The "Zagray!" goes from prototype to product

Discussion in 'Builder's & Retailer's Forum' started by VacuumVoodoo, Jun 25, 2010.


  1. VacuumVoodoo

    VacuumVoodoo Member

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    I'm still a bit undecided myself, it's why I wrote "about to lock design". off to study the font catalog.
     
  2. fyrwyr

    fyrwyr Member

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    Zagray in Comic Sans font
     
  3. VacuumVoodoo

    VacuumVoodoo Member

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  4. v-verb

    v-verb Member

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    MUCH easier to read - I like it!

    Cheers

    nigel
     
  5. Babaji

    Babaji Member

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    ...Much Mo bettah! What Splatt said...
    I'm looking forward to playing it in Nashville...That is... after I loose my fear of being in the same room with the guys in the photo above!
    Bill
     
  6. tubeinmyhead

    tubeinmyhead Member

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    Who... us?!

    nothing to fear here, nothing at all... just step inside, that's it... close the door behind you, yeah,yeah, that's it... nothing to fear here...

    ... now close your eyes real tight...
     
  7. fyrwyr

    fyrwyr Member

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    Yeah Roger, hold him long enough for me to get away with his vintage Park head!!!:hide2
     
  8. v-verb

    v-verb Member

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    Apologies if I've missed it, but has there been an approx price on this amp?

    Thanks!

    nigel
     
  9. VacuumVoodoo

    VacuumVoodoo Member

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    page 2 post #28 specs & rough price estimate.
    link to youtube clips in my signature.
     
  10. v-verb

    v-verb Member

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    Thanks Alex! Pretty fair IMO

    Cheers!

    nigel
     
  11. voojo

    voojo Gold Supporting Member

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    Sounds good Alex, I'll go with 78787, since I like symmetry and palindromic numbers! :love:
     
  12. VacuumVoodoo

    VacuumVoodoo Member

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    Nice one :banana I wonder if the man who loved only numbers ever presented any conjecture on palindromic primes.

    Here is chassis puzzle coming together, the top cage is next. Box & bottom in 1.5mm steel sheet, front and rear panels in 3mm aluminium (yes, my spell checker is set to Queens english :D)
    Since you are a mechanical engineer working daily with CAD - any comments?

    [​IMG]
     
  13. voojo

    voojo Gold Supporting Member

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    Looks very good Alex! I'm by no means a sheetmetal expert but it looked good to me, so I showed it to a guy in our shop who is a sheetmetal expert and he said it looks great, and said that should be no problem for any competent sheetmetal shop, quite easy as far as chassis go.
     
  14. voojo

    voojo Gold Supporting Member

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    Alex, I'm wondering if you could comment on your choosing of the 7591 Power Tube, and also your thoughts on current production versions.

    I'm just curious as it's not as common of a power tube for guitar amps, though I know that Ampeg and Gibson used the tubes, and probably others.

    It appears that Electro Harmonix and JJ make new versions of this tube.
     
  15. Bobby D

    Bobby D Member

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    i can't wait to hear the demos of this amp from the upcoming show! :bow
     
  16. VacuumVoodoo

    VacuumVoodoo Member

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    Ehrrmm, you coming to Nashville, aren't you? You're more than welcome to drop in and have a go at it. In fact, I'll be really disappointed if we don't get together.
    BTW, did you check the current demos on youtube? click "Zagray" in my signature.

    A dissertation on why 7591 tubes are my darlings will be forthcoming shortly.
     
  17. VacuumVoodoo

    VacuumVoodoo Member

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    7591 was developed rather late in tube history (ok, several years before many of you were born, but still...). Tube designers listened not so much to the amps as to engineers designing the amps.....and then bean counters got in a word or two.
    What were they saying? Probably something like this:

    "Look at any power tube application data, as soon as we change plate supply voltage we need a different plate load (Raa). This means we have to stock either a variety of tubes or several different output transformers or both. Why don't you make a tube that will work optimally with same plate load over a wide supply range?
    Oh, and when you're at it give it steep characteristic i.e. give it high sensitivity. We like high input sensitivity and gain (that's when engineers got to utter one sentence)"

    Voila, they came up with 7591. It's optimum Raa load is 6k6 over 300V to 450V supply giving output power from ca 20W to almost 50W. Your circuit needs to allow 15-25V bias adjustment range and same basic design covers a wide product range.
    This in combination with high sensitivity means you don't need a 100V swing on the control grid so it's easy to drive and easy to overdrive.
    Now, stocking only one OT model for 50W output in production quantities and being able to use it for anything between 20 to 50W was a LOT cheaper than 2 or 3 models with different winding ratios and output power ratings. No wrong OT in an amp coming off the line. A 25W amp with 50W OT - that was HIFI and these amps still sound phenomenal even today. Find an old Scott or Fisher stereo amp, preferably 3 of them and hook them up to run your home cinema surround system.

    Ok, so what do they give a guitar amp that other tubes won't and is that Raa thing important today?
    Well, maybe not the Raa but the high sensitivity combined with high power capability makes it interesting. Imagine an EL84 that you gave input sensitivity of an EF86 and power capability of the 6L6GB with kink less plate characteristics like KT66. This tube looks like result of some kind of inter species genetic engineering, a really inspired design IMO.
    So what does it contribute to guitar amp power stage? We're talking almost unforgiving touch sensitivity with a smooth compression before it breaks up. Ride a well designed PA close to headroom ceiling and you can leave the note hanging in the air till the cows come home...

    Now, is there something they screwed up with the 7591? Yes, big time, and it's what stopped it from becoming a massive hit. The pin-out. THE PIN OUT!!! They didn't make it pin compatible with standard power tetrode/pentode pin out. Bad decision by creative marketing person backed up by a finance guy. A subject for an off topic discussion over a beer or two.

    NOS 7591 supply diminishing quickly with prices going up - what are the choices?
    As mentioned, new production from JJ in Slovakia or EHX Made in Russia. Are they real 7591? They are, almost...

    5 years ago I tested a batch from JJ when they just started making them and they were everything else but not 7591. A heated discussion with JJ bosses took place at Frankfurt Musikmesse in 2006. They have improved since then and are ok now.
    EHX 7591A measure and perform like a 7591 but are in taller bottle than genuine ones were.
    Like any power tube, both JJ & EHX need a good pre-delivery burn-in at full output power into dummy load to weed out early demise, bias drifters and screen grid rattlers. So I'm watching daily electricity price and do the burn-ins when it's cheap.....not really, but it sounded cool while I was writing it.
    Otherwise no complaints, amps will be delivered with a spare pair of new production 7591 for those who order standard 23W version.
    The amp can also be configured with same OT etc but with 6L6GC tubes if it'll make you feel more at ease re tube replacement availability.

    That's it on 7591 tubes. I like other tubes too :)
     
    Last edited: Jul 16, 2010
  18. fyrwyr

    fyrwyr Member

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    That was quite a good lesson in tube history and mechanics!!!

    You are my hero;)
     
  19. voojo

    voojo Gold Supporting Member

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    Thanks for the interesting dissertation Alex! Had I not seen this I would have probably leaned more towards the 6L6 tube with the stock transformers, but now I've become a believer in the 7591 tube!
     
  20. VacuumVoodoo

    VacuumVoodoo Member

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    Guys, this is what it will look like in its cage. Colors aren't real - my rendering SW doesn't have correct textures. For example, control panel will not look like gold-plexi acrylic, it will be very light caffe-latte etc.
    Question: I designed handle bars for 2 handed carrying.
    How important for you is a single handle on top of the cage to allow one handed carrying? Would absence of top carrying handle be a deal breaker? The weight will be around 13-15kg (ca 28-32 lb) depending on some material choices.

    Front view
    [​IMG]

    Birds eye view, front & top
    [​IMG]

    Better view of sun symbol air vents
    [​IMG]
     

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