Rewiring my cabs?? Ohmage switch??!!

Discussion in 'Amps and Cabs [BG]' started by jja412, Sep 12, 2004.

  1. jja412

    jja412 Fine gear enthusiast

    Feb 2, 2004
    St. Louis
    Okay - here's the deal...

    I have an SVT from the 70's, and I run it through my Eden 410XLT, or my aguilar gs210. Sounds magnificent through both.

    However, I have read several posts yesterday and today that warn against running all tube amps at HIGHER than recommended ohmage. My SVT is rated for 2 or 4 ohms, and both my speakers are 8 ohms.

    Obviously, the easiest solution is to run them both together, making a 4 ohm load. BUT... in most of my gigs, this is frowned upon. (Church with some volume issues and ignorant sound men.)

    So... I am looking to manipulate each cabs to run 4 ohms or 2 ohms. I assume that both can be wired in series to make a 2 ohm load each. But I would like the option of running them both on big gigs. So I need a selectable ohmage switch or somekind of dummy load?? I know Accugroove has load switching, is it possible for me to do this - or should I sell my cabs and get some new ones??

    I would prefer to mod them for financial purposes. But I need some advice on how to do this before I try to proceed either way.

  2. Thunderfunk


    Mar 27, 2004
    McHenry, IL
    It's OK to run a tube amp off by one ohmage step (8 ohms on a 4 ohm tap). Only a slight loss of power. If you're off by two steps (16 ohms on a 4 ohm tap) it will affect the tone. Running an 8 ohm cab on a 4 ohm tap is better than running a 2 ohm speaker on a 4 ohms tap, so it's better to run a higher impedance. Don't confuse a higher impedance with a higher load. A higher load is a lower impedance.

    Dave Funk
  3. jja412

    jja412 Fine gear enthusiast

    Feb 2, 2004
    St. Louis
    I've read in several places here that running an all tube amp like my SVT at higher ohmage rating (meaning 8 ohms instead of 4) is bad because it raises the plate voltage and can fry the tubes and output tranny. I believe the posts are by Psycho Bass Guy, who has been a tube tech for a long time.

    But Dave - you make tube amps too (not the Thunderfunk, but guitar amps) so which is correct?

    I'll try to find the posts I read for clarification. I may have misunderstood PBG's post.
  4. jja412

    jja412 Fine gear enthusiast

    Feb 2, 2004
    St. Louis
    Quote of Psycho Bass Guy:

    "Your amp repairman is telling you right. HIGHER impedances than rated are bad for tube amps (or s/s output transformer coupled amps, but since they're not around anymore , I digress..) While a 100% mismatch is generally OK, it is NOT healthy for the amp and will stress the output tubes and transformer. How dangerous this is depends entirely on how close to tolerance the tubes are being run. Considering the SVT already runs above the 6550A's rated maximum B+, it's not a good idea. That huge output tranny will get you by for awhile, but when the amp goes, it will be a spectactular failure."

    click here

    and here
  5. Thunderfunk


    Mar 27, 2004
    McHenry, IL
    You mean Thunderfunk tube amps? It's the same brand name.

    So 6550A's are rated at 600 volts, and the SVT is 660 volts. I wouldn't worry about it. That's still only 10%. And as you play through it, that voltage drops. So when the amp is "stressed" it's actualy lower. And if you're using real American made 6550's, they don't suddenly blow up at a few volts over. The real problem with 6550's is the control grid is very close to the cathode and can be shorted out by vibration. If this happens, you'l lose the bias voltage and the tubes will go cherry and be ruined. Tube amps are high maintenance these days.

    And the ohmage switch is a patented AccuGroove device. I don't know how it works, and my guess was wrong. You could rewire the cabs with a switch to change it's impedance, but I doubt that would work out to be practical.

    Dave Funk
  6. Eric Moesle

    Eric Moesle Supporting Member

    Sep 21, 2001
    Columbus OH
    My understanding: There is no such word as "ohmage", the proper term is "impedance". No?
  7. Lyle Caldwell

    Lyle Caldwell

    Sep 7, 2004
    You *could* install an 8 ohm Weber Mass in series inside each cab (or two in one cab, if you're using a 2 speaker cab), with a on/off switch. That would safely change the resistance of your speakers/cabs (as long as you don't change it while the amp is on).

    I don't know how cost effective that would be, but it would work.
  8. notanaggie

    notanaggie Guest

    Sep 30, 2003

    In any case, with 600 volt plate voltage, the tubes see at least 1200 that isn't a huge issue.

    In fact, in manufacturing, tubes are "flashed off" with a much higher voltage to vaporize and clean up crud. As high as 3000 volts.

    If that ISN"T done, they can fail very quickly.

    SVTs have been run at 8 a long time....and the 4 ohm speaker goes to 30 or so at resonance.....of course 8 goes to over 50......whatever......if you don't push it to max you should be fine. Since teh whole point was the lower volume, you are OK.

    Thought of a dummy load of 8 ohms at 200W or so? That would fix you up and keep everything happy if in parallel with your 8 ohm speaker
  9. Munjibunga

    Munjibunga Total Hyper-Elite Member Gold Supporting Member

    May 6, 2000
    San Diego (when not at Groom Lake)
    Independent Contractor to Bass San Diego
    Isn't that voltage applied to the "getter" and not the elements?
  10. You are correct Munji; it's the getter that gets the voltage surge. I stand by my previous posts although the points about keeping the volume low DO make a difference.
  11. notanaggie

    notanaggie Guest

    Sep 30, 2003
    NO, that is NOT about the getter.

    The getter has no external connections. It is usually activated by induction heating. (note most getter elements are circular, you can see them right under the silver spot on the glass)

    The high voltage is applied as I said. It isn't for gas, like the getter, it is for bits of crud that are left after assembly.

    This is not for debate...its a fact....I have spoken to engineers who worked in the tube manufacturing business. Statement was that if you don't do that, you will have failures in service under conditions within the ratings.

    If you use 8 on the 4, and drive the amp very hard, I would expect damage. But if you play at the lower volume originally described, 8 on the 4 isn't deadly. It just isn't as good an idea.
  12. The question still remains: Is there a way to re-wire an 8-ohm cabinet to 4-ohm? Any way to do this with...say...a 4x10 Acme cabinet?
  13. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Not really. The typical 8 ohm 4-10 is set up with either 32 ohm speakers all in parallel, or 8 ohm in series/parallel.

    Now, 2 ohms you can easily get by wiring all the 8 ohm speakers in parallel.
    Since the SVT in the original post has a 2 ohm output, a 2 ohm cabinet would be fine.

    There have been several manufacturers who have made speakers that were switchable to either 4 or 16 ohms. It is a simple switch connection. The 8 ohm/2 ohm change is the same system.

    If you have 32 ohm speakers in parallel already, you can't do that, so in that case you are out of luck on re-wiring.

    About the 8 ohms on the 4 ohm tap.....I agree it isn't instant death to a decent amp. But it WILL increase the voltage stresses somewhat. We don't recommend it, and I wouldn't do it on an amp like the SVT. Certainly not if I expected to drive it hard.

    It is true that the tubes are/were generally "cleaned" or flashed-off with high voltage in the manufacturing process. it is a separate thing from vaporizing the getter.

    We got a shipment once that hadn't had that done. They would fail with spectacular arcing internally when driven hard into clipping.

    We didn't know about the flash-off cleaning process, but we eventually put two and two together and did something like it ourselves. We found that the tubes then worked fine.

    When we finally found a smart old-time tube engineer at one of our suppliers, he told us we had reproduced what the manufacturer should have done, but skipped! It is done to get rid of vaporizable crud, by vaporizing it in a safe way and letting the "getter" take care of it without power on the tube.

    Incidentally, that story should indicate the truth of the "high voltages at clipping" theory. What was going on was that the high spike voltages at clipping drew an arc in the tube, and the high available power caused the arc to destroy the tube, partly because the crud vaporized and provided conducting gas.

    Would it happen without a piece of crud somewhere to help the arc? Yeah, possible. Might or might not destroy something. I'd avoid doing that with a nice vintage SVT, but YMMV. :eek:
  14. jja412

    jja412 Fine gear enthusiast

    Feb 2, 2004
    St. Louis
    Can you suggest some "dummy loads" I could hook up. I have no experience with this... :confused:
  15. jja412

    jja412 Fine gear enthusiast

    Feb 2, 2004
    St. Louis
    Yes, I meant Thunderfunk tube Amps. I'm just so used to calling the BH420 "the Thunderfunk" that I had to clarify.
    Sorry bout that - and thanks for the replies.
  16. No.
  17. Petebass


    Dec 22, 2002
    QLD Australia
    Dummy loads are not a good idea. Say for example you put an 8 ohm resistor in your 8 ohm cab to create an overall 4 ohm load. As you play along, the wattage produced by the amp will be split 50/50 between the 8 ohm resistors. In this case, the speaker gets half the power, and the 8 ohm resistor gets half the power. In other words, the speaker is probably recieving less watts that it was before, and the resistor will generate a lot of heat via wasted watts from the amp. It's very difficult to find resistors with a high enough wattage rating.

    jja412, I wouldn't worry so much. You mentioned that your church requires low volume playing. All this talk about amp failure is only applicable if your driving the amp fairly hard. From what I can tell, that's a situation where you're likely to use both cabs anyway (and hence a 4 ohm load).
  18. jja412

    jja412 Fine gear enthusiast

    Feb 2, 2004
    St. Louis
    Yeah... good point. I guess I'll just combine for the louder gigs. Or buy a Schroeder 1210.
  19. Didn't argue that; matter of fact, I agree completely and know that you'll not find a "getter pin" on a tube base. It's the voltage surge that heat the tubes and "fires" the getter. I'll also go one further and wage an educated guess which supplier gave Ampeg the unfired tubes, as related by Jerrold: they're OLD Svetlana stock from the 80's (back when they were just "Saratov Russian tubes"), sold as 6550B's. Since their getter were never fired, they de-rated their performance in order to try and minimze field failures. The "unfired" getter pill disc is VERY visible in every example I have seen and there is NO silver getter deposit anywhere on the glass because the getter was never fired.

    All that still doesn't change the fact that too high a load is NOT a desirable condition, but at low volumes, it shouldn't be dangerous. That 3kv+ "fire" surge is a MOMENTARY pulse, not a sustained operating condition, but again, I'm NOT arguing with you and I agree that jja412's SVT should be OK.