Some tube advice on 12AT7WC-EH vs 12AT7EH

Discussion in 'Amps and Cabs [BG]' started by therealbigsteve, Jan 25, 2015.


  1. ThisBass

    ThisBass

    Aug 29, 2012
    Germany
    V1a consists of a cathode capacitor C1 that bypasses cathode resistor R2. Therefore the initial input stage is not AC feedbacked
    To reduce the gain I'd remove C1 or at least provide a serial R that feeds C1 to ground. May be 2.7k for cathode resistor R2 helps to enlarge headroom capability.

    I'd assume the grid voltage to around -1.2 .. -1.5 Volt. Max. input swing around +/- 0.7 Volt peak to peak at most.

    IMO 1 Volt rms input level will easily exceed the negative grid voltage and therefore start to distort the signal due to grid current.

    An active electronic is capable to drive signals of +/- 4 Volt peak to peak. I have never seen of standard cathode basis stage that can cope with those strong levels at the grid, independently which tube is stuck.

    The design obviosly intends to yield extended even Harmonics even on smaller input signals due to bypassing R2. R2 reads to 1.5k and very likely the bias point is close at the edge of asymmetrical output biasing.

    IMO the design is very well sophisticated if you look for a bluesy sound, but in my experience a schematic design like that must not sound "clean" with enlarged headroom.

    I'd remove C1 and would try 2.7K for R2.
    Disadvantage will be that different tubes don't hit its characteristics into the overall sound of the amp. The input stage will sound more linear, deminishing a little bit the different characteristics of different tubes.
     
    Last edited: Jan 27, 2015
  2. I really appreciate the advice and the time alot of you spent on trying to help me out, but what's the the difference between the Electro-Harmonix 12AT7WC-EH and the Electro-Harmonix 12AT7EH. Lets pretend my amp came with a 12AT7 in the V1 or V2 spot and these are the last tubes on earth. Which would you suggest?
     
  3. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    In a V1 position, you want low noise so, if I had to choose one, I would go with the short plate. But it isn't that simple. You should try them both and see which one sounds best in you amp. This is because the tubes are slightly different in terms of their specs. The design of the amp will determine how well it copes with these different specs. A different capacitance spec will affect how the tube performs at high frequencies for example. This can add a different shimmer, for the lack of a better term, on the harmonics. This would become more evident when you turn up the treble. If you never do that you might not notice a difference.

    Your ears will tell you which you prefer. But how tubes differ can be very complicated.
     
    therealbigsteve likes this.
  4. teemuk

    teemuk

    Mar 1, 2011
    Well, that in relation to volume pot dial. For actual volume and headroom the swap doesn't help at all. The headroom is still defined by the power tubes and power supply and with an incorrect tube there you only lower open loop gain (and precedingly the power tubes practically make up that loss) and bias the tubes incorrectly because 12AT7 is a totally different tube than 12AX7 and requires a totally different circuit to work optimally. I could estimate that the swap creates more distortion in the PI section, and decrease in open loop gain makes damping weaker and therefore changes frequency response of the power amp. If there are tone controls implemented to the negative feedback loop (e.g. resonance & presence) their operation and range is compromised as well.

    In preamp slots you again lower overall gain and misbias the gain stage to very different operating conditions. Different amounts of gain and different internal impedances will also result to entirely different frequency responses. If a gain stage based on 12AX7 has a low value cathode bypass to accentuate high frequencies swapping in a 12AT7 will negate it due to lower gain and result is darker and muddier tone. Different loadline from misbiasing the tubes will make overdriving characteristics different.

    Although they are pin and heater voltage compatible do acknowledge that 12AX7, 12AU7, 12AT7, etc. are all actually entirely different tubes and not any kind of replacements for each other. Replacement with an incorrect tube type will generally skew the designed operation of the circuit. You may like it, or you may not. But it's not just about higher or lower gain. The swap will affect several other characteristics too.
     
    Last edited: Jan 28, 2015
    ThisBass likes this.
  5. SteveCS

    SteveCS

    Nov 19, 2014
    Hampshire, UK
    The 'W' designates military specification in terms of reliability - shock resistance, temperature/climate change, and so on. The 'Short Plate' reduces the mass of the internal components, creates space for more robust internal fixtures which in turn reduce movement of the internal parts. Stuff can function when being driven down a dirt track and doesn't break when thrown in the back of a jeep. The 'W' does not guarantee anything other than ruggedness. For a touring band, yes. For anyone else, save your money.
     
    therealbigsteve likes this.
  6. Thank you for y'all's input.
     
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  7. BurningSkies

    BurningSkies CRAZY BALDHEAD

    Feb 20, 2005
    Syracuse NY
    Endorsing artist: Dingwall Guitars
    Also, lets take a moment and consider that while W may have designated 'military' it's highly questionable that EH is sourcing tubes for any military organization and/or that it is meaningful in any sense in this day and age.
     
    B-string likes this.
  8. waltdogg

    waltdogg

    May 24, 2014
    I honestly doubt a new production New Sensor tube is actually military grade. Go with the JAN Philips 12AT7WC if you're gonna swap a 12AT7 in.
     
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  9. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    The US military has a stockpile of tubes but they do occasionally reorder tube based equipment.

    I have one of these to keep the room warm when it's cold out.
     
    Last edited: Jan 28, 2015
  10. Anything wrong with EH tubes?
     
  11. SteveCS

    SteveCS

    Nov 19, 2014
    Hampshire, UK
    Huh? Who said that EHX is sourcing tubes for any military organisation? Mil. spec. components have always been available in the consumer market - it is the equipment they are integrated into that is controlled, and the 'W' would have meant that the tube met certain criteria over and above those of the 'stock' 12AX7. Either way, the 'W' still designates high reliability regardless of who the end user is/was/may be.
     
  12. Passinwind

    Passinwind I know nothing. Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
    + a zillion. Of all the common preamp tube types, 12AT7s are probably the the cheapest and easiest to source as NOS. I bought some really nice GEs fairly recently for ~10 bucks apiece. I think the GEs are gone, but they came from this eBay vendor, who has JAN Philips ones for sale currently.
     
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  13. B-string

    B-string Supporting Member

    Nothing more than any other new production tubes. No guarantee of consistency with any remaining new production factories.
     
  14. Passinwind

    Passinwind I know nothing. Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
    Yep, and the "lightly used/NOS" market is a crapshoot too, IME. Buy two, hope one ends up being a keeper.
     
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  15. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    The factories used to perform a higher level of quality control than do now. If you pay them, they will screen the tubes to any level that you specify. Unfortunately, it seems to be more cost effective for resellers to buy the in bulk and screen them on their own. Double unfortunately, some resellers adopt higher standards than others.
     
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  16. B-string

    B-string Supporting Member

    There are old processes that also are skipped these days that brought out much higher failure rates (better stability of end product). Let's face facts though, we are lucky we have new production valves to complain about ;).
     
  17. waltdogg

    waltdogg

    May 24, 2014
    I use Tung-Sol 12AX7s made by New Sensor, so nothing is wrong with the EHXs. But Modern tubes just don't even come close to an NOS mil-spec tube.
     
    BurningSkies likes this.
  18. waltdogg

    waltdogg

    May 24, 2014
    Also just go through tubedepot.com for your NOS JAN Philips 12AT7s. Don't risk getting burned by eBay sellers that are just testing tubes on Heathkits and not going the extra mile like retailers such as Tube Depot, The Tube Store and Doug's Tubes to name a few.
     
    beans-on-toast likes this.
  19. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    I love this image of a dump of rejected tubes. This is what they used to do when the tubes did not meet their standards. Today, unless they are dead, they get passed on to resellers or they rebrand them as some other make or model.

    [​IMG]

     
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  20. B-string

    B-string Supporting Member

    Thumbs up Beans! IIRC there was a Mullard pic out there also? Wish I had a pic of the scrap from Raytheon in CA were a family friend worked.
     
    beans-on-toast likes this.
  21. Primary

    Primary TB Assistant

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