Selmer question--Unquiet, anyone

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by 94prs22, Aug 7, 2003.


  1. 94prs22

    94prs22 Member

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    After considering it for several weeks I just pulled the plug on a deal to trade my Maven Peal Zeeta head for a Selmer Zodiac Twin 30 (including what appear to be it's original oyster shell Celestion g12 speakers, number T731). Aside from being astounded at how great this amp sounds I have a couple of questions for anyone whose owned/messed with one. First are there any specific problems with this amp model that I ought to be aware of? This particular amp has had a professionally installed Mercury Magnetics replacement power tranny so that the amp will run on 110 volts with a transformer, at the same time the filter caps were changed and any burnt/damaged components were changed as well, right now it sounds great. Second, do I have any risk of blowing these original speakers by running the amp full bore? The previous owner removed the originals and used reissue g12h30s so I don't think these originals have seen much abuse recently. Third, is it cool to use more than 1 push button control at once? The amp lets me do it and sounds fine but I'm not sure if it's gonna cause problems. Fourth and last, I think the 2xEL34s in this amps power section run in a self-biasing, ultra-linear manner right? I guess the real question is when I change output tubes I don't have to worry about biasing problems do I? Oh, one more, can this amp use Siemens EL34s without burning them up, anybody know what the plate voltage is supposed to be?
    Thanks
    Brandon
     
  2. John Phillips

    John Phillips Member

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    1. No, there are no specific problems (but see '4'). They're very well built and very reliable. A retro 'beat' group (The Kaisers) who I know and worked for used them as their working backline for many years with virtually no problems, and they weren't exactly babying their stuff... quite the opposite, it got hammered. Very little trouble at all.

    2. Probably not. Same applies - The Kaisers never blew any speakers apart from the 50W 18" Goodmans in their bass cab (driven with a Selmer Treble'n'Bass 50 of the same Snakeskin era), and they really cranked those things.

    3. AFAIK if you use more than one buttons at once, all you get is the same effect as the lower of the two (or more) buttons you've got pressed. Certainly it won't do any damage (although it can be tricky to get them up again if you've pressed them all!)

    4. Be careful with power tubes. This is the one area where you can't just expect to plug'n'go. The plate voltage is around 500V, and the amp runs the tubes very hot. You really need top-quality EL34s (eg Mullards - I would not recommend Siemens) that will handle both the voltage and a dissipation of a bit over the rated maximum of 25W. You also may need to change the bias resistor (or at least check that it is giving the correct results). The amp runs at very close to true Class A, so you can get away with setting the idle dissipation to almost 25W (50mA per tube, or 100mA the pair). The last one I worked on was over this with the stock 220-ohm (or 250, I couldn't tell since the writing was burned off! - it measured about 240 I think) resistor. The Mullards seemed to handle it, but with a tiny amount of plate glow. I changed the resitor to 270 ohms which cured it and didn't seem to change the sound, although it dropped the power output very slightly (not a bad thing if you want to preserve the speakers).

    Make sure that the cathode-bypass cap was changed as well as the fliter caps. It's often overlooked but if it shorts out those Mullards will be history.

    Great amp BTW - and until very recently totally overlooked and undervalued.
     
  3. 94prs22

    94prs22 Member

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    Thanks John, excellent info as always.
    Brandon
     
  4. teleharmonium

    teleharmonium Member

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    I have one of those, it's a favorite. I run it with a stepup transformer and a GZ37 instead of the GZ34 rectifier in order to get a bit lower plate voltage on the EL34s. I have been running Siemens EL34s this way for the last year or so with no problems. I like they way they sound in that amp better than Mullards, which are a little too fat sounding and hard to control in the midrange with my guitars.

    If you play vintage type single coil pickup guitars and don't overdrive the input with external effects I think you will be fine. On the other hand if you play the amp at full blast with lots of low end and heavy distortion I could easily see you blowing those 15 watt speakers. It would sound incredible until they go, mind.

    I have a pair of later silver Celestion Vox alnico speakers, supposedly rated for 25 watts, that I plan to try out in my Zodiac, to give me some insurance against blown speakers hopefully without sacrificing the tone hardly at all compared to the early 15 watters, which are great speakers. Unquiet, do you happen to know what colored wire on the OT would be the 16 ohm tap ? The silver Celestions I have are 8 ohm, so I would be looking at running a total load of 16 instead of the 8 ohm load now with a pair of 16 ohm speakers.
     
  5. John Phillips

    John Phillips Member

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    I can't remember... but the stock setup is for 8 ohms (2 16s in parallel). If your amp hasn't been messed with you should be able to identify which is which.

    If it has been, just meter the DC resistance of the taps (to ground) with the amp off and no speakers connected. Although they will both be close to zero, you should be able to measure one as higher than the other. The high one is the 16-ohm tap.

    BTW, that's a good idea about the GZ37... Siemens (and a lot of other EL34s) don't seem to survive well at 500V. But I have to say I do like the huge sound of the Mullards better anyway... ;)

    I'm not sure the Kaisers used a lot of bass distortion of course... Kaiser Matt's favorite setting was 'treble' and Kaiser George's 'high treble', both with fairly trebly-sounding guitars too. George had a Burns Sonic (with the Tri-Sonic pickups) though which is not a 'quiet' guitar. And no FX of course, there weren't any in 1960...
     
  6. trisonic

    trisonic Member

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    That was my first guitar too in the early sixties - it still exists, in England though the neck joint is getting a little loose. Burns didn't make many fixed neck guitars. Funny looking things really. Still love the P'ups, of course.

    Pete.
     
  7. 94prs22

    94prs22 Member

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    One more thing, pushing 2 buttons at once on this particular amp does very clearly sound different than either alone, for example bass + high treble does not sound like bass alone. I don't know if thats how it's supposed to be or if it just works out that way on this amp. Right now with singles on my tele (vintage 1967 originals) or with humbuckers (SD antiquities) my favorite push button setting is bass or pushbuttons bypassed using the rotary tone control. While I'm at it are the volumes on this amp interactive? There seems to be some midrange fullness added when using one channels volume and turning up the volume on the other channel.
    Thanks,
    Brandon
     
  8. teleharmonium

    teleharmonium Member

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    I haven't noticed the volumes interacting in that way, but maybe John can tell us if that would make sense electronically. The amp sounds great with the channels jumped, that I do know.
    I too like the bass and low bass settings with bright guitars, but I always play through the non selectortone channel unless I am going to be using the tremelo, I think the basic tone is a bit better in that channel. I hadn't tried pressing two selectortone buttons at once, I'll see what it does.

    What kind of tubes are you using in it ?
     
  9. 94prs22

    94prs22 Member

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    When I got it a couple of days ago it had all Russian current production tubes. Since I've had it I've broke out the tube stash and put a Mullard GZ34, a matched pair of Sylvanie 6ca7s (the only NOS EL34s I have are Siemens and I was afraid to put them in until new they could take the amps voltages), a JJ 12ax7 in the oscillator, a pair of Amperex Bugle Boy EF86s and a pair of sylvania 12ax7s in the preamp. I like that GZ37 rectifier idea but just briefly looking around looks like those are pretty rare tubes.
    Brandon
     
  10. 94prs22

    94prs22 Member

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    By the way, the volume interaction seems to be subtle, not like on a fender deluxe or anything like that.
    B
     
  11. teleharmonium

    teleharmonium Member

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    I think you'll find it is an amp that shows the differences between preamp tubes well. I like Tungsram and Mullard 12AX7s in it, I think I have Tungsram EF86 in there as well.
     
  12. Garey

    Garey Member

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    Steve at Angela sells GZ37's for $75....

    I like them...

    Aloha, Garey
     
  13. 94prs22

    94prs22 Member

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    While we're talking about these Zodiac amps if anybody knows the functions of the various tubes I'd appreciate knowing that too. In the top chassis the first 12ax7 looks like it's maybe a gain stage after the instrument inputs and the volume control. I assume that the next 2 tubes, which are EF86s, are being controlled by the respective volume controls and maybe being utilized by the rotary tone controls as well. The last tube in the top chassis is at least the oscillator tube for the tremolo but does this tube contribute any gain or function in any way in the non-tremolo channel. On the bottom chassis the first tube is a 12ax7 which I assume is the phase inverter. One more question, does anybody know what the middle transformer is? On the lower chassis there are from left to right 3 transformers, the output tranny, the middle one, then the power tranny. Is it some sort of a choke? It looks pretty big for a choke.
    Thanks again to any and all who can help.
    Brandon
     
  14. John Phillips

    John Phillips Member

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    Yes, that's basically right.

    The first ECC83 is the input gain stage for both channels; the EF86s are the second gain stage for each channel (not connected with the tone controls though, those are passive and come after the first stage); the one on the tremolo channel is also how the tremolo is applied to the channel, by modulating the screen grid - clever, and only possible with a pentode like the EF86; the other ECC83 is the tremolo oscillator (I think only half of it is used); the EM84 'Magic Eye' (if your amp has it, not all models do) is purely an indicator... but SO cool!

    The middle 'transformer' is the choke. It needs to be large because this amp has the feed to the OT from after the choke, not before as on most amps. This means the choke has to handle a lot more current - the whole current for the power stage as well as the preamp. This was the 'traditional' design for hi-fi etc and is quieter, but requires a more expensive choke. If the amp isn't pure Class A (which has constant current draw so no sag), this position of the choke also causes more sag and reduces available power, but produces a very smooth overdriven sound.
     
  15. 94prs22

    94prs22 Member

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    Thanks again John, this amp does have the flashing Magic Eye and I didn't include it because it looked like it was purely for show. Speaking of the tremolo, isn't this screen bias voltage fluctuation used on power tubes as well in some Fender amps?
    Brandon
     
  16. John Phillips

    John Phillips Member

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    No, they do it by modulating the normal signal-grid bias supply voltage to the power tubes - crude, but it sounds very nice too. The other main Fender method (neon/LDR) is even cruder - it simply partially shorts the audio signal to ground - but nonetheless effective, although it also significantly affects the tone on that channel (which is why you see many people mod their Fenders with a switch to take the tremolo out of the circuit entirely).

    I like the Selmer method because it is an 'elegant' solution - the tremolo circuit is still very isolated from the signal path and doesn't introduce noise, interfere with the power tubes or degrade the tone, but directly controls the gain in that second stage.
     
  17. Robal

    Robal Supporting Member

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    I am reviving this ancient thread because I have a Zodiac Twin 30 and wonder what other tips folks have come up with in the last 12 years to enjoy these great amps. My neighbor Tracy Norton has one as well and we put them side by side and played them once. That was fun.

    Here's my tip: I use a Lehle Dual SGoS switcher to send the guitar signal to both channels of the Zodiac, then I can switch between them or blend them. The variable gain for two outputs on the Lehle makes this particularly effective in fine tuning how hard the preamp stage of the Zodiac is hit when driving both channels. This opened up a whole new range of useful sounds for me, because the amp reacts a lot to how hard you drive the preamp. Pushing down more than one button simultaneously on the Zodiac is less useful to me, by comparison.
     
  18. UsableThought

    UsableThought Supporting Member

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    Just an idea - not sure if it's useful or not -

    I often find myself discovering old threads that I am interested in. However I also notice that forum etiquette (to the extent it exists) frowns on reviving very old threads. Not sure why but maybe because people won't have been following the thread, so it forces them to backtrack & read a lot of old messages. No one enjoys doing that, it seems.

    SO - maybe an alternative for you, if you don't get a lot of responses, would be to post this as a new topic, and just put a link to the old thread in so people can read that if they want to or skip it completely if they don't? Anyway just a thought.
     
  19. Rob s

    Rob s Member

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    Great thread though
     
  20. splatt

    splatt david torn / splattercell Gold Supporting Member

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    @Robal :::

    maybe you should start a new Selmer thread in the "amps & cabs"-section, not here in the amps fix-it section?
     

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