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Can you use 7189 tubes in place of EL84's all the time?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Chi, May 12, 2005.

  1. Chi

    Chi Member

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    I was lead to beleive that you couldn't from several sources, But I have had a few who have told me you could....does anybody know?
     
  2. Jeff West

    Jeff West Member

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    Chi- The answer to your specific question is yes, you can always consider 7189 a direct sub for EL84.

    However, you might also be interested in the following, some of which can cause confusion:

    EL84/ 6BQ5 and 7189 have identical pinouts, so can substitute 7189 for EL84 w/o problem, but 7189 has 33% higher palte voltage rating, so the converse may not be true in some applications w/o risking damage to the EL84s.

    Also, note that 7189A does not have identical pinout to 7189 (!); there's a lot of overlap, but 7189A has additional internal connections of pins 1 to 2, and 6 to 9, which EL84/6BQ5 lacks. 7189A has still higher plate and screen ratings than 7189, as well. So there are numerous amps (e.g., certain Pilots, Magnatones) calling for 7189As where 7189s and especially EL84s typically would not long survive, and there are amps calling for EL84s that are wired up in a way that would not tolerate shorting pins 1 and 2 and/or 6 and 9 as would occur by insertion of 7189As.

    So if your question had said 7189A insead of 7189, the answer would be "no, not always" instead of yes. That may be what's led to apparent contradictory advice.

    Jeff W.
     
  3. Chi

    Chi Member

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    THANK YOU JEFF!!!!!!!

    One question tho- is there anyway to identify the difference physically between the 7189A and the 7189? Is there a visible difference?
     
  4. Blue Strat

    Blue Strat Member

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    To add further confusion, some NOS EL84s have internal connections to pin 1 as well.
     
  5. Jeff West

    Jeff West Member

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    Hey Mike- You're quite right, in fact I think most if not essentially all EL84/6BQ5s do have pins 1 and 2 internally connected, just like 7891A always is supposed to. But I believe that pins 6 and 9 will typically distinguish the others from 7581As.

    Strictly speaking, most manufacturers seemed to list pins 1, 6, and 8 on EL84/6BQ5/7581 as "IC" not "NC"; IIRC "IC" or internal connection essentially means "don't connect any other external circuitry here, we might connect it to something else internally as we see fit". As opposed to, say pin 6 on 6L6 or EL34, which is true NC and may be MIA entirely!

    I was just checking a few EL84s kind of at random to refresh myself on this; 12/60, 4/69 and 3/74 USA RCA specimens all have pins 1 and 2 connected right at the base, with 6 and 8 each totally unconnected and free standing. '62 and '71 USA Sylvanias are the same in this regard. Dutch Amperex rX3s from 6/59, rX4s from 7/64 have pins 1 and 2 connected to opposite sides of the control grid support rods (so directly connected), pin 6 goes up to the bottom mica (for structural support during manufacture, maybe? or drainage of any developing leakage paths?, no other connection to metallic element), and pin 8 is free standing. '59 USA RCA and '62 GEs are also exactly like this. 12/58 Blackburn Mullard rX1s have 1&2 connected directly at the button, 6 goes up to the mica like other Philips, and 8 is totally unconnected. I quit at that point. I've heard that there are GE 6BQ5s that have pin 8 connected to an internal element, but I haven't found one offhand. Haven't checked a lot of 7189s for specifics, and I'm also outta touch w/modern production Russian, etc. that call themselves EL84s . . .

    Since you can generally see exactly where the pin connections go on these unless the glass is dirty, I guess the simplest answer to Chi's last question would be to look at pin 6 and if its not connected to pin 9 or a rod to beam plate/equiv screen grid it's likely not a 7189A.

    Jeff
     
  6. Jeff West

    Jeff West Member

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    As a postscript, I looked at a bunch of ~'67-'69 ("W" and "Z" date codes) GE 6BQ5s and 7189As. Those designations with "USA" and the dot patterns are etched on the glass in all cases..

    Internally, inc plates, grid rods, micas these all look virtually identical except that the 7189As have halos that are more canted. All these of both types have pins 1&2 connected to opposite sides of the control grid rods. The one difference is that the 7189As do have pin 6 connected to one of the screen grid support rods (pin 9 connects to the rod on the other side). On the 6BQ5s, pin 6 ends abruptly above the button, NC, just like pin 8 on both. So at least in this time frame with this brand, that's how you could tell 'em apart.

    One other observation: although both types have identical-looking support rods in the suppressor grid position, connected to pin 3 and the cathode, in the 6BQ5s I think I can see a single loop of silver grid wire visible through small cutouts at the very top and bottom of the plates that is not observable at least by my eyes in the 7189As. So I suspect the latter are beam type construction rather than true pentodes, just like you usually hear about 7189As.

    Jeff W.
     

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