. Bought it used ( billy dickens was the previous owner...maybe i should call him haha)

Colors - No red lights on the back, tubes are all @ green lights. No blue haze in any of the tubes.

p.craig - this MAY have been happening before, but like i said, i've just left the amp running @ 8ohms and it's just seemed to work better?
I run at Ch 2 volume @ like 3, and the Master vol at about 3-4, but I'm running the Ch2 gain @ 10(yeah yeah i know...) but i run it like that all the time, this isn't something new as far as the gain is concerned.


No sound difference between 4 & 8 ohms that i can distinguish...just heat & smell(rbonner knows what's up with it). and i keep smelling it to make sure nothing's BURNING, and it's all good, but the bugger gets hotter than hell.

as far as tech goes, i'm in chicago, and i've never really had to bring my amp in anywhere before for any problems... i'm going to assume that going back to guitar center(where i picked up the amp) would be a pretty large waste of my time? if so, any advice on where to go to get it checked out at a reasonable price(just lost my job...so cash is kinda tight :()?

Thanks a ton everybody. I love this forum more now that i've actually joined it instead of just reading. you guys are a tremendous help to people like myself who know a little bit about what they're doing, but not all the nitty gritty that could make or break situations.

J
 
#14
+1 check cabs impedances ARE what they're s'posed to be. I had a Fender 300PRO(basically-same amp) & mine DID run HOT , but IIRC not any hotter in 8/4/2 ohms. prety much same in each. = tech time poss. I DID install a old PC fan-mounted it onto the inside of the perforated back panel... BUT as stated - it shouldn't run HOTTER in one impedance setting. I'd def. check that yr cabs ARE the correct impedance & if they ARE- worth getting it checked over- BY a TUBE AMP tech- not GC!! :-) Good luck
 
#15
If the amp works poperly it should handle 4 Ohms with ease. The OT has even a 2 Ohm tap - so I guess something may be wrong (with amp OR cab) when amp overheats at 4 ohm.

On the other hand I remember a band we played with and their bassplayer used a sunn 300t that he had equiped with 3 large fans mounted on the inside of the frontgrill - because of overheating problems that he had in the past as he stated. It was worth it, cause he had a killer tone this evening. :smug: So maybe the design itseld is a little prone of overheating if the amp is driven hard.

I d let a good tech check it and then build in some Rotron 115V fans - maybe switchable for studio use. Just to be on the save side.
 
#16
It looks like you run your controls very much like I do, except Channel 2 gain I run about 7. I noticed this weekend mine seemed hot. I think it's time to check the bias (both you and me). It is easy to do if you have a multi-meter and the amps manual.

BTW, I have only ever run mine a 8 ohms. Never needed more than one cab. As others have suggested, you mifgt want to verify your cabs are 8 ohm.
 
#17
Yeah, i think im going to have to get it checked out...just to be sure.
I'l have to look into installing some fans. that sounds like a great idea...anybody have any schematics/ideas for that?

as far as my cabs, i was thinking the EXACT same thing...so i checked them the other day at practice and they were both labeled @ 8 ohms.
I'm using an SVT-115HE(which is listed at 8 ohms) and an SVT 410HE(also listed as 8 ohms).

I'm practicing tomorrow, so what i'm going to do is try to run the head @ 4 ohms and see if this could completely be my imagination(doubt it, but can't be too sure). I'm not going to run the bugger until it burns, but I want to be absolutely sure that it'll be correct.

Thanks everybody.
Anybody know any tube amp geeks/magicmen in the chicagoland area that I can go talk to? Like I said before, I'm new to the whole amp repair stuff.
J
 

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