Sunn 300t - is there a doctor on the plane?!??

Discussion in 'Amps [BG]' started by freshchops, Apr 6, 2014.


  1. freshchops

    freshchops

    Joined:
    Apr 6, 2014
    I recently bought a used Sunn 300t (ebay). Unpacked it and all seems to be in excellent shape (cosmetically speaking). Before powering it up, after it's long trip, I went over it all making sure the tubes were well set in as well. I pulled the fuses out and noticed that the AC fuse was blown to hell... charred, fried, scrambled - all that! This immediately drew red flags however I have read enough about the fuses blowing on these series of amps for various reasons.

    Before proceeding, I ordered new fuses, including the 3 fuses at the pairs of power valves.

    Now the concern: The AC fuse that blew was rated at twice Volt rate. The blown fuse was a 10A / 250V fuse when the specs call for a 10A / 125V fuse. So it blew a fuse with a higher voltage rate than required!! What else could hva possible blown.

    Also, two of the three valve fuses were 500mA when the specs call for 100mA. These were all OK with the exception of the actual 100mA fuse appearing to have a little dark browning in the center of the glass (connector within still in tact though).

    Now that I have replaced the fuses, it will power into standby, no problem. The only problem is that of all of the power valves, the first one in the series doesn't light up or even warmup in the slightest when in standby.

    Figuring, I was going to be returning it anyway, I tried to connect to a cab and take it out of standby to see what may happen. When I take it out of standby, the power valved begin to glow and the amp is giving off a loud low freq. hum. One of the power valves (in the last pair) will continue to glow brighter and brighter as if it's about to explode. Soon into the out-of-controll valve glowing, I shut it off.

    The tubes look very old and I can assume one may already be dead (unless it's an issue with the socket or even a more internal flaw).

    I have committed to going with a high watt, all tube head and would really like to get this one going. I guess the question would be, based off of what you're hearing or have maybe even experienced yourself, I'm considering all possibilities for flaw and to what degree of damage or measure of doctoring will it take to get it back to peachy?

    If I am going to have to buy a new set of power valves ($200-300) and then even take to the local pro amp tech... I may be looking at the price of a new Ampeg SVT-CL Heritage by the time I'm done. I went with the Sunn because it was a cheaper alternative (assuming it'd work) and I'm pretty partial to Sunn / Fender amps.

    One thing to note, when I switch out of standby and the lone tube begins to glow to the degree of becoming the next closest star to our planet - the bias led indicators are all glowing green, as if the amp thinks they're all within proper bias range and just fine.

    If this (loud hum, one tube glowing excessively, etc.) could simply be the result of a shotty set of valve tubes, I don't mind replacing them with a new set. If it is more likely or from an amp tech's perspective, seems it'll require more internal work (like capacitors or worst), I don't know if it's worth the gamble of trying to get it going?

    If there are any amp tech's that can chime in on the issue (from an instinctual or professional point of view) or those who own these an may have experienced something similar, I'd be curious to hear alls opinions.

    I'd like to think it has hope, but not to drop $5-600 into it to find the cost would no longer make it a reasonable.deal.

    Any and all input and theories welcomed and appreciated....
    Thx!
  2. Mr. Foxen

    Mr. Foxen

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    Jul 24, 2009
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    Bristol, UK
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    Voltage on fuse doesn't mean a lot, its a max rating. Amp needs fixing, don't assume its new valves. Its never a whole set of valves, something in the amp has to be wrong before a whole set of valves would be damaged, so that will always need fixing first. Chances are if its a valve problem, its going to be only some or just one valve. Mains fuse going is not generally a valve sort of issue though.
  3. Mr. Foxen

    Mr. Foxen

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    Jul 24, 2009
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    Also, if it was sold as working on ebay, and the mains fuse is blown before you have even powered it, then open a case right away, they are being deliberately dodge but hoping you'd power it and blaming you for the damage.
  4. B-string

    B-string Supporting Member

    Joined:
    Nov 21, 2008
    Location:
    Lake Havasu City, Az USA
    Sounds like you have trouble in two output tubes, one with no heater operation and last one "red plating". It "could" be just valves but could also already have more serious troubles. If you have two old known good tubes to try in those positions (don't worry about matching) you "could" try them to know if there are more serious troubles in the head. Don't keep pushing things though, you can cause some really expensive damage. This is only a one time to assess the unit.
  5. BassmanPaul

    BassmanPaul Gold Supporting Member

    Joined:
    Aug 25, 2007
    Location:
    Toronto Ontario Canada
    If the plate of one tube glows red you will hear a hum from the output. If the power fuse was blown when you got it you should return the amp and retrieve your money.
  6. jastacey

    jastacey Supporting Member

    Joined:
    Feb 8, 2009
    Location:
    Houston,Tx
    I had one of those amps and they are something you don't want to do self repairs, you really need to get the ball rolling on the E-Bay Resolution .... I have used the E-Bay Resolution and they do make it right for you, but it takes about 2 weeks to get your money back .... the fact that they shipped a non-working amp raises questions
  7. B-string

    B-string Supporting Member

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    Nov 21, 2008
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    Lake Havasu City, Az USA
    I guess the big question is: Was the amp listed as "for parts or repair" or as a functional unit?
  8. ThirteenGlobes

    ThirteenGlobes

    Joined:
    Apr 29, 2012
    Hate to spam this forum with posts about my own 300T, but I had a somewhat similar experience - power tube fuses blowing, tubes running cherry-red, the works.

    To make a long story relatively short (and I'd be happy to expound upon it if you want to read and see how comparable our situations are), I took mine to a guy nearby who really knows his **** about the 300T specifically. I think that's the biggest trick to getting (and keeping) them working. There are a couple manufacturing updates that can be done to them, as well as hard-soldering some things together, moving some components around so they don't overheat each other, etc that will make the amps way more reliable. I had even taken the amp to a guy who was a Sunn expert (worked for Sunn for decades) who didn't have specific expertise with the 300T and he wound up doing more damage to it than he did good. As much as it sucks to have to bite that kind of financial bullet (I think my bill for the manufacturing updates, some repairs, fixed a "mod of unknown origin" that caused bias failure and made a pair of tubes run way hot, new fuses, etc was $270) it'll be worth it if you want to keep it and have it working. In good condition it's an absolutely astounding amp, all it takes is a little bit of wading through ******** to get there.
  9. freshchops

    freshchops

    Joined:
    Apr 6, 2014
    Thanks for all the input, and heeds of advice! I am hopeful and intrigued by what this amp offers. My local amp service tech noted that this could be just a power valve issue if one or more is failing or dead, the resulting red-plating and hum would be expected, but also could be some stuff on the board (such as resisters)... wouldn't know until he got into it. I wouldn't attempt to open it up myself. Way too much voltage to pry around with.

    I have proposed to the seller (ebay) to help me cover the repair cost / parts in leu of returning it and absorbing ebay fees, return shipping cost, etc. He seems to be favoring a return which I will likely end up doing, but I really do have hope for this head and would be willing to go a little further to see the extent it'd take to get it back to peachy.

    If I return it for a refund, I will be going the safe route with a new Ampeg SVT-CL (possible Heritage), but wanted to try something a little different and cheaper for that matter. I'm open to suggestions on that note too. If some think it's absolutely worth it to try at least give the Sunn 300t a chance (even over the Ampeg) or have other suggestions aside from the Ampeg. I am, at this point though, certain I want to go high watt, all tube. I just sold a Genz Benz 1200 on an Orange 410 & 115 cab rig. It had pretty broad tone and all but overall sounded very boxy (for most simplistic description). I realized what was missing.... the TUBES!!! I'm such a tube fiend as I've become accustomed to the tube warmth, fullness and musicality enjoyed through my guitar amps. Also, to note, I have been shopping for heads foremost, but I'm about dead set on replacing my rig with the Ampeg 810. I am looking for "That Sound" with the bass rig... the full, rich, warm lows / the growl, bite and punch as needed mid and high, etc. etc.... Pretty set on the Ampeg 810, so can assume whatever head I end up with will sit atop one of those.

    Thanks again for alls input, I've thoroughly read all and taken all into consideration.
  10. freshchops

    freshchops

    Joined:
    Apr 6, 2014
    Mr. Foxen, Yeah the amp tech noted the same thing about the voltage and just put the emphasis on the Amp rating per fuse. Could not be as bad an indication as I thought.
  11. freshchops

    freshchops

    Joined:
    Apr 6, 2014
    Unfortunately don't have any spare tubes, so it's be a $200-300 investment to get a new matching pair for me to try. May consider, as I'd replace with the SVT and could use the tubes later anyway. Also, yes, it was sound as "excellent condition and sounds great"
  12. freshchops

    freshchops

    Joined:
    Apr 6, 2014
    Otherwise, the seller has a pretty impeccable rating and seller history. Benefit of the doubt, he may not have known, but he is offering to refund the sell including return shipping cost.

    I wanted to ask you.... you mentioned you had one of these amps. What was your thoughts on it? What cab were you playing through and what did you end up replacing it with and to what benefit or noted difference in tone?
  13. freshchops

    freshchops

    Joined:
    Apr 6, 2014

    Yes, I'm very intrigued by this amp and it's potential so may go through the ropes with a good tech work-over. I just don't know if it's worth the cost of new tubes and service/parts in total when I could have ended up with an SVT Classic for the price total spent. You like the 300t... have you compared it to the Ampeg SVT Classics?
  14. Bill Hughes

    Bill Hughes

    Joined:
    Oct 17, 2013
    See if you can get it to function with two known good 6550's, assuming that you have 2 of the 6 in good shape.

    The three to the left and the three to the right are opposite in phase.

    Don't play it hard with only 2 tubes.

    Verify that all the sockets are working. If so, it may be worth re-tubing.

    Bill Hughes
  15. freshchops

    freshchops

    Joined:
    Apr 6, 2014

    Bill, I'm pretty certain that at least 3 of the tubes are fine. I'll give this a try. I've heard of this for guitar amps but know nothing about this for bass amps.... "super high wattage" bass amps.

    So, if the tubes are opposite phase, and I'm counting off the 6 valves, left-to-right (facing back panel), 1 - 6 - would they match up like 1+6, 2+5, 3+4. Or would it be 1+4, 2+5, 3+6? ...or something completely different. If I understand correctly, it'd be like 1+6, first socket and last socket, eh?

    Granted I don't push much signal through, you're implying it's capable of functioning with only 2 tubes at a time (matching phase pairs)? No risk of blowing anything? I also assume you mean to ONLY have the matching phase pair in at a time correct?

    Hypothetically, If I take a good pair of tubes and power up trying each matching set of sockets, experience no dead-plating or red-plating, no excessive hum (would the signal still hum via under-supplied valves?), and all pair configurations work fine... this would likely point to the flaw being a dead tube(s)? .... that the idea?

    Don't mean to be dense about it, just want to make sure I understand correctly.... Thanks for the idea/tech tip. I forgot, I've done this before some time ago (Fender Twin Reverb) to eliminate a shotty valve.
  16. Bill Hughes

    Bill Hughes

    Joined:
    Oct 17, 2013
    No...one in EACH phase! Make sure that all sockets work, by using both phases.
  17. jastacey

    jastacey Supporting Member

    Joined:
    Feb 8, 2009
    Location:
    Houston,Tx
    I had the Bassman 300 Pro, which I believe is the same amp ..... I liked it ... was able to get an Ampegish tone out of it, but I liked the clean tone I could get with it .... it is an amp that can get pretty loud cleanly, I used it with an Ampeg 810 (with Jensen MODS in the upper top two, this was one of those 90"s dark sounding cabinets ), a Berg NV425, Berg NV610 and a Red Star Audio 412 .... one of the several reasons I moved on, is I just got tired of moving 100 pound amp, in a road case and I like having an amp that has parametric EQ vs. Fixed point EQ ..... and at the time, I had GAS for other gear

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