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I can only put my musicman on low power? (help tube techs)

Discussion in 'Amps and Cabs [BG]' started by AckAckAttack, Nov 3, 2006.

  1. AckAckAttack


    Aug 27, 2006
    So about 2 weeks ago my musicman hd130 amp started to kind of crackle out on me, and I bought it a few months ago and never brought it to a tech (I know, a mistake, but the guy I bought it from worked on amps himself so I wasn't too worried.) I took out the chassis and noticed some of the tubes defenitely looked bad and when I took them out 2 were in rough shape and 2 looked good so I ordered 2 EH 6CA7's off the bay and tried to get in touch with my normal tech, but I couldn't reach him for a week so I decided to try a guy I had heard about that repairs amps and specializes in mods.
    I brought the amp to him and he seemed to really know his stuff, but he said a few things that I am worried about, not to question his knowledge, but it just seemed a little weird.

    First off... the two tubes that I thought were good werent so I have to order another pair, but he said I could now only run the amp at low power (the hd130 has a switch for high or low power, I used it on high before). He said the amp runs at about 700+ volts on high which is way more than any modern day tube would handle and it would possibly ruin the amp. Does this sound right? The low has about 300-350 volts, he told me never to run it on high, and I'm pretty sure he meant when it had all the tubes in it as well.

    Also, he was running the amp with just the two tubes (two middle tubes) in it out of 4, but he said it was fine for the amp, but I've heard its bad for it, is he right?

    Also, it obviously didn't sound as loud as it did before on high, but I'm looking for a really loud sound, and I asked him if it would be louder once the two other tubes were put in and biased, and he said probably a little, but not much, but it seems like this should double the watts going out the cab, since I am doubling the amount of power tubes. Is he right though? Will the full set of tubes not make that much of a difference?

    I do trust this guy, but I just wanted to check with some of you to see if he knows his stuff. He did a great job with the mods though, and he's real cheap (4 tubes biased, pots cleaned, and 2 mods to the amp for $25 flat). If anyones in the MA area, you should go to this guy.

    Any and all help is greatly appreciated.
  2. BbbyBld


    Oct 13, 2005
    Meridian, MS
    No. Quality, modern tubes can handle that kind of voltage. I would go with JJ EL34s...I know for a fact they can handle the voltage.

    That's fine, you should remember that if you run half the tubes, you need to go to the next impedance setting down...in other words, if you have an 8 ohm cabinet, the amp will see it as a 4 ohm cabinet. I'm not sure if your amp has a selector or seperate jack for different impedances, but if not, that could cause a problem.

    The impedance should change when you cut the voltage in half as well...hmmm, I wonder if they switch that as well...

    You'd double the watts, but not the volume. You'd need 10 times the power to double the volume.

    You would definately notice a difference with the two other tubes. That amp should be loud as hell with a 700 volt plate and 4 EL34s (6CA7). You should get over 150 watts like that.
  3. AckAckAttack


    Aug 27, 2006
    Thanks a ton for the help... so you're saying with quality tubes (like JJ's or Groove Tubes), that this could handle with it being set on high defenitely? Even though its at around 700 volts?

    Also, when you said that it possibly cuts the ohm load in half when you run the amp at low I think that would be normal on some of them, but with this amp when you run it on low it doesn't just run 2 tubes out of 4, it just runs all 4 at a lower level.

    Thanks for all help, its greatly appreciated.
  4. Here's your first problem:
    First and foremost, a tech's job is to get an amp running to spec, THEN see if any mods are needed. MOST of the guys who "specialize" in mods are hacks who have read a couple of amp how-to books by some less than reputable people (cough... Gerald Weber... cough... Dan Torres) and know very little about how an amp is SUPPOSED to run. It's a sad fact that in this day and age, it's not just enough to find someone willing to work on an amp; you need one qualified to do so. It sounds as though you haven't.

    No, it is NOT right. Yes, the Music Man does have very high voltage, but it runs its tubes near Class B most of the time. That means they are MOSTLY in cutoff and literally pulse for output power. Modern 6CA7's have been used very effectively in them as reputable techs who also sell tubes can attest. People like Willie Whitaker (Lord Valve) and Ned Carlson (Triode Electronics) have been happily supplying Music Man amp owners with output tubes for years.

    That depends on the load connected to it. When you remove the two tubes and change the primary output impedance, you must compensate with a corresponding change in secondary impedance. Otherwise, you run too low a load and can age the tubes prematurely. It won't hurt the amp per se, but isn't a great idea either.

    I can't find a schemtic for the HD130 without knowing its chassis number, but what he told you does not sound right. Typically, a half power switch disconnects the tubes, and does not reduce operational voltage. It's been so long since I've even seen an HD130 much less been in one, that I didn't even remember them having a half power switch! Sorry I can't be more helpful there.

    What mods?

    If you're only running two output tubes, how did he bias four?
  5. BbbyBld


    Oct 13, 2005
    Meridian, MS
    I would not suggest GT because I've had bad experiences with them in the past. The JJ is solid. I would stay away from the current EH/Sovtek EL34 too.

    That amp could probably stand some new caps.

    Do you know the year by any chance?
  6. AckAckAttack


    Aug 27, 2006
    Well the half power switch doesn't just use two tubes instead of four, it runs all 4 tubes at a lower level.

    He put a couple mods on it, and he made the first channel a higher gain channel, and the second channel he made a sound a bit more full sounding, almost like a boost in the eq or something, I don't really know circuitry well so I don't know exactly what he did, but he said it took him years of experimenting to find the right spot to mod them, and it sounds good to me.

    He hasn't biased four yet, but he said if I bring him the amp with two more tubes he'd bias them for me no extra charge, and he told me the caps should probably be replaced too, he showed me a little spot where it was bubbled.

    So it sounds like I can run this at high? I got the fat bottle EH 6CA7s for this, because I wasn't even sure if it was the tubes that were creating a problem in the first place, but will they be able to handle the amp on high? Or will only high quality tubes be able to handle it?

    Oh and for the load impedence, I run this with an 8 ohm cab, so should I set the ohm load to 4 since it only is on 2 out of 4 tubes?
  7. AckAckAttack


    Aug 27, 2006
    Is the el34 fine for this amp? I've heard that the fat bottle 6CA7 handles high voltages the best, but then again, a lot of what I've been hearing lately hasn't been exactly true.

    I'm getting some new caps put in when I bring it back to get the other 2 tubes biased.

    Unfortunately I don't know the year off hand, it was originally the 210 combo model but then at some point the cabinet was rebuilt and now its only the head.
  8. BbbyBld


    Oct 13, 2005
    Meridian, MS
    That's what you normally do.

    Originally, the 6CA7 was pretty much the same tube as an EL34. From what I've read, the new EH 6CA7 is not a true pentode, they call it a beam tetrode--so it doesn't even sound like a real 6CA7. I have no experience with the new tube tube, so you're on your own with that one. Once again, I would use the EL34.

    That's probably a good idea. I would ask to hold on to the old ones. Don't forget to check the bias supply caps. Make sure the amp has the right fuse.
  9. AckAckAttack


    Aug 27, 2006
    Ok, thanks a ton for the help, I'm kind of wishing I had known this before I spent $60 on those EH tubes.

    But do you think the EH 6CA7's will be able to handle the voltage on high power without damaging the amp at all?
  10. NO- I've heard that the EH 6CA7's will NOT handle the 700+ V I've got an ol' Aussie "Eminar" amp-based on a MArshall, made in '78- & I just wasted some good $$$$ on EH EL34's- have also been doing some enquiries on what Tube WILL actually handle this type of V.( mine puts 700+v also) I've been told the EH6CA7 will POSSIBLY handle it BUT......
    THE ONLY tube I've been told is the strongest, that will handle it is the JJ KT77( haven't heard re the JJ EL34's- if anyone can tell me DEFINATELY that they'll handle the V's then I'll go with 'em, but I've heard that even tho some tubes CLAIM to be able 2 handle the voltage they do so BUT not for long-as what happened to mine-BTW I bought EH EL34's-6 of 'em- one blew after 1 & 1/2 gigs, 3 blew at the same time a week later- NOW I've got the OLD tubes in-the ones I originally replaced-they're about 15-20 yrs old Philips 6CA7's- & even tho a couple are MICROPHONIC!!!!- they at least let me use the amp!!)
    SO I'm gonna order me some JJ KT77's( a matched sextet) ASAP unless anyone can reccomend me something stronger- AS I've heard these are the strongest tube that'll fit into this amp in place of EL34/6CA7's.
    I'd love it IF anyone CAN give me some insight into some OTHER options.... eg.. using 6550's or some other tube- would I have to change the amp?????? apart from biasing?????
  11. If you care to send me your chassis number via PM or post it (there's no risk in it) I can confirm that through Music Man's own schematic. I don't consider what your tech told you reliable.

    Modifying a preamp is relatively easy compared to some of the other stuff in the amp and is by no means a demonstration of competence with tubes.

    If he showed you that but didn't fix it, that's ANOTHER strike against him. A bubbled cap is almost certainly bad and should never be left unchecked.

    My experience with EH/Sovtek beam tetrodes shows that they will most likey be fine and are in fact high quality tubes.

    In general, the 6CA7 IS a better tube for high voltages and specifically Music Man amps.

    This isn't quite correct. The 6CA7 is an equivalent tube to the EL34, which is an older design. There are important differences though; a 6CA7 has better current capability which means better low end and overall stability. It is also a beam tetrode which means instead using another control grid for high output, it uses a beam former which is more efficient, as do the 6550, 6L6, 6V6, KT88, KT66, KT90, and KT77. A 6CA7 is NOT supposed to be a true pentode. KT77's (which you all seem so hot for) are simply British copies of American 6CA7's in smaller bottles and are themselves beam tetrodes. The EH6CA7 is a REAL 6CA7.
  12. BbbyBld


    Oct 13, 2005
    Meridian, MS
    There must be different versions of the 6CA7. The ones I've come across were pretty much identical to EL34s.

    Every 6CA7 datasheet I've ever seen has the seperate suppressor grid just like an EL34 with pin 1 not connected to the cathode.

    The MM schematic shows pin one tied to ground and the 6CA7s are drawn like pentodes.

    Pete Traynor even used the negative suppressor trick on the YBA-3 and those came with 6CA7s.

    Am I missing something?
  13. andertone

    andertone Supporting Member

    Mar 22, 2000
    Southwest USA

    Listen to Pyscho bass guy, he's right on!

    I have a 76 HD130 and have tried every tube in the book.
    Best I tried was Sylvania 6CA7, worst was Sovtek.

    Right now I have Electro Harmonics 6CA7s, and my tech
    was impressed by how they handle the big plate
    voltage. Been happy for two years now. They are
    touchy and need to be biased.

    Big bottle 6CA7 good, EH34 doesn't provide a nice low
    volume bottom IMHO

    Phil Anderson
    Tucson, AZ
  14. AckAckAttack


    Aug 27, 2006
    Thanks for all the help from everyone.. its greatly appreciated. So it sounds like the EH6CA7 can actually take the high voltage, so I'll try it out on high once I get the other tubes in and see how it goes.
  15. There are and the confusion is the tube makers' fault. The earliest Amperex 6CA7's were nothing more than EL34 copies, meant to cut into Mullard's huge sales of the type. After the success of the 6550A, GE redesigned its "6CA7" as a beam tetrode. (It is rumored that GE 6CA7's were just 6550's that didn't meet spec.) Sylvania soon followed suit.

    The answer to this is twofold: the reason the 6CA7 works in place of an EL34 is that the beam former acts a virtual control grid and can be connected as pentode. The reason all the tube pinouts diagrams are wrong is that they were made from the oldest "6CA7" datasheets, which are simply renumbered EL34 datasheets. It's VERY rare that you'll come across a correct pinout for a 6CA7 because of this; even the Duncan amps datasheet is wrong:


  16. Interesting!!! So U reckon I'd be safe with the EH BIG BOTTLE 6CA7's???? The tube sellers that I've talked to reckoned the JJ KT77 was a STRONGER tube?? So I'm back 2 not knowing which one to get! :bawl: I DO like the tone/power the EL34/6CA7 's put out, but does anyone have ANY experience with the JJ(or other sort of) KT77???
    THANX to all for input here-tis VERY interesting for me. AS right now I'm running the old amp with 3 old PHILIPS 6CA7's, 1 Ruby EL34, 2xEH EL34!!!!!!!!!!!!!!!! :eek: :eek: NOt how I want it to run at all!!!!! Can I risk hurting the amp in doing this???????????????Its just until I get a matching set of EH EL34's BIG BOTTLE's or JJ KT77's!
  17. AckAckAttack


    Aug 27, 2006
    One last question... if the guy biases the other two EH 6CA7's into my Musicman with the thought that it will only be played on low, would this change his biasing job? Or is a biasing job the same either way?
  18. ANYONE???????? Bump!!!;-)
  19. It makes a HUGE difference.
  20. AckAckAttack


    Aug 27, 2006
    so should I bring it to someone else to bias it? or just tell the guy that it's ok to run it on high and bias it that way?

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