6550 tubes for SVT et al.

Discussion in 'Amps and Cabs [BG]' started by Decedo, Apr 29, 2012.

  1. Decedo


    Nov 23, 2008
    I'm going to have my old MTI SVT retubed. I was talking to a tech about the process and he was telling me about the 6550s. Also want to state I'm not a tech, but just trying to re-iterate what he told me.
    He asked what my amp was 'rated' at. I told him 300 watts. He said the specs for the 6550 at 100% only put out 35 watts each, so 6 would be a total wattage of 210 watts. He showed me a website calculator for tube wattage per volt drive website (and milliamp I think). I'll post it when he sends it, couldn't find it now. He said that was at pushing the tubes at 100% (hot) with 600V. It's also typical to set them between average and cold, so this calculated their wattage to around 24 watts each, to a total of 144 watts. I went what?!??!?! :confused: The amp is rated at 300, and it's typically LOUD. What am I missing here? Any Ampeg tech able to 'fill in the blanks', and let me know where the (Ampeg) watts are coming from?

  2. 96tbird

    96tbird PLEASE STAND BY

    Amp questions are supposed to be over in the amps forum. ;) Tube watt rating is "dissipation in watts", and is only one part of the final resulting output of an amp. For this discussion, just know that the output transformer and the manner in which tubes are wired and voltages fed into them for operation are the determining factors. Your tech should know this and surely should have worked on an SVT by now and know that 300 watts is their rated output. Find a new tech who knows what's up, I wouldn't let this guy touch my V4-B! But I'm no expert, I just can spot a guy who knows what he's talking about when providing a service.
  3. Plucky The Bassist

    Plucky The Bassist ZOMG! I'm back from the dead! Supporting Member

    Jul 30, 2010
    BIG + 1 to this

    If this were an odd or boutique amp I could see the confusion, but many people regard the SVT as THE bass amp for a classic bass rig tone. If he's this confused with an SVT, I would look for another tech who knows something about quintessential tube amps for bass guitar.
  4. Hi.

    With the right tubes a SVT will put out 300W, 100Hz (IIRC), 5%(IIRC) THD. Some combinations can go higher.

    With generic CP tubes the figure Your tech quoted is in the ballpark for the same THD figure.

    If someone tells You that any SVT with any tubes will put out 300W they're just BS'ing You.

    "Rated output" by itself means about as much as Peak-, Program- or Music watts on a loudspeaker enclosure from a questionable manufacturer.

  5. Decedo


    Nov 23, 2008
    Thanks Guys,

    I guess I'll be looking for another (Bass) amp tech. I went to this guy because I heard he's a local tube amp wizzard, and I've heard guitar amps he's hot rodded, and they sound pretty wild. Having said that, I'm currently living in Sweden and old tube Ampeg SVTs are very few and far between. There's not alot to choose from as amp guys go. In Vancouver it was easy to find capable hands. I could send it to a guy in Stockholm, but the freight would be huge :(
  6. rickdog

    rickdog Gold Supporting Member

    Mar 27, 2010
    It sounds like this tech may have some good practical skills troubleshooting and repairing tube amps, but may be a little short on theory of how they work. Perhaps he was looking at the plate dissipation rating on a JJ 6550, it's 35 watts. But that is not what the output power of a well-designed amplifier like the SVT would be. This doesn't mean he's a bad tech, he doesn't have to be able to design an amp to fix one.

    You could buy yourself a good digital voltmeter and learn how to bias the new tubes yourself. Search the forums, you'll find the information you need. Look especially for posts by Jerrold Tiers, BassmanPaul, Johnk_10, Beans-on-toast, and many other tube amp gurus here whose names don't come to mind immediately.

    BTW, I have Tung-sol reissue 6550s in my SVT-CL, and I've been happy with them so far (with the caveat that I haven't had much opportunity to run them at significant output power).
  7. Lo-E


    Dec 19, 2009
    Brooklyn, NY
    A little short is being kind. Any amp tech should know the difference between max plate dissapation and Class AB output.
    He might be very handy indeed with a soldering iron, but I wouldn't trust my vintage amps to someone who didn't understand what was going on under the hood.
  8. Alex1984


    Jan 16, 2010
    Actually, a well-respected local tech here from an authorized Ampeg repair centre also reports the same thing. He said the output of an SVT on some of the service manuals was around 180 Watts or so.
  9. coreyfyfe

    coreyfyfe Supporting Member

    Nov 19, 2007
    boston, ma
    This one says 300


    And bassgear mag measured the output of Tom B's SVT at 300/330 watts per channel at <5% THD (issue 2 or 3 I believe?).

    Edit: it was issue #4, channel 1 was 319 (323 burst) at 4.06% THD, channel 2 was 328 (340 burst) at 4.06% THD. It would be interesting to know how the amp was biased and what tubes were running for anyone who has spec'd out their SVT.
  10. johnk_10

    johnk_10 vintage bass nut Supporting Member Commercial User

    Feb 16, 2008
    Washington, Utah
    John K Custom Basses
    i've measured mine at full output and they ranged from 288 to 300 watts with 5% distortion.
  11. Well my Mesa Bass 400 does about 245 watts from a sextet of 6550.
  12. timv


    Jun 7, 2000
    Chandler, AZ
    How much distortion????

    There is no, I mean NO way a bass 400 puts out that much.
    They are not made to do so.
    I've ran a 400, d-180 and m-190 power amp with several types of 6550's including GE's and 6L6's and max clean no distortion was about 180 watts.
    Adjustable bias tweaked for each set.

    Back to topic on hand.
    My old 70 SVT with 6146's does about 360 clean but all 6550 SVT's I have tested do max clean right around 270 watts.
  13. beans-on-toast


    Aug 7, 2008
    Here is the data sheet for the GE 6550A tube.

    At the top, it states that the tube is designed to provide up to 100W output for two tubes in a push-pull amplifier. That would be 300W for a sextet of tubes. This is based on the published specs. See the box on page two where they state that these specs provide an acceptable service life of the tube. What they aren't telling you is that the tubes can be pushed a bit harder. Ampeg ran higher plate voltages than the 660V maximum that is specified in the data sheet. My '71 SVT reads 673V at the plates with a 117 VAC line voltage.

    Also note on the data sheet that the tube has a 42 watt plate dissipation. This is how much heat the plate inside the tube can dissipate. There is another spec for screen dissipation. These wattage specs are not the same as the output power which the tube is capable of producing. This can confuse people.
  14. beans-on-toast


    Aug 7, 2008
    Check this out to see what a sextet of GE6550A's can do. They claim 560 clean RMS watts from a Fender 400-PS amp with six tubes. The amp was designed to put out 435W.

    One reason why you used to see a lot of GE tube sets in SVT's.
  15. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Worked on SVTs in the 1970's, had "a little more involvement" since....and from then to now, I have never seen one with decent tubes that didn't put out anywhere from 275 to 330W.

    100W a pair is right for 6550. It's in the tube manuals, at 600V, 5000 ohms plate-to-plate.

    You could get 100W a pair out of regular old-time 6CA7 also, at a significantly higher voltage. (But the new substitute for 6CA7 isn't even close).
  16. NOVAX


    Feb 7, 2009
    just do not go tinkering around and touching anything in there or you could kill yourself

  17. B-string

    B-string Supporting Member

    Can't remember testing any (TUBE) SVT that made less than 280 watts @< 5%THD.
  18. timv


    Jun 7, 2000
    Chandler, AZ
    I do not own a distortion meter so always test to the point right before the wave form tops start to clip.

    I'm trying to find a good distortion meter to have in my tool arsenal but all the 334a's on ebay are always pretty expensive.
    Any other recommendations?
  19. beans-on-toast


    Aug 7, 2008
    The HP334A does a fairly good job but you have to manually turn dials a couple of times to balance the bridge. Once you get the hang of operating it, it can be done fairly quickly. It is a nice analog instrument. If you are patient and shop around, good deals can be found. I recently saw one for $250.

    The HP8903A audio analyzer is quite nice because it calculates the results automatically. I've seen good deals on them recently for around $500.

    The Tektronix AA501A is easy to use but requires a low noise signal generator and powered rack. Costs can add up.

    Some spectrum analyzers can be programmed to calculate THD and noise.

    The modern digital audio analyzers like those from Audio Precision are expensive.
  20. rickdog

    rickdog Gold Supporting Member

    Mar 27, 2010
    Holy something-I-can't-say-on-TB! They don't make 'em like they used to, do they? I have datasheets for JJ and Svetlana 6550s, they only claim 35W plate dissipation. That amp would eat them alive....